Analysis On Imbalance Of Un-transposed Double-circuit Lines On The Same Tower

With the rapid development of economy in China, the EHV Transmission Lines of high-capacity and long-distance are increasing, which will inevitably lead to shortage of the line corridor.In order to solve this problem, the high voltage transmission lines of double or more-ircuit in the same tow er came into being. Generally the high-voltage transmission lines should have adopted transposition n measures to reduce the asymmetry of the current and voltage in the power system during the normal operation, but transmission lines in some areas may have no condition to transposition completely, since shortage of the line corridor, and the experience of design and operation has shown that the EHV Transmission Lines which completely transposed would have weakened the electrical and mechanical strength of transmission lines. But the transmission lines that have no transposition would have caused the asymmetry of electrical three-phase parameters, which would result to the asymmetry of three-phase current and voltage in the power system during the normal operation,increasing the loss of the element in transmission lines.Although the serious power system crash accident has not happened in our country, owing to asymmetric three-phase parameters of transmission line, but sometimes the asymm-etric three-phase parameters of transmission line would lead to the protection devices unwanted operation in transmission lines. For example, an accident of closed loop failure due to asymmetry of three-phase current had happened in220kV grid of China Southern Power Grid in the middle of2009, which caused the grid could only work in open-loop operation, and the reliability of power system is forced to lower. Presently our country is in the critical period of rapid development of economy, and power system is starting to step into the period of large power grid, extra high voltage and large unit. And the asymmetry of three-phase parameters in transmission lines has created a significant negative impact on the normal and stable operation of tpower system.To solve this problem, the formulas of calculating the current imbalance on double-circuit lines are presented in this paper. Using PSCAD/EMTDC,the Zero-sequence current unbalance and Negative sequence current unbalance of a750kV double circuit lines without transpositi-on are calculated in different type of tower, different phase sequence arrangement, different distance between the two loops and different line length. According to the simulation results, when the transmission line is short, the current (voltage) can be controlled within a certain range by adopting with the reverse phase sequence in the same tower window, which loops distance is adjusted properly. To avoid bad effect on the power system, transmission line must be transposed when the line is long.