Power Frequency Withstand Voltage On-site testing of 400 kv GIS D. Anaraki Ardakani, A. Omidkhoda, M. Solati High Voltage Engineering Center ACECR Tehran, Iran Da_ardakani@yahoo.com Paper Reference Number: Name of the presenter: D. Anaraki Ardakani: Abstract Using Gas Insulated Substation in urban areas in the world has been widely popular. After installation, and before being put into service, the GIS shall be tested in order to check the correct operation and the dielectric integrity of the equipment. Since it is especially important for GIS, the dielectric integrity shall be checked in order to eliminate fortuitous causes (wrong fastening, damage during handling, transportation, storage and installation, presence of foreign bodies, etc.) which might in the future give rise to an internal fault. Because of their different purpose, these tests shall not replace the type tests or the routine tests carried out on the transport units and, as far as possible, in the factory. They are supplementary to the dielectric routine tests with the aim of checking the dielectric integrity of the completed installation and of detecting irregularities as mentioned above. In this paper, a novel test set-up and the measurement results of a 400 kv GIS has been installed for the Sheykh Bahai Substation, Tehran, Iran, are presented. The study of test conclusion shows that it is done successfully in three phases. Key words: Dielectric on site test, GIS, Cascade transformer, Power frequency withstand voltage test, MATLAB 1. Introduction Gas Insulated Substation (GIS) has been in service for more than 40 years and they have shown a high level of reliability with extremely small failure rates even for modern GIS showing a high compactness.gis was first developed in various countries between 1968 and 1972. After about 5 years of experience, the use rate increased to about 20% of new substations in countries where space is limited. In other countries with space easily available, the higher cost of GIS relative to AIS has limited use to special cases. For example, in the U.S., only about 2% of new substations are GIS (Bolin, 2003). Each GIS in compare with Air Insulated Substation (AIS) with similar condition need to 10% space. In many countries the safety of electrical installations against military and terrorist attack has prime importance. This kind of substation is a proper choice since it can be concealed and protected easily. In Iran, also many GISs have been installed successfully up to extra high levels of voltage such as 400 kv and they are in use presently. Although the reliability of GIS is high, any internal breakdown that does occur invariably causes extensive damage and an outage of several days
duration is needed to affect the repair. During this time the associated circuit may be out of operation and the consequential losses can be high, especially if the GIS is operating at 420 kv or above. If in addition the GIS is connecting the output of a nuclear station to the transmission network and the breakdown leads to a reactor shutdown, the financial penalties could be most severe (A.Hadda, 2007).Therefore after installation, and before being put into service, the GIS shall be tested in order to check the correct operation and the dielectric integrity of the equipment. These tests and verifications comprise dielectric tests on the main circuits, dielectric tests on auxiliary circuits, measurement of the resistance of the main circuit, gas tightness tests, checks and verifications, and gas quality verifications ( IEC 62271-203,2012). Because of their different purpose, these tests shall not replace the type tests or the routine tests carried out on the transport units and, as far as possible, in the factory. On-site high-voltage tests are required (IEC 60060-3, 2006): 1- As withstand tests as part of a commissioning procedure on equipment to demonstrate that transport from manufacturer to site, and the erection on-site complies with manufacturer s specification; 2- As withstand tests after on-site repair, to demonstrate that the equipment has been successfully repaired, and is in a suitable condition to return to service; 3- For diagnostic purposes, e.g. PD measurement, to demonstrate if the insulation is still free from dangerous defects, and as an indication of life expectation In this paper, the quality of performing the power frequency test in 400 kv GIS substation of Sheikh Bahai and the measured results are demonstrated. In the following sections, we will review the possible solutions to perform the test; the method which is used and the voltage measurement are desired in these segments. 2. The Possible Solutions Generally the normal laboratory testing equipment is not proper for transportation and Special designs of portable systems are necessary. These may take the form of custom-built vehicles containing, for example, High Voltage (HV) testing transformers and standard capacitors. It is obvious that an important factor in test equipment design is the need for greater mechanical strength than their counterparts in the laboratory and increased awareness regarding safety aspects while setting up temporary test areas. (A.Hadda, 2007). The power frequency voltage may be produced by (IEC 62271-203, 2012): test sets with a test transformer, test sets with a variable resonant reactor for constant frequency, test sets with a constant resonant reactor for variable frequency, energizing power or voltage transformers from the low-voltage side which entails no dismantling after testing. 3. APPLIED SOLUTION: CASCADED VOLTAGE TRANSFORMERS
In this test the method which is used is test set with a test transformer. High voltage transformers are not usually proper for portable test system due to their big dimensions and high weight. Many experiments are performed for dielectric test on GIS, and systems with different power and energy are employed for on-site tests. In order to perform the AC test we need a voltage source with a high power due to Load capacitances of GIS installations are relatively high (10 or even 20 nf per bus bar). For instance, an 8 nf capacitor in kv voltage needs 1 A current in output which is considered a high power. Two high voltage transformers are made and designed for the test in this study. 3.1. The structure and components of system set This system includes following components (Fig. 1) 1) Regulating Transformer 2) Compensating Reactor 3) Cascade Transformers 4) Voltage Divider 5) Control Panel 6) Damping Resistor 7) High Voltage Connections and Earth System Insulating case transformers are well suited for cascade connection. These transformers are made as the standard essentials IEC for the test. HV transformers are supplied by a Regulating transformer. These regulating transformers are designed in different ranges. Regulating transformer supply the desired voltage by a motor which has a standard rate. According to standard after that voltage reaches to 75% of test voltage, it would increase by 2% of test voltage per second. In order to compensate the reactive power made by capacitor currents of substation, compensation reactors are used in low voltage side. These reactors are placed in low voltage side and its preference is to need a low power for regulating transformer. In this test, three reactors (two air core and one oil reactor) by different capacities are used. According to every capacitance of each test level, its connections are added to the circuit manually in parallel forms. In order to limit the current and increase short circuit impedance of transformer when possibility of discharge occurs, the damping resistor is placed in low voltage circuit in series mode on the regulating transformer output. 3.2. Test procedure As regards the aim of these tests is to offer a final check before energizing. It is very important that the chosen test procedure does not jeopardize sound parts of the GIS.
