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Protective Relay

Protective Relay

Electricity is our need today. We use Electrical Energy Everywhere. This Energy is first generated and then transmitted to the users. This huge power system consists of Generators, Transformers, Transmission lines etc. In this huge system there can be failure at any time and at any place. There can failure due to the breakage of Transmission Lines, fault in transformer and generator etc. The fault can be very severe, it can damage the equipment of users and it can be the cause of fatalities. We cannot eliminate the fault but we can detect it. Our main concern is to detect the fault and isolate that faulty system from the whole system (healthy system). If there is fault at the transmission line, then we will isolate that transmission line from the system to minimize the interruption of supply and damage.

Protective Relay:

Relay is a device which detects the fault and abnormal condition in power system. It takes some inputs from the sensing devices. On the basis of received input it decides whether there is fault or not. If there is fault, then it commands the Circuit Breaker (CB) to isolate the faulty element or subsystem. As it is used for protection of the power system hence it is referred as Protective Relay.

A relay is usually connected with C.T (Current Transformer) for the measurement of current and V.T (Voltage Transformer) for the measurement of Voltage. It then takes decision and command the Circuit Breaker (CB).

Characteristics of Protective Relay:

A protective relay has some attributes and characteristics. On the basis of these characteristics we do grading of relay, whether the relay is good or suitable for the desired application or not. The attributes are given below,

  • Selectivity
  • Reliability
  • Sensitivity
  • Speed
  • Economy


The purpose of Relay is to isolate the faulty part of the system only.  Selectivity is the ability of relay to choose the faulty part of the system without disturbing the healthy part. If a transformer is faulty then it should be taken out from the system while the other system should remain undisturbed. If the relay is not selective then it will cause unnecessary interruptions. For Instance, consider the following picture,

There is fault at Bus C, then this bus should be taken out from the system so that the healthy system remains unaffected.


Reliability means a relay should operate whenever it is supposed to operate, and should not operate falsely. We rely on the relay that it will protect from the faults. The relay is of no use if it is not reliable. There is still a backup protection provided to deal when the relay fails.


It is the ability to operate at minimum fault. The apparent power of relay gives the idea of sensitivity. A 1VA relay is more sensitive than 5VA relay. A relay should be sensitive so that it can work on minimum apparent power.


It is desired to isolate the faulty system as soon as possible. If the system is not isolated early then the fault will sustain for more time which can damage the equipment.


Relay is to protect the equipment or system. Therefore, its cost should be not more than 5% of the total cost of the system. It is uneconomical to spend $100 on the relay for protecting an equipment worth $120.Its feasible protection cost will be less than $6.

Zone of Protection of the Relay:

The area which is protected by relay is known as Zone of protection of Relay. The farthest point that the relay can protect is called Reach point and the distance between Relay and Reach point is referred as Reach of the Relay.

The zone of protection of Relay A and Relay B is indicated in red and blue respectively. The point to be noted is that there is some area which is in both the zones. It is known as overlapping in zones of protection. It is done so that no area remains unprotected between the zones.

Terminologies used for Relay:

There are some common terminologies used with relays. The terminologies are,

  • Pick-up Current
  • Current Setting
  • Plug-setting Multiplier
  • Time-setting Multiplier

Pick-up Current:

It is the minimum amount of current at which the relay operates. When the current in the relay reaches this value then the relay operates. The relay doesn’t respond to current below this value.

Suppose that the Pick-up current of a relay is 5A. If any current below 5A passes through it, the relay will not operate. It will respond to 5A and the current above this value means the trip signal (output of relay is 1) will be given to Circuit Breaker.

Current Setting (C.S):

The adjustment of Pick-up current to a desired value is called as Current Setting. There are several tapings taken out of the relay coil according to application of the relay. These tapping caries the different number of turns in the coil. The Pick-up current depends on the number of turns of the coil. If the number of turns are changed then the Pick-up current will also change. It is denoted as C.S.

Current Setting is given in percentage of the secondary current of CT which is attached to the relay. Suppose we have a CT of ratio 100/5, If we set the Current Setting to 100% then the Pick-up Current will be 5A.

 If we set the Current Setting to 150% then the Pick-up Current will be 7.5A.

It basically changes the reference of fault. If we set Current Setting to 100% with CT of 100/5 then we want to trip the Circuit Breaker if the current through load is 100A or above. While, if we set Current Setting to 150% with the same CT then we want to trip the Circuit Breaker if the current through load is 150A or above. So by this we change the reference of fault.

Plug-setting Multiplier (P.S.M):

It is a measure of severity of fault and defined as the ratio of Current through the relay to the Pick-up Current. It is written as P.S.M.

As the current in the relay coil comes from CT, we can also write as,

For Example, we have a 100/5 C.T and the Pick-up Current of the Relay is 5A. We set Current Setting to 100%. If 100A current passes through the load, then there will be 5A at the secondary of C.T which will move to the Relay Coil. If we calculate P.S.M, then it will be 1.

 It indicates that the load current is 1 time of the nominal current or the Relay Coil current is 1 time of the Pick-up Current.

Now if the 1000 amperes pass through load, then C.T will give 50A to Relay and P.S.M will be 10.

It means the Load current is 10 times the nominal current or Relay Current is 10 times the Pick-up Current. It is telling the severity of the fault.

Time-P.S.M Curve:

It is a curve which tells the relation between the Operating Time of Relay and P.S.M.

It is very clear that the operating time depends on the value of P.S.M. At higher severity the relay takes less time to operate. For example, the relay operating time at P.S.M = 10 is nearly 0.6 sec and at P.S.M = 30 is 0.4 sec.

Time-setting Multiplier (T.S.M):

It is the adjustment of operating time of Relay. A relay is provided with a dial having T.S.M values from 0 to 1 with step size of 0.05. There are 20 values of T.S.M that are available on Relay to set the Operating Time. The new Operating time can be given as,

For instance, we have a relay which is following the above Time-P.S.M curve. The Time-P.S.M curve is actually defined at T.S.M=1. At P.S.M=10 it will operate in 0.6 sec. Now if we select T.S.M= 0.5 from the dial then the operating time can be given as,

Hence the operating time can be reduced by selecting T.M.S less than 1.

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