APPLICATION GUIDELINES. Overvoltage protection Metal-oxide surge arresters in medium-voltage systems

Transcription

1 APPLICATION GUIDELINES Overvoltage protection Metal-oxide surge arresters in medium-voltage systems

2 First published November nd revised edition: September rd revised edition: May th revised and expanded edition: February th revised edition: May th revised edition: June 2018 All rights reserved. Neither the complete brochure nor parts of it are to be copied, reproduced, transmitted in any way or translated into other languages without the express written consent of ABB Switzerland Ltd. ABB Switzerland Ltd. Surge Arresters Wettingen, Switzerland

3 FOREWORD 3 Foreword to the Sixth Edition The first edition of our guidelines for the dimensioning, testing and application of metal-oxide surge arresters (MO surge arresters) for use in medium-voltage systems appeared in A number of developments in technology and application of MO surge arresters as well as in standardization have taken place in the past years. The standards produced by TC 37 of IEC have undergone radical changes, based on recent research work initialized and supervised by Cigré working groups WG A3.17 and WG A3.25 of SC A3 High Voltage Equipment. Edition 3.0 of IEC contains important changes to the definitions and test requirements of the energy-handling capability. All related standards of the IEC series were adapted accordingly, and new standards were published. Consequently, it was necessary to completely revise the selection principles and application recommendations. In principle, this revised brochure keeps the concept of the previous editions. The design, function and application of MO surge arresters are described, taking into consideration the new definitions and test procedures. Some chapters have been shortened and are more concentrated, for better readability. Additional and more detailed information will be given in separate documents with respect to theory, background information, and specific applications. We hope that you as a reader will be satisfied with the new appearance of our new edition, and that you will find it useful for your purposes. We welcome amendments and suggestions that help us to better understand and meet all possible customer needs. Further, we would like to thank everyone who contributed their valuable comments on this brochure. Bernhard Richter ABB Switzerland Ltd. Surge Arresters Wettingen, May 2018

4 4 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION Table of contents Foreword to the Sixth Edition 3 1 Introduction 6 2 Surge arrester technology General Arrester design Metal-oxide resistors High-field MO resistors and GIS arresters Influence of different frequencies and DC voltage on MO resistors Different frequencies DC voltage Microvaristors and field grading 15 3 Function and performance of MO surge arresters General Currents and voltages Charge transfer and energy absorption capability Cool-down time Stability of an MO surge arrester Thermal stability Long-term stability Protective characteristics Temporary overvoltage 22 4 Service conditions Normal service conditions Special service conditions Overload behavior Mechanical stability Elevated ambient temperature Pollution and cleaning Altitude adjustment of the arrester housing 25 5 Tests General Type tests (design tests) Routine tests Acceptance tests Special tests Commissioning and on-site tests 30

5 TABLE OF CONTENTS 5 6 Neutral earthing methods and determination of U c General considerations Systems with insulated star point or with earth fault compensation Systems with high-ohmic insulated neutral and automatic earth fault clearing Systems with direct or low-ohmic star point earthing Systems with direct star point earthing Systems with low-ohmic star point earthing Four-wire, multi-earthed-wye systems Distribution systems with delta connection Arresters between phases Six-arrester arrangement Neptune design Operating voltage with harmonic oscillation 36 7 Coordination of insulation and selection of MO surge arresters General considerations Selection of nominal discharge current, charge and energy Protection level Selection of arrester housing 39 8 Protective distance of MO surge arresters General considerations Traveling waves Protective distance Induced voltages 43 9 Equipment protection General considerations Protection of transformers Protection of cables Cable sheath protection Arresters in metal-enclosed medium-voltage substations (cubicles) Generator connected to a lightning-endangered MV line Protection of motors Arresters parallel to a capacitor bank Line traps (parallel protection) Line arresters MO surge arresters in parallel connection General considerations Parallel connections to increase the energy handling capability Coordination of parallel-connected MO surge arresters Accessories Spark prevention unit Disconnectors Indicators Brackets, ground plates and clamping devices Monitoring of MO surge arresters Overload and failure analysis Summary and developments 55 Acronyms/Abbreviations 56 Literature 57

6 6 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 1 Introduction ABB in Switzerland produces MO surge arresters for the protection of equipment against transient overvoltages. Decades of experience in design and development gives the competence for specific solutions. Overvoltages in electrical supply systems result from the effects of lightning incidents and switching actions and cannot be avoided. They endanger the electrical equipment because, for economic reasons, the insulation cannot be designed to withstand all possible cases. An economical and safe on-line system calls for extensive protection of the electrical equipment against unacceptable overvoltage stresses. This applies generally to high-voltage systems as well as to medium- and low-voltage systems. Overvoltage protection can be basically achieved in two ways: Avoiding lightning overvoltage at the point of origin, such as through shielding wires in front of the substation that intercept lightning. Limiting overvoltage near the electrical equipment, for instance through surge arresters in the vicinity of the electrical equipment. In high-voltage systems, both methods of protection are common. Shielding wire protection of overhead lines in medium-voltage systems is not generally used. The most effective protection against overvoltages in mediumvoltage systems is therefore the use of surge arresters in the vicinity of the electrical equipment. A surge arrester is a protective device for limiting surge voltages on equipment by diverting surge current and returning the device to its original status. A surge arrester is capable of repeating these functions a large number of times as specified. Today s technology for surge arresters intended for use in medium-voltage systems is the gapless metal-oxide surge arrester (MO surge arrester) with a synthetic housing. Therefore, this brochure concentrates only on MO surge arresters without gaps with silicone housing, as developed and produced by ABB in Switzerland. ABB in Switzerland has concentrated all surge arrester activities under one roof in Wettingen. This ensures that all steps in development and production, from raw material qualification up to shipment of the final product, are under the same management and quality control.

7 INTRODUCTION 7 Close cooperation with ABB s corporate research center, which is located a short distance from the surge arrester factory, ensures that the current state of the art is considered in material technology and processing of MO resistors and surge arresters. This, and decades of experience in the design and development of MO surge arresters gives the competence for specific solutions and applications in overvoltage protection. Due to the variety of MO resistors and surge arresters produced, applications in AC and DC power systems, e.g. traction systems and high- voltage direct current (HVDC) systems are possible with products adapted to the system and environmental requirements. In this brochure, the basics of MO surge arrester technology are described, covering the function and performance of MO material and MO surge arresters. Service conditions and tests according to the current international standard IEC , Ed. 3.0 are listed and briefly explained. Then follows a section on neutral earthing methods in medium-voltage power systems, which is important for the selection of the power frequency voltages that can be applied to the MO surge arresters. Installation principles and the protective distance of MO surge arresters is addressed, followed by a more detailed section describing the protection of various equipment in mediumvoltage systems. Special applications, like parallel connection of MO resistors and surge arresters, and coordination of surge arresters, are addressed in a separate chapter. Accessories like disconnectors and indicators, etc., are mentioned. Finally some remarks are made on the overload performance of MO surge arresters and failure analysis. An overview of ongoing developments in MO surge arrester technology and standardization closes this brochure. Acronyms, abbreviations and a list of literature are given at the end. In the large number of publications on MO resistors and MO surge arresters, different terms are used for basically the same object: ZnO varistor, ZnO resistor, MO varistor, MO resistor, varistor, ZnO or MO arrester, MO surge arrester, etc. This has historical reasons, and also depends on the technical community or the kind of research and development performed. In this brochure, the technical terms MO resistor and MO surge arrester are mainly used, following the wording in the international standards of IEC TC 37, which are directly related to the subjects of this brochure.

8 8 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 2 Surge arrester technology Based on experience in surge arrester design and application from the very beginning of arrester technology, ABB in Switzerland produces today MO surge arresters, MO resistors and microvaristors for various applications in overvoltage protection and field grading. 2.1 General ABB in Switzerland has a long history in MO technology and surge arrester design. In the 1980s, BBC (now ABB) started producing MO resistors and gapless MO surge arresters. In 1986, the first MO surge arresters with patented direct molding for medium-voltage systems were delivered. In the same year, the first gapless MO surge arrester for SF 6 gas insulated substations (GIS) came to market. Since the beginning, continuous development of products and process technology has taken place. High-field MO resistors have been developed, along with MO resistors for DC applications and microvaristors for field grading applications, to name a few. Applications in specific fields, like traction systems, power electronics and wind power parks, are possible with products adapted to the specific requirements. 2.2 Arrester design Generally, an MO surge arrester is made up of two parts: the active part, consisting of one or more piled up MO resistors, and an insulating housing, which guarantees both the insulation and the mechanical strength. Fundamentally, there are three different possibilities for construction: The active part is held mechanically together with glass-fiber reinforced loops or straps. The polymeric material (such as silicone) is directly molded on to the MO resistors. This direct molding has the advantage that no gas volume remains in the arrester. Sealing problems and inner partial discharges are thus out of the question. There are no interfaces between the polymeric materials into which humidity can penetrate. The danger of violent shattering of the housing is negligible. MO surge arresters designed according this principle belong to Group I, see Figure 1a. The active part is wrapped with glass-fiber material and is soaked with resin, which turns the whole into a rigid body. The insulating polymeric housing is then slipped over the resin block or shrunk onto it. This construction has the disadvantage that it forcibly breaks apart when the MO blocks are overloaded. Another disadvantage is the fact that there are different insulating materials, which also means that there are more boundary layers. Therefore, it is necessary to take special measures for sealing. This principle of design belongs in Group II, see Figure 1b. In a glass-fiber reinforced tube made of synthetic material, which is covered with an insulating polymeric material, the active part is installed, similarly to insulators made of porcelain. These hollow insulators have the same disadvantages as the porcelain insulators: they need a sealing and pressure relief system and they can have internal partial discharges. This is considered Group III, see Figure 1c. Silicone rubber (usually simply referred to as silicone ) is an excellent insulating material for high-voltage insulators. In high-voltage technologies, silicone has been successfully used for about 50 years for long rod insulators and bushings, for example. The first MO surge arresters with the typical ABB direct molding were used in Millions of these arresters have been, and are still being, used trouble-free all over the world and under all climate conditions. The basic Si-O-Si-O matrix with additional CH 3 - groups (methyl) is characteristic of silicone. The filling materials and special additives cause the arcs and creep resistance necessary for use in high-voltage technology. The qualities of silicone include very high elasticity and resistance to tearing, high temperature stability, very low combustibility (silicone is a self-extinguishing material) and high electrical disruptive strength. Besides all these qualities, the most remarkable one is hydrophobicity: water simply rolls off the silicone surface. Silicone insulators are water-repellent even if they are polluted. This means that the hydrophobicity is also transmitted into the

9 SURGE ARRESTER TECHNOLOGY 9 Figure 1: Design principles for MO surge arresters: a) Group I, b) Group II, c) Group III. For explanation see text above. Figure 2: POLIM-D type MO surge arrester (design Group I). Left: active part before molding. Middle: schematic design. Right: complete arrester. Figure 3: Range of MO surge arresters developed and produced by ABB in Switzerland. pollution layer on the surface. All this provides excellent performance properties for high-voltage equipment insulated with silicone. The hydrophobicity of the silicone can be diminished under the influence of a long period of humidity or electrical discharges on the surface; it is however completely restored in a short period of time (from a couple of hours to a couple of days). As much as we can say today this mechanism works for unlimited time. All MO surge arresters produced by ABB in Switzer land used in medium-voltage systems are designed according to the same principle. This construction concept of silicone direct molding, which was patented by ABB, consists of two electrodes connected together through two or more glass-fiber reinforced elements. It results in a stiff cage or frame, which guarantees the mechanical strength. The MO resistors are arranged within this frame. Additional metal cylinders with the same diameter as the MO resistors fill the inside completely, forming a uniformly round active part. The MO resistors are pressed together with a bolt in the center of the lower electrode; the bolt is secured in the end position, thereby providing each arrester with the same contact pressure. The active part is placed into a mold and completely sealed with silicone. As a result, the surge arrester, which is completely sealed and tight, has no internal void. Figure 2 shows an MO surge arrester of the POLIM-D type manufactured according to this technique. It is shown before and after being molded in silicone. The flexible method of construction (modular concept) makes it possible to change the form of the arrester to meet any necessity. The demands on the arresters depend on the operational conditions and the type of the electrical equipment to be protected. Figure 3 gives an overview of the variety of MO surge arresters developed and produced by ABB in Switzerland, covering station and distribution types intended for use in medium-voltage systems, as well as arresters for application in traction systems (AC and DC) and for special applications. Figure 1 a) Figure 1 b) Figure 1 c) Figure 2 Figure 3

10 10 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION The MO resistor stack of the surge arrester behaves in an almost pure capacitive manner with applied continuous operating voltage U c. The stray capacitance of each resistor against the earth causes an uneven voltage distribution along the arrester axis under applied U c. This unevenness increases with the length of the resistor stack. High-voltage MO surge arresters therefore need grading elements, such as grading rings, which mostly compensate the unfavorable influence of the stray capacitance. The resistor stack with medium-voltage arresters is, however, so short that the uneven voltage distribution can be neglected. Therefore, medium-voltage arresters do not require any grading elements. As a rule, the mechanical loads are low with medium-voltage arresters. All ABB medium-voltage arresters can be installed in regions where earthquakes occur. Horizontal and hanging installation is possible. If the arresters have to bear additional mechanical loads, besides their own weight and the normal wind and ice loads, that exceed the guarantee data, then the manufacturer should be contacted. 2.3 Metal-oxide resistors MO resistors are made of different metal-oxides in powder form, which are compressed and sintered in the form of round blocks. Figure 4 shows the principle of the manufacturing process. The diameter of the MO resistors produced by ABB in Switzerland lie between 38 mm and 108 mm. The height of the MO resistor blocks is typically between 23 mm and 46 mm. For special applications, the MO resistors can be sliced to a height as small as 0.8 mm. The diameter of the MO resistors determines the current; the height of the MO resistors (or resistor stack) determines the voltage in continuous operation and the volume of the blocks determines the energy handling capability and charge transfer capability. Figure 4: Manufacturing process of MO resistors 1 Mixing of the metal-oxide powders 2 Spray-drying of the powder mixture 3 Pressing of the MO resistors 4 Sintering 8 MO resistors ready to be installed in the arrester 7 Final tests of the MO resistors 6 Laser cleaning, activation and metallizing 5 Coating the surface passivation with glass

11 SURGE ARRESTER TECHNOLOGY 11 The lateral surface of the MO resistors is passivated with glass, the contact areas are laser cleaned and activated before metallizing with soft aluminum. The metallization reaches up to the edge of the MO resistors. In this way, the MO material of the MO resistors produced by ABB in Switzerland is completely covered. Figure 5 shows a selection of MO resistors. Figure 6 shows in an enlarged form the inner structure of the MO material. It is absolutely necessary to obtain a very homogeneous structure of the material in order to achieve a high specific energy handling capability for the MO resistor. The energy handling capability of an MO resistor and of an MO surge arrester respectively, depend on the volume of the active part, the design (heat transfer) and the electrical dimensioning. Metal-oxide resistors have an extreme non-linear voltage-current characteristic, which is described as I = k U α α is variable between α 5 and α 50. k is a material depending factor. Figure 5: Range of MO resistors produced by ABB in Switzerland. An exact value for α can only be provided for a very restricted range of the current in the characteristic curve. Figure 6: Surface electron microscope image of the MO structure. Fracture surface, enlarged The MO grains and the boundaries between the single grains can clearly be seen.

