Bernhard Richter*, Alexander Bernhard*, Nick Milutinovic** SUMMERY Based on the system voltages for AC and DC railway systems the required voltage ratings for modern gapless MO surge arresters are given. The origin of overvoltages occurring in railway systems are explained, and some typical figures are presented. VDV Recommendation 525 gives a protection concept for railway systems with U n = 600 V and U n = 750 V DC, including requirements for MO arresters. This protection concept is becoming common praxis in other DC railway systems as well, e.g. in 1500 V DC and 3000 V DC systems. The described concept provides protection of equipment against overvoltages originating from lightning and switching. For the protection against dangerous touch voltages low voltage limiters are used. These devices, generally based on gap technology, have the disadvantage of short-circuiting when stressed with impulses due to lightning. To overcome this problem a new hybrid voltage limiter, a combination of MOsurge arrester and low voltage limiter was developed and introduced. Tests with LVLs prove that these devices go in short-circuit conditions after the first impulse stress. LVLs with claimed lightning resistance withstand impulse stresses originated by lightning phenomena. Anyhow, the discharge voltage is changing after applied impulse stresses, being then in some cases out of the declared tolerances. The hybrid voltage limiter HVL turns out to be the only device that protects against overvoltages and touch voltages and provides reliable and safe protection. 1 INTRODUCTION Overvoltages in electrical supply networks of AC and DC railway networks result from the effects of lightning strokes and switching actions and cannot be avoided. Furthermore, the power supply of electrical railways is endangered by direct or nearby lightning. Considering additional the safety aspects for people (touch voltages) in railway systems, one has to deal with three aspects: direct or nearby lightning, induced voltages, and unacceptable voltages due to failures in the system (e.g. breaking of overhead line). Depending on the insulation of the rails against earth, different aspects have to be considered for a general protection concept. A new device, combining protection against overvoltages and touch voltages is introduced. 2 VOLTAGES IN RAILWAY SYSTEMS 2.1 System Voltages The supply voltages for railway networks are given in the European Standard EN 50 163: Railway applications Supply voltages of traction systems. In the following some definitions and values are given, which are necessary for the dimensioning of the MO-surge arresters and the protection concept in general. Nominal voltage U n : the designated value for a system. Highest permanent voltage U max1 : the maximum value of the voltage likely to be present indefinitely. Highest non-permanent voltage U max2 : the maximum value of the voltage likely to be present for maximum of 5 min. Table 1 gives the voltage values for the relevant DC and AC systems. U n in V, DC U max1 in V, DC U max2 in V, DC 600 720 770 750 900 950 1500 1800 1950 3000 3600 3900 U n in V, AC U max1 in V, AC Table 1 U max2 in V, AC 15 000 17 250 18 000 25 000 27 500 29 000 The values in Table 1 for U max2 can become 800
V in the 600 V system, and 1000 V in the 750 V system in case of regenerative breaking. The AC values are given in rms values. The system with U n =15 kv has a power frequency of f=16,7 Hz, the system with U n =25 kv a power frequency of f=50 Hz. 2.2 Overvoltages The dimensions (height, insulators, etc.) of overhead AC and DC power supply systems in railway networks are somewhat similar to medium voltage (MV) distribution networks. Therefore, with respect to overvoltages in AC and DC power supply systems the same considerations as for MV systems may be used for an estimation of the occurring stresses. The expected number of direct and indirect lightning events depends on the exposure of the line and on the screening provided by nearby objects such as trees, buildings, towers, etc. Overvoltages due to direct and indirect lightning strikes are characterized by complex and very different wave shapes. Overvoltages due to a direct strike have an overall wave shape similar to that of the incident lightning current, the fast rising portion being superimposed with several spikes which are due to insulation breakdown. Induced overvoltages are characterized by pulse width significantly shorter than the channel-base current of the lightning and their wave shape is strongly affected by the ground electrical conductivity. 2.2.1 Overvoltages due to direct lightning A lightning striking a phase conductor injects current waves in both directions. The corresponding voltage equals half of the current multiplied by the line characteristic impedance, which is about 400 Ω. Since more than 90% of the strokes have a peak current of at least 10 ka, the overvoltage will exceed 2 MV for 90% of the strokes. These voltages are far above the lightning withstand of the line. For this reason flashovers between phase and earth will occur. Depending on the type of pole and the earthing conditions this voltage can still reach values up to the MV range. 