Failure analysis of Onload tap changers using static / dynamic resistance measurement test method

Abstract

On-load tap-changer (OLTC) failures occur year after year in Uganda’s electricity transmission industry. The OLTC is a popular component of power transformers in the electricity transmission industry and its failures contribute to 40% of power transformer failures worldwide. In Uganda Electricity Transmission Company Limited (UETCL), 65% of failures are attributed to OLTC failures and as of 31.10.2022 the entire 66/11kV Lugazi sub-station in central Uganda is still down due to failed OLTCs. The failure of such components often leads to voltage regulation problems in the affected power transformers. In most cases, this leads to the failure of the power transformers or the operation of the transformer at half power capacity, which affects the supply of quality electricity to customers. Failure of the OLTCs with the transformers running can result in catastrophic outages and loss of equipment and revenue for the Government of Uganda. Quantitative results were obtained after performing the static/dynamic resistance measurement (SDRM) tests and evaluating the measurements by comparing one phase to another. This was done under the following standard IEEE C57.152-2013 (IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors) which states that the resistances between phases are within 5% of each other. The results show fluctuations in different resistances of the different phases of the power transformer and for each tap position. These were mainly analysed with the primary test manager and Microsoft Excel and conclusions were drawn about a suspected failure of the OLTC contacts. In addition, an algorithm was developed that helps to detect failures in the OLTC before such failures occur. To do this, the arc which always occurs in the OLTC when voltage fluctuations occur in the transmission network was monitored. This causes the tap selector to move from one tap to the other to compensate for the loss or rise in transmission line voltage.