12 12 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION The typical U-I characteristic of such an MO resistor (or MO surge arrester) is shown in Figure 7. Some important terms are explained below. Region A describes the part of the U-I characteristic curve relevant to the power frequency voltage. It is also considered to be the pre-breakdown or low-current region. The continuous operating voltage U c is the power frequency voltage that can be applied to the MO surge arrester (or MO resistor) continuously without any restrictions. The current flowing through the MO surge arrester is the leakage current i c, which is almost purely capacitive (see Figure 8). The power losses at U c can be neglected, assuming standard ambient conditions and the correct choice of arrester. The rated voltage U r is the voltage value that is applied for t = 10s in the operating duty test in order to simulate a temporary overvoltage in the system. The relationship between the rated voltage and the continuous operating voltage is generally U r /U c = This is understood as a given fact, but it is not defined anywhere. Other ratios are possible. The rated voltage has no other importance, although it is often used in type designations or when choosing an arrester. MO resistors or MO surge arresters at the measurement of the reference voltage is negligible. Therefore, the reference voltages, which are measured at single MO resistors, can be added to give the reference voltage of the entire arrester. The measurement of the reference voltage is a routine test for each MO resistor and each MO surge arrester produced by ABB in Switzerland. The measurement of the reference voltage U ref at i ref, and the residual voltage U pl at I n ensures a control of the U-I characteristic of each MO resistor. It is important to note that in Region A, the resistive part of the current, and therefore the power losses, depend strongly on the temperature of the MO resistors. Due to the negative temperature coefficient in this region, there is a strong increase in power losses with increasing temperature. This may be critical for the thermal stability of the MO surge arrester in service, and it has to be considered in the relevant type tests, as well as in applications at elevated ambient temperatures. The reference current i ref is the peak value of the resistive component of a power frequency current, and is chosen by the manufacturer. Usually, the same current density is used for all MO resistors in production. The reference voltage U ref is the peak value of the power frequency voltage divided by 2, which is applied to the arrester to obtain the reference current (see Figure 9). Because of the dominant ohmic component of the reference current, the influence of stray capacitances of the Region B is the breakdown region. It is the part of the U-I curve in which even minimal voltage increases lead to a significant rise in the current. Only transient events in the time range of milli- and microseconds (switching overvoltages) can be handled by the arrester. A continuous application of power frequency voltage in this area of the characteristic would destroy the arrester in a fraction of a second. Figure 7: Non-linear voltage- current characteristic of an MO resistor (principle) A: Region relevant to power frequency voltage. B: Region with the highest non-linearity. C: Region describing the protection characteristic. U U pl A B C α 5 I = k xu α with α 50 α 5 U U ref r Uc i iref In c log I

13 SURGE ARRESTER TECHNOLOGY 13 Figure 8: Continuous operating voltage U c and leakage current i c of an MO surge arrester. Figure 9: Reference current i ref and reference voltage U ref. Figure 10: Nominal discharge current I n = 10 ka and residual voltage U pl. Region C is the area of currents greater than about 100 A, and it describes the protective characteristic of the MO surge arrester. It is considered to be the high-current region. The most important parameter is the lightning impulse protective level U pl. This is the maximum permissible peak voltage on the terminals of an arrester subjected to the nominal discharge current I n. The amplitude of the nominal discharge current I n, with a wave shape of 8/20 µs, together with the arrester class prescribe the test parameters, see also section 3.3, Table 2. Figure 10 shows as an example the nominal discharge current I n and the residual voltage U pl of an MO resistor. It needs to be mentioned that in Region C we have a positive temperature coefficient. The influence of the temperature on the residual voltage of the MO resistors is in the range of only a few percent and can be neglected in standard applications. 2.4 High-field MO resistors and GIS arresters The field strength (voltage per unit height of the MO resistor) is generally in the range of 2 kv/cm at a given current i B in the breakdown range, considered to be the normal field strength. The field strength of MO resistors is determined by the number of boundary layers per unit height. By increasing the number of boundary layers, i.e. reducing the size of the grains in a given MO resistor, the field strength can be increased up to 4 kv/cm, considered to be high-field (HF). ABB in Switzerland developed a specific recipe for high-field MO resistors that provides lower power losses, especially at higher temperatures. This opens advantages in the design of MO surge arresters with high-field MO resistors. In arrester designs with SF 6 gas as insulating medium (GIS arresters) the use of high-field MO resistors can bring really big advantages. As the MO resistor stack can be reduced by up to 50 percent, the size of the vessel can be reduced accordingly. This reduces the volume of the vessel and the amount of SF 6 gas needed. Further, high-field MO resistors can find their application in liquid-immersed arresters and in arresters with solid insulation, e.g. completely encapsulated arresters. High-field MO resistors for DC applications are also available. The use of high-field MO resistors in standard applications, e.g. air-insulated MO surge arresters (AIS), brings little or no benefit, because the height of such a surge arrester is given by the external flashover withstand capability of the housing., Figure 8 Figure 9 i u U c i, u U ref i c i ref t ms t ms Figure u [kv] i [ka] t µs

14 14 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION Figure 11 shows the relation between an MO resistor with normal field strength and a high-field MO resistor, developed and produced by ABB in Switzerland, as used in GIS arresters. The highfield MO resistor has the same rated voltage of 8.8 kv as the normal-field MO resistor. The diameter of the high-field MO resistor is, at 108 mm, the same as for the MO resistors with standard field strength. The height of the MO resistor with normal field strength is 46 mm, the one for the highfield MO resistor is 24 mm. ABB in Switzerland has developed and produces SF 6 gas-insulated (GIS) MO surge arresters for all transmission system voltages. Figure 12 illustrates the reduction in volume of the arrester vessel if high-field MO resistors are used instead of MO resistors with standard field strength. It has to be understood that doubling the field strength means doubling the energy under a given current impulse, and consequently the temperature rise. Therefore, the increase in field strength means that the energy absorption capability, thermal stability and voltage withstand capability is decreased. These disadvantages can be technically covered by increasing the diameter of the MO resistors or by the use of heat sinks in an arrester design. As mentioned above, ABB in Switzerland developed a specific recipe for highfield MO resistors that avoids such drawbacks in the design of MO surge arresters for gas insulated substations (GIS). 2.5 Influence of different frequencies and DC voltage on MO resistors Different frequencies Beside the system frequency of f = 50 Hz and f = 60 Hz, the railway frequency of f = 16.7 Hz also has technical importance. MO surge arresters without spark-gaps can be used without any problem with these frequencies. It is to be noted that the continuous current i c will change with the frequency, because the MO surge arrester behaves in an almost purely capacitive manner considering the continuous operating voltage. Because of 1 X C = ffff ff ω C X c = capacitive impedance ω = 2 π f = angular frequency C = capacity of the MO surge arrester the capacitive impedance becomes smaller with increased frequency, and consequently the capacitive current increases with increasing frequency. Table 1 shows typical values as examples. Table 1: Power losses P v and continuous current i c for an MO surge arrester of class SL with U c = 20 kv. Frequency f in Hz Power losses P v in W Continuous current i c in ma, rms The dimensioning and application of MO surge arresters for railway systems with f = 50 Hz and f = 16.7 Hz is precisely described in a separate brochure. The manufacturer must be contacted if the MO resistors or arresters are to be used for frequencies rated higher than 60 Hz. A special case are test transformers and resonance circuits with 450 Hz, which are sometimes used for on-site insulation tests. In this case the capacitive current of the arrester is approximately nine times higher than with 50 Hz DC voltage In principle, in DC voltage systems, there also appear overvoltages produced by lightning or switching activities, which may endanger the equipment and the insulation. In this case, it is also necessary to use an arrester as protection against overvoltages. MO surge arresters without spark gaps are particularly suitable, because they do not conduct any follow current after the limiting of the overvoltage, except a leakage current of a few μa, and therefore it is not necessary to extinguish any DC current arc. Figure 11: MO resistor with normal field strength (left, field strength approximately 2 kv/cm) and a highfield MO resistor with approximately 4 kv/cm field strength (right). It is to be observed that only MO resistors with proved DC long-term stability are to be used for MO surge arresters in DC voltage systems (see section 3.5.2). It goes without saying that all the type tests using continuous voltage should be performed with DC voltage. Typical DC voltage stresses are to be found in the high-voltage DC transmission (HVDC). The various voltage stresses in HVDC systems and the relevant tests are given in the standard

15 SURGE ARRESTER TECHNOLOGY 15 IEC , Ed. 1.0, Surge arresters Part 9: Metal-oxide surge arresters without gaps for HVDC converter stations, published in June DC voltage systems are broadly used for traction systems. The nominal voltages in the public DC traction systems lie between U n = 600 V (urban traction systems) and U n = V (long-distance trains). It is necessary to observe both the high electrical requirements for MO surge arresters in the traction systems, as well as the mechanical and safety-relevant requirements. Additional DC voltage applications are to be found in converter stations, drives and photovoltaic systems. It is absolutely necessary to get into contact with the manufacturer if MO surge arresters are to be used in such installations. 2.6 Microvaristors and field grading Microvaristors (µvar) are small spherical particles that behave like a varistor (see Figure 13). The materials of the microvaristors and the production process are similar to the materials and production of MO resistors. Microvaristors are used in polymeric materials, e.g. silicone, for field grading purposes. Typical applications can be in HV and MV terminations, long rod insulators for AC and DC, grading tape for stator windings, or semi-conducting varnish. The advantage of microvaristor filled polymers (compounds) is that the non-linear characteristic of the compound is given by the U-I characteristic of the microvaristors and not by the filling grade only, as is the case with other functional filler materials, like e.g. carbon black. This makes it possible to adjust the properties of the compound to the required applications. The large variety of the field strength of the compound allows tailor-made solutions for different products. ABB in Switzerland produces microvaristors for the various applications mentioned and consults on the development and production of field grading products. Figure 13: Photograph of sintered microvaristors determined by electron microscopy. The diameter of the microvaristors is in the range of 20 to 150 µm. Figure 12: GIS arresters. The arresters have the same ratings. Left: design with MO resistors with standard field strength. Right: with MO resistors with high-field strength.

16 16 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION

17 ARTICLE OR CHAPTER TITLE 17 Metal-oxide resistors at the heart of modern surge arresters.

18 18 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 3 Function and performance of MO surge arresters MO surge arresters are devices that protect electrical equipment and installations by limiting surge voltages and diverting surge currents to earth. 3.1 General The function of a surge arrester with an active part consisting of a series connection of MO resistors is very simple. In the event of a voltage increase at the arrester s terminals, the current rises according to the characteristic curve (see Figure 14) continually and without delay, which means that the arrester skips over to the conducting condition. After the overvoltage subsides, the current becomes smaller according to the characteristic curve. The subsequent current after the MO surge arrester protected is an almost pure capacitive leakage current i c of about 1 ma. I n is the nominal discharge current, and U pl is the lightning impulse protection level of the surge arrester. It is defined as the maximum voltage between the terminals of the surge arrester during the flow of I n. The following paragraph shows, and briefly explains, typical current and voltage waveforms in the high-current region (protection characteristics) of the characteristic curve. For the lowcurrent region, see Figure 7 in section Currents and voltages Residual voltage U res Peak value of voltage that appears between the arrester terminals during the passage of discharge current. The residual voltage of an MO resistor or MO surge arrester is determined with surges having different wave forms and current heights, and it is given in tables or as a voltage-current characteristic on a curve (see Figure 14). The measurements are generally performed on MO resistors. As the measurement is mostly performed in regions of the characteristic where the ohmic part of the current is dominant, the capacitive stray influences can be ignored. The residual voltages measured on single MO resistors can be summed as the residual voltages of the whole arrester. High current impulse I hc Peak value of discharge current having a 4/10 μs impulse shape. The high current impulse represents not only an energetic stress, but also a dielectric one, taking into consideration the high residual voltage that occurs with a high current impulse with a peak value of 100 ka. However, it is necessary to strongly emphasize that a high current impulse with an amplitude of 100 ka is not the same as a real lightning current of the same amplitude. The real lightning current of this amplitude measured during a thunderstorm lasts longer than several hundred microseconds. Such strong lightning currents and impulse shapes are very rare and appear only under special conditions, such as during winter lightning in hilly coastal areas. Switching current impulse I sw Peak value of discharge current having a virtual front time greater than 30 μs and less than 100 μs, and a virtual time to half-value on the tail of roughly twice the virtual front time. The switching current impulses are used to determine the voltage-current characteristic. The current amplitudes lie between 500 A and 2 ka for station class arresters, and roughly reproduce the load of an arrester caused by overvoltages due to circuit breaker operation. Steep current impulse Current impulse with a virtual front time of 1 μs and a virtual time to half-value on the tail not longer than 20 μs. The steep current impulses are used to determine the voltage-current characteristic. They have amplitudes up to 20 ka and roughly reproduce steep current impulses like those which may appear with disconnector operation, re-striking, back flashovers, and vacuum circuit breakers. All the current impulses described above (except the high current impulse) are used to determine the voltage-current characteristic of an MO surge arrester. It is to be considered that only the virtual front time and the amplitude of the current impulses are decisive for

19 FUNCTION AND PERFORMANCE OF MO SURGE ARRESTERS 19 Figure 14: Voltagecurrent characteristic of an MO surge arrester with I n = 10 ka, type SL. The voltage is normalized to the residual voltage of the arrester at I n. The values are given as peak values for the voltage (linear scale) and the current (logarithmic scale). Shown are typical values. Figure 15: Long-duration current impulse I ld = 506 A with a virtual duration of the current of t 90% = 2.15 ms. The residual voltage is U res 10.8 kv. Figure 14 the residual voltage and not the virtual time to half-value on the tail. That is the reason why the tolerance for the virtual front times is very tight, and contrastingly, the virtual times to half-value on the tail are very broad. Long-duration current impulse I ld Also called rectangular wave (I rw ) or square wave, a long-duration current impulse is a rectangular impulse that rises rapidly to its peak value and remains constant for a specified period of time before it falls rapidly to zero. The length of the current pulse duration is correlated to the line length in transmission and distribution systems. Rectangular impulses are used in laboratories during type tests. The current amplitudes are typically up to 2 ka and reproduce the load of an arrester when a charged transmission line discharges into the arrester in case of an overvoltage occurrence. See Figure 15 for an example of a rectangular current impulse with a virtual time duration of 2.15 ms. For comparison of different MO surge arresters, it is regarded as a matter of course to use a rectangular wave of 2 ms duration, although there is no norm established for doing so. Specified is either the amplitude of the rectangular wave for a specific MO surge arrester, or the energy transferred into the arrester during the flow of the rectangular current. 3.3 Charge transfer and energy absorption capability With Ed.3.0 of IEC , a new concept of arrester classification and energy withstand testing was introduced: the line discharge classification was replaced with a classification based on repetitive charge transfer rating (Q rs ), as well as on thermal energy rating (W th ) for station class and thermal charge transfer rating (Q th ) for distribution class arresters. Station and distribution class arresters are classified as indicated in Table 2. The letters H, M and L in the designation stand for high, medium and low duty, respectively. In medium-voltage systems, distribution arresters are mainly used. For specific applications, where higher energy requirements apply, such as protection of cables, rotating machines or capacitor banks and other important equipment, station class arresters may also be needed for medium-voltage systems. The repetitive charge transfer rating Q rs is defined as the maximum specified charge transfer capability of an arrester, in the form of a single event or groups of surges that may be transferred through the arrester without causing mechanical failure or unacceptable electrical degradation to the MO resistors. This rating is verified in a type test on single MO resistors in open air and, therefore, is an MO resistor-related material test. 1,5 pl U/U 4/10 µs 1,0 1/9 µs 8/20 µs 30/60 µs 0,5 U U U ref r c AC DC i I I ref sw n Figure u [kv] i [A] I A ,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 t ms 0

