Benefits and limits of advanced methods used for transformer diagnostics

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 due to insulation breakdown can generate substantial costs for 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. For the "in depth" analysis not only the service conditions but also experience in the application of different diagnostic methods and the knowledge of the design of the transformer play an important role for the correct judgment of the actual condition of the insulating system.