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Distance protection relay

Distance protection relay is the name given to the protection, whose action depends on the distance of the feeding point to the fault. The time of operation of such protection is a function of the ratio of voltage and current, i.e., impedance.

There is one type of relay which functions depending upon the distance of fault in the line. More specifically, the relay operates depending upon the impedance between the point of fault and the point where relay is installed. These relays are known as distance relay or impedance relay.

Distance Relay or Impedance Relay Working Principle Types


There is one type of relay which functions depending upon the distance of fault in the line. More specifically, the relay operates depending upon the impedance between the point of fault and the point where relay is installed. These relays are known as distance relay or impedance relay.

Working Principle of Distance or Impedance Relay


The working principle of distance relay or impedance relay is very simple. There is one voltage element from potential transformer and a current element fed from current transformer of the system. The deflecting torque is produced by secondary current of CT and restoring torque is produced by voltage of potential transformer. In normal operating condition, restoring torque is more than deflecting torque. Hence relay will not operate. But in faulty condition, the current becomes quite large whereas voltage becomes less. Consequently, deflecting torque becomes more than restoring torque and dynamic parts of the relay starts moving which ultimately close the No contact of relay.


Hence clearly operation or working principle of distance relay depends upon the ratio of system voltage and current. As the ratio of voltage to current is nothing but impedance so a distance relay is also known as impedance relay. The operation of such relay depends upon the predetermined value of voltage to current ratio. This ratio is nothing but impedance. The relay will only operate when this voltage to current ratio becomes less than its predetermined value. Hence, it can be said that the relay will only operate when the impedance of the line becomes less than predetermined impedance (voltage / current). As 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 fault is occurred with in a predetermined distance or length of line. 


Types of Distance or Impedance Relay


There are mainly two types of distance relay

  1. Definite distance relay.
  2. Time distance relay.

Let us discuss one by one.

Definite Distance Relay

This is simply a variety of balance beam relay. Here one beam is placed horizontally and supported by hinge on the middle. One end of the beam is pulled downward by the magnetic force of voltage coil, fed from potential transformer attached to the line. Other end of the beam is pulled downward by the magnetic force of current coil fed from current transformer connected in series with line. Due to torque produced by these two downward forces, the beam stays at an equilibrium position. The torque due to voltage coil, serves as restraining torque and torque due to current coil, serves as deflecting torque.

Under normal operating condition restraining torque is greater than deflecting torque. Hence contacts of this distance relay remain open. When any fault occurs in the feeder, under protected zone, voltage of feeder decreases and at the same time current increases. The ratio of voltage to current i.e. impedance falls below the pre-determined value. In this situation, current coil pulls the beam more strongly than voltage coil, hence beam tilts to close the relay contacts and consequently the circuit breaker associated with this impedance relay will trip.

Time Distance Impedance Relay

This delay automatically adjusts its operating time according to the distance of the relay from the fault point. The time distance impedance relay will not only be operated depending upon voltage to current ratio, its operating time also depends upon the value of this ratio

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