20 20 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION Table 2: MO surge arresters made by ABB in Switzerland, classification according to IEC , Ed Arrester class Designation Station SH Station SM Station SL ABB Type (choice) POLIM-H..N POLIM-S..N POLIM-I..N MWK Station SL POLIM-K Distribution DH POLIM-D Nominal discharge current I n 20 ka 10 ka 10 ka 10 ka 10 ka Switching impulse discharge current 2 ka 1 ka 0.5 ka 0.5 ka Q rs (C) W th (kj/kv rated) Q th (C) 1.1 The thermal energy rating W th is the maximum specified energy, given in kj/kv of U r that may be injected into an arrester or arrester section within three minutes in a thermal recovery test without causing a thermal runaway. This rating is verified by the operating duty test for station class arresters. This test is a thermal stability test for MO surge arresters of classes SH, SM and SL. The thermal charge transfer rating Q th is the maximum specified charge that may be transferred through the arrester or arrester section within one minute in a thermal recovery test without causing a thermal runaway. This rating is verified by the operating duty test for distribution class arresters. This test is a thermal stability test for MO surge arresters of classes DH, DM and DL. The operating duty tests are performed on thermally prorated sections representing the arrester being modelled. The purpose of this test is to verify the arrester s ability to thermally recover after injection of the rated thermal energy W th or transfer of the rated thermal charge Q th under applied temporary overvoltage and following continuous operating voltage conditions. 3.4 Cool-down time The arresters in the system can work reliably and safely if their energy absorption or charge transfer capability is greater than the energy strain expected in the system operation. In case of multiple surges, one after another, the injected energy accumulates in the arrester, and therefore an intermediary cool-down time can be ignored. But if the energy reaches the guaranteed value, which is applied in the operating duty test, the arrester must have enough time to cool down. The necessary cool-down time for the arrester depends on the construction, the ambient temperature and the applied voltage. The cool-down time typically lies between 45 and 60 minutes, depending on the arrester type and the ambient conditions. 3.5 Stability of a MO surge arrester There are two situations to take into account: the thermal stability of the MO surge arrester after adiabatic energy absorption (sometimes known as short-time stability ) and the long-time stability of the MO surge arrester in system operation Thermal stability In Figure 16, P represents the power losses of the MO resistors in an arrester when U c is applied. It is evident that P exponentially increases with the MO-temperature T, which also results in increased heating of the active component. The cooling down of the MO resistors occurs with the heat flow Q from the active part of the arrester to the exterior. P is greater than Q at temperatures above the critical point (thermal stability limit). Here, the cooling is not sufficient to dissipate the heat produced by the power losses to the exterior. The MO resistors would continue to heat up and the arrester would be destroyed by overheating. This occurrence is called thermal run-away or thermal instability. If the power losses P stay under the critical point (i.e. P < Q ), it is possible to eliminate the warmth faster than it is produced, and the active part cools down until it returns to the stable working condition after the cool-down time (stable operating point). This is the area of thermal stability. As long as the critical point is not exceeded, the arrester can branch off the loaded energy as often as is necessary, which means that it can limit the overvoltage just as often as is required. It is possible to raise the critical point to such a level, that even if the highest energies are likely to occur during the operation, this critical point cannot possibly be reached. This can be achieved through suitably dimensioning of the MO resistors and through design measures that enable them to cool down Long-term stability An MO surge arrester can operate absolutely reliably if the voltage-current characteristics curve of the MO resistors under applied continuous voltage does not change. The continuous current i c should not be allowed to shift to higher

21 FUNCTION AND PERFORMANCE OF MO SURGE ARRESTERS 21 Figure 16: Power losses P of the MO resistors and the heat flow Q from the active part of an arrester to the exterior, as a function of the temperature (T) of the MO resistors at continuous operating voltage U c. Figure 17: Example of a long-term stability test (type test over 1,000 h). values to also prevent increases in power losses. A change of the electrical characteristic curve due to applied continuous voltage U c is not to be expected with MO resistors that are produced by leading international manufacturers, considering the present state of technology. Under certain circumstances, a change (or, more precisely, deterioration) of the voltage-current characteristic curve can occur due to extreme stresses, such as very high or very steep current impulses. Another cause that can lead to a change of the electrical characteristics close to the rim may be different components of the materials in which the MO resistors are embedded. This is the reason why the surface area of the MO resistors is passivated, which means that they are coated with a gas-proof glass that is also highly robust. All these reasons make it indispensable to permanently control the long-term behavior of MO resistors during their manufacture. This is achieved with the long-term stability test according to IEC (Ed. 3.0). In addition to type tests of over 1,000 hours, there are also accelerated ageing tests according to internal manufacturer instructions to be conducted on each production batch. It should be emphasized that the long-term stability test must be performed with the same kind of voltage that is applied to the MO surge arrester in the system. Thus, the MO resistors for AC systems must be tested with AC voltage, and the MO resistors for DC systems must be tested with DC voltage. Experience shows, however, that DC-stable MO resistors are usually also stable under AC loads, but AC-stable MO resistors are not necessarily stable under DC loads. That is why it is particularly important to use DC-stable MO resistors with MO surge arresters in DC systems. Figure 17 shows an example of a long-term stability test. Temperature and test voltage are to be kept constant over the whole test time. The power losses P are recorded and should decrease constantly or remain constant. The test duration of 1,000 h at 115 C is considered to correspond to an operating time of 110 years in the system at an environmental temperature of 40 C. Figure 16 P, Q. thermal runaway W thermal stability limit Q. P T Q. stable operating point T C Figure 17 1,1 1,0 P 0,9 0,8 0,7 0,6 0,5 0, t h

22 22 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 3.6 Protective characteristics The protective characteristic of an arrester is given by the maximum voltage U res at the terminals of an arrester during the flow of a current surge. Generally, a lightning impulse protective level of U pl 4 p.u. is considered acceptable. This is a value that is generally accepted for the insulation coordination. The real residual voltage with nominal discharge current I n (thus U pl ) can lie above or below that, depending on the type of arrester. If U pl is set in a relationship with U c of an arrester, it is possible to get very good information about the quality of the arrester performance with regard to the protective level. The smaller the U pl /U c ratio, the better the protection. In addition to the residual voltage at I n, the residual voltages at steep current impulse and at switching current impulse are also important. The residual voltage increases slightly with the current, but also with the steepness of the current impulse, as can be seen from the data sheets of each arrester, and also from the voltage-current characteristic. Depending on the application, the residual voltage at the steep current impulse and at switching current impulse must be taken into account, apart from the residual voltage at I n. 3.7 Temporary overvoltage Temporary (short-time) overvoltages U TOV are power frequency overvoltages of limited time duration. They appear during switching operations or earth faults in the system and they can stay in medium-voltage systems with insulated transformer neutrals for several hours. Their height depends on the system configuration and the treatment of the star point. The duration is given by the time that elapses until the registration and the switching off of the system failure. MO surge arresters are able to withstand an increased operating voltage for a certain period of time. The factor of resistance (T ) of the arrester against such temporary overvoltages can be seen as an example in Figure 18. T = U TOV /U c is the extent of the permissible height of U TOV. The following example should explain the use of TOV curves in Figure 18. An arrester with U c = 24 kv is operated with U c in a normally functioning, undisturbed system for an unlimited period of time. At time t = 0 the arrester is stressed with an energy of W th = kj/kv Uc. Immediately afterwards, the temporary overvoltage U TOV = 31 kv occurs. Therefore, it is T = U TOV /U c = 31 kv/ 24 kv = T = 1.29 results in a time of t = 20 s according to curve b. That means that the arrester can withstand an increased voltage of 31 kv for 20 s without becoming thermally instable. After 20 s, the voltage must go back to U c so that the arrester does not become overloaded. If the arrester is not loaded with the energy W th before the appearance of the temporary overvoltage, it is curve a that counts, and the arrester can withstand U TOV for 90 s. Therefore, the height and duration of the admissible temporary overvoltage directly depend on the previous energy load of the arrester. Figure 18: Resistance T = U TOV /U c against temporary overvoltages depending on the time t. Curve a is valid for an arrester without energy pre-stress, curve b with a pre-stress of the guaranteed energy W th, and t is the time duration of the overvoltage at power frequency. Example for type MWK T = U TOV/ Uc a b t s ,

23 Long term tests of the MO resistors in modern automated test stations ensure reliable performance. 23

24 24 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 4 Service conditions MO surge arresters must perform reliable under normal and special service conditions. Adaptations in design may be necessary to meet specific applications. 4.1 Normal service conditions The service life of an arrester made by ABB in Switzerland is 30 years or more under normal operating conditions and if it is correctly chosen according to the system voltages and the expected electrical and mechanical loads. The normal service conditions for an arrester are listed in IEC (Ed. 3.0). Ambient air temperature within the range of -40 C to +40 C Solar radiation of 1.1 kw/m 2 An altitude not exceeding 1,000 m above sea level Frequency of AC voltage between 48 Hz and 62 Hz A power frequency voltage at the arrester terminals not higher than the continuous operating voltage U c of the arrester Wind speed 34 m/s Vertical erection, not suspended All arresters made by ABB in Switzerland meet, or even exceed, these operating conditions. For example: The ambient air temperature can be up to 55 C (at derated thermal energy capability) The AC power frequency can be between 15 Hz and 62 Hz The altitude can be up to 1,800 m without altitude correction The arresters can be mounted in any position, including hanging 4.2 Special service conditions The following examples are typical special service conditions (referred to as abnormal serviced conditions in IEC (Ed. 3.0)) that may require special consideration in the manufacture or application of surge arresters and should be called to the attention of the manufacturer. Ambient temperatures in excess of + 40 C or below 40 C Service at altitudes above 1,000 m Fumes or vapors that may cause deterioration of the insulating surface or mounting hardware Excessive contaminations by smoke, dirt, salt spray, or other conducting materials Excessive exposure to moisture, humidity, dropping water or steam Live washing of arresters Areas with a risk of explosion Unusual mechanical conditions Voltage distortions or voltages with superimposed contents of high frequencies that are caused by the system Further special conditions are listed in IEC (Ed. 3.0). The following paragraphs illustrate a few special cases. It is advisable to contact the manufacturer should conditions appear that are not covered here. 4.3 Overload behavior Any arrester can be overloaded. The causes can be extremely high lightning currents, lightning currents with a very large charge, or a voltagetransition. This is to be understood as a shortcircuit between two different voltage levels. In all these situations, there is in fact an energy overload. In the case of an overload, the MO resistors either spark-over or break down and tend to create a permanent short-circuit. An arc results inside the arrester, and the current in this arc is defined by the short-circuit power of the system. ABB arresters with directly molded silicone housings do not face the risk of explosion or violent shattering in the case of an overload. There is no air space between the active part of the arrester and its silicone insulation: thus, there is no space for the pressure to build up. The occurring arc (or sparks) escapes the silicone insulation as soon as it occurs and is freed. Because of their special construction, the arresters are protected from violent shattering up to the highest short-circuit currents. 4.4 Mechanical stability ABB s arresters are operationally reliable even in areas of high earthquake activity. The arresters

25 SERVICE CONDITIONS 25 may partially take on the support function or serve as line arresters, or they may have the function of suspension insulators. The manufacturer should be informed about such operational situations. The values given in the data sheets of the individual arresters are not to be exceeded. Arrester types that are to be applied for rolling stock are delivered with a reinforced base plate and are tested under vibration and shock conditions. 4.5 Elevated ambient temperature ABB arresters (AC and DC voltage) are guaranteed to function flawlessly up to 40 C ambient air temperature. This also includes maximum solar radiation of 1.1 kw/m 2 for outdoor arresters. If there are heat sources in the vicinity of the arrester, the increased ambient temperature has to be taken into account, and the value of U c increased if necessary. If the ambient temperature exceeds 40 C, U c must be increased by 2 percent for every 5 C of temperature elevation. This correction is possible up to maximum of 80 C ambient temperature. If it is not acceptable to increase the continuous operating voltage U c, and consequently the protection level U pl, in a specific application, then a reduction of the thermal energy rating has to be considered. 4.6 Pollution and cleaning Silicone is the best insulating material in case of pollution. This is mainly because the material is water-repellent (hydrophobic). Silicone arresters behave more favorably under conditions of heavy pollution than porcelain-housed arresters or other polymeric insulation materials, e.g. EPDM. Decisive for the long-term behavior under pollution of an insulation made of a polymeric material is the dynamic behavior of the hydrophobicity, which is originally always very good. Depending on the material, a loss of hydrophobicity can be permanent or temporary. In contrast to other polymeric materials, silicone is able to regain its hydrophobicity after losing it temporarily. In our operation instructions the best way to clean silicone surfaces, if needed, is described. 4.7 Altitude adjustment of the arrester housing Correction factors for altitude adjustment of external insulation are given in several IEC and IEEE standards. The correction factors differ from standard to standard, depending on the type of equipment and mainly due to assumptions and safety margins considered. In IEC standards the normal service conditions are valid up to 1,000 m above sea level, while in IEEE standards 1,800 m are mentioned. Above these standard altitudes an adjustment of the arrester housing has to be considered. MO surge arresters made by ABB in Switzerland can be used without any housing adjustment up to a height of 1,800 m above sea level. At higher altitudes, the air density may be so low that the withstand voltage of the arrester housing (external flashover) is no longer sufficient. In this case, the unaltered active part of the arrester (same protection level) must be placed in an elongated housing with a longer flashover distance. As a reference value, one may consider that for every 1,000 m above 1,800 m above sea level the flashover distance must be increased by 10 percent. For example, at an altitude of 3,300 m above sea level the flashover distance of the housing must be 15 percent longer than that of a standard arrester. It is necessary to observe here that the flashover distances of surge arresters for lower voltage levels are initially relatively large, exceeding the minimum requirements of the withstand voltage. Thus, in each individual case it should be checked whether the normal housing possesses a sufficient withstand voltage for application at higher altitudes.

26 26 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 5 Tests Constant quality and the guaranteed performance of the products is ensured by a number of tests performed during development and production of MO resistors and surge arresters. 5.1 General Tests have to demonstrate that an MO surge arrester can survive the rigors of reasonable environmental conditions and system phenomena, while protecting equipment and/or the system from damaging overvoltage caused by lightning, switching, and other system disturbances. Arresters manufactured by ABB in Switzerland are tested according to the current international IEC standards. IEC : 2014 (Ed. 3.0) is applicable for the MO surge arresters with polymer housings. Below, the main tests relevant for MO surge arresters with polymeric housings for medium- voltage systems are addressed in brief. If in doubt, only consider the text in the current edition (English version) of the relevant standard. 5.2 Type tests (design tests) The development of an arrester design ends with type tests. They are the proof that the arrester construction fulfills the applicable standards. These tests need be repeated only if changes in the design also cause changes in the properties or characteristics. In such cases, only the affected tests need be repeated. The type tests that are to be performed on MO surge arresters with polymer housing are listed and briefly explained in the following paragraphs. Insulation withstand tests The insulation withstand tests demonstrate the voltage withstand capability of the external insulation of the arrester housing. The withstand values to be proved are calculated from the residual voltages of the arrester. The withstand values of arresters intended for use on systems of U s 245 kv (this means all arresters used in medium-voltage systems) are tested with the lightning impulse voltage (wave shape 1.2/50 μs) under dry conditions, and with a one-minute AC voltage test. The AC voltage test is performed under wet conditions for arresters intended for outdoor use; arresters intended for indoor use are tested in a dry environment with the AC voltage test. Naturally, the tests are performed with arrester housings without active part inside. Residual voltage tests These tests determine the voltage-current characteristic in the high current range. The residual voltage for steep current impulse, lightning current impulse and switching current impulse at different amplitudes is determined and given either in tables or in a curve form. The residual voltage tests are generally performed on MO resistors. Test to verify long term stability under continuous operating voltage The test is an accelerated ageing test performed on individual MO resistors to provide insurance that they will exhibit stable operating conditions in terms of power loss over the anticipated lifetime of the arrester. The test is performed on MO resistors including all material (solid or liquid) in direct contact with them, e.g. in air, SF 6 gas or molded in silicone. Therefore, MO resistors of directly molded arresters also have to be molded with the same material during the accelerated ageing test. The test is passed if the power losses of the MO resistors during the 1,000-h test under elevated conditions (115 C and increased voltage) do not increase above 1.3 time the lowest power losses, P min, and all measurements of power losses are not greater than 1.1 times the power losses at the start of the test, P start. MO resistors made by ABB typically show a constant decrease in power losses over the whole test time and, therefore, have long-term stability under AC and DC conditions. Test to verify the repetitive charge transfer rating, Q rs The purpose of this test is to verify the maximum impulse charge (and, indirectly, the maximum energy) that can be handled by an arrester in an event that may be repeated many times over its lifetime. The charge has been chosen as a test basis for the purpose of better comparison between different makes of MO resistors and arresters.

