To get accurate performance of the distance protection function, Must be all influencing quantities and limitations on the measuring accuracy of distance relay that imposed by power system in considered. This research includes study and analysis of the problems which faces the conventional distance relay which was designed for single circuit distance protection when applied double circuit of over head transmission line, due to mutual coupling zero - sequence system in this case, For various operational conditions and fault location,This work had been conducted with (I.N.R.G. 132 kV) transmission lines systems under to ground fault condition, The current and voltage at the relay and fault location were calculated.
The results show that zero sequence mutual coupling may cause the distance relay to seriously over reach under reach for different operational situations and fault locations, Values of short circuit levels ratio and power flow for feed sources which connected to the line ends have major effect in this problem.
This research indicate the possibility of zero sequence coupling compensation by injecting proportion of the zero sequence current flowing in the parallel feeder into the relay to reduce this effect, This method is successful when relay and fault location are in the a same circuit, Also when relay and fault location are in different circuits but with limits .
Keywords: Protection, Distance Relay, Influence of Mutual Coupling.