Research On Relay Protection And Unbalanced Current Of A 220kV Double-circuit Ultrashort Line

Rapid development of China’s economy put forward higher demands for the construction of the power system. The increasingly tense transmission corridor and the decreasing distance between adjacent regions in the power intensive area results in the emergence of more and more ultrashort 220 kV double-circuit lines. Despite of their advantages,ultrashort lines bring up with new challenges for the power system as well. A serious three-phase imbalance occurred recently in an ultrashort double-circuit line of Jiangxi grid, which may threat the stable operation of the power system,and the following parts do some research on this double-circuit line.This paper summarizes the research status for the three-phase imbalance at home and abroad, then analyzes the calculation theory of current imbalance with comparison of existing methods. A promotion of an existing decoupling method also has been made. The research lines is modeled with ATP-EMTP based on the equivalent PSASP system afterwards.The following research work is divided into two parts.First part concentrates on imbalanced current of ultrashort line.Taken the research line as an example, the effects of several factors on current imbalance between the ultrashort lines and the long lines are compared. Then the unbalanced current of the studied line is analyzed, and some possible reasons are put forward.Second part is the research of relay protection. The simulation analysis has been made to study the influence of several factors on zero-sequence current and measured impedence. Those factors are zero-sequence mutual inductance, fault resistance and the current imbalance of double-circuit line. Finally, a simple verification of setting value and sensitivity of the backup protection device has been made.With all those research work mentioned above, this paper hopes to provide some reference so as to improve current imbalance and operation stability of the studied line.