27 TESTS 27 Because surge arresters are subjected to current impulses from lightning and switching events, it is necessary to know the capability of the arresters in terms of the charge transferred by the arresters during such events. In addition, the withstand capability of MO resistors is a statistical parameter, and a high-voltage arrester can contain a significant number of MO resistors. If a single MO resistor fails, the probability is high that the complete arrester would fail. Distribution arresters contain only a few MO resistors, but the installed number of distribution arresters is very high. Heat dissipation behavior of test sample In the operating duty test and the power frequency voltage-versus-time test, the behavior of the tests sample is to a great extent dependent on the ability of the sample to dissipate heat, i.e. to cool down after being stressed by a discharge. Therefore, the thermal equivalency between the complete arrester and the arrester section shall be demonstrated by a test. Operating duty tests The purpose of the operating duty tests is to verify the arrester s ability to thermally recover after injection of the rated thermal energy W th or transfer of the rated thermal charge Q th under applied temporary overvoltage and following continuous operating voltage conditions. If an arrester absorbs energy from a system event (e.g. lightning impulse, switching surge, temporary overvoltage), the temperature of the MO resistors may rise to a point that is beyond the arrester s ability to thermally recover to its previous steady state condition. Arrester manufacturers are required to specify values for energy (W th for station class arresters) or charge (Q th for distribution class arresters) that represent the thermal limit for each arrester type. This implies that the arrester must always remain thermally stable after duty while in service over its expected lifetime. It is also the purpose of this test to ensure that the protective characteristic is not significantly changed by such duty. The test consists of two parts: The characterization and conditioning part of the test, which may be performed at an ambient temperature of 20 C (± 15 K) on the MO resistors or pro-rated sections in still air. The thermal recovery part of this test shall be performed on thermally pro-rated sections. With station class arresters, the thermal energy W th is brought in within three minutes by one or more long-duration current impulses or by unipolar sine half-wave current impulses. Station class arresters have to absorb energy that may be stored in the system in the moment the overvoltage occurs. Line arresters without gaps (NGLAs) may be tested with lightning current impulses. With distribution class arresters the thermal charge transfer Q th is injected with two lightning current impulses of 8/20 µs within one minute. Distribution arresters are mainly stressed by lightning events, meaning that they have to absorb a charge, which is why they are tested with lightning current impulses. Power frequency voltage-versus-time test (TOV curve) The purpose of the test is to demonstrate the temporary overvoltage (TOV) withstand capability of an MO surge arrester. The test is performed with prior duty (injection of rated thermal energy W th or rated thermal charge Q th ) and without prior duty. The test procedure is the same as the thermal recovery tests in the operating duty tests (second part of the operating duty tests). But instead of applying only the rated voltage U r, the TOV test has to be performed with four different overvoltages at different time durations each (test with prior duty), and two overvoltages at different time durations (without prior duty). The published data must cover the time range between 0.1 s and 3,600 s. Tests of arrester disconnector This test applies principally to distribution arresters and non-gapped line arresters. The test is to verify that the disconnector of an arrester can withstand all stresses related to their application in arresters without operation such as charge transfer and operating duty. The test also demonstrates that the disconnector will perform according to the time-current characteristic published by the manufacturer. Furthermore, the water tightness and the mechanical strength of the disconnector have to be verified. Short-circuit tests Surge arresters are not allowed to fail with violent shattering in case of overloading and should self-extinguish any open flames within a defined period of time. This is to be proved with a shortcircuit test. The way the short-circuit is initiated in the arrester depends on its construction. Directly molded medium-voltage arresters are electrically pre-damaged, that is they are made low-ohmic by applying an increased voltage, and afterwards they are connected to the actual test so that the short-circuit develops inside the arrester. This is a form of overload that looks very much alike the one taking place in the arrester under real conditions in service. The short-circuit ratings for MO surge arresters are tabled in IEC , Ed. 3.0 and must be declared by the manufacturer.

28 28 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION Test of the bending moment This test demonstrates the ability of the arrester to withstand the manufacture s declared values for bending loads. As a rule, an arrester is not designed for torsional loading. If an arrester is subjected to torsional loads, a specific test may be necessary by agreement between the manufacturer and the user. A test in two steps (for U s 52 kv) shall be performed one after the other: a thermomechanical test, and a water immersion test. These tests demonstrate the ability of the arrester to resist ingress of moisture after being subjected to mechanical stresses. Radio interference voltage (RIV) test This test is applicable only for arresters intended for use in systems with U s 72.5 kv. For arresters in medium-voltage systems this test is performed as a routine test (internal partial discharge test) on each complete arrester. Test to verify the dielectric withstand of internal components The purpose of this test is to verify the internal dielectric withstand of an arrester even under impulse currents of amplitudes higher than the nominal discharge current. The test is required only, if the conditioning part of the operating duty test was not performed on a dielectrical prorated section. If the dielectrical prorated section is identical to the thermal prorated section as used in the operating duty test, this test can be omitted. Residual voltage tests The residual voltage is measured on each MO resistor at a current value of 10 ka with a current rise time of 8 μs, which is normally a lightning current impulse (or the nominal current). The residual voltages of the MO resistors inside an arrester can be directly added up, and they represent the total residual voltage of the arrester. Internal partial discharge test In case of medium-voltage arresters, the test is normally performed on each complete arrester. This test is performed at 1.05 U c after the rated voltage was applied for 2 to 10 s. The measured value of the internal partial discharges is not allowed to exceed 10 pc according to the IEC. ABB s internal guidelines require a value less than 5 pc, which means virtually no partial discharges. During this test, the arrester can be screened off from the external partial discharges. Tightness test (leakage check) This test demonstrates that the construction of the arrester is tight. The manufacturer must choose a procedure that is sensitive enough. This test is not applicable for arresters that are completely molded in silicone. Current distribution test The current distribution test is to be performed on MO surge arresters with parallel MO resistors or parallel columns of MO resistors. Arresters with one column only are naturally not to be subjected to such a test. Weather ageing test This test demonstrates the ability of a polymeric-housed arrester to withstand specific climatic conditions. The test consists of two parts: 1,000 h test under salt fog conditions and a 1,000 h UV test. The former must be performed on the highest electrical unit with the minimum specific creepage distance and the later on shed and housing materials. As a rule, the largest arrester is tested with the medium-voltage arresters. 5.3 Routine tests Routine tests are performed on each arrester or parts of an arrester (for example, on MO resistors). According to the IEC, at least the following tests must be performed: Measurement of reference voltage The reference voltage is measured with the reference current specified by the manufacturer, and should be within the range specified. This measurement is performed on each MO resistor and on each MO surge arrester. Proper assembly of disconnectors The proper assembly of each disconnector has to be demonstrated by either measurement of resistance/capacitance or partial discharges. Apart from the routine tests considered as the minimum requirement by the IEC, ABB performs additional routine tests on MO resistors and arresters to ensure high quality. These include: Measurement of the total leakage current on each arrester at U c Regular measurement of the power losses on the MO resistors and arresters Examination of the energy handling capability of MO resistors with current impulses A reduced accelerating ageing test on some MO resistors from each production lot

29 TESTS 29 Figure 19: Internal leakage currents of MO surge arrester design principles during a long-term humidity test Leakage Current (µa) Group III Group II Group I Testing Time (days) 5.4 Acceptance tests Standard acceptance tests performed include: Measurement of the reference voltage on the arrester Measurement of the lightning impulse residual voltage on the arrester or arrester unit Test of internal partial discharges The acceptance tests are to be agreed upon when the products are ordered. The tests are performed on the nearest lower whole number to the cube root of the number of arresters to be supplied. The proof of the thermal stability of an arrester as part of the acceptance test requires additional agreement between manufacturer and purchaser, and it is to be explicitly specified in the order. This is necessary, because proof of thermal stability means that part of the operating duty test has to be performed. This test is expensive and can be performed only in laboratories that have the necessary equipment, and they have to be booked in advance. 5.5 Special tests As part of the development of the arresters, additional tests were performed in cooperation with users and research institutes. These tests were performed to examine the behavior of MO surge arresters with silicone housings under special conditions. Temperature cycles The construction and also the materials used for the MO surge arresters manufactured by ABB in Switzerland tolerate temperatures up to -60 C and extreme changes in temperature between -40 C and +40 C without any changes to the mechanical or electrical qualities. The construction of the arrester, and especially the surface of the silicone, were not harmed in any way by ice during cyclic freezing. Humidity tests The electrical behavior of the arresters directly molded with silicone were not influenced by humidity during long-duration tests that lasted more than two years, and during which the arresters were subjected to a relative humidity of more than 90 percent, and also to regular rain. Figure 19 shows the results of a long-term test at high humidity. Design principle Group I (directly molded arresters, see Figure 1) behaved best in this test. No significant increase in the internal leakage current was observed during the test period of more than two years. Behavior in fire Silicone is a self-extinguishing material. If silicone catches fire as a result of a flame or an electric arc and the cause of the fire is removed or switched off, then the burning silicone extinguishes itself in about one minute. Only non-toxic burnt silicone remains in the burned patch, which is in fact nothing but fine quartz sand. Smoke analyses show no toxic gases occur as a result of fire. Shock and vibration Surge arresters of type POLIM-I/S/H 36 N were subjected to a vibration test according to IEC 61373: 2010, category 1, class B. Due to the variable installation position of the surge arresters, testing was performed with the highest longterm test level (z-direction) and shock test level (x-direction). No significant damage was found by visual examination after the vibration test (lifetime test) and shock test in the z-, y- or x-coordinate directions.

30 30 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION Finally, repeated electrical tests showed no significant changes compared to the premeasurements. The surge arresters of type POLIM-I/S/H 36 N meet the requirements of IEC 61373: Wind tunnel test MO surge arresters for application in traction systems on rolling stock, e.g. high-speed trains, may be subjected to very high wind loads. To ensure the mechanical integrity of the MO surge arrester and the stiffness of the silicone housing under extreme wind loads, MO surge arresters of type POLIM-S (class SM) and POLIM-H (class SH) were subjected to aerodynamic forces in a wind tunnel test. As a result of the high-speed visualization, it can be stated that at a wind speed of 100 m/s (360 km/h), one does not observe oscillations of the sheds. This proves that the MO surge arresters for application on rolling stock can be used without restrictions on high-speed trains. 5.6 Commissioning and on-site tests All MO surge arresters undergo a routine test (factory test) before shipping. The routine test report contains all relevant results and is delivered together with the MO surge arresters. No on-site or commissioning test is necessary, and it is not advised. It has to be noted that on-site tests, e.g. insulation tests on cables or gas-insulated substations (GIS), cannot be correctly performed if MO surge arresters are connected to the system under test. This is because MO surge arresters will carry a current in the ma range and will limit the test voltage. In a worst case the MO surge arresters can be destroyed by the application of AC withstand voltage for a prolonged time. For this reason, MO surge arresters must be disconnected when on-site tests are performed. If in any doubt, the manufacturer must be contacted before such tests are performed.

31 ARTICLE OR CHAPTER TITLE per cent factory tests are performed on each MO resistor and MO surge arrester in automated test stations.

32 32 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 6 Neutral earthing methods and determination of Uc For correct choice of MO surge arresters, the system preconditions must be known. The handling of the transformer neutral and the failure conditions in the system determine the continuous operating voltage U c. 6.1 General considerations The earthing method of the star point of a transformer has a direct influence on the choice of the continuous operating voltage U c of all MO surge arresters to be installed in the system. The manner in which the star point is treated affects directly the height of the current, which occurs in cases of failure with the earth connection, on temporary overvoltages with power frequency and transient overvoltages. Singlephase-to-earth faults (earth fault, earth shortcircuit) are the most frequent failures in mediumand high-voltage systems. Low currents at the failure point tend to be connected with high and long existing temporary overvoltages of the sound phases. This is the case with systems having an insulated star point or earth fault compensation. The single-phase earth fault is registered and quickly switched off by the system protection in systems with low-ohmic star point earthing. In Figure 20, the basic circuit of a medium-voltage transformer with a star connection with open star point (Mp) is shown. Specified are the voltages and currents in case of a symmetrical load, i.e. in an undisturbed service case. All line-toearth voltages U LE are equally high. The voltage of the star point U Mp-E relative to earth is zero. The voltage triangle is provided on the right side for better understanding. If a single earth fault occurs in the described system, e.g. line L3 touches the earth, an asymmetry occurs, the voltage at L3 becomes almost zero, and the voltage at the sound phases shifts to the voltage U LL, which is the system voltage U s. The consequence is that a failure current I Ce flows through the failure point back into the system. The value of the failure current is determined by the impedance in the current path. Further, a TOV occurs on the sound phases as long as the failure lasts, which the installed equipment (the MO surge arresters) has to withstand. Figure 21 reflects this situation. In the following chapters, different star point treatments are briefly explained and the choice of the continuous operating voltage U c as the most important characteristic for a safe application of the MO surge arrester in the system is specified. While choosing the continuous operating voltage U c, it is necessary to ensure that the arrester will not be overloaded under any circumstances due to the voltage with power frequency. In this way, the arrester meets the requirements of the operating system. Therefore, U c of the arrester is to be chosen in such a way that the arrester cannot become unstable either through the continuous applied voltage coming from the system, or through temporary overvoltages that may occur. In selecting the U c of an arrester in a three-phase system, the location of the arrester plays the deciding role: between conductor and earth, between the transformer neutral and earth or between two phases. The maximum operating voltage at the arrester terminals can be calculated with the help of the maximum system voltage U s. As mentioned above, in medium-voltage systems special attention must be paid to potential temporary overvoltages U TOV. They occur during earth faults, and they depend on the treatment of the star point of the transformers and the system management. Thus, generally the demand for the continuous operating voltage results are as follows: k x U s U c T x 3 T is the factor given in the TOV curves, supplied by the manufacturer.

