A Study on Progress of Accuracy in Estimation of Sparkover Rate of Medium-Voltage Line

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A model of a pole-type transformer is developed by taking account of resonance, and the insulation characteristics of a solid-core insulator are investigated. In the analysis of the sparkover rate of a medium-voltage line, it is recommended that a model of a transformer developed by taking account of resonance is used. For the accurate estimation of the sparkover rate, it is better to take account of V-t characteristics of the insulator for steep-front and short-tail lightning impulse, however, the 50% sparkover voltage for the impulse might be reasonable criteria for the estimation since calculation by using the V-t characteristics is time-consuming. The sparkover rate of the line over lossy ground caused by an indirect stroke is more than 10 times higher than that over perfectly conducting ground. The sparkover rate due to a direct stroke to the line is more than 10 times higher than that due to an indirect stroke even when the line is over lossy ground. Therefore, it is confirmed that the sparkover associated with a direct stroke to the line is more frequent than that associated with an indirect stroke. The fact that the estimated sparkover rate is 10 times higher than the practical rate indicates the necessity of the further study on the striking distance and the mechanism of the outages. The waveform parameters of negative strokes of natural lightning estimated based on measured electric field waveforms are in good agreement with those evaluated from direct measurement of the current waveform. This infers the possibility that that the return stroke current parameters including the parameters on wave tail can be estimated from the electric field measurement.