Experience in Testing Metal-Oxide Surge Arresters on Bosnia & Herzegovina Transmission Network IGOR DJOKIC
Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network Authors: Igor Djokic M.SC.E.E., Evelin Sokolovic M.SC.E.E., Goran Skelo B.SC.E.E. Elektroprenos BiH – Sarajevo, Bosnia and Herzegovina
Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network I. Djokic, E. Sokolovic, G. Skelo From 1996 to 2000 virtually all transformer bays in 400 kV, 220 kV and 110 kV Elektroprenos Sarajevo network were equipped with metal-oxide surge arresters In 2001 it was decided that Elektroprenos would start a campaign of condition monitoring and testing of metal-oxide surge arresters. For practical and economic reasons it was desirable that testing of metal-oxide surge arresters would be done in normal working conditions. The testing is performed by measurement of leakage current resistive component based on leakage current harmonic analysis with compensation of third harmonic present in network voltage. This method is also called the third harmonic method with compensation. The purpose of surge arrester is to limit overvoltage and protect other electrical devices in transmission network when overvoltage occurs yet for most of its life the surge arresters are expected to act as an insulator. Any deterioration in the insulating characteristics of surge arrester will result in an increase of total leakage current and especially its resistive component at given network voltage and ambient temperature.
Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network I. Djokic, E. Sokolovic, G. Skelo LCM TESTING EQUIPMENT PRINCIPLES AND MEASUREMENT RESULTS Total leakage current of MO surge arrester contains higher harmonics due to its nonlinear resistance. The amount of harmonics contained in the resistive leakage current depends on the amplitude of resistive leakage current and degree of nonlinearity of the MO varistors resistance (also depends on temperature and voltage), that is the reason why the harmonic content of leakage current can be used as an indicator of the level of resistive leakage current. Harmonics contained in total leakage current are also created by the transmission network harmonics. Harmonics in capacitive leakage current generated by the harmonics in network voltage may be of the same order of magnitude as leakage current harmonics generated by surge arresters nonlinear resistance. During determination of third harmonic of leakage current one can make a big mistake which is why it is necessary to perform compensation (elimination) of harmonics in leakage current generated by the harmonics in network voltage. LCM testing equipment principle of operation is based on the harmonic analysis of total leakage current and compensation technique where the third harmonic component of current generated by harmonics in network voltage is eliminated in order to obtain third harmonic component of current generated by the surge arrester.
Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network I. Djokic, E. Sokolovic, G. Skelo LCM TESTING EQUIPMENT PRINCIPLES AND MEASUREMENT RESULTS CORRECTION AND ANALYSIS OF MEASUREMENT RESULTS The measurement results by LCM testing equipment are always shown as if the network voltage of 0.7 Un was applied on surge arrester and varistor temperature was 20°C. The correction of the measurement results is carried out based on correction curves provided by the manufacturer of testing equipment. The result of the measurement results correction is resistive leakage current corrected to the actual ambient temperature and network voltage applied to the surge arrester. Due to the fact that relation between resistive leakage current third harmonic and total resistive leakage current in the LCM testing equipment is determined for surge arrester produced by ABB company exact measurement results are only valid for ABB surge arresters. However, LCM testing equipment can be used for testing surge arresters manufactured by other producers by taking. This fact does not have a large impact on the analysis and interpretation of measurement results if measurements are made at equal time intervals, while recording monitoring the increase of resistive leakage current between the two measurements
Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network I. Djokic, E. Sokolovic, G. Skelo LCM TESTING EQUIPMENT PRINCIPLES AND MEASUREMENT RESULTS CORRECTION AND ANALYSIS OF MEASUREMENT RESULTS The analysis of measurement results can be divided in two: the analysis of measurements performed on ABB surge arresters and the analysis of measurements performed on surge arresters of other manufacturers. Correction and analysis of measurement performed on ABB surge arresters The analysis, correction and interpretation of the results of measurements performed on ABB surge arresters is made based on limit values for different types of surge arresters, supplied by the manufacturer in LCM operating manual. Correction and analysis of measurement performed on surge arresters of other manufacturers When testing the surge arresters of other manufacturers it is necessary to follow the increase of resistive leakage current from the moment of surge arrester energization, and then at specified time intervals. Ohio Brass Company the manufacturer suggests that if the resistive leakage current increases more than 3 times its initial value, then additional testing of surge arrester should be performed, or contact the manufacturer or finally replace the surge arrester.
Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network I. Djokic, E. Sokolovic, G. Skelo LCM TESTING EQUIPMENT PRINCIPLES AND MEASUREMENT RESULTS CORRECTION AND ANALYSIS OF MEASUREMENT RESULTS Correction and analysis of measurement performed on surge arresters of other manufacturers If the measurements are made after surge arrester energization the analysis of measurement results is done by comparing the results with the results obtained on the surge arresters of the same type, same year of production, same catalog number and similar serial number. It is necessary to take into account the exploitation conditions (time in service, exposure to surges, number of surges registered on surge counter etc.). In order to compare the results of two measurements on the same surge arrester or on the surge arrester of the same type, it is necessary to perform correction of measurements results to the reference value (usually to 20°C and 0,7Un). Correction curves can be obtained from the manufacturer of surge arresters.
Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network I. Djokic, E. Sokolovic, G. Skelo LCM TESTING EQUIPMENT PRINCIPLES AND MEASUREMENT RESULTS CORRECTION AND ANALYSIS OF MEASUREMENT RESULTS Correction and analysis of measurement performed on surge arresters of other manufacturers The variation of resistive leakage current with voltage and temperature for Ohio Brass surge arresters.
Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network I. Djokic, E. Sokolovic, G. Skelo RESULTS OF SURGE ARRESTERS TESTING IN ELEKTROPRENOS TRANSMISSION NETWORK Around 80% of surge arresters installed in Elektroprenos transmission network are produced by Ohio Brass company. When the measurement campaign started in 2001 most of the installed metal-oxide surge arresters were in service between two and five years, so it was not possible to know the value of resistive leakage current at surge arrester energization. The analysis of measurements results was performed by comparison of measurements results of with the results obtained on surge arresters of the same type, same year of production, same catalog number and similar serial number. The highest number of surge arresters tested was installed in 110 kV network. After first testing campaign which lasted three years there were enough data to talk about expected values of resistive leakage current for surge arresters manufactured by Ohio Brass company type: PVN, catalogue number: 314076. The total number of surge arresters tested was 165. During testing it was noted that the results of measurements obtained on the middle phase (phase "4") were 10% - 40% lower than the results obtained on outer phases (phases "0" and "8").
Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network I. Djokic, E. Sokolovic, G. Skelo RESULTS OF SURGE ARRESTERS TESTING IN ELEKTROPRENOS TRANSMISSION NETWORK
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