Applicability of Stockwell transform in diagnosing impulse faults of a transformer

Lightning impulse test is one of the mandatory tests that every transformer has to undergo after assembly. This is done for assessing the condition of insulation of its windings. In the case of any fault, the neutral current and/or tank current contain(s) typical signatures depending on the nature and location of the fault. Since these current signals are non-stationary in nature hence each of these faults can be classified from such a transformation technique that has the time-frequency localization property. This paper investigates the performance of Stockwell Transform, popularly known as S-transform, for identification of insulation failures of a transformer winding during impulse test. The present study is restricted only to identify failure of winding insulation due to some defects that are already present in the winding and have not been developed due to the application or during the propagation of impulse voltage applied during the test. The results presented in this paper are for simulated models of a 3 MVA and a 5 MVA transformer. From the results, the prospect of S-transform in identifying the nature and the location of winding faults in distribution transformers is reflected.