Experimental investigation of internal short circuit faults leading to advanced incipient behavior and failure of a distribution transformer

Transformer fault detection and diagnosis is becoming more important due to the restructuring of the electric power industry. In this era of deregulation, loading transformers to their optimum capacity is becoming normal practice, which in turn applies high stresses on the insulation of the transformers and increases the probability of occurrence of internal short circuit winding faults. Such faults can lead to catastrophic failure and hence cause outages. Utilities and other entities in the electric power business are therefore exploring ways of detecting these faults in transformers in the incipient stage. Terminal values, primary and secondary currents and voltages of the transformer convey information that can be used to detect internal transformer failures before developing a detection method. The behavior of these terminal values should be understood. In an effort to characterize the behavior of the terminal values of a transformer during internal short circuit and incipient faults, short circuit faults were staged on a 25 kVA, 7200 V/240 V/120 V two winding custom-built transformer. This paper discusses the results of the field experiments performed over a 19-month period. It presents time domain results of selected short circuit experiments. It also presents recordings of advance incipient-like behavior during the last set of experiments.