354 / 2017-04-24 17:18:58

The Statistical Analysis of Waveform Characters for VFTO in GIS Substation

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The very fast transient over-voltage (VFTO) waveforms of gas-insulated switchgear (GIS) in different voltage grades are complicated with a great deal of influence factors. The standard waveform of VFTO has not been provided in the current international and national standards. It is difficult to examine the insulation level of GIS under VFTO. So it is necessary to investigate the standard waveform of VFTO, providing the basis of insulation test and insulation coordination.
The existing extraction methods of standard waveforms for lightning impulse and switching impulse voltages were analyzed in this paper. The double index wave was proposed to fit the waveform curve of VFTO for obtaining its standard waveform. The characteristic parameters of typical VFTO waveform were determined as the magnitude and the steepness in the first peak, the maximum peak, and duration, oscillation coefficient, principal oscillation frequency etc. VFTO generated by disconnectors switching under different operation modes were simulated for 15 220~1000kV GIS substations in typical engineering. The distributions of VFTO characteristic parameters were obtained as the magnitude of 1.5~1.8p.u., steepness of 3~8 kV/ns, duration of 3~5µs, oscillation coefficient of 0.3~0.75 and the principal oscillation frequency of 0.4~1.45MHz.
The simulation results have shown that the magnitude in the maximum peak of VFTO waveform is the largest, however, the steepness in the first peak is the largest. The probability distribution of characteristic parameters in VFTO simulation waveforms were gotten. The statistical values of magnitude, steepness and duration of VFTO were achieved with the probability of 75%, 80%, 85%, 90% and 98%. For strictly examining the VFTO effect on GIS insulation, the VFTO characteristic parameters with probability of 90% were selected as those of its standard waveform. The wave front time of VFTO was 0.06~0.2μs, and its wave tail time was 4~5μs. The error ranges of VFTO standard waveform were calculated through the probability distribution of its characteristic parameters. The peak, wave front time and wave tail time were determined for VFTO standard waveform in 252~1100kV GIS. The results have provided the important reference for the test waveform to examine GIS insulation under VFTO.