The Study On Effects Of Lightning Activity And Terrain On Transmission Lines Shielding Failure Characteristics

Power grid is continuously developing toward Extra-high Voltage (EHV), long-distance and High-capacity with the high speed development of Chinese economy, thus the shielding failure protection issues of transmission line are gradually outstanding and have been widespread concern at home and abroad. Shielding failure characteristic of transmission line is under affected by many factors, such as, activity regularity and characteristic parameters of lightning, structure parameters of transmission line, topography and geomorphology, etc. At present, lightning activity regularity, such as influence mechanism of discharge selectivity is not sufficiently studied. The probability of lightning current amplitude is one of the most important parameters of characterization, while its calculation result from regulation method existing large difference compared with actual measured value. Different topography and geomorphology leads to the different shielding effect of shielded wire to conductor and this work is necessary to carry out. In addition, because the conductor height in arbitrarily point alone span direction is neglect, the error is existed between the calculation results of current shielding failure model of transmission line and actual data. Thus the key in this paper is to research how lightning activity and topography influence shielding failure characteristic of transmission line, which enriches the lightning shielding failure theory of transmission line and has important practical value for the lightning shielding failure protection. This paper commences around the following aspects:Test apparatus are made to produce spark discharge in short gap and identify spark paths. According to the analysis to some spark discharges photos which high-speed video camera take, several stages in spark discharge are discovered and cause of these phenomenons is explained. It also showed comparability of spark discharge and lightning discharge. Then a large number of experiments are fitted on the relationship between the location of point electrode and probability of discharge. The function relationship between the location and discharge probability of point electrode is obtained. Lightning model of transmission line is established, and the influence mechanism of height change of transmission line and voltage polarity to lightning probability is analysed.Lightning activity distribution in a certain area has been investigated and lightning strike condition of different span of two500kV transmission lines is analyzed. The distribution of lightning amplitude between actual data and calculation result of recommend formula in regulation DL/T620-1997is compared, and the data showns great difference between them. Formula about Probability of lightning current amplitude is fit based on recommend formulas of CIGRE、IEEE Std. and measured data about lightning parameters through the past10years. The effect of coefficient β on the formula on curves is analyzed and value range of coefficients is given.The concepts of protection globoidal and exposure globoidal are presented based on electro geometric model, and a three-dimensional electro-geometric model is built.The conductor height is made variable, change rules of shielding failure rate is obtained which changed with distance between the section vertical to transmission lines and tower. By means of integral to this formula, the formula about shielding failure rate of the whole span spatial model is obtained. The change regularities of conductor height variation, protection angle and lightning current amplitude by taking these calculation formulae are comparatively analyzed with electric geometry method (EGM).When transmission lines are installed on the following terrains, such as:the top of the hill, hillside and installed from the mountain foot to mountain top, model analysis model on transmission lines shielding failure trip-out rate is built. Calculation formulae for corresponding shielding failure trip-out rates are derived. Taking220kV transmission line with two overhead ground wires for example, both maximum shielding failure lightning current of transmission line located at different positions of the mountain range and the variation regularity of shielding failure trip-out rate with the dip angle of ground are researched. Finally, the change regularity of shielding failure performance at different positions of single span of transmission line is researched when single span is erected upon valley.