1) Regulating Transformer 2) Compensating Reactor 3) Cascade Transformers 4) Voltage Divider 5) Control Panel 6) Damping Resistor Fig 1: General schematic of test circuit In choosing an appropriate test method for each individual case, a special agreement may be necessary in the interest of practicability and economy, e.g. the electrical power requirements and the dimensions and weight of the test equipment may need to be considered (James, 2007). Before the test starts, is considered the following points: The GIS shall be installed completely and gas-filled at its rated filling density. Some parts may be disconnected for the test, either because of their high charging current or because of their effect on voltage limitation, such as high voltage cables and overhead lines; power transformers and, occasionally, voltage transformers; surge arresters and protective spark gaps. The sections which, in such cases, are not being tested, and which are isolated by a circuit breaker or a disconnector from the section under test, shall be earthed For 3-phase enclosed GIS, the specified test voltage shall be applied between each phase conductor, one at a time, and the enclosure, the other phase conductors being connected to the earthed enclosure. The insulation between phase conductors shall not be subjected to any other separate dielectric test on site. The test voltage source may be connected to any convenient point of the phase conductor under test.
It is often convenient to divide the whole installation of GIS into sections by opening circuit breakers and disconnectors for following reasons (IEC 62271-203, 2012): to limit the capacitive load on the test voltage source; to facilitate the location of disruptive discharges; to limit the discharged energy if a disruptive discharge occurs. In this test, the substation which is installed is divided to three separate sections (section1, section2, section3) which is shown in Fig. 2. Considering that transport units have normally been subjected to routine tests, the probability of disruptive discharges is higher for the complete installation than for individual functional units, disruptive discharges in correctly installed equipment shall be avoided and agreed between the manufacturer and user applied voltage are done by the following test sequence (IEC 62271-203, 2012): The testing voltage is increased to 230 kv and it is kept to 15 (min). Testing voltage is being increased again to kv and is kept to 1 (min). Then the voltage level would return to zero as soon as possible (Fig. 3). The switchgear shall be considered to have passed the test if each section has withstood the specified test voltage without any disruptive discharge. Fig 2: The GIS is divided to three parts to perform the test, which is shown in different layouts.
