Research On Reliable Coefficient And Its Adaptability Of Zone1Phase-to-phase Distance Protection

With the development of power grid construction, there are more and more high-voltage short lines. When applied to short lines, distance protection often acts improperly because of the overreach of the protection. Currently, in order to avoid the over-reaching problem, we usually quit the distance protection unconditionally in short lines, but it will greatly reduce the reliability of protection system.Reliable coefficient of zone1distance protection is set typically to reach80%to85%of a line segment, and it is usually determined by relative error of each factor that effects the distance protection. However, it is only suitable for middle long lines in general sense. The margin of0.15-0.2can not cover all of the errors as to short lines or the middle long lines with large system impedance, and the absolute errors of transformers, protection devices and other factors will be larger than relative errors. If we still set reliable coefficient of zone1distance protection by relative errors, it will easily act improperly because of the overreach of the protection when there is a fault. In view of this situation, based on the maximum impedance measurement errors caused by factors that influence distance protection, this paper proposes an adaptive method of setting reliable coefficient of zone1distance protection. This method sets the reliable coefficient adaptively according to the system impedance and the current after a fault, which makes the setting of zone1distance protection more reliable. It effectively avoids the over-reaching problem when a phase-phase short fault occurs at the end of a short line and it also ensures the selectivity of distance protection.Firstly, the paper analyzes the error characteristics of factors that influence the setting of reliable coefficient of zone1distance protection based on the worst conditions in detail. In order to ensure the selectivity of distance protection and avoid the over-reaching problem, the paper accumulates the impedance measurement errors in the same direction to determine the maximum impedance errors of distance protection. Finally, based on the total impedance measurement errors, the paper proposes a functional relationship among reliable coefficient of zone1, the length of transmission line, the system impedance and the current after a fault. Simulation results show that this method can effectively avoid the over-reaching problems of distance protection when a fault occurs at the end of the line and ensures the selectivity of distance protection.