33 NEUTRAL EARTHING METHODS AND DETERMINATION OF U C 33 As a rule, in medium-voltage systems the withstand voltage values of the insulation are rather high in relation to the system voltage (see Table 3). This means that the distance between the lightning impulse withstand voltage (LIWV) and the lightning impulse protection level (U pl ) of an MO surge arrester is in most cases sufficient. On the other hand, the system conditions and the maximum system voltage U s are not always clearly known. That is why it always makes sense to set the continuous operating voltage U c of an MO surge arrester somewhat higher than the calculated minimal value that is required. This safety margin contributes to a secure and reliable operational system. A safety margin of 10 percent is recommended when choosing the U c unless there are explicit technical reasons for not doing so. The thermal stability of the surge arrester in the system is always to be preferred over a fully optimized protection level. The examination of the residual voltage of the chosen arrester and eventually the examination of the resulting protection distance is necessary in any case. The earth fault factor k is the ratio of the highest power-frequency phase to earth voltage U LE,f on a healthy phase during an earth fault to the power frequency phase to earth voltage U LE in absence of the fault at the same location in the system. The earth fault factor only refers to a particular point of a three-phase system, and to a particular system condition. The magnitude of the earth fault factor depends on the way the neutrals of a system are earthed. U LE, f k = U LE Figure 20: Basic circuit of a medium-voltage transformer with connected lines (star connection with isolated star point). Trafo i L1 L1 L2 Mp U LL = Us L2 U LE L3 UMp-E = 0 U LE U LL L3 Figure 21: Situation in a medium-voltage system with insulated star point and single earth fault (line L3). Trafo i L1 L1 L2 Mp L2 U L1-E U L2-E C k L3 U Mp-E UMp-E = ULE I Ce U = 0 U = Us = 3 ULE I Ce U U LL= s ICe 30A R E L3

34 34 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION A system is considered effectively earthed if the earth fault factor (k) does not have a value higher than 1.4 anywhere in the system. This is the case in systems that are described as solid or directly earthed. If the earth fault factor is higher than 1.4 at any point in the system, then this is considered ineffectively earthed. In such systems, the star point is insulated (also described as open) or compensated. 6.2 Systems with insulated star point or with earth fault compensation As a rule, systems with an insulated star point are systems of small extension, auxiliary power systems for power stations or station services. A capacitive earth failure current I Ce of about 5 A to 30 A flows in case of failure. The earth fault factor is: k 3 In case of intermittent earth faults, the earth fault factor can reach values up to k = 1.9. The duration of the failure lies between several minutes and several hours. Systems with earth fault compensation are mostly overhead line systems with system voltages between U s = 10 kv and U s = 110 kv. One or more transformer star points in these systems are earthed with high-ohmic Petersen coils. An earth fault current of approximately 5 A to 60 A can flow in case of an earth fault. The earth fault factor is in this case k ( ) 3 In both cases, the voltage increases in the healthy phases to a maximum of U s under earth-fault conditions. This results in: U c U S for the arrester between phase and earth. The voltage at transformer neutral can reach a maximum of U s / 3. This results in: U s U c for the arrester between transformer neutral and earth. In every system, there exist inductances and capacitances that produce oscillating circuits. If their resonant frequency is close to that of the operating frequency, the voltage between the phase conductor and earth could basically become higher than that of U s in single-pole earth faults. The system management should avoid the occurrence of such resonances. If this is not possible, then the U c should be correspondingly increased. In systems with earth fault compensation the earth fault factor can reach a value of 1.9 in unfavorable conditions. This is to be taken into account by increasing the continuous voltage by 10 percent. 6.3 Systems with high-ohmic insulated neutral and automatic earth fault clearing. The same voltages occur as described in section 6.2 in the case of an earth fault. However, immediate automatic fault clearing enables a reduction of U c by the factor T. Naturally, it is decisive to know the level of the possible temporary overvoltage, as well as the maximum time for the clearing of the earth fault. Making use of the TOV curve, this results in: U s U c T for the arrester between phase and earth. U s U c T x 3 for the arrester between transformer neutral and earth. 6.4 Systems with direct or low-ohmic star point earthing A system with low-ohmic star point earthing is provided if the star point of one or more transformers are directly earthed or through current limiting impedances. The system protection is set up so that even a single line-to-earth fault at any place in the system causes an automatic fault clearing. These are typical cable systems in towns with system voltages between 10 kv and 110 kv. In the case of a failure, the earth short-circuit current (I k ) flows, which leads to an immediate automatic clearing of the fault. As a rule, the duration of the failure is limited to t k < 0.5 s. In unfavorable situations, the duration of the failure can last up to 3 s in medium-voltage systems Systems with direct star point earthing Direct or solid star point earthing (earth conductor) is principally used for all systems with system voltages of 220 kv and above, but it can also be found in medium-voltage systems. In these types of systems, there are so many transformers with direct neutral earthing that during an earth fault, the phase voltage in the complete system never exceeds 1.4 p.u. Therefore, the earth fault factor is: k = ( ) 3, that is k 1.4 In these systems a short-circuit appears in case of a failure. In medium-voltage systems the short-circuit current can be as high as I K = 20 ka, and consequently, the failure has to be cleared in

35 NEUTRAL EARTHING METHODS AND DETERMINATION OF U C 35 less than 0.5 seconds. However, under worst case conditions, and considering some safety margins, it can be assumed that in medium-voltage systems the clearing time of the earth fault is t = 3 s at the most. In Figure 18 the described TOV curve for an MO surge arrester with class SL (e.g. an arrester type MWK) lists T = as a result, so that it may be written k U s 1.4 U s 1.05 U s U c = T for arresters between phase and earth. This simple equation can be generally used as a rule of thumb for systems with direct earthed neutral. The voltage of the neutral of the unearthed transformers in the system reaches a maximum of U TOV = 0.4 U s This results in: 0.4 U s 0.4 U s U c = = 0.3 U s T for arresters between transformer neutral and earth Systems with low-ohmic star point earthing In case of low-ohmic earthing, one has to distinguish between inductive earthing (neutral reactor) and resistive earthing (earthing resistor). The fault current is in the range of 500 A to 2,000 A. The fault duration is in the range of a few seconds maximum. The earth fault factor is: k = ( ) 3 For arresters in the vicinity of low-ohmic earthed transformers, an earth fault factor of k 1.4 is applicable, and the same equations for U c as for direct earthed transformers can be chosen, see Care is required if the arresters are located just a few kilometers from the transformer. In unfavorable earthing conditions, e.g. desert regions or mountains, the earthing resistance can be very high, and consequently the earth fault factor higher than 1.4. In the case of single pole earth faults with resistive current limitation, earth fault factors of 2.0 can appear. In such cases the procedure described in Section 6.3 should be followed. 6.5 Four-wire, multi-earthed-wye systems In some countries, a four-wire system is used in special cases. In this system, a fourth wire is connected to the earthed neutral point of the transformer and connected additionally to earth at several points along the line. In such systems, an earth fault factor of k = 1.25 can be assumed. The continuous operating voltage should be chosen according to U c 1.25 U s / 3 = 0.72 U s for the MO surge arrester between phase and earth. 6.6 Distribution systems with delta connection Transformers in delta connection naturally have no neutral or star point. In the case of an earth fault of one of the phases in such systems, the arresters connected to the sound phases will be stressed with the system voltage U s. An earth fault factor of k = 3 = must be considered. The continuous operating voltage should be chosen according to U c U s 6.7 Arresters between phases Considerable overvoltages between the phase terminals of transformers or reactors may occur when a reactor or a reactive loaded transformer is switched off. The withstand voltage of the reactor or transformer between the phases may be exceeded without operation of the phase-toearth arresters. If such switching overvoltages are expected, surge arresters should be applied between phases in addition to the phase-to-earth arresters Six-arrester arrangement In special cases, such as in arc furnace installations, switching overvoltages occur that are insufficiently limited by arresters between phase and earth. In such cases, it is necessary to install additional arresters between the phases. The arresters between the phases should have a continuous operating voltage of U c 1.05 U s. The continuous operating voltage U c of the phase-to-earth arresters depends on the earthing of the transformer neutral. A typical arrangement for systems with insulated transformer neutral is given in Figure 22a. In this case all six arresters should have a continuous operating voltage of U c 1.05 U s The factor 1.05 takes account of possible harmonics in the system voltage U s. The continuous operating voltage U c of the phase-to-earth arresters depends on the earthing of the transformer neutral. In case of a system with low-ohmic star point earthing (directly earthed) of the transformer the continuous operating voltage can be chosen to U c (1.05 U s ) / 3 for the phase-to-earth arresters.

36 36 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION Neptune design A variation of the six-arrester arrangement is the Neptune (or candle ) design, because of its arrangement of arresters. It consists of four similar arresters. Two arresters in series are fitted between the phases and the earth and also between the phases, as shown in Figure 22b. This arrangement permits overvoltage protection both between the phases, and between the phases and the earth. This kind of arrangement however, has a fundamental disadvantage in comparison to the six-arrester arrangement. Since the arresters behave in a capacitive manner at continuous operating voltage, if there is an earth fault, all four arresters form an asymmetrical system. In the case where each arrester has identical capacitance (meaning that the arresters have identical ratings), arresters A1 to A3 would be stressed with U s and A4 with U s. However, a simple solution is to use four arresters of the same type and rating. For this case: U c U s The protection level of this arrangement, which has always two arresters in series, is therefore similar to that offered by the arrester with U c U s. The residual voltage of this arrester combination is therefore also 32 percent higher than that of the six-arrester arrangement. If a lower protection level is required between phase and earth, a lower continuous operating voltage for arrester A4 may be chosen compared to A1 to A3. Since the arrester capacitance is inversely proportional to the arrester U c, the final steady-state voltages need to be calculated individually for each specific case. 6.8 Operating voltage with harmonic oscillation Harmonic currents generate harmonic oscillations superimposed upon the power frequency voltage. For this reason, it is possible that the peak value of phase-to-phase voltage (U s ) can be higher than 2 U s. If this difference is less than 5 percent, then a correspondingly higher U c must be used. On the other hand, if due to the harmonics the voltage increase is higher than 5 percent, the choice of U c should be discussed with the arrester manufacturer. The same applies for forms of voltage that can often be seen in the vicinity of thyristor converters: voltage steps, ignition peaks, and asymmetries in the two half cycles. Commutation overshoots with a high repetition rate, or other voltage spikes, which are common in drives and converters, cannot generally be limited by gapless MO surge arresters. This is not a typical application for MO surge arresters. In the case of commutation overshoots and other superimposed voltage spikes, special criteria for the dimensioning of MO surge arresters have to be considered. This makes close cooperation and detailed discussion between the user and manufacturer necessary. U U c > 1.05 x s L1 T L1 T L2 L2 L3 L3 U U c > 1.05 x s A1 A2 A3 A4 U U c > x s Figure 22 a: Six-arrester arrangement with U c 1.05 U s Figure 22 b: Neptune design. A1, A2, A3 and A4 are four similar arresters, each with U c U s

37 COORDINATION OF INSULATION AND SELECTION OF MO SURGE ARRESTERS 37 7 Coordination of insulation and selection of MO surge arresters Insulation coordination is a balance between stresses from the system vs. strength of the equipment. The MO surge arresters are matched to the system preconditions and the insulation levels of the equipment to be protected. 7.1 General considerations The principle of insulation coordination for an electricity system is given in the IEC and IEC standards. It is the matching between the dielectrical withstand of the electrical equipment taking into consideration the ambient conditions and the possible overvoltages in a system. For economic reasons, it is not possible to insulate electrical equipment against all overvoltages that may occur. That is why surge arresters are installed to limit the overvoltages up to a value that is not critical for the electrical equipment. An MO surge arrester ensures that the maximum voltage that appears at the electrical equipment always stays below the guaranteed withstand value of the insulation of an electrical device. Therefore, an arrester has to fulfill two fundamental tasks: It has to limit the occurring overvoltage to a value that is not critical for the electrical equipment. It has to guarantee a safe and reliable service in the system. The choice of the continuous operating voltage U c is described in detail in section 6. The following paragraphs briefly deal with the necessary energy handling capability and the protection characteristic of MO surge arresters in mediumvoltage systems. Note: Ferromagnetic resonances are the exception. They can become so high and exist for so long that they may not be taken into consideration by the dimensioning of the continuous voltage if the arrester should still be able to fulfill its protection function in a meaningful way. If ferromagnetic resonances appear, then this generally means that the arrester is overloaded. The system user should take the necessary measures to avoid ferromagnetic resonances. An MO surge arrester can fulfill its function of protection properly if the lightning impulse protection level U pl lies clearly below the lightning impulse withstanding voltage (LIWV) of the electrical equipment to be protected, the safety factor K s is also to be taken into consideration. The continuous operating voltage U c is to be chosen in such a way that the arrester can withstand all power frequency voltages and also temporary overvoltages without being overloaded in any possible situation. This means that T U c must be always higher than the maximum possible temporary overvoltages U TOV in the system.

38 38 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION Table 3: Typical values of the lightning impulse withstanding voltage (LIWV) and the lightning impulse protection level U pl = 4 p.u. U m in kv rms LIWV in kv pv U pl in kv pv LIWV/U pl The point is to set the voltage-current characteristic of the arrester in a way that both requirements are met. Figure 23 shows in a simplified way the principle of insulation coordination. The lightning impulse withstand voltage (withstand voltage of the insulation) is relatively high compared to the system voltage, as can be seen in Figure 23. This automatically results in a large distance between the maximum admissible voltage at the electrical equipment to be protected and the lightning impulse protection level; see also Table 3. Note: The acronym BIL, which is often used for basic lightning impulse insulation level, is exclusively to be found in the US standards (IEEE/ANSI standards). It is similar to the lightning impulse withstand voltage (LIWV) as used in the IEC definition. As mentioned above, it makes sense to choose a continuous operating voltage U c higher than was calculated (10 percent). As a rule, there is enough distance between the maximum admissible voltage at the electrical equipment and the protection level of the arrester. 7.2 Selection of nominal discharge current, charge and energy The lightning current parameters are taken from lightning statistics. The expected magnitude and probability of lightning discharge currents are correlated to the repetitive charge transfer rating Q rs and the thermal charge transfer rating Q th. The thermal energy rating W th results from the energy that may be stored in a loaded transmission line, or in other energy sources like capacitor banks etc. Therefore, it is necessary to know the possible energy stores in a system, such as cables, capacitors or capacitor banks and inductivities. Figure 24 shows a statistical evaluation of all the measured lightning currents. The curve of the mean value shows the probability of the occurred lightning current peak values. The probability of reaching or exceeding 20 ka is 80 percent, whereas lightning currents with peak values of over 100 ka are very rare. The specified lightning currents and the high current impulses are derived from these lightning current statistics. Assuming that in medium-voltage systems a lightning current diverts in the case of a far distance direct stroke, and that flash-overs between phases and at insulators will occur, one can get a nominal discharge current of I n = 5 ka. A wave shape of approximately 8/20 μs results for the lightning current if a flashover occurred at one of the insulators. The worst case to be considered is Figure 23: Comparison of the possible occurring voltages in a typical medium voltage system, the withstand voltages of the electrical equipment and the parameters of the MO surge arrester. The lightning overvoltages are decisive in mediumvoltage systems. That is why are shown only the parameters for the lightning overvoltages. LIWV unprotected, endangered area Ks T U c Upl Uc U p.u lightning overvoltages U TOV 1 U L-E Requirements of equipment, related to Um Design parameters of MO arresters System preconditions, related to Us 1 p.u. = Us 2 / 3

