저압용 전기전자기기의 절연성능 평가를 위한 부분방전시험에 관한 연구

Abstract

This dissertation deals with the application of partial discharge(PD) test on low-voltage electrical and electronic devices, which is recently being accepted as a non-destructive and an effective dielectric test method. The withstand voltage test(WVT) on low-voltage electrical and electronic devices may cause degradation of insulation performance by applying the test voltage.

However, the PD test does not occur insulation degradation of the device under test because it is carried out at a relatively low AC voltage compare to that of WVT. The PD test also provides much more detailed information about the insulation such as proper fabrication of the insulation system, selection of materials, and precise manufacturing of any piece.

Occurrence and characteristics of PD were studied in solid insulation system, and a PD measurement system which can detect charges below 1pC was fabricated. PD pulse detection was performed by a coupling network with a discharge free capacitor and a RLC detection impedance. The low cut-off frequency of the detection circuit was set at 1MHz(-3dB) to attenuate AC voltage by 270dB and to pass discharge pulses without any distortion. A low noise wide-band amplifier having a gain of 40dB was designed since the magnitude of PD pulse propagated to the detection circuit was as low as the order of ㎶.

A shielding enclosure of a Ni-Cu double structure and a HV filter with a high cut-off frequency of 3.5kHz was designed to get a better measuring condition against radiation and conduction noises from the outside.

The PD measurement system was evaluated to noise interference because the maximum sensitivity of the system depends on noise level, and the peak noise level appeared 3mVp-p. Calibration to calculate the sensitivity of the PD measurement system was performed according to the IEC standard on low-voltage insulation transformers and induction motors. The sensitivities in them were 38.4mV/pC and 11.4mV/pC, respectively. The system is possible to measure apparent charge of 0.2pC in the transformers and 0.5pC in the motors considering the noise level.

Comparative experiment combined with the WVT was carried out on the transformers and the induction motors to propose a PD test requirement for low-voltage electrical and electronic devices. Discharge inception voltage (DIV), discharge extinction voltage (DEV), apparent charge, and phase distribution of PD pulses were analyzed in the experiment.

The results showed a decrease in DIV, DEV and an increase in apparent charge, and demonstrated that degradation of insulation proceeded during the WVT and aging. Also, no changes in PD parameters appeared up to 70% of the test voltage specified in the WVT. Therefore the PD test for the types of transformers can be completed under 70% of the test voltage by measuring apparent charges below 1pC.

As a replacement of the WVT for low-voltage electrical and electronic devices, the PD test requirement can be standardized through the same procedure proposed in this dissertation. However, the PD test should be performed in ranges from 30% to 50% of the test voltage not to make any degradation of insulation during the test.

The PD test that can measure apparent charges of 0.1pC order is expected to be widely used electronic components with short dielectric distance such as photo-couplers, ICs, and PCBs.