Influence of Cavity Geometry on Partial Discharge Measurement at Very Low Frequency

As a promising alternative to conventional high voltage AC testing at PF (power frequency 50/60 Hz), VLF (very-low-frequency, typically 0.1 Hz) testing is nowadays used in practice. The increasing application of VLF testing for condition assessment of power apparatus necessitates understanding the partial discharge (PD) behaviors at this frequency. In this paper, the influence of cavity geometry on PD activities is investigated. A comparative experimental study is carried out with PF and VLF voltage excitations for different void structures (cylindrical, block, prism). Measurement results are presented with phase-resolved discharge patterns and integrated parameters such as average void discharge, repetition rates, etc. Under the same applied overstress relative to inception voltage, measurement results show that void discharges are strongly dependent on the cavity geometry as well as excitation frequency. With increasing cavity volume, PDs are more likely to occur and in particular, VLF excitation yields lower discharge magnitude, repetition rate and the phase range of occurrence.