39 COORDINATION OF INSULATION AND SELECTION OF MO SURGE ARRESTERS 39 Figure P% Figure 24: Statistical evaluation of lightning measurements all over the world. Described is the probability of occurrence above the lightning current s peak values (adapted from Cigré survey). a direct stroke in a phase wire in front of a substation without an insulator between the point of stroke and the substation. In this case it can be assumed that a lightning current of e.g. 20 ka diverts in both directions of the line and half of the lightning current (10 ka) travels undamped into the substation. The nominal discharge current can be chosen according to the thunderstorm activity in a region or the expected threat of lightning to a substation. In this way, the requirements for the arresters can be clearly specified together with the repetitive charge transfer rating Q rs and the thermal charge transfer rating Q th or the thermal energy rating W th. MO surge arresters with I n = 10 ka and classification DH are used in applications in medium-voltage systems. Higher nominal discharge currents (I n = 20 ka) and higher classifications like SL, SM and SH are chosen in special cases in medium-voltage systems, such as: Regions with extreme thunderstorm activities and the danger of direct lightning strikes Overhead lines at concrete poles or wooden poles and cross arms that are not earthed Arresters placed at locations where people are often to be found (for instance in electrical traction systems) Lines that demand exceptional high safety standards for the working process For protection of motors, generators and cables Arc furnace protection Capacitors and capacitor banks Very long cables Rotating machines 7.3 Protection level The switching impulse protective level U ps is decisive for the coordination of the insulation in transmission systems of higher system voltages. I ka It is less important in the medium-voltage systems discussed here. Of prime importance here is the lightning impulse protection level U pl and, if necessary, the protection level at steep current impulse, such as when vacuum breakers are in the system. Generally speaking, the protection level should be as low as possible to ensure optimal protection. As previously emphasized, the operational safety of the arrester in the system is always to be preferred to the complete exploitation of the protection level. These opposing requirements are mainly non-critical in the medium-voltage systems, see Figure 23 and Table 3. The protection ratio U pl /U c is fundamentally important. The smaller the ratio, the lower the protection level with the same U c, and the better the protection. If a very low protection level is technically absolutely necessary in a specific case, it is possible to choose an arrester with a better protection ratio. As a rule, this is an arrester with a higher energy rating, because these arresters have MO resistors with a larger diameter as an active part. The choice of an MO surge arrester with the same U c but a higher energy rating offers better protection in the system, although the operational safety stays the same, and it also provides a higher energy handling capability. Moreover, an MO surge arrester with a lower protection level always provides a larger protection distance. Therefore, the choice of arrester or the comparison of different products should also take into consideration the protection ratio U pl /U c in addition to the nominal discharge current and the charge or energy handling capability. In this context, the temporary overload capability of an MO surge arrester with temporary overvoltages should also be observed. A high resistance towards temporary overvoltages generally means that the voltage-current characteristic of an MO surge arrester was set so high that all power frequent overvoltages that occur do not fundamentally exceed the knee point of the U-I characteristic. However, this means that the residual voltage of an MO surge arrester lies correspondingly high, which causes an unfavorably high protection level. 7.4 Selection of arrester housing Silicone or EPDM are almost exclusively used today as housing material for medium-voltage arresters. Silicone is preferred due to its excellent behavior, especially in regard to pollution. The choice of the housing for MO surge arresters in medium-voltage systems is not critical. The flashover distance of the arrester housing and the creepage distance along the surface of the housing are to be taken in account.

40 40 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION The minimum flashover distance is determined by the required withstand values of the test voltages which have to be applied in the relevant withstand tests, the lightning voltage impulse test and the AC withstand test with power frequency for one minute performed with an empty housing or with the MO resistors replaced by insulating material. The height of the test voltage to be applied is related to the protection characteristic of the MO surge arrester. The test voltage during the test with lightning voltage impulse must be 1.3 times the residual voltage of the arrester at I n. The housings for 10 ka- and 20 ka station class arresters intended for use in systems with U s 245 kv, must withstand for one minute an AC voltage test with a peak value of the testing voltage 1.06 times the switching protection level. Housings of distribution class arresters must withstand a power-frequency voltage with a peak value equal to the lightning impulse protection level multiplied by 0.88 for a duration of one minute. The resulting values for the arrester housings are, as a rule, lower than the insulation values for insulation of devices and installations. This is proper because the voltage at the arrester is determined by the voltage-current characteristic curve of the active part, and the arrester naturally protects its own housing against overvoltages. The real provable withstand values of the housing are generally higher than the demanded minimum values corresponding to IEC, especially with arresters for lower voltage levels. The behavior of the external insulation under pollution and applied operating AC voltage is important and determines the creepage distance. The pollution classes and the corresponding reference unified specific creepage distances (RUSCD) are specified in IEC 60507: 2013 and IEC/ TS : 2008, see Table 4. IEC/TS : 2008 refers to polymer insulators for AC systems. For the purpose of standardization, five classes of pollution characterizing the site severity are qualitatively defined. It is possible, however, to specify the reductions of the creepage distances for synthetic materials that have a regenerative hydrophobicity, such as silicone, towards ceramic insulations. These reductions (shown in Table 4) are based on general recommendations given in IEC , results from tests and field experience. Note: The creepage distance for a MO surge arrester is sometimes specified in relation to the continuous operating voltage U c. Therefore, it is important to carefully consider the voltage to which the creepage requirements are related. Table 4: Correlation of pollution class and creepage distance. Pollution class Minimum recommended specific creepage distance in mm/kv* a Very light % b Light % c Medium % Possible reduction of the creepage distance with silicone insulation d Heavy 43.3 No reduction recommended e Very heavy 53.7 No reduction * the shortest specific creepage distance for insulators between phase and earth.

41 PROTECTIVE DISTANCE OF MO SURGE ARRESTERS 41 8 Protective distance of MO surge arresters The place of installation of an MO surge arrester is, besides the correct choice, critical for an optimized protection of the equipment. The MO surge arrester must be as close as possible to the equipment to provide best protection. 8.1 General considerations The higher its lightning impulse withstand voltage (LIWV) lies above the residual voltage of the arrester at nominal discharge current I n, the better the equipment is protected against lightning overvoltages. Modern MO surge arresters with a residual voltage of U res 3.33 x U c at I n maintain a value of U pl 4 p.u., even in systems with high-ohmic earthed or insulated transformer neutrals. The U pl is the lightning impulse protection level of the arrester. A summary of the typical values are given in Table 3. It should be noted that the specified residual voltages U res from the data sheets apply for the terminals of the arrester, which means they are valid only for the place where the arrester is installed. The voltage at the devices that are to be protected is always higher than the voltage that is directly at the arrester terminals in view of the reflections of the overvoltages at the end of lines. Further, inductive voltages drops along the connections from line to the arrester terminal and the earth conductor have to be considered. Therefore, the overvoltage protection no longer exists if the arrester is placed too far from the device to be protected. The protective distance L is understood to be the maximum distance between the arrester and the equipment at which the latter is still sufficiently protected. 8.2 Traveling waves Voltage and current impulses with a rise time shorter than the travel time of an electromagnetic wave along the line, travel along the line as traveling waves. This means that, disregarding damping, the current and voltage impulse travels along the line without changing its form. Therefore, it is in another place at a later time. capacitance per unit length of the line, disregarding the ohmic resistance per unit length and the conductivity of the insulation. L Z = C L = Inductivity per unit length in H/km C = Capacitance per unit length in F/km Only the voltage impulses are important when analyzing the overvoltages. When a voltage traveling wave on a line reaches a point of discontinuity, i.e. a change in the surge impedance, part of the voltage is reflected backward and part is transmitted forward. This means that voltage decreases and voltage increases appear on the connections of the overhead lines to the cable, and at the end of the line. Especially at the end of the line, such as at open connections or transformers, reflections appear that lead to a doubling of the voltage. The height of the voltage for each moment and for each place on the line is the sum of the respective present values of all voltage waves. In the moment an MO surge arrester is limiting the voltage by conducting the charge to earth, it is very low-ohmic and can be considered a shortcircuit. The consequence is that the travelling voltage wave is reflected back and forth between a very high impedance (positive reflection) and a short-circuit (negative reflection). Further, in the moment the MO surge arrester limits the voltage, all overvoltage is reflected negatively. Thus, the MO surge arrester protects in both directions. To simplify matters, a funnel-shaped voltage increase results from the arrester, as can be seen in Figure 25. Current and voltage are connected to one another because of the surge impedance of the line. The surge impedance results from the inductivity and

42 42 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION Considering that the voltage at the end of an open line (e.g. transformer) can reach a maximum of twice the residual voltage U res of the arrester, it can clearly be seen that the distance between the point of installation of the arrester and the equipment to be protected should be as short as possible to ensure good protection. As seen in Figure 25, an MO surge arrester installed in position X A1 will not protect the transformer, because the voltage at the transformer will be higher than the withstand voltage (LIWV) of the transformer insulation. If the arrester is installed at position X A2, the voltage at the transformer is well below the LIWV and provides very good protection. 8.3 Protective distance On the overhead line in Figure 26 an overvoltage U travels as a traveling wave with speed v towards the line end E. At point E is the equipment to be protected. For the following analysis, it is considered that the equipment to be protected is highohmic (transformer, open circuit breaker). When the traveling wave reaches E, it is positively reflected and the voltage increases to 2 x U. The function of arrester A is to prevent unacceptably high voltage values at the equipment to be protected. Under the simplified assumption that the front, with wave steepness S, of the incoming overvoltage wave is time-constant, the following relationship applies for the maximum value U E : 2 S (a + b) U E = U res v v = 300 m/μs A protection factor K s is recommended between the LIWV of the equipment and the maximum lightning overvoltage that occurs (see also Figure 23). This protection factor takes into consideration, among other things, any ageing of the insulation and the statistical uncertainties in defining the lightning impulse withstanding voltage of the equipment. For outdoor insulation a safety factor of K s = 1.15 is recommended (IEC 60071). This results in: LIWV 2 S L U E = U res L = a + b K s v The required equation for the protection distance is: v LIWV L = ( U res) 2 S K s It should be mentioned that the given approximation for L is valid in the strict sense only for b = 0, in practice, however, it gives sufficiently precise values. The steepness S of the incoming overvoltage wave must be known in order to determine the protective distance as it is above described. A general value for the steepness S cannot be given, because it depends on various parameters and statistics. Values between S = 800 kv/µs and S = 1,550 kv/µs are to be expected in mediumvoltage systems, depending on the pole construction and insulators used. As a rule of thumb, a steepness of S = 1,000 kv/µs can be used for rough calculations. It is certainly to be assumed that the arrester and the equipment to be protected are connected to the same earthing system. The connections on the high-voltage side and the earth side must be short and straight. Especially connection b should be as short as possible. In this way, it makes sense to lead the overhead line first to the arrester, and from there directly to the bushing of the transformer, for example. 8.4 Induced voltages As mentioned above, the connections between the arrester terminals and the equipment to be protected must be as short and straight as possible. This is because inductive voltages appear at each conductor due to the self-inductivity during the flowing of an impulse current. These induced voltages are considerable during high rate of changes di/dt, such as when lightning currents occur. The induced voltage is calculated as: U i = L di/dt For example: an approximate inductive voltage of U i = 1.2 kv per meter connection line results from an inductivity of L = 1 µh for a straight wire of 1 meter length and a lightning current of 10 ka peak value of the wave shape 8/20 µs. The specified residual voltages, which are to be found in the data sheets, are always the voltages between the arrester terminals only. Especially in case of steep current impulses (e.g. rise time of 1 µs) the induced voltages have to be considered in a protection concept.

43 PROTECTIVE DISTANCE OF MO SURGE ARRESTERS 43 U T U 1 LIWV U 2 U res X A1 Z L = 450 Ω XA2 XT Z T => Figure 25: voltage increase from the point of arrester installation in both directions. U v S a U E E b Figure 26: Assumption for the calculation of the voltage at the end of a line and for the determination of the protective distance L. A U res

44 44 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 9 Equipment protection To reach an optimized protection each equipment needs special attention and must be treated separately. 9.1 General considerations To reach an appropriate overvoltage protection in medium-voltage systems, it is necessary to find the best compromise between the costs and the benefits of the protection devices to be used. An ideal technical-economic balance is to be striven for. Overvoltage protection that is accurately applied reduces: Outages of lines and substations Interruptions of critical manufacturing processes that demand high voltage stability Costs due to interruptions in the energy supply Costs for the replacement and repair of electrical equipment Ageing of the insulation (e.g. cables) Maintenance work The aim of overvoltage protection is to guarantee an uninterrupted supply of electrical energy with high voltage stability to the greatest degree possible. Therefore, the costs for a set of surge arresters are not the primarily consideration, but the costs that may arise on a long-term basis if adequate overvoltage protection is not used. In fact, all electrical equipment and installations in high-voltage and medium-voltage systems need overvoltage protection. In particular, the following equipment needs protection: Transformers Cables and cable sheaths Capacitors and capacitor banks Overhead lines Rotating machines (motors and generators) Power electronics Coils and line traps Traction equipment (rolling stock and power supply) AC and DC It is sometime insufficient to install only one arrester per line in the substation, considering the limited protective distance of the arresters and the spatial distance between the equipment in the substation. If the various pieces of equipment are installed too far from one another, it is necessary to consider where to place an additional arrester. Some typical cases are described in the following paragraphs. 9.2 Protection of transformers Generally, all transformers that are directly linked to lightning-endangered lines have to be equipped with arresters between phase and earth. As described in section 6, the occurring power frequency voltages in a power system depend on the system voltage U s and the handling of the neutral of the transformers in the system. It is obvious that the continuous operating voltage U c of the MO surge arresters, which have to protect the transformers and the neutrals of the transformers, have to be chosen to be equal to or higher than the calculated values in section 6. It was explained that all MO surge arresters have a limited protective distance, which has to be considered when choosing the place of installation. At distribution levels (U s 52 kv), arresters can often be located very close to the equipment to be protected, e.g. the transformers. In this case, and where possible, the earth terminal of the arrester and the equipment, in this case the transformer, should be bonded with a very short, straight conductor. Figure 27 gives hints for good and poor connection principles. Low earth resistance is essential, and it should be as low as possible in order to limit the earth potential rise at the earth terminal, and hence mitigate safety hazards and flashover on the low-voltage side of the transformer. A value for earth resistance of 10 Ω or less is considered to be sufficient.

45 EQUIPMENT PROTECTION 45 Figure 27: Examples of good and poor connection principles for MO surge arresters in distribution systems. overhead line C T C T C T 1 1: Poor. The connection leads are too long and the transformer and the MO surge arrester do not have the same earthing point. 2 2: Good. Common earth of MO surge arrester and transformer. The connection leads are much shorter. 3 3: Very good. The MO surge arrester is earthed directly at the transformer tank. The loop is very short. In this way the inductance is kept to a minimum. Figure 28: Coupling of a lightning overvoltage through a mediumvoltage transformer. MV C LV i U res 0.4 U res U R E U If a transformer connects a high-voltage system with a medium-voltage system and only the line on the high-voltage side is lightning-endangered, it is necessary to install an arrester on the medium-voltage side as well. Transient overvoltages can be transmitted up to 40 percent capacitive from the primary (high-voltage side) to the medium-voltage side. That is why it is also necessary to install an arrester on the medium-voltage side, even though the medium-voltage side is not directly endangered by lightning. The situation is similar with transformers that connect a medium-voltage system to a low-voltage system. The high-frequency overvoltages from the medium-voltage side are capacitive transmitted to the low-voltage side here as well. Thus, in principle arresters should also be installed on the low-voltage side of the medium-voltage transformers. It is always advisable to install arresters on both sides of all the transformers, particularly in regions with high thunderstorm activity. Figure 28 shows in principle the resistive and capacitive coupling from the medium-voltage to the low-voltage side of a distribution transformer.

