Method for estimating unbalanced currents in untransposed double-circuit lines on the same tower

Due to the technique and economic considerations, transmission lines in the power grid are usually untransposed, which leads to the result that the line parameter is asymmetric. For double-circuit lines on the same tower, asymmetric line parameters may cause unbalanced currents even under normal operation, which will have a negative impact on the line temperature, line loss and even relay protections. This paper firstly investigates the line parameters and current characteristics of the double-circuit lines on the same tower under normal operating mode. Then, we provide a method to analyze the current characteristics including circulating current and the net through current, and effects of phasing arrangement on each kind of unbalanced components are analyzed. Finally, based on the actual parameter of tower and transmission line in power grid, unbalanced currents of a certain double-circuit line are analyzed with full consideration of different phasing arrangements. With the help of analysis results, some useful conclusions and suggestions to limit the current unbalance are listed in the last part of this paper.