Time-frequency analysis of VHF wave form for precise location of partial discharge in small-scale high voltage apparatuses

This study is aimed to develop the technique for locating partial discharges in small-scale high voltage apparatuses like rotating machines and transformers. Conventionally for such a purpose, discharge signals detected at different points are compared in time domain to determine the time lag. However, for small-scale apparatuses, a high precision of the location is required. In addition, as the inspection is usually performed in a small room, the waveform may be highly interfered by reflected waves. In the experiment, radio frequency signals in VHF range radiated from the discharge point were carefully acquired the time lag between these signals was determined by more than two whip antennas. Direct comparison in time domain was not suitable since the signals were not completely the same. Time-frequency analysis was then applied to the waveform. Specific part of the waveform was extracted by using a proper window function and subjected to phase analysis. Location of the discharge point was performed using the phase spectrum. It was proved that the technique was suitable to avoid the influence of the interference by the reflected waveform. The precision of location was as good as several centimeters.