Analysis of cavity PD characteristics’ sensitivity to changes in the sinusoidal supply voltage frequency
Date
2021
Authors
Venge, Tapiwa
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Abstract
Most high voltage equipment operates at power frequency (50/60 Hz), yet their testing is often done at different supply voltage frequencies for various reasons. Therefore, the study of how partial discharge (PD) phenomena respond to the changes in the supply voltage frequency has continued to draw research interest. Literature has shown that there is a mixture of agreements and inconsistencies on the reported conclusions on how PD characteristics respond to supply voltage frequency variations. However, there has been consensus on the recognition of the mechanisms that influence how and why PD characteristics respond to the changes in the supply voltage frequency. These mechanisms that influence the frequency dependency are; statistical time lag, cavity surface conductivity, and the residual charge effect. The present research work used a PD model implemented in MATLABR© SimulinkR© to simulate the mechanisms of interest. A 3-capacitor model (ABC) is modified to simulate the extent to which the mechanisms influence PD characteristics and respond to changes in the supply voltage frequency. The PD characteristics studied in the present research are the cavity spark-over voltage, PD repetition rate (PDRR) and the phase-resolved PD patterns (PRPDP).Three configurations of the ABC model are developed to study each phenomenon that influences PD frequency dependency. Simulations are performed on three supply voltage frequencies: 1 Hz, 50 Hz and 200 Hz. The findings are that the phase-resolved partial discharge pattern (PRPDP) and PD repetition rate (PDRR) characteristics are more sensitive to variations in the statistical time lag of the seed electron availability at higher frequencies of the supply voltage. The opposite trend is observed for the cavity surface resistance. At lower resistance of cavity surface, the PRPDP and PDRR characteristics are more sensitive to changes in the supply voltage frequency than at higher resistances. The residual charge effect is more sensitive at higher frequencies of the supply voltage. At higher residual charge, the PRPDP and PDRR are more sensitive to changes in the supply voltage frequencies than at lower residual charge. The trend is in opposition to cavity surface conductivity’s response but in compliment to the statistical time lag’s response. The study also confirms that incorporating equivalent resistances with the equivalent residual capacitor in the ABC model makes it more authentic than the model comprising of equivalent capacitors and resistances or equivalent capacitors only
Description
A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Electrical Engineering, 2021