Research On Calculation And Transposition Method Of Unbalance Of 1000kV Single And Double Loop Transmission Lines

The three-phase voltage balance of the power system is one of the main indicators of power quality.According to the requirements of the national standard "Power Quality Three-phase Voltage Unbalance"(GB/T 15543-2008),the unbalance of the system voltage needs to be controlled.The 1000 kV transmission line has the characteristics of long transmission distance,large transmission power,and important position in the system.At the same time,it has the characteristics of large tower,large area,and wide corridor.Two parallel single loop is used in sparsely populated areas with difficult transportation conditions,and double loops on same tower is adopted in crowded areas of corridors while for double power transmission.Due to obstacles and planning,it is difficult to open new high-voltage corridors,and it is also common to use the original corridors to arrange multiple loops in parallel.When single and double transmission line loops are mixed,the asymmetry of the line is increased.So it is necessary to research the unbalance and transposition method of the line.The unbalance degree of the line parameters had been simulated and calculated in this paper,establishing the parameter coupling model of the double-loop on the same tower and the two-parallel single-loop,as well as the voltage drop equation and the leakage current equation.Combined with the definition of 1000 kV unbalance degree with single-loop and double-loop on same tower,the influence factors of the unbalance degree of line parameter and the relationship between the unbalance degree of line parameter and the voltage are analyzed.The results show that the negative sequence electromagnetic unbalance Mt2 of unbalance degree in the line parameter has a good correlation with the voltage unbalance.The ATP-EMTP simulation calculations also had been carried out in this paper regarding the voltage unbalance of single loop,two parallel single loops and double loop lines on the same tower.The results show that compact towers are better than cup type towers in two parallel single loop,and the reverse phase sequence of V strings suspension is better than the one of I strings suspension in double loops on the same tower.The factors which affect the unbalance of the line are analyzed,the length of the line,the transmission power,and one loop outage have a greater impact on the unbalance of the line voltage among them.In order to improve the voltage unbalance degree of the double-loop line on the same tower mixed connecting with two parallel single-loop lines,it should be avoid middle phase connection from double-loop line on the same tower to two parallel single-loop lines through simulation calculation of unbalance degree in above mixed transmission lines.The middle phase of double loop arranged in reverse phase sequence can be connected to the outer phase of the parallel single loop to obtain the smallest voltage unbalance.There are several types listed in combined connection between double loop on the same tower and two parallel single loop.The control function of voltage unbalance degree under two common connection modes is obtained.The equal-pitch transposition of the I strings suspension of double-loop on same tower with reverse phase-sequence connection to two parallel single-loop cup type towers,and the corresponding transposition mode of the middle phase of the double loop on the same tower to two parallel single loop are given.After a full cycle of transposition,the voltage unbalance is greatly reduced,meeting the regulatory limit.