Charging Current Compensation for Distance Protection

One difficulty of the application of line protections to long transmission lines is the influence of the large capacitance of the line. When a fault occurs on a long line, relatively large magnitude harmonic components of current and voltage are superimposed on the fundamental frequency component due to the large capacitance of the line. This harmonic component can cause oscillation of the distance measurement. It is difficult to remove the harmonic component completely because the order of the harmonic component varies with several conditions. Our research has shown that the degree of oscillation can be reduced significantly by compensating for the charging current in the impedance calculation. This proposed charging current compensation (CCC) method provides an excellent result under all conditions. In this paper, the calculation method of CCC is introduced and good performance of CCC for the application to long overhead lines and to the long cable line is proven based on Alternative Transient Program simulations.

I. Introduction

When a fault occurs on a transmission line, transient harmonic components are superimposed onto the fundamental frequency. When the transmission line is long, in other words, when the line capacitance is large, the influence of the harmonic component becomes significant. The main problem of large line capacitance for current differential relays is the high charging current, and charging current compensation using voltage is widely accepted as the solution to this problem [4]. On the other hand, the effect of large line capacitance on distance protection has rarely been studied in detail. In our studies, we have found that the lower harmonic component caused by large line capacitance affects the distance measurement and that the influence appears as an oscillation in the impedance measurement. In severe cases, it can cause distance protection to overreach or to operate slowly. In this paper, this undesirable effect of large line capacitance on the distance measurement is explained. In our research, it has been found that the oscillation can be reduced significantly by compensating charging current in the calculation of impedance. The method for taking line capacitance into account for the mathematical model of the distance measurement is described in this paper. And the excellent performance of the charging current compensation (CCC) method is proven by simulations using ATP. The performance of the CCC method for cable circuits is also proven. Furthermore, by investigating the case when the current and voltage are distorted by the cables behind the relays, the importance of the CCC method is explained.