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.