Low Cut-Off Frequency Extension and Amplitude Decoupling Analysis in Non-Contact Transient Measurement

For the non-contact measurement system, the low frequency measuring and uncertainty analyzing are critical to obtain the accurate amplitude of the transient overvoltage. In this article, the circuit parameters that affect the low cut-off frequency of the measurement system were analyzed. Then, based on impedance converter, the ±1 dB low cut-off frequency was reduced from 9.8 to 0.25 Hz. Moreover, the transient overvoltage generated by the air-insulated disconnector operation in the 500 kV ac switchyard of a ±800 kV high-voltage direct current (HVdc) converter station was measured by the developed system. The time and frequency domain characteristic of the transient overvoltage were obtained. The measured three-phase transient overvoltage voltages were recovered using the decoupling matrix. The uncertainty analysis was carried out based on the field waveforms. After that, the impacts of average times and voltage step amplitudes on the uncertainty were compared. And the optimal average number of times is 10 and the voltage step amplitude selection should be greater than 0.5 p.u. The analysis results indicated that the measurement uncertainties for both high-frequency overvoltage and power frequency are below 10%.