| In some areas the land for transmission corridor was in short supply. To raise the power transmission capability of unit corridor, triple-circuit DC transmission lines in the same tower would be applied. For there still had been no design and application precedents of such lines in the world, it was necessary to study the valid calculation method for their total electric field at ground level to satisfy the requirements in engineering design and environment protection study.The main contents of this paper are summarized as follows:(1)A new calculation method for the total electric field at ground level of triple-circuit DC transmission lines in the same tower was proposed. Because of the large amount of computation and instability during the calculation process, the following improments were made:the optimization method of CSM was improved to consider calculation accuracy of both potential and electric field; during the process of seeking upstream element in the reverse process, for node whose upstream element cannot be found, its charge density was approximated by averaging the space charge density of surrounding nodes; the traditional iterative method was optimized, making the adjustment coefficient and charge density convergence precision adjustable according to the error of conductor surface field strength.(2)The total electric fields at ground level of triple-circuit DC analog transmission lines in the same tower were tested. The validity of the calculation method was verified by experiments. Considering the difference between the corona onset electric fields of positive and negative pole conductor bundles, calculation results for the total electric field at ground level obtained with this method were close to corresponding experiment results, so the method can be used for engineering design.(3) The factors such as horizontal distance, vertical distance, the minimum height and wind speed, which could affect the total electric fields at ground level of triple-circuit DC analog transmission lines, were analyzed. The electric field of bundle conductor surface and the ground nominal electric field were resorted to explain how those factors affect the total electric fields at ground level of triple-circuit DC transmission lines.(4) Under the same conductor dimension and line height, the electric field at ground level of triple-circuit DC transmission lines, double-circuit DC transmission lines and single-circuit DC transmission lines were compared. Results indicated that the maximum electric field and the corridor of rationally configured triple-circuit DC transmission lines are far less than that of double-circuit and single-circuit DC transmission lines.(5) In a case of a DC transmission line converted from a typical 500 kV double-circuit AC transmission line, the electric field of bundle conductor surface and the ground total electric field in different pole conductor bundles arrangement schemes were calculated and analyzed. The results shown that in most cases the electric field of bundle conductor surface of this DC transmission line is smaller than that of±500kV typical DC trsnmission lines, and at the minimum height of pole conductor bundles of DC transmission lines in residential and non-residential area, corridor width of DC transimission line is highlt close to the corridor width of AC transmission line. |
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