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A method is proposed for recognizing PD events within a winding during impulse tests using a lock-in amplifier technique. The resonant frequencies of the winding serve as the basis of selection of the lock-in amplifier reference frequencies. It is implemented with a virtual instrument and validated with an independent stand-alone lock-in amplifier. A physical model of a winding with discrete elements is used to demonstrate the results. A theoretical basis for the instrument is established using an 'abc' model of PD along with a linear discrete winding model. The influence of location of PD is also addressed. The method will provide online recognition of PD during the impulse test and can be adapted for onsite measurements.Index Terms -Impulse test, transformer, partial discharge, lock-in amplifier, virtual instrument.

An instrumentation scheme is proposed for the acquisition of waveforms during the lightning impulse test on transformers. Three filters segregate the response into distinct frequency bands. As partial discharge signals primarily influence the high frequency behaviour, experiments are reported on disc, interleaved disc and layer windings.