The operating principle of distance relay or impedance relay is very simple. There is a voltage element of the potential transformer and a current element fed by the current transformer of the system. The deviation torque is produced by the CT secondary current and the restoration torque is produced by the voltage of the potential transformer.
Under normal operating conditions, torque restoration is more than a pair of deviation. Therefore, the relay will not work. But in defective condition, the current becomes quite large while the voltage decreases. As a result, the deflection torque becomes more than just the torque restoration and the dynamic parts of the relay start to move, which finally closes the contact Without contact of the relay. Therefore, the operation or operating principle of the distance relay clearly depends on the relationship between the voltage and current of the system. Since the ratio of voltage to current is nothing more than impedance, a distance relay is also known as an impedance relay.
The operation of said relay depends on the predetermined value of the voltage / current ratio. This relationship is nothing more than impedance. The relay will only work when this voltage to current ratio is lower than its default value. Therefore, it can be said that the relay will only work when the impedance of the line is lower than the predetermined impedance (voltage / current). Since the impedance of a transmission line is directly proportional to its length, it can easily be concluded that a distance relay can only operate if the fault occurs within a predetermined distance or line length.