(V (kv)) 230 15 1 (t(min)) Fig 3: Test sequence 4. Simulation Before doing the practical test, simulation is performed with MATLAB (version 7.0.4) and the results are used for the practical test.the simulation is according to maximum load (capacitance of the largest sections) 6 nf, technical specification of test equipments (for example Transformer 1 (T1):0.5/400 kv, Transformer 2 (T2):0.4/300 kv) and existing reactors in this test that have different reactance (L1=0.5 mh, L2=1 mh, L3=4 mh). Due to the table I the max of current in reactors is about 1260 A (L1) and the network current with maximum 300 A and regulating transformer with maximum 400 A is needed. 5. Practical Test According to standard it is often convenient to divide the whole installation of GIS into sections so this substation by opening circuit breakers and disconnectors is divided in to three sections in the test and test sequence applied to each section. In table II different current of each reactor and secondary transformer and regulating transformer are shown. Currents are relatively high that supplied by reactors. Without these reactors demanded power and subsequently the reactor dimension would increase. Also the network with a high power is needed. The reason of diversity in capacitor current of each section is the length of bus bars and their different capacitance. Section 3 has a higher capacity according to the extra Bus duct than other sections and thus, the longer length. Then it would have higher capacitor current (charging current). The used transformers in the test have the transformation ratio of 400 V/300 kv, 500 V/400 kv, so we get 700 kv for the whole set. Max. of Load (nf) Reactors Current (A) L1=0.5(mH) L2=1(mH) L3=4(mH) Capacitor Current (ma) Output Voltage (kv) Network Current (A) Secondary Current of Regulating Transformer (A) 6 1260 630 157 0.795 417 310 407 TABLE I. CURRENT OF REACTORS, REGULATING TRANSFORME AND CAPACITOR IN SIMULATON OF TEST CIRCUIT In order to get the final testing kv voltage, the primary 238 V is needed. In sections 1 and 2 only one reactor (L1) is used according to load capacitance. As shown in table II even by
the compensation reactors, a relatively high current flowed in the secondary regulating transformer. If the reactors didn t use, the maximum current in low voltage side reached to 1400 A instead of 400 A, which is a high current and it actually would make the test impossible and impracticable. Because this regulating transformer definitely would have big dimension and volume, they cannot be moved easily and there were no low voltage network available having these feature. So one of the essential parts of this test was to lower the transformer current to 400 A, based on available tools and equipment. Hence, using the reactors to meet this essential need was one of the most important parts of the test (each reactor have different maximum current, L1=1500 A, L2=700 A, L3=600 A). The other of essential parts of the test was power limitation of T2, according to simulation the maximum capacitor current in section3 (the largest section) is higher than power of T2 and it could cause problem to do the test. But, considering the short time (1 minute) of the maximum apply test voltage, it is negligible. A capacitive divider was used in the test to measure the HV output. In order to remove the corona and proper distribution of voltage a top electrode is placed on its upper cover of divider (Fig. 4). The top electrode is either an aluminum toroid. To measurement of output voltage is use Digital Peak Voltmeter (DPV). This voltmeter is made in the Academic Center for Education, Culture and Research (ACECR) and measures the output voltage. The earth connections between the elements are considered star mode. For each element, the set is connected to the earth independently. Fig 4: A view of HV capacitors for voltage divider (left),and cascade transformers (right) No. of Sections 1 1 1 2 2 2 3 3 3 Phase Name A B C A B C A B C Output Voltage (kv) Reactors Current (A) L1 L2 L3 1080 - - 1089 - - 1089 - - 1080 - - 1079 - - 1050 - - 980 600 190 890 600 200 950 600 210 Secondary Current of regulating transformer (A) 145 133 136 136 134 139 353 365 398 Capacitor current (Charging current) (ma) 560 540 546 550 544 542 761 771 794 TABLE II. CURRENT OF REACTORS, REGULATING TRANSFORME AND CAPACITOR IN TEST CIRCUIT
In this test tried to consider the shortest route for the earth connection especially in GIS substation, in order to prevent the voltage differences if possible flashover happens (James, 2007). In order to apply high voltage on every Bushing s head, an HV connection with diameter of 8 cm is used to remove corona. 6. Test result The test with kv is indicated by cascade transformer (two transformers) with displayed currents in table II is performed at 400 kv substation of Sheikh Bahai-Tehran. The highest current lead to the highest power of 330 kva. During the test no breakdown is seen. Finally all the sections were tested in the applied voltage of 230kv in 15 (min) and kv in 1 (min) without any problems or errors. 7. Conclusion Dielectric testing on GIS substation is one of the most important indicators to confirm the accuracy of these systems before initiation. According to the large dimensions of the substations it is impossible to test whole GIS before its installation. On the other hand, because of relatively large capacitance of such systems, especially in cases where the total length of the installed GIS exceeds some tens of meters, the high voltage source has to be capable of delivering relatively high levels of current. In this paper, a high voltage test set is proposed to perform in-situ insulation tests on a 400 kv GIS installed for the Sheikh Bahai- Tehran Iran.Different solutions are discussed for the test and its results. The method used in the test includes two cascade transformers with 1 A output current which fed by a regulating transformer. In this method three low voltage reactors used to compensate the capacitive current and minimize the input current of the test transformers.gis is divided into sections and the applied voltage of each step was kv. The results show that the test was successful and the GIS was confirmed in the dielectric test. References Bolin, P. (2003). Electric Power Substations Engineering. CRC Press. Feser, k. (1980). High Voltage of Testing ofmetalenclosed,gis on-site Oscillating switching Impulse Voltages. HAEFELY. Hadda, A., D. W. (2007). Advances in high voltage engineering. London: IET Power and energy Series 40. (2012). High-voltage switchgearandcontrolgear Part 203: Gas-insulated metal-enclosed for rated voltages above 52 kv. Switzerland: IEC. (2006). High-voltage test techniques Part 3:Definitions and requirements for on-site testing. Switzerland: IEC. James, R. (2007). Condition Assessment of High Voltage Insulation in Power System Equipment. IET Power and Energy.