Research on the unbalance degree of four-circuit overhead lines on the same tower with different phase sequence arrangements

The phase sequence arrangements of multi-circuit overhead lines on the same tower, commonly used in extremely high voltage/ultra-high voltage (EHV/UHV) transmission lines, directly affects the symmetry of power transmission systems, which influence the operation of power grid and relay protection. It is difficult to achieve complete transposition of four-circuit overhead lines on the same tower because of their complex structure, and it is a practical and effective way to select a proper phase sequence arrangement to improve the symmetry of the line parameters. This paper presents an evaluation index based method for evaluating the symmetry of four-circuit overhead lines on the same tower and selecting the optimal phase sequence arrangement. First, the unbalance degrees of the single-sequence component and composite-sequence component are defined as the basic evaluation index of the symmetry of the parameters and obtained by the calculation of the line sequence parameters. Furthermore, the unbalance degree comprehensive evaluation index is constructed by the weighted sum based on the ranking of basic evaluation indexes. The weight distribution factors of various unbalance degrees are calculated according to the exponential (0,2) scale algorithm. The unbalance degree comprehensive evaluation index is used to evaluate the symmetry of transmission line parameters and select the optimal phase sequence arrangements of four-circuit overhead lines on the same tower. Finally, the optimal phase sequence arrangements for the different towers with typical structures, such as vertical, horizontal, d-type and e-type towers are obtained by using the above analysis method and verified by simulation of PSCAD/EMTDC.