46 46 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 9.3 Protection of cables Disruptive breakdowns in cable insulations lead to grave damage and require expensive repairs. Flashovers along the cable bushings can damage them and lead to the same consequences as insulation breakdowns. It is well known that repeated overvoltage stresses negatively influence the ageing behavior of the cable insulation, which means that the service life of the cable is shortened. Cables must therefore be treated like station equipment and protected against transient overvoltages (e.g. lightning) with arresters. The arresters are to be placed directly next to the cable bushings here as well. The junction lines should be as short as possible. It must be noted that the earth connection of the arrester is directly attached to the cable sheath. Longer cables require arrester protection at both ends. For short cable sections, protection on one side can be sufficient. This is possible because the protection range of an arrester at one end of the cable can still offer sufficient protection at the other end. A cable that connects an overhead line with a substation is often only endangered by lightning on the side of the overhead line. Therefore, the arrester must be installed at the junction between the overhead line and the cable. 9.4 Cable sheath protection The cable sheath of a single-conductor cable in high-voltage systems is earthed on one side only for thermal reasons. This procedure is increasingly used in medium-voltage cable systems as well to avoid additional losses in the cable sheath. If the cable sheath stays open on one side, the sheath can take up to 50 percent of an incoming overvoltage on the inner conductor on the nonearthed side. The sheath insulation is not able to cope with this overvoltage stress. Breakdowns between the sheath and the earth can occur, which damage the external insulation of the sheath. That is why it is necessary to protect the cable sheath against overvoltages on the unearthed side with an arrester. The voltage induced along the cable sheath in case of a short-circuit is decisive for the continuous operating voltage U c. The induced voltage is dependent on the way the cable is installed and can at most amount to 0.3 kv per ka of short-circuit current and kilometer of cable length. The continuous voltage to be chosen for the arrester which protect the cable sheath results from: U i U c I K L K in kv T I K = Maximum 50 Hz short-circuit current per phase in ka L K = Length of the unearthed cable section in km U i = Induced voltage occurring along the cable sheath in kv T = Resistance of the arrester against temporary overvoltages according to TOV curve. With U i = 0.3 kv and T = for a maximum fault clearing time of t < 3 s (from TOV curve for the MWK) of the short-circuit current, the result is: 0.3 U c I K L K = 0.22 I K L K in kv Arresters in metal-enclosed medium-voltage substations (cubicles) It is often necessary to install arresters in a metal-enclosed medium-voltage substation. If a cable connects the substation with a lightning-endangered line, an arrester with a nominal current of I n = 10 ka should be installed at the cable bushing. The conditions are different if the arresters must limit switching overvoltages instead of lightning overvoltages. The former could occur during switching if the inductive current is interrupted before it reaches its natural zero crossing. In addition, vacuum breakers can produce high and very steep overvoltages. 9.6 Generator connected to a lightningendangered MV line If a loaded generator is suddenly disconnected from the system (load rejection), its terminal voltage increases until the voltage regulator readjusts the generator voltage after a few seconds. The relationship between this temporary overvoltage and the normal operating voltage is called the load rejection factor δ L. This factor can reach a value of up to 1.5. In the worst case, the arrester could be charged with a temporary overvoltage of U TOV = δ L x U s, which must be taken into account when choosing U c. δ L U s U c T The duration t of U TOV determines T and lies in a range of 3 to 10 seconds. The high operational safety requirements for generators make it advisable to use arresters with low residual voltage U res and high energy handling capability W. That is why the arresters of the type POLIM-H..N are recommended for generator protection. With the help of an example the U c of an arrester for the generator protection should be determined. With U s = 24 kv, load rejection factor δ L = 1.5 and t = 10 s results for the type POLIM-H..N kv U c = 27.8 kv In this way, the type POLIM-H 28 N can be chosen for this case (worst case scenario considered). Generators as important equipment need special attention in overvoltage protection. Therefore, it is especially important to place the arresters close to the generator terminals.

47 EQUIPMENT PROTECTION Protection of motors High-voltage motors can be over-stressed by multiple restrikes resulting from being switched off during the run-up. This is especially critical when the cut-off current is less than 600 A. In order to protect these motors, it is necessary to install surge arresters directly at the engine terminals or alternatively at the circuit breaker. The dimensioning of U c is to be carried out according to the recommendations in section 6. It is necessary to use an arrester with a residual voltage U res as low as possible because of the insulation of the motors, which is generally sensitive to overvoltages, especially if it is aged. That is why arresters should be chosen with an especially favorable U pl /U c ratio. Under certain circumstances, it is possible to use the lowest allowable arrester limit of U c. However, in no case is U c allowed to be lower than U s / 3. Typical arresters used for the protection of electrical motors are MWK, or MWD for indoor applications. 9.8 Arresters parallel to a capacitor bank Normally, no overvoltage occurs when a capacitor bank is switched off. The circuit breaker interrupts the current in the natural zero crossing, and the voltage in the capacitors to earth reaches a maximum of 1.5 p.u. As a result of the network voltage varying at the power frequency, a voltage across the open circuit breaker of 2.5 p.u. is caused. A high-frequency transient effect takes place between the capacitor voltage and the operating voltage if the breaker re-strikes. During this process, the capacitor is charged with a higher voltage. This overvoltage at the capacitor between the conductor and the earth reaches a maximum of 3 p.u. If the capacitors are connected in a star, then they are discharged by the arrester parallel to bank between conductor and earth. During the discharge up to the voltage of 2 x U c, the arresters are loaded in terms of power with: S K W c = [ 3 - (U c /U s ) 2 ] ω S K = 3-phase reactive power of the capacitor battery W c = The discharge energy taken up by the arrester Assuming that the arrester must carry out this process three times without any cool down phase, it follows with U c U s that W c 6 S K U c ω U s The thermal energy rating W th of the arrester with U c must thus be adjusted to the reactive power of the battery. The maximum admissible reactive power values of the parallel capa citor battery for different arrester types can be found in Table 5. If the reactive power of the parallel capacitor bank for a certain arrester type exceeds the limiting values from Table 5, an arrester with higher thermal energy rating must be selected. For systems that are not operated with a standard voltage, the limiting values for S K are found in the c olumn with the lower standard voltage. If the reactive power is very large, arresters connected parallel are to be chosen. In this case, the manufacturer has to be informed in order to take the necessary measures to guarantee a sufficient current distribution between the parallel arresters. The manufacturer should also be consulted when arresters with U c < U s are being used. Table 5: Arrester parallel to a capacitor bank. Maximum admissible reactive power S K of the capacitor battery for the indicated arrester type. Three discharges of the battery are allowed without a cool down phase for the arrester. W/U c : The arrester energy absorption capability in relation to U c. Arrester type U c U s POLIM-D POLIM-K POLIM-I MWK/MWD POLIM-S POLIM-H W th /U c in kj/kv 3.6* U s in kv S K in MVA r S K in MVA r S K in MVA r S K in MVA r S K in MVA r * Equivalent to Q th = 1.1 C

48 48 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 9.9 Line traps (parallel protection) Line traps are air-core coils that are installed in high-voltage lines. Their inductivity L is in the range of mh. If no measures are taken, the lightning current in the conductor must flow through the line trap. Even relatively small current rates of rise of several ka/µs would produce overvoltages on the line amounting to several million volts and would lead to a flashover. Arresters are connected to the line trap to prevent this. These arresters take over the lightning currents and limit the overvoltage to its residual voltage U res. When an earth fault to earth or a short-circuit occurs in a high-voltage system, the fault current I K flows through the conductor. This power frequency current would overload the arrester. U c must therefore be selected so that the short-circuit current flows through the line trap. It induces a temporary overvoltage of U TOV = ω x L x I K, which determines U c at the line trap. U TOV ω L I K U c = T T I K = Maximum fault current through the line trap. L = Inductance of the line trap It may be assumed T = for the duration of short-circuit current of t < 3 s (from TOV curve for the MWK) Line arresters Line arresters are arresters that are installed parallel to insulators on poles along an overhead line. The reason for the use of line arresters is the necessity to avoid short interruptions or outages of the overhead lines due to lightning overvoltages or the necessity to reduce the frequency of their occurrence. As a rule, the line arresters are installed in connection with an earthed shielding wire. In this application the arresters are called NGLAs (non-gapped line arresters). Line arresters are used in regions with high thunderstorm activity and a very poor earthing situation. The continuous voltage U c for MO surge arresters that are used as line arresters is to be determined in exactly the same manner as those used for the protection of substations or transformers. Since the line arresters should protect especially against the effects of lightning strokes, it is necessary to dimension them according to the lightning parameters of the respective region (probability, current steepness, charge, footing resistance, a.s.o). As a rule, the line arresters are equipped with disconnectors, so that an arrester that is overloaded can disconnect itself from the system and no earth fault appears. A special usage of line arresters is MO surge arresters with an external series gap. These EGLAs (externally gapped line arresters) are to be found in several countries. Figure 29 shows in principle the arrangement of an EGLA. The challenge is the coordination of the spark-gap in series with the MO surge arrester and the sparkgap parallel to the insulator to be protected, and also the residual voltage of the MO surge arrester used. In IEC and application principles and test procedures are given in detail. Figure 29: Possible execution of line arresters (description in principle). Earth wire Phase wire NGLA EGLA Insulator with arcing horn Insulator with arcing horn Tower Tower R E MO arrester parallel to an insulator in an overhead line. These so-called NGLAs (Non Gapped Line Arresters) are installed as desired with or without disconnector. R E,M MO arrester with an external spark gap in series parallel to an insulator in an overhead line (EGLA = Externally Gapped Line Arrester). R E,M

49 Continuous improvement of the products and process technology require specific test equipment to perform development tests in house. 49

50 50 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 10 MO surge arresters in parallel connection There are two reasons to connect arresters in parallel: to increase the energy handling capability, or optimize protection using MO surge arresters with different U-I characteristics and energy ratings General considerations Arresters are generally considered as single devices, i.e. they fulfill their task in the place in which they are installed according to their specified data, independent of other nearby devices. That is why it is possible in principle to install different kinds of arresters close to one another on a phase wire in the system. However, it is necessary to take into consideration that according to different ways of functioning, some arresters may become useless, while others may become overstressed, such as in cases when arresters with spark-gaps and without spark-gaps are installed in parallel, or when MO surge arresters with different voltage-current characteristics are used in parallel. Deliberate parallel connections of MO surge arresters are made if the energy absorption needs to be increased, the residual voltage should be reduced, or the energy absorption and the residual voltage should be deliberately dimensioned in a different way Parallel connections to increase the energy handling capability Two or more MO surge arresters can be connected in parallel in order to increase the energy handling capability if during an application the energy occurring cannot be handled by a single MO surge arrester. The requirement for equal current sharing, and consequently even energy sharing, between the arresters is the fact that the arresters have to have almost identical voltagecurrent characteristics. In view of the extreme non-linearity of MO resistors, small differences in the residual voltage in the area of switching current impulses bring big differences in current. With a nonlinearity coefficient of α 30 in the region of switching current impulses on the voltage-current characteristic, a difference of 5 percent in the residual voltage would lead to a current sharing ratio of 1:4 between the surge arresters. Therefore, it is absolutely necessary to perform a current sharing measurement on all MO surge arresters that are to be intended to work in parallel. The manufacturer must be informed when the order is made if the user intends to intentionally connect more MO surge arresters in parallel. It is also to be noted that the arresters are to be installed close to one another and are to be connected together with short connections of low inductance. If this is not taken into consideration, then separation effects may appear, which lead to uneven current sharing and consequently to an overstress of one of the arresters. The parallel connection of MO surge arresters has, besides the sharing of the current over more arresters, the positive effect of a better (i.e. lower) protection level. This is because the current density per arrester becomes lower in view of current sharing, and consequently a lower residual voltage occurs. It is to be strongly emphasized that it is always better to use a MO surge arrester with a larger MO resistor diameter than to connect more MO surge arresters in parallel with smaller MO resistor diameters Coordination of parallel-connected MO surge arresters In some cases, it is necessary or advantageous to use two arresters in an installation separated from one another in space, but electrically parallel on the same line. This is, for instance, the situation when in view of the distances in a substation, one of the arresters is installed at the entrance of the station and another arrester is placed directly in front of the transformer, at a certain distance. In such a case, two arresters of the same type and with the same continuous voltage may be used. In case of an incoming voltage, both arresters will discharge a part of the current towards the earth and will provide very good overvoltage protection. However, it is not to be assumed that the energy occurred will be uniformly shared.

51 MO SURGE ARRESTERS IN PARALLEL CONNECTION 51 MO surge arresters of different types or of the same type with different characteristics that are matched to one another are used deliberately if uneven sharing of the energy absorption is intended. This is the case, for example, in stations in which the transformer is connected through a cable to the overhead line, see Figure 30. An arrester is installed on the pole at the junction of the overhead line to the cable, and this arrester has a higher energy absorption capability and a lower residual voltage characteristic than the arrester in the station in front of the transformer. The effect of this is that the largest part of the energy is absorbed by the arrester outside on the pole, and at the same time, the voltage is limited as much as possible. Thus, the arrester in the station has to discharge only a small part of the current, and at the same time protects the transformer against overvoltages due to reflections. In practice, this principle can be used by choosing two MO surge arresters of the same type, such as MWK (class SL): the arrester in the station has a continuous operating voltage U c of about 10 percent higher than the arrester outside on the pole. The same result is reached if two MO surge arresters with the same continuous operating voltage U c but of different types are installed, such as a MWK (class SL) on the pole and a POLIM-D (class DH) in front of the transformer. Taking into consideration the smaller cross-section of the MO resistors of the POLIM-D compared to the MWK, its residual voltage characteristic lies automatically higher than the one of the MWK. In English-speaking countries, the arrester on the pole is described as a riser pole arrester. This is not a type description for an arrester, but specifies the installation place, which is the place where the cable rises up on the pole and where it is connected with the overhead line. Figure 30 shows an example. The MO surge arrester on the pole directly at the cable bushing is, for example, an MWK 20 with U pl = 61.4 kv, and the arrester in the station is, for example, a POLIM-D 20 with U pl = 70 kv. This coordination of residual voltage and the energy handling capability makes it possible that the larger amount of the current is discharged against the earth on the exterior of the station. In case of an unfavorable ground situation or in extremely lightning-endangered regions, the installation of an earth wire for some span width in front of the station is recommended. earth wire phase wire cable bushing tower tower cable substation R E,M R E,M R E,S Figure 30: Arrangement of two MO surge arresters to protect a station with cable entry.

