Condition assessment of power transformers

Large power transformers belong to the most expensive and strategically important components of any power generation and transmission system. Their reliability is of key importance for the availability and profitable operation of such systems. A serious failure of a large power transformer can generate substantial costs (transport to the factory, repair and financial losses due to power outage). Therefore, utilities have a clear incentive to assess the actual condition of strategically important transformers, with the aim to minimize the risk of failures and to avoid forced outages. Special attention in this respect is given to large power station step-up transformers, large substation transformers or any large autotransformers (phase shifter) in a HV transmission system. In case of a problem during operation of a new or service-aged transformer, utilities are challenged with the following questions: (1) what is the origin of the problem? (2) does the problem affect the reliable operation of the transformer? (3) is it possible to do a repair on-site? (4) is a replacement with a new transformer necessary? In this contribution it will be shown on practical examples, that it is possible to identify and localize the reason for the alarm or spontaneous outage of the transformer by consequent application of all existing methods (conventional and advanced) and by final "in depth" analysis of all available results.