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The transfer function of a transformer winding is deconvoluted in the frequency domain from the digitally recorded neutral current and high voltage applied during impulse tests. The integrity of the winding insulation is determined by comparing the transfer function obtained at full (BIL) and reduced test voltage.Differences between the transfer function plots reveal local breakdowns in the winding that can be dissociated from partial discharges. Thus the method permits unambiguous acceptance or rejection of the transformer and, since the transfer function is theoretically immune to changes in the applied impulse, also allows evaluation o f the chopped-impulse test.Some 100 windings of large HV power transformers have been tested so far using the transfer function method, which on several occasions has revealed transformer faults as well as test set-up problems that would have been missed or misinterpreted by the conventional technique.

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