Experience in Testing Metal-Oxide Surge Arresters on Bosnia & - - PowerPoint PPT Presentation

Experience in Testing Metal-Oxide Surge Arresters on Bosnia & Herzegovina Transmission Network

IGOR DJOKIC

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

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

• ccurs 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. Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network

• I. Djokic, E. Sokolovic, G. Skelo

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.

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. Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network

• I. Djokic, E. Sokolovic, G. Skelo

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

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. Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network

• I. Djokic, E. Sokolovic, G. Skelo

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.

LCM TESTING EQUIPMENT PRINCIPLES AND MEASUREMENT RESULTS

CORRECTION AND ANALYSIS OF MEASUREMENT RESULTS

Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network

• I. Djokic, E. Sokolovic, G. Skelo

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.

LCM TESTING EQUIPMENT PRINCIPLES AND MEASUREMENT RESULTS

CORRECTION AND ANALYSIS OF MEASUREMENT RESULTS

Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network

• I. Djokic, E. Sokolovic, G. Skelo

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.

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. Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network

• I. Djokic, E. Sokolovic, G. Skelo

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").

RESULTS OF SURGE ARRESTERS TESTING IN ELEKTROPRENOS TRANSMISSION NETWORK 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 Cases with significant increase of resistive leakage current Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network

• I. Djokic, E. Sokolovic, G. Skelo

During regular testing it was discovered that resistive leakage current of two surge arresters was over ten times higher than expected. In both cases the testing of current transformers insulation was performed by measurement of the insulation resistance, the capacity, the dielectric dissipation factor (tgδ), and dissolved gases analysis of oil samples . A significant insulation weakening of these current transformers was diagnosed and they were taken out of service Consequently, it was decided to test circuit breakers in these transformer bays. In these 110 kV transformer bays circuit breakers with single pole driving mechanism were installed. In the first case circuit breaker testing have shown that during closing operation one pole was lagging by 260 ms first pole to close, and in the second case one pole was lagging by 140 ms first pole to close, and it was the same pole where the increase of resistive leakage current was measured The reason for the increase of resistive leakage current, and the weakening of insulating properties is apparition of overvoltages caused by circuit breakers poles lagging during switching operation of unloaded transformer.

RESULTS OF SURGE ARRESTERS TESTING IN ELEKTROPRENOS TRANSMISSION NETWORK Cases with significant increase of resistive leakage current Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network

• I. Djokic, E. Sokolovic, G. Skelo

CONCLUSION Experience in Testing Metal-Oxide Surge Arresters on Bosnia and Herzegovina Transmission Network

• I. Djokic, E. Sokolovic, G. Skelo

By initial measurements carried out in the first three years of measurement campaign the expected values of resistive leakage current were determined for OhioBrass surge arresters type: PVN, catalogue number: 314076. Based on the measurement results gathered in the database, as well as from contacts with manufacturers of surge arresters and test equipment, considerable experience was gained in testing, limit values ​for the surge arresters in good condition and specific criteria for the assessment of the surge arresters were specified. The testing results from the last measurement campaign showed that there was an increase in surge arresters resistive leakage current which was expected having in mind the age of surge arrester at the end of 2014. For those surge arresters where resistive leakage current increased more than twice the expected value it was recommended to decrease the testing interval to one year. The testing results showed that tested metal-oxide surge arresters were extremely reliable element of the power system. This is emphasized by the fact that during period of 13 years only three surge arresters were taken out of service due to extremely high resistive leakage current. That increase of resistive leakage current all three cases was the result of long term exposure to

• vervoltages.