A Multi-parameter Geometric Model Of Shielding Failure And Its Application In Differentiated Lightning Protection In Transmission Lines

With the rapid development of high voltage power grid in our country, the voltage grade of transmission lines is higher and higher and the transportation capacity is larger and larger. Thus, the operating safety of the grid in power system is becoming more and more important. In addition, tower height and the size of line corridors also increase, as well as thunder attractive radius. Therefore, the damage probability of the grid suffered from natural thunder is bigger. At present, the exposed weak ability to resist lightning risk of the power grid has been a threat to the safety of the operation. Therefore, a accurate and comprehensive evaluation method of power line lightning protection performance has great guiding significance to power line designing and lightning protection measuring. It is the theoretical basis to reducing the lightning accidents, ensuring the safety of power supply,and providing a powerful guarantee to the construction of strong power grid.Shielding failure is one of the key links in evaluating lightning protection performance of the transmission lines. Analysis and researches are taken in this paper on some key problems existed in differentiated lightning protection performance of shielding failure at present. And on this basis, an evaluation model is set up for the differentiated lightning protection performance of shielding failure in power lines, which considers various parameters. The main research contents in this paper include:1) Based on the traditional electrical geometric model(EGM), a two-dimensional geometric coordinate system is established for the transmission lines. In this coordinate system, the calculation formulas in traditional model is modified, which include the angle of lightning incidence, the lightning strike distance and the lightning exposure distance.Thus, an improved electrical geometric model is born, which is called multi-parameter geometric model in this paper. Based on the new coordinate system, the formulas of maximum lightning strike distance and the lightning exposure distance. Connecting all the parameters mentioned above, a lightning trip-out rate calculation model is established which considers various parameters. Afterwards, taken the foreign transmission line at 345 kV as an example, the model is used to calculate the line shielding failure trip-out rates of each tower in the line. A comparison is made between the calculated results, the operation experience values and the values calculated through traditional method. The result validates the rationality of the model. What’s more, compared to the calculating error of traditional method, the error of new model presents a 56.15% reduction.2) The median filtering theory and its algorithm are introduced to solve the problem of “noise” which exists in classical grid statistical method for ground lightning density.And then, an improved grid statistical method for ground lightning density is born. By using the improved method, an attempt is done to obtaining ground lightning densitydistribution map in simulated environment. After that, a comparison is made between the third order filtering results, the second third order filtering results and the classical grid statistical results. It turns out that the filter is effective. After obtaining the ground lightning densities by median filtering, the densities of power line corridors can be calculated through the transmission line corridor grid statistical method. Then using the multi-parameter geometric model proposed above, the shielding failure trip-out rate of each tower along a transmission line can be calculated. Based on all the above, a evaluation model is set up for the differentiated shielding failure lightning resisting performance of transmission lines.3) Based on all the above conclusions, the lightning strike risk assessment system is developed for transmission lines. The system can realize calculating lightning protection levels of power lines accurately. Also, it can find out lightning strike weak spots. That is to say, the system can provide decision-making foundations for lightning protection works in power lines. Taken the ZhuMing transmission line at 110 kV in Sanmenxia area as an example, the shielding failure lightning resisting performances of the towers are evaluated,which applied the proposed evaluation model. At last, a comparative analysis is made between the results which are evaluated by the proposed model and the actual operating values. It turns out that the proposed model applies to the real project.