Medium-size distribution transformer diagnostics

An improved method of degradation diagnosis for medium‐size distribution transformers is described. The proposed method uses refined criteria of degradation for specific application for 200‐ to 1000‐kVA range transformer diagnostics. The new criteria were established with a 95 percent confidence limit of data after extensive case studies focusing on the relationship between the amounts of CO and CO2 generated in transformer oil and the length of operating time of the transformer. Another feature of this diagnostic technique is the provision of a capability to estimate the thermal history of the insulation material by examining the amounts of certain trace products of decomposition present in the transformer oil. For this purpose, a thermal history discriminant chart was devised by application of a canonical discriminant analysis method.