Abstract: A method for determining transient partial discharges in the insulating medium of an electrical power transformer, for example, oil and paper, uses in one embodiment a circular plate inserted in the interior of a transformer tank which picks up partial discharge signals. These are high frequency transients which are processed by use of DC notch filters, blocking capacitors and amplified by high frequency amplifiers. The peaks of the partial discharge signal are detected by peak amplifiers, sorted by relative magnitudes, and either displayed or further processed by computer techniques for indicating incipient transformer failure.