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This paper includes a study and analysis of an earthed out of service for one of double circuit transmission lines impact on a Distance Measuring Relay (DMR) setting due to zero sequence system mutual coupling. The protection in a remaining line measures the fault impedance correctly, except when the line that is not in service is earthed at both ends. In this case a DMR performance which installed in this configuration at the ground faults occur is much less reliable and accuracy to get an ideal tripping protected zone for its. This study has been Modeling and simulting by using (Matlab\Simulink) program to analyze a steady state of ground fault on single circuit to calculate the DMR voltage,current and apparent impedance of sequences and system at various fault locations under operational status for the Iraqi North Regional Grids (INRG) 132Kv system. The paper results shows that mutual coupling by zero sequence current of earthed line may cause the DMR to seriously over-reach under different operational situations. The DMR first zone reach may extend to its second zone, So a DMR will give a trip signal for an external faults. [ABSTRACT FROM AUTHOR]

This paper investigates a study of the influence of an earthed transformer in a double circuit transmission lines have intermediate tapped to provide to loads on a measuring accuracy and performance of the conventional Distance Protection Relay (DPR) installed in this configuration when the ground faults occur beyond tee point. The protection of three-terminal lines is not as simple as that of two-terminal lines. They usually experience problems caused by the zero sequence current in-feed of an earthed transformer from the third terminal as well as mutual coupling due to this current. This study has been modeling and simulating by using (Matlab\Simulink) program to analyze a steady state of ground fault on single circuit to calculate the DPR voltage, current and apparent impedance with various fault locations under operational status for the Iraqi North Regional Grids (INRG) 132 kV system. The paper results shows that current in-feed and mutual coupling by zero sequence of earthed transformer may cause the distance relay to seriously under-reach or Under Reach according to zero sequence current direction. The values of short circuit levels ratio of system have major effect in this problem, So an apparent impedance as seen by DPR and length of protective zone will change. [ABSTRACT FROM AUTHOR]