Insulating materials used for high voltage equipment such as transformers and power transmission cables may suffer from various deteriorations for long-term usage. In particular, the deterioration due to the partial discharge (PD) occurring in a place where the electric field concentrates locally occupies an important position among insulation deterioration mechanisms. Therefore, understanding the characteristic of the PD and conducting tests may lead to prevent accidents and to long life of power equipment. However, PD may have small discharge magnitudes and may not lead to dielectric breakdown immediately. Consequently it may be necessary to continue to apply voltage for a long time for the test. The purpose of this study is to investigate the validity of the frequency acceleration test for reduction of test time by measuring PD characteristics under a.c. voltage with various frequency such as 50 Hz or more higher frequency using a needle-plain electrode system and polyethylene sheets as specimen films. Firstly, we investigated the initial characteristics of the PD, which is shortly after the discharge started. The applied a.c. voltage was set to be among 2 kVp and 4 kVp with a sinusoidal wave form. The applied voltage frequency was changed from 50 Hz to 10 kHz at each voltage. We measured the maximum PD magnitude q(max), phi-n distribution and pulse-average phi-q distribution. It was found that 2 kVp is close to the discharge inception voltage. Each PD characteristics became stable for each frequency. However, at higher voltages than 2 kVp the dispersion of PD characteristics became larger due to the applied voltage frequency. Secondly, we investigated the PD characteristics at each fixed cycle of each frequency at an applied voltage of 3 kVp. The measured parameters are the same as for the initial characteristics. It was found that Each PD characteristics became stable for each frequency.