2.2.2 Lightning-induced overvoltages A cloud-to-ground lightning flash generates a transient electromagnetic field which can induce overvoltages of significant magnitudes on overhead power lines in the vicinity. The return stroke of the lightning discharge is considered to be the major responsible for the induced voltages, because the highest di/dt in the lightning channel occurs during this phase. The analysis and calculation of lightning-induced overvoltages require the use of elaborate models. The complexity of these models calls for an implementation into computer codes because they require a numerical integration of the relevant equations. For more details see for instance, where the different models are described and the influencing lightning parameters (peak value of the current and steepness di/dt) are explained. Table 2 gives typical parameters for induced overvoltages (calculated values, considering the point of flash in 50 m distance from the line, adopted from [1]) First stroke I peak = 30 ka Subsequent stroke I peak = 12 ka U peak (kv) 74 53 du/dt max (kv/µs) 52 153 Front time (µs) 2,8 0,8 Time to half value (µs) Table 2 5,8 1,7 For illustrating the influence of the peak value of the current the following examples for induced overvoltages U peak are given (considering a constant di/dt of 40 ka/µs) I peak U peak 4,6 ka 28 kv 12 ka 66 kv 30 ka 133 kv For a constant current peak value of I peak = 12 ka the steepness di/dt of the current influences the peak value U peak of the induced voltage as follows. di/dt U peak 12 ka/µs 50 kv 40 ka/µs 66 kv 120 ka/µs 72 kv
2.3 Touch Voltages The earthing concepts for AC and DC railway systems are different. In AC systems the rails are directly connected with all metallic parts along the rails, e.g. the poles, and the common earth. In DC systems the rails are isolated against the earth to avoid stray currents. Especially in DC systems dangerous potential differences can occur due to normal train service, and especially in case of failures like broken overhead lines or derailed pantographs. All this leads to endangering of persons and has to be avoided by the use of low voltage limiters. The acceptable values for touch voltages U t and admissible voltages U c1 are given in different standards, for instance in EN 50 122-1:1997 [2]. The acceptable touch voltages U t are depending on the time duration. The longer the time, the lower the value. The touch voltages depend on the impedance of the human body, the resistance of the shoes etc., and therefore it is complicated to estimate exact values for all cases. As long as the effects of AC and DC currents are somewhat different in the human body, the values for the touch voltages are a bit different for AC and DC. In the time range of t=0,02 s the touch voltages can be up to U t =940 V, for t=0,5 s the voltage is U t = 395 V (for DC systems), and for t=1 s and longer time durations the voltage has to be in any case below 80 V or even 25 V depending on the degree of safety required. 3 PROTECTIVE DEVICES 3.1 MO Surge Arresters The wish to increase the reliability and safety of the arresters, and correspondingly of the energy supply, brought about the development of the arresters with polymeric housing in the middle of the eighth decade of the last century. Silicon, as an insulating material with excellent properties, especially under severe pollution conditions, is used in the high voltage area since about 35 years. Due to the various possibilities of application in very difficult electrical, mechanical and environmental conditions special designs are increasingly recognized globally, especially in medium voltage systems and for specific applications as for instance DC systems. When designing arresters for different network voltages, three main characteristics are to be considered: the highest continuous voltage that arises in the network, the protection level, and the energy absorption capability of the arrester. The strong voltage fluctuations in the railway systems make it necessary to lay the maximum continuous operating voltage U c of the MOarrester above the highest continuous voltage U max1 of the system. The highest non-permanent voltage U max2 of the system can appear for 5 min, but it is not known how often and in which time intervals this can happen. As long as the modern MO-surge arresters have very favourable protection levels combined with a high energy absorption capability, it is possible to lay the MO-arresters continuous operating voltage U c equal or higher to U max2, so that follows U c U max2. It follows for the system voltages acc. to Table 1: U n = 600 V => U c 800 V U n = 750 V => U c 1000 V U n = 1500 V => U n = 3000 V => U c 1950 V U c 3900 V U n = 15 kv => U c 18 kv U n = 25 kv => U c 29 kv 3.1.1 MO surge arresters for AC application AC networks with 15 kv, 16,7 Hz. With the SBB and other Swiss Railways MO surge arresters with U c =18 kv are used on electrical locomotives and in overhead lines. Due to safety aspects completely in silicon moulded MO surge arresters with line discharge class 3 are used for the overhead lines, and line discharge class 4 for the use on locomotives. The German DB adopted a coordination concept with the locomotive BRE 101, which is used in the interregional trains, and installs on the roof of the locomotive a MO surge arrester with U c =18 kv and in the locomotive one with U c =20 kv. Again, the arrester on the roof of the locomotive is of line discharge class 4, the one in the locomotive has a lower line discharge class. AC networks with 25 kv, 50 Hz. MO surge arresters with U c =29 kv to U c =31 kv are used in the German National Railways (DB). With modern E-locomotives the high voltage from the collector is brought into the inner part of the locomotive through a cable, and this requires again a coordination of the MO arresters on the locomotive and in the locomotive. On the roof of the locomotive are two arresters with U c =29 kv and line discharge class 4 are installed, at each pantograph one, and in the locomotive an
