| To save the power transmission line corridor, the number of domestic construction of high voltage tower with multi-circuit transmission lines is increasing. In multi-circuit transmission lines, the space between the lines is small, the circuit with a high voltage level will induce very large voltage and current. In earthing switch selection calculation of the new construction or renovation transmission lines, it has been found that the induced voltage and current values are close to or more than the rated capacity of the earthing switch. How to make a reasonable and economical selection of the earthing switch has became a serious production problem. At present, when solving the induced voltage and current, the EMTP software, which is based on the circuit-model, is commonly used, and the results on the transmission lines calculated by EMTP are treated as the induced value in the substation. The actual situation can not be fully reflected by this method, and the result is different with the measured value. Although the versatility of the method, solving the electromagnetic model with finite element method, is not as good as the circuit method’s, it helps to find the reason why the error occurs, which provides practical guidance for the engineerings.Aimed at this topic, we have done intensive research and achieved the following results:(1) By analysis and calculation of the method based on circuit-model, which is aimed at solving induced voltage and current of transmission lines, it pointed out that EMTP is not suitable to solve the induced value on the line when finite size electrical equipments exist(like in substation). It is the primary cause of error between the actual measured value and calculation results.(2) Using finite element method(FEM) analysing the electromagnetic problem, we calculated the induced voltage of the infinite and finite(including electrical equipments) transmission lines, and measured the induced values in Rong Jiang substation. The results show that the calculation results of two dimension electromagnetic model and the circuit model are the same. For the model in this paper, the result of three dimension model with finite size is smaller than the value obtained by EMTP, which is more close to the measured result.(3) The result calculated by three dimension electromagnetic field FEM shows that the supported frame affected the induced electrostatic voltage greatly, further study to improve the partical electrical field situation will facilitates the selection of earthing switch. The result also indicates that we can adjust the size of grading ring to improve the electric field enviroment. And a three dimension model to calculate surface electric field on the maintenance personnel body provides a theoretical basis for the selection of security value.(4) Use the two dimension electromagnetic model and theory to study the effect of phase sequence. the result shows that, for induced voltage, after changing the phase sequence, the induced value stays the same because the phase difference of the charge on the transmission line is approximately equal to 120 degree. But the value changes for the induced current after changing the phase sequence. This helps engineer to optimize the phase sequence.(5) Write the computer program based on circuit model. Different from the EMTP software, this software obtains the numerical solution for the distribution equation set without symmetrical component reduction. It has friendly user interface and it is easy to use.(6) The outcomes in this paper have been used on the 500 k V Jia Rong transmission line in practice, partly solving the problem that earthing switch is lack of capacity to connect and disconnect the power line. |
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