Research on circulating unbalance degree of double-circuit transmission line

Circulating current unbalance of double-circuit transmission line (DCTL) on the same tower has a vital effect on relay protection, especially in the short line. In this paper, a mathematical method based on phase-mode transformation is proposed to calculate unbalance of DCTL and explain the phenomenon that circulating unbalance degree is nearly insusceptible by the line length and load level. The effect of the equivalent system impedance, load level and the overhead ground wire are analyzed. According to the simulation results, reverse phase sequence is recommended as the best phase sequence arrangement to abate the circulating current unbalance of DCTL. And the paper also indicate that using overhead ground wire with high resistivity and improve height of overhead ground wire are efficient measures to reduce circulating unbalance degree.