In overcurrent protection, the relay depends on the magnitude of fault current and source impedance. Hence, the relay operation may result in under-reach or over-reach. To solve the drawback of overcurrent protection, distance protection is introduced in the protection of transmission lines.
In distance relay, the operation depends on the ratio of voltage and current that is expressed in terms of impedance. Therefore, the distance relay is also known as the impedance relay.
The impedance is defined as an electrical measure of distance along a transmission line.
The impedance relay operates when the ratio V/I (impedance) is less than a predefined value.
As the performance of distance relay affects by the ratio, these relays are also known as ratio relays.
According to the ratio of V and I, distance protection is classified into three types;
- Impedance relay (depends on measurement of impedance Z)
- Reactance relay (depends on the measurement of reactance X)
- Admittance relay (Mho relay) (depends on the measurement of admittance Y)
Therefore, we can say that the distance relay is one whose performance depends on the measurement of impedance, reactance, and admittance of line between the location of a relay and the point where a fault occurs.
The impedance relay works corresponding to the ratio of voltage V and current I of the circuit to be protected.
To measure the ratio of voltage and current, two elements are used in the impedance relay. One produces torque proportional to the voltage and the second produces torque proportional to the current.
The torque produced by the current element is balanced with the torque produced by the voltage element.
Therefore, the torque produced by the current element is operating torque or pickup torque which is said to be positive torque. And the torque produced by the voltage element is restraining torque or reset torque which is said to be negative torque.
This relay is also known as a voltage restrained overcurrent relay.
The current element is energized by the current through CT. And the voltage element is energized by voltage through PT.
The ratio of voltage V and L under normal conditions is denoted as ZL which is the impedance of a line. Under normal conditions, the ratio measured by the impedance relay is lower than ZL. Hence, the relay remains inoperative under normal conditions.
When the fault occurs, the voltage drops, and the current increases. Therefore, the ratio V/I reduces drastically.
So, the relay operates when the impedance decreases below the predefined value.
In a reactance relay, the operating torque is obtained by current while the restraining torque is developed by a current-voltage directional relay.
The overcurrent element develops the positive torque and the directional unit produces negative torque.
Hence, the reactance relay is an overcurrent relay with directional restraint.
The directional element is designed so that the maximum torque angle is 90˚.
A separate unit is needed to make an impedance relay directional. But in the mho relay, it is made inherently directional by adding a voltage winding called polarizing winding.
The mho relay works on the measurement of admittance Y ∠ θ. This relay is also known as angle impedance relay.
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