WHAT IS A PROTECTIVE RELAY? | Types and Working

WHAT IS A PROTECTIVE RELAY

A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.

The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency, and phase angle. Through the changes in one or more of these quantities, the faults signal their presence, type, and location to the protective relays. Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the opening of the breaker and the disconnection of the faulty circuit.

A typical relay circuit is shown in Figure. This diagram shows one phase of a 3-phase system for simplicity. The relay circuit connections can be divided into three parts viz.

  1. The first part is the primary winding of a current transformer (C.T.) which is connected in series with the line to be protected.
  2. The second part consists of the secondary winding of C.T. and the relay operating coil.
  3. The third part is the tripping circuit which may be either ac or dc it consists of a source of supply, the trip coil of the circuit breaker, and the relay stationary contacts.

PROTECTIVE RELAY Working

When a short circuit occurs at point F on the transmission line, the current flowing in the line increases to an enormous value. 

This results in a heavy current flow through the relay coil, causing the relay to operate by closing its contacts. 

In turn, closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of the system. 

This way, the relay ensures the safety of the circuit equipment from damage and the normal working of the healthy portion of the system.

When a short circuit occurs at point F on the transmission line, the current flowing in the line increases to an enormous value. This results in a heavy current flow through the relay coil, causing the relay to operate by closing its contacts. This in turn closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of the system. In this way, the relay ensures the safety of the circuit equipment from damage and the normal working of the healthy portion of the system.

Requirements of Protective Relaying

The principal function of protective relaying is to cause the prompt removal front service of any element of the power system when it starts to operate in an abnormal manner or interfere with the effective operation of the rest of the system.

In order the protective relay system may perform this function satisfactorily, it should have the following qualities :

  1. selectivity
  2. speed
  3. sensitivity
  4. reliability
  5. simplicity
  6. economy

Basic Protective Relays Types

Most of the relays in service on the electric power system today are of the electromechanical type.

They work on the following two main operating principles :

  1. Electromagnetic attraction
  2. Electromagnetic induction

Working

There are several types of protective relays, each with a different function and working principle. Some of the most common types of protective relays include:

  1. Overcurrent Relays: These relays are used to detect excessive current levels in a circuit. They work by comparing the current flowing in the circuit to a set point, and if the current exceeds the set point, the relay will trip and open the circuit breaker to isolate the fault.
  2. Distance Relays: Distance relays are used to protect transmission lines from faults that occur along the line. They work by measuring the impedance of the line and comparing it to a set value. If the impedance exceeds the set value, the relay will trip and open the circuit breaker to isolate the fault.
  3. Differential Relays: Differential relays are used to protect transformers and generators from faults within the winding. They work by comparing the current flowing into the winding to the current flowing out of the winding. If there is a difference between the two currents, the relay will trip and open the circuit breaker to isolate the fault.
  4. Directional Relays: Directional relays are used to protect transformers and generators from faults that occur in a specific direction. They work by detecting the direction of the current and comparing it to the expected direction. If the direction is not as expected, the relay will trip and open the circuit breaker to isolate the fault.
  5. Earth Fault Relays: Earth fault relays are used to detect faults that occur between a conductor and the earth. They work by measuring the residual current in the circuit and comparing it to a set value. If the residual current exceeds the set value, the relay will trip and open the circuit breaker to isolate the fault.

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