Analysis of partial discharge pulse distribution along transformer windings using digital filtering techniques

Abstract

The capacitive network often used for the calculation of initial voltage distribution along a transformer winding can be in error for steep impulses. As such pulses are produced by partial discharges (PDs) the application of the concept could contribute to inaccuracies in methods developed for the location of PDs in windings. By means of theoretical analyses, together with experimental measurements on several disc-type windings, it has been found that a capacitive ladder network can be a good representation within a certain frequency limit, the latter being related to the particular winding. Terminal measurements, involving both impulse response and sinusoidal voltage methods, and digital filtering techniques, were employed to determine the appropriate frequencies. Results are presented for tests on 66 kV and 132 kV interleaved windings and a conventional 66 kV winding. For the interleaved windings greatly improved accuracies with steep pulses were achieved, compared with normal impulse voltage distribution measurements. An explanation of the differences between the winding characteristics is given through analysis of the behaviour of two hypothetical double-disc coils (interleaved and conventional), allowing for capacitive intercoupling and series inductance. It is concluded that the studies described will assist in the long-term development of more accurate electrical PD location systems.