Sensitivity analysis of the surge test applied to AC machines

The surge test is a well-known offline method to determine the integrity of the turn-to-turn insulation of coils as they are found in transformers or induction machines. There are different methods used to evaluate the waveform obtained during the surge test. These methods include the examination of the zero crossings of the waveform, i.e., its ringing frequency, the differential error area ratio (EAR) also known as the pulse-to-pulse EAR (P-P EAR) or a corona evaluation. A semi-analytical derivation of the P-P EAR under healthy and faulty conditions as well as a definition of the frequency and EAR sensitivity is given in this paper to gain more insight into the evaluation of the ringing frequency and the P-P EAR. The derivations shown in this paper are supported by simulations and experimental results.