Discharge characteristics in long air gaps follow certain statistical regularities. Statistical methods and finite element simulation were both employed to deeply study discharge characteristics. The statistical results indicate that the height of grounding rod has a slight influence on the 50% breakdown voltage of rod-rod air gaps and that the breakdown time is longer than the wave front time when 20/2,500 μs switching impulse is applied and shorter than the wave front time when 80/2,500 μs switching impulse is applied. Furthermore it can be shown that the distribution of the breakdown voltages in 10 m rod-plane gap follow the normal distribution and that the V-t characteristics of rod-rod gaps is lower than that of rod-plane gaps. Calculation results indicate that the non-uniform coefficient of the electric field increases with increasing gap distance and that the ratio of gap distance to the non-uniform coefficient also increases with the gap distance. The work described in this paper can be used for the insulation coordination of UHV transmission lines.

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In actual operation, the shielding failure rate of UHV transmission line is much higher than that in simulation, so it is crucial to study the lightning shielding performance by experimental research. The testing UHV transmission line model is scaled down in the ratio of 12.5 which is relatively large comparing to other testing models. Two kinds of negative switching impulse waveform were applied to HV rod to simulate the lightning descending leader. The probability of transmission line being stroked was calculated statistically by recording more than two thousand discharges and the factors that affecting the discharge probability were studied through contrast tests. The results show that the grounding condition, DC bias, the protection angle, the switching impulse voltages have an influence on the discharge probability, breakdown voltage and breakdown time. The spatial boundary of shielding failure area was determined in the middle of the span. The upper and lower spatial boundary and the distance between them increased with the increasing of the lateral distance between HV rod tip and transmission line model simultaneously.Index Terms -Large scaled model test, negative switching impulse, spatial boundary of shielding failure area, discharge probability, breakdown voltage, breakdown time.

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