arrester with U c =31 kv and a lower line discharge class in front of the main power breaker. The ratio U p /U c can be chosen 3, as long as the thermal stability of the MO-arrester in the system is not endangered. A ratio U p /U c >3 is not acceptable. Most of the modern MO-surge arresters on the market to be used as A1-arresters have a lower protection level than the here proposed 3,0 kv, for instance U p =2,4 kv. It should be taken advantage of the better protection level when choosing an A1-arrester. Fig.2 shows examples of A1- and A2-arresters. Fig. 1: MO surge arrester type POLIM-H 18 N on the roof of a locomotive (left). 3.1.2 MO surge arresters for DC application DC networks of 600 V and 750 V. In [3] a protection concept for DC railway systems with a nominal voltage U n =600 V and U n =750 V, and the requirements for MO-surge arresters to be used, are proposed. The protection concept defines two arresters according to their place of installation and their application, a type A1, between the overhead line, or the feeder-line, and the return line (may be the rail), and a type A2 between the return line (rail) and the so called station earth (equipotential bar). The details of this concept are given in the following chapters. Requirements on A1-arresters: - Maximum continuous operating voltage U c 1,0 kv - Protection level U p 3,0 kv - Nominal current I n (8/20 µs) 10 ka - Rectangular current, 2 ms I rw 1200 A - Short circuit current 20 ka Requirements on A2-arresters: - Maximum continuous operating voltage 120 V U c 300 V - Protection level 360 V U p 900 V - Nominal current I n (8/20 µs) 10 ka - Rectangular current, 2 ms I rw 1200 A - Short circuit current 20 ka Fig. 2: A1-arrester with U c =1,0 kv (left) and A2- arrester with U c =0,29 kv (middle). Both the arresters are completely moulded in silicon. On the right a HVL is shown. 3.2 Low Voltage Limiters In order to avoid stray currents in DC railway systems it is common practise not to have a direct earthing of the rails, as far as possible. In case of a broken overhead line (falling down of the conductor) an unacceptable high touch voltage may occur. Normal train service may also produce a voltage rise of unacceptably high values. In such cases personal protection is given by the use of low voltage limiters (LVL). Basically, LVLs consist of two opposite copper electrodes, insulated from each other, which produce a remaining short-circuit (by melting together) after energy absorption beyond a specified maximum value. The insulation is implemented by a ring of insulating material. Alternatively a gas discharge tube is used to provide insulation and to ensure a certain degree of protection against surges in addition. LVLs produce a short-circuit between the two potentials whenever an unacceptable touch voltage occurs. The short-circuit persists because virtually all LVLs existing in the market are not reversible once after being in shortcircuit condition. Low voltage limiters, installed in substations and along the rails, are stressed with surges originated by lightning.
3.3 Hybrid Voltage Limiter HVL 120-0,3 The HVL can be considered a fully reversible low voltage limiter with very high energy handling capability, combining the required personal protection against touch voltages with the protection of electrical equipment against lightning overvoltages. The HVL is made up from a metal oxide (MO) resistor in parallel to two thyristors in anti-parallel configuration. The MO resistor functions as a surge arrester and limits lightning and switching overvoltages (time range between some microseconds to some hundred microseconds). The thyristors short-circuit potential rises of the rail and provide protection against dangerous touch voltages (time range app. 400 µs up to minutes). The MO-resistor in the HVL is of the same diameter and the same electrical characteristics as the MO-resistor in the A1-arrester. Therefore, by combining an A1-arrester with a HVL an optimised protection can be achieved. This protection concept can be used in overhead lines and in substations as well, just by replacing the A2-arrester by a HVL. Figure 2 shows a HVL with accessories as well as examples of A1 and A2 arresters. U U I MO-resistor no trigger Triggerelektronik trigger unit thyristor triggered antiparallel thyristors overvoltage protection protection against touch voltages Fig. 3: Hybrid voltage limiter type HVL. Figure 3 shows on the top the principle circuitry of the HVL, in the middle the lightning currents and corresponding overvoltages, handled by the MO resistor, and at the bottom the potential rises of longer duration, which lead to triggering of the thyristors. The failure current is then commutated to the thyristors and the voltage is virtually zero. Fig. 4: Hybrid voltage limiter of type HVL, practical installation 4 PROTECTION CONCEPT A protection concept for railway systems, especially for DC systems, has to consider the protection of equipment (against overvoltages) and the protection of persons (against touch voltages) in the same time. Of great importance is the safety aspect in public places, as for instances train stations, rail crossings, etc.. Based on the VDV publication 525 [3] a protection concept for DC railway systems is discussed which considers the overvoltage protection of the supply lines against the return line (generally the rail) and the protection of the return line against the station earth. The protection against touch voltages is included in this concept. In [3] a protection concept for DC railway systems with U n = 600 V and 750 V is proposed, as already mentioned. This concept considers the protection of overhead lines and substations. 4.1 Protection Measures On The Overhead Lines It is recommended to install gapless arresters for outdoor use at each service entrance, at the end of feeding sections, and at the coupling point as well as at points of power demand (e.g. for point heaters). For track sections with frequent lightning strokes, for instance at bridges or free overland routes additional arresters are