52 52 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 11 Accessories To meet worldwide all different installation and performance requirements a large number of accessories is available Spark prevention unit The spark prevention unit (SPU) is a device to avoid wildfire hazards caused by thermally overloaded surge arresters. The SPU is installed in the earth connection of a medium-voltage arrester, as shown in Figure 31. The SPU monitors the load and thermal behavior of the surge arrester and interrupts the current in case of overload. Comparing to other solutions, e.g. arc rotators, the concept of the SPU prevents the spark production instead of controlling it. In this way, violent arrester failures and related arcing, sparking or emission of hot particles do not occur. The SPU is approved for application with classes DH and SL arresters up to 36 kv continuous operating voltage and that includes a trip indication clearly visible from ground level Disconnectors Disconnectors are used for automatically disconnecting a surge arrester that has been overstressed. Disconnectors are generally placed on the earth side directly under the arrester. The earth connection must be flexible, and it is necessary to have sufficient distance beneath the arrester, so that the disconnected earth connection can hang freely and the applied operating voltage that occurs at the foot of the arrester does not lead to spark-over. The purpose of disconnectors is to prevent overstressed arresters from leading to a permanent short-circuit resulting in the system switching off. It is thus possible to continue to supply consumers with electrical energy. This is surely an advantage in inaccessible areas or if the overstressed arrester cannot be quickly replaced. The disadvantage is that there is no overvoltage protection as long as the arrester is disconnected. That is why it is important to replace arresters that are out of order and were disconnected from the system as quickly as possible. If high-voltage fuses are installed in the same current path as the disconnectors, the response characteristics of both protection devices have to be matched to one another. The disconnector has to respond in time before the fuse or at the same time as it. This concept prevents the switching on an existing short-circuit when a new fuse is installed. Figure 31: Mediumvoltage arrester of class SL with spark prevention unit. Left: SPU installed below surge arrester. Right: SPU tripped after overload of surge arrester Indicators Indicators are devices that clearly indicate an overstressed arrester, i.e. a short-circuited arrester. Such devices are installed either on the overvoltage side or on the earth side directly at the arrester. In the event of an overstress, the short-circuit is permanent and the system is switched off, but the damaged arrester can clearly be detected and in this way can be quickly replaced. Indicators are used in lines or stations with arresters that cannot easily be visually controlled Brackets, ground plates and clamping devices A variety of different installation material like brackets (hangers), ground plates and clamping devices is available.

53 MONITORING OF MO SURGE ARRESTERS Monitoring of MO surge arresters The performance of modern MO surge arresters does not change under normal system conditions over the whole life time, assuming correct application. Monitoring of events, like surges due to lightning and/or switching, give valuable information about activities in sub-stations. Different methods of diagnosis and indicators were discussed and developed in the past for the condition monitoring of MO surge arresters. Surge counters can be installed if there is interest in monitoring the occurrences of the discharges of an arrester in the system. These surge counters count all discharges above the threshold value of the surge counter. Some modern products classify the current surges according their height. A milli-ampere-meter (ma-meter) can be installed if the continuously flowing leakage current of an MO surge arrester is to be monitored. If monitoring devices are used, for example, for measuring the continuous current that flows through an arrester, it is important to watch the current tendency. The momentary value cannot provide enough information about the condition of an arrester. For this, it is necessary to make the first measurement directly after the arrester installation and to record the conditions during the measurement (voltage, ambient temperature, pollution of the arrester housing, etc.) Thermal vision cameras can be used to detect the temperature of MO surge arresters. Here again, the absolute temperature is not really important, but for instance differences in temperature of arresters in the same sub-station, or a steady increasing temperature over time. Besides monitoring the MO surge arresters, the number and magnitude of counted surges gives valuable information about the events in a sub-station. It can also provide helpful statistics for the performance, potential malfunction and stresses seen by the system. In high-voltage applications, especially for GIS arresters monitoring devices like ma meters and surge counters are frequently installed.

54 54 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION 13 Overload and failure analysis The analysis of failed or overstressed MO surge arresters can give information about the reason of the failure. However, the information received from failed arresters is rather vague, because of secondary effects due to arcing. The reliability of modern MO surge arresters is very high. The probability of high-voltage arresters breaking down is virtually zero. With medium-voltage arresters, it is approximately 0.1 percent throughout the world; however, there are considerable differences regionally. The sealing system was the weak point in some older products with porcelain housings. Humidity was able to enter the housing after years of operation due to corrosion of the metal parts or due to deterioration of the sealing rings, which eventually led to the arrester breaking down. For modern MO surge arresters direct-sealed with silicone, there are only a few reasons for overstress. These include: extreme lightning strokes in the line directly at the arrester or unexpected high power-frequency overvoltages because of earth failures, ferromagnetic resonances, or a short-circuit between two systems with different system voltages. As a rule, the MO surge arrester builds a permanent earth or short-circuit in case of an overstress. If an arrester breaks down in the system, it is, in principle possible to get an idea of the cause of the failure from the failure mode. However, the information received from overstressed arresters is rather vague, because it is generally not possible to differentiate between the cause of the failure and the secondary effects due to the arc. If an overload case is to be examined, the following information should be available: All the lightning strikes that occurred close to the arrester before the breakdown and, if possible, also the height of the lightning current All the circuit breaker operations before the affected line broke down. The existing voltage at the arrester terminals before the breakdown and, if possible, a recording of the voltages Any earth faults at other points in the affected system A line diagram of the line or the installation with the position of the arrester before the breakdown Counting data of the surge counter, if any Ambient conditions at the time of the breakdown If an arrester breaks down in a phase and it is replaced, the other two arresters in the other phases should be also replaced, or they should at least be examined to determine if they have also been damaged. It is thus recommended that all three arresters be sent to the manufacturer for examination. It bears mentioning that an MO surge arrester fulfils its protection function even in a case of overloading. The voltage decreases towards zero due to the fact that an earth or short-circuit is produced, and in this way, the devices connected in parallel to the arrester are protected against excessively high voltages. The protection that takes place in an overload case is deliberately used in some special cases as the last possibility to protect very important and expensive electrical equipment. If the aim is to overstress an MO surge arrester at a predetermined point such as the exterior of a building this arrester is dimensioned to be deliberately weaker, from the voltage point of view, than the other arresters in the installation. These so-called victim or sacrificing arresters can be seen as an electrically predetermined breaking point in the system.

55 SUMMARY AND DEVELOPMENTS Summary and developments Ongoing research and development in material, design and production technology of MO surge arresters ensure reliable performance under very different and specific applications. Modern MO surge arresters with direct silicone molding are to be found in a large number of varieties, covering every necessity. In recent decades, they have proved to be very reliable as protection elements in the system. They protect electrical equipment that is much more expensive than themselves, and thereby guarantee high reliability and a good energy supply. They act as insurance against breakdowns in regard to high overvoltages. Integrated solutions are in discussion, and corresponding installations, devices and concepts are being developed for systems that are becoming more complicated. At the same time, the available space is shrinking. This means that a device has to perform more functions under certain circumstances. Correspondingly, an arrester could perform, in addition to the function of overvoltage protection, the function of a support insulator as well. Therefore, it is necessary to continue developing and optimizing MO surge arresters and all other electrical equipment. At the same time, it is necessary to continuously revise the standards and the application guidelines, because the requirements and the tests are changing, as well as the field experience is growing. The application of MO surge arresters, MO resistors or MO material in general (e.g. microvaristors) is increasing due to new applications in DC systems, DC circuit breakers, and power electronics. Questions about lightning and overvoltage protection are dealt with in different committees and working groups in IEC (standardization and application recommendations), Cigré and CIRED (field experience and trends).

56 56 APPLICATION GUIDELINES OVERVOLTAGE PROTECTION Acronyms/Abbreviations AIS Air insulated substation ANSI American National Standards Institute BIL Basic lightning insulation level (peak value). Similar to the LIVW according to IEC. The term BIL is used exclusively in US standards. CENELEC European Committee for Electrotechnical Standardization Cigré International Council on Large Electric Systems CIRED International Conference on Electricity Distribution DH Distribution High (arrester class) DL Distribution Low (arrester class) DM Distribution Medium (arrester class) GIS Gas insulated substation IEC International Electrotechnical Commission IEEE Institute of Electrical and Electronics Engineers I n Nominal discharge current of an arrester, i.e. the peak value of lightning current impulse which is used to classify an arrester. k Earth fault factor, k U s / 3 is the maximum voltage between phase and earth in case of an earth fault Lightning current impulse 8/20 current impulse with rise time of 8 µs and time to half-value of 20 µs. LIWV Standard rated lightning impulse withstand voltage of an equipment or insulation configuration (generally given in kv). MCOV Maximum continuous operating voltage (= U c ). Defined and used in US standards. MO Metal-oxide p.u. Per unit, 1 p.u. = 2 U s / 3 RUSCD Reference unified specific creepage distance SC A3 Study committee A3 of Cigré, responsible for high-voltage Equipment. SH Station High (arrester class) SiC Silicon carbide SIWV Standard rated switching impulse withstand voltage of an equipment or insulation configuration (generally given in kv) SL Station Low (arrester class) SM Station Medium (arrester class) SPU Spark Prevention Unit TC 37 Technical Committee 37 in IEC, responsible for surge arresters TOV Temporary overvoltage with power frequency of limited time duration U c Continuous operating voltage of an arrester, i.e. the designated permissible r.m.s. value of power-frequency voltage that may be applied continuously between the arrester terminals. U m Highest voltage for equipment, i.e. highest value of the phase-to-phase voltage (r.m.s. value) for which the equipment is designed in respect of its insulation. U n Nominal voltage of a system, i.e. a suitable approximate value of voltage used to identify a system. U pl Arrester lightning impulse protective level LIPL, i.e. the maximum residual voltage of the arrester at the nominal discharge current In. U ps Arrester switching protective level SIPL, i.e. the maximum residual voltage of the arrester for the switching impulse discharge current specified for its class. U r Rated voltage of an arrester, i.e. maximum permissible r.m.s. value of power-frequency voltage between its terminals at which it is designed to operate correctly under TOV conditions (t = 10 s). U ref Reference voltage of an arrester, i.e. the peak value of power-frequency voltage divided by 2 which is obtained when the reference current flows through the arrester. U res Residual voltage of an arrester, i.e. the peak value of voltage that appears between the terminals of an arrester during the passage of discharge current. U s Highest voltage of a system, i.e. highest value of the phase-to-phase operating voltage (r.m.s. value) that occurs under normal operating conditions in the system. Varistor Variable resistor ZnO Zinc-oxide

57 LITERATURE 57 Literature Consult the following literature for further information of the fundamentals of MO surge arresters and specific applications: Cigré TB 60: Metal Oxide Arresters in AC Systems, April 1991 by WG 06 of SC 33 Cigré TB 287: Protection of MV and LV Networks against Lightning. Part 1: Common Topics by CIGRE-CIRED JWG C4.402, 2006 Cigré TB 440: Use of Surge Arresters for Lightning Protection of Transmission Lines by CIGRE WG C4.301, 2010 ISBN: Cigré TB 441: Protection of MV and LV Networks against Lightning. Part 2: Lightning Protection of Medium Voltage Networks by Cigré WG C4.4.02, 2010 ISBN Cigré TB 455: Aspects for the Application of Composite Insulators to High Voltage ( 72 kv) Apparatus by CIGRE WG A3.21, 2011 ISBN: Cigré TB 544: MO Surge Arresters Stresses and Test Procedures by CIGRE WG A3.17, 2013 ISBN: Cigré TB 549: Lightning Parameters for Engineering Applications by CIGRE WG C4.407, 2013 ISBN: Cigré TB 550: Protection of MV and LV Networks against Lightning. Part 3: Lightning Protection of Low Voltage Networks by CIGRE WG C4.408, 2013 ISBN: Cigré TB 696: MO Surge Arresters Metal Oxide Resistors and Surge Arresters for Emerging System Conditions by CIGRE WG A3.25, 2017 ISBN: Hinrichsen, Reinhard, Richter (on behalf of Cigré WG A3.17) Energy Handling Capability of High-Voltage Metal- Oxide Surge Arresters Part 1: A Critical Review of the Standards Cigré SC A3 Technical Colloquium, Rio de Janeiro, September 12/12, 2007 Reinhard, Hinrichsen, Richter, Greuter (on behalf of Cigré A3.17) Energy Handling Capability of High-Voltage Metal- Oxide Surge Arresters Part 2: Results of a Research Test Program Cigré Session 2008, Paris, Report A3-309 Richter, Schmidt, Kannus, Lahti, Hinrichsen, Neumann, Petrusch, Steinfeld Long Term Performance of Polymer Housed MO surge arresters Cigré Session 2004, Paris, Report A3-110 Greuter, F., Perkins, R., Rossinelli,M., Schmückle, F.: The metal-oxide resistor at the heart of modern surge arresters; ABB Technik 1/89 W. Heiss, G. Balzer, O. Schmitt, B. Richter: Surge Arresters for Cable Sheath Preventing Power Losses in M.V. Systems. CIRED 2001, Amsterdam, June 2001 M. Darveniza, L.R. Tumma, B. Richter, D.A. Roby: Multipulse Lightning Currents and Metal-Oxide Arresters. IEEE/PES Summer Meeting, 96 SM 398 PWRD, W. Schmidt, J. Meppelink, B. Richter, K. Feser, L. Kehl, D. Qiu: Behavior of MO-Surge Arrester Blocks to Fast Transients. IEEE Transactions on Power Delivery, Vol. 4, No 1, January Richter, B., Krause, C. and Meppelink, J.: Measurement of the U-I characteristic of MO resistors at current impulses of different wave shapes and peak values. Fifth Int. Sym. on High Voltage Engineering, Paper 82.03, Braunschweig (Germany), 1987 W. Schmidt, B. Richter, G. Schett: Metal oxide surge arresters for gas-insulated substations (GIS) Design requirements and applications; CIGRE Paris Session 1992, Report L. Gebhardt, B. Richter: Surge arrester application of MV-Capacitor banks to mitigate problems of switching restrike; 19 th International Conference on Electricity Distribution (CIRED), Paper 0639, Vienna, May B. Richter New Test Requirements for Distribution Arresters; 32 nd International Conference on Lightning Protection (ICLP), Shanghai, China 13 th 17th Oct T. Christen, L. Donzel, and F. Greuter: Nonlinear resistive electric field grading part 1: Theory and simulation, IEEE Electr. Insul. Mag., vol.26, no. 6, pp , Nov./Dec L. Donzel, F. Greuter, and T. Christen: Nonlinear resistive electric field grading part 2: Materials and applications, IEEE Electr. Insul. Mag., vol.27, no.2, pp.18-29, March/April IEC standards relevant for MO surge arresters The selection describes the current state of the most important IEC standards on MO surge arresters and associated topics. IEC , Edition 3.0, Surge arresters Part 4: Metal-oxide surge arresters without gaps for a.c. systems IEC , Edition 3.0, Surge arresters Part 5: Selection and application recommendations IEC , Edition 1.0, Surge arresters Part 6: Surge arresters containing both series and parallel gapped structures Rated 52 kv and less IEC , Edition 1.0, Surge arresters Part 8: Metal-oxide surge arresters with external series gap (EGLA) for overhead transmission and distribution lines of a.c. systems above 1 kv IEC , Edition 1.0, Surge arresters Part 9: Metal-oxide surge arresters without gaps for HVDC converter stations IEC , Edition 8.1, Insulation coordination Part 1: Definitions, principles and rules IEC , Edition 3.0, Insulation coordination Part 2: Application guide IEC , Edition 3.0, High-voltage test techniques. Part 1: General definitions and test requirements IEC 60507, Edition 3.0, Artificial pollution tests on high-voltage ceramic and glass insulators to be used on a.c. systems IEC/TS , Edition 1.0, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions Part 1: Definitions, information and general principles IEC/TS , Edition 1.0, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions Part 2: Ceramic and glass insulators for a.c. systems IEC/TS , Edition 1.0, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions Part 3: Polymer insulators for a.c. systems IEC 60038, Edition 7.0, IEC standard voltages US standards relevant to MO surge arresters IEEE C IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (> 1 kv) IEEE C IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating Current systems

58 Additional information We reserve the right to make technical changes or modify the contents of this document without prior notice. With regard to purchase orders, the agreed particulars shall prevail. ABB AG does not accept any responsibility whatsoever for potential errors or possible lack of information in this document. We reserve all rights in this document and in the subject matter and illustrations contained therein. Any reproduction, disclosure to third parties or utilization of its contents in whole or in parts is forbidden without prior written consent of ABB AG.

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