Examining faulty transformer tap changer using frequency response analysis

Failure of tap changer is a significant issue in reliability of a distribution transformer. For this reason, the tap changer faults requires detail investigation in order to understand the main causes. This paper presents an investigation to determine the applicability of using frequency response analysis measurement for diagnosing faults on tap changer. Two faults have been investigated were coking (carbon build-up) and pitting which are commonly occurred on tap changer contact. These two faults have been fabricated and experimented on a transformer in the laboratory. The measurement has been conducted at a local power utility on a 3 phase de-energized load tap changer (DLTC) transformer of rating 500 kVA. Based on the results, it was found that the frequency response was indeed affected. Therefore, the frequency response demonstrating complete influence of capacitive element throughout the measured frequency range. This significant finding might be due to the increases of tap changer resistance because of considerable faults on the tap changer. Based on the findings in this paper, it shows the possibility of determining faults occurred on tap changer using frequency response analysis test even though it is typically used for investigating faulty core and winding.