advisable. These arresters are called A1- arresters and should fulfil the electrical requirements as given in chapter 3.1.2. Figure 5 shows the principle arrangement. Power demand isolated rails (open ballast). The remaining potential on the pole may lead to touch voltages of unacceptable value. The remaining of the failure voltage can be avoided with the installation of a second arrester. It is proposed to install a type A2-arrester as used in the substations, between the earthing terminal and the rail. A hybrid voltage limiter (see chapter 3) could be used alternatively to the A2-arrester, as it is shown in Figure 7. Feeder lines Fig. 5: Installation of A1-arresters along the track with reference to VDV Recommendation 525 [3]. b The arresters should be installed at the level of the overhead line and connected in the shortest possible way with insulated conductors. In case the rails are isolated against earth in open ballast (to avoid stray currents) it is recommended to connect the A1-arrester to a separate earth connection with a resistance less or equal than 10 Ω. In case the rails are in closed ballast not isolated against the earth, the A1-arrester is to be connected via an insulated connector directly to the rail. Figure 6 shows the two possibilities. a c l vom Unterwerk Fig. 6: Connection of the A1-arrester to the separate FFF earth termination (left), and to the rail (right). a: insulated cable to earth termination, b: insulated cable to rail, c: feeder line from substation. b c Fig. 7: Connection of an A2-arrester, or alternatively a hybrid voltage limiter, between the earth terminal and the rail in case of isolated rails. b: insulated cable. It has to be pointed out, that the installation of a hybrid voltage limiter is the better solution in all respects. Due to the limited protection distance arresters are protecting only a distance of some meters. This means that the arresters generally have to be as close as possible to the equipment they have to protect. The connections should be as straight and short as possible. Due to the length of the earth connection a remarkable inductive voltage drop will occur along the earth connection, which has to be added to the residual voltage of the arrester. The additional voltage drop can be calculated with the equation u(t) = L di/dt. For a straight connection L can be roughly estimated with 1 µh/m. In the very rare cases of an overload of the A1- arrester a current of some 10 A would flow against earth, which would not be detected as a failure current by the protection relays in case of 4.2 Protection Measures For Substations Arresters installed along the track can protect only the equipment in the vicinity, and not electronic or other equipment in the substations.
Feeder lines Speiseleitungen vehicles and directly connected to the pantograph. The arresters have to fulfil the same requirements as the A1-arresters on the poles of the overhead lines. Special mechanical requirements as shock and vibration resistance have to be fulfilled in addition. Safety aspects, as explosion resistant design and the use of inflammable materials have to be considered. Figure 9 shows a practical installation. L1 L2 L3 A1 A1 Return line Rückleiter A2 A2 PB PAS R E R E;UW Fig. 8: Protection of a DC substation with A1- and A2-arresters. PB: potential bonding bar, R E : earthing resistance. For the protection of the DC-substations it is proposed to install A1-arresters directly at the breakers in the substation between the feeder line and the return line (rail). Additional an A2- arrester should be installed between the return line (rail) and the equipotential bonding bar. The A2-arrester reduces overvoltages travelling along the rail, originated by lightning, and protects against potential rise of the rail against the earth of the building. The use of A1- and A2- arresters in the substations is nowadays essential, because electronics are more and more used. Nowadays A1- and A2-arresters are on the market, which are adapted to the special requirements of DC railway systems. Figure 8 shows the principle of the substation protection. Alternative to the A2-arresters hybrid voltage limiters can be installed as well. This should be done in general, if besides the overvoltage protection, personal protection against touch voltages is required. 4.3 Protection Of Traction Vehicles It is state of the art to protect the cable bushings and the equipment downstream with arresters of type A1, installed on the roof of the traction Fig. 9: A1-arrester installed on the roof of a traction vehicle at the base of the pantograph. 5 CONCLUSION Modern MO surge arresters with silicon housing have been adapted to the special requirements in AC and DC railway systems. The installation of AC and DC MO surge arresters in substations, overhead lines and on locomotives is state of the art and worldwide practice. Coordination concepts and newly developed hybrid voltage limiters allow new protection concepts. 6 REFERENCES [1] Cigré Technical Brochure 287: Protection of MV and LV networks against lightning Part 1: Common Topics. Cigré JWG C4.4.02, 2005 [2] EN 50122-1:1997 Railway applications Fixed installations Part 1: Protective provisions relating to electrical safety and earthing. [3] VDV Recommendation 525 01/06: Protection of DC Traction Power Supply Systems in case of a Lightning Strike. Association of German Transport Undertakings (VDV), Köln.