Complex diagnostic methods for lifetime extension of power transformers

Power transformer, the most expensive equipment in the power networks, is one of the key components from the point of view of reliability. A failure of the transformer can result in high cost due to repair or replacement and outage costs. Ageing is often thought to be the most important cause of the component failures. That is why the proper determination of the degree of ageing is so important. A significant part of the typical transformers population is 20-30 years old, when the reliability of the transformers comes questionable. Asset managers are moved from time based maintenance (TBM) to condition based maintenance (CBM), or even reliability centered maintenance (RCM). In RCM decisions are no longer driven by an average time-frame defined by observation and statistical experiences, but the actual conditions of the equipment and the level of the reliability are taken into account. The paper shows the main elements of the complex diagnostics of the transformer used by the National Power Line (Ovit) Co. Ltd. and by the Hungarian Transmission System Operator (Mavir) Co. Ltd. According to the experts suggestions and the statistical results of the "fingerprint" data, the different refurbishment and repair methods (in field or in factory) for high voltage transformers are shown.