Combination of withstand voltage testing by frequency-tuned resonance and partial discharge testing under damped AC for power cables in field

The denser power grids in Chinese cities have increased the demand for secure power cable systems.Withstand voltage (WV) testing has been used to evaluate the insulating state of power cables in field. However, most of the existing mobile AC frequency-tuned resonant test systems (ACRF) are superimposed by pulse interference. In the past five years, damped alternating current (DAC) generated by direct current circuit has been extensively applied in on-site partial discharge (PD) tests in China. WV and PD tests both conducted under DAC generated by DC were controversial, because the voltage didn't last long enough, whereas the respective conduction of WV and PD tests was work-consuming, so a new method is proposed in this paper. In this method, PD test under DAC voltage automatically follows AC WV test by the frequency-tuned resonance. PD test was executed under DAC voltage, which was automatically initiated by cutting off the inverter power supply circuit. The impulse interference from the power electronic devices could be successfully avoided at the same time. When WV was supplied, cable capacitance was resonated with the fixed inductive reactance by adjusting the frequency of the excitation source. The parameters of the test system were as follows: the maximum voltage was 80 kVrms, the resonant frequency range was 30Hz to 400Hz, the capacitance range was 0.05 μF to 2 μF, and the duration for the WV test was 60 mins. After the completion of the WV test, the inverter power supply was cut off, DAC voltage was initiated automatically, and an on-site PD test was immediately performed. The frequency band of the PD detector, which ranged from 30 kHz to 40 MHz, could be combined with the filter because of the difference in the types of interference in field. PD testing sensitivity, which was 1 pC in the laboratory, was significantly dependent on the noise level in field. The measurement apparatus developed in this work is under trial on a 10 kV cable in Beijing.