The Effect of Tap Changer Coking and Pitting on Frequency Response Analysis Measurement of Transformer

The tap changer in a power transformer plays a vital function in regulating the voltage. It has been reported that the tap changer contributes to a large percentage of failures in the power transformer. This paper investigates a faulty tap changer using the frequency response analysis (FRA) test. All measurement configurations in FRA were employed to verify the effectiveness in detecting the fault. The test configurations are the end-to-end open circuit, the end-to-end short circuit, the capacitive inter-winding, and the inductive inter-winding. The coking and pitting of the tap changer are fabricated in several pieces of copper. These pieces are installed on the transformer tap selector. The FRA measurement was then performed on an 11/ 0.433 kV, 500kVA transformer at normal and faulty conditions. The variation between the FRA measurements is then calculated using the correlation coefficient (CC). This study improves the understanding of the FRA method by exploring its ability to detect tap changer faults. It is realized that the FRA method is sensitive to the tap changer coking and pitting. The results show that the tap changer coking effect at low frequencies. The response drop in magnitude at low frequencies between 20 Hz and 2 kHz. For pitting, it shows the response shifting towards higher frequencies.