Cavities are one of the typical defects resulting in partial discharges (PDs) in extruded power cables. This paper focuses on studying the PD characteristics of arbitrary dielectric bounded cavities and electrode bounded cavities embedded in EPR insulation, such as PD distribution, PD inception and extinction voltage, under different type of testing voltage. During the experiment, the applied voltage is sinusoidal waveform at frequencies of 20, 50 and 200 Hz and damped AC waveform at frequencies of 20, 50, 200 and 500 Hz. The amplitude of applied voltage, the peak value, is 1.25, 1.5 and 2 times of the amplitude of PD inception voltage (PDIV). The diameter of cavities is 1.2, 1.5 and 2.0 mm. A batch of five simples has been tested during PDIV test. Each sample has been tested five times so as to obtain accurate data. It has been found by the experiments that the PD inception voltage increases with increasing frequency of applied voltage. The symmetrical PDs distribution can be observed in the time lag. It seems that the PD distribution depends on the frequency. The maximum PD amplitude at higher frequency is slightly larger than at lower frequency. PD distribution spreads more widely at higher frequency. It is also found that the cavity diameter and the amplitude of applied voltage influence the frequency dependence of PD distribution. The speed of degradation of EPR insulation is related with the frequency of applied voltage. PDs can also corrode the surface of dielectric bounded cavities embedded in EPR insulation, whereas there is not obvious change on the surface of electrode bounded cavities.
