Principle of Operation

The basic principle involved in the above figure explains the division of the voltage at the relay by the measured current. The absolute impedance is compared with the reach point impedance If the measured impedance is less than the reach point impedance, it is assumed that a fault exists on the line between the relay and the reach point.

The reach point of the relay is the point along the line impedance locus that is intersected by the boundary characteristics of the relay. Distance relay is the broader name of the different types of impedance relay.

 The relay is connected at position , R and receives a secondary current iF, equivalent to a primary fault current , IF. The secondary voltage , VF, is equivalent to the product of the fault current “IF” and impedance of the line up to the point of fault, ZF. The operating torque o f this relay is proportional to the fault current “IF”, and its restraining torque is proportional to the voltage “VF”.

Taking into account the number of turns of each coil, there will be a definite ratio of V/I at which the torque will be equal. This is the reach point setting of the relay. The relay will operate whe n the operating torque is greater than the restraining torque .Thus any increases in current coil ampere - turn s , without a corresponding increase in the voltage coil ampere - turns , will unbalance the relay. This means the V/ I ratio has fallen below the reach point. Alternatively if the restrain torque is greater than the operating torque , the relay will restrain and its contacts will remain open[8] . In this case the V/I ratio is above the reach point. The reach of a relay is the distance from the relaying point to the point of fault. Voltage on the primary voltage transformer , VT , is :

V = EZ_F/Z_S +Z _F

The Relay compare the secondary values of V and I as to measure their ratio which is called measured impedance Zm

Z_M =  Z_F * C T Ratio/ P T Ratio