Types of Relays and design Requirements. Part
2b. This is the second part of Topic 2 in the series of “Types of Protective Relays and
design requirements.” In this topic we continue our discussion on Design, mechanism and application of different
types of relays And in this part we will be discussing
Overcurrent relays and Directional relays. Now, 1)OVER CURRENT RELAYS:
What is an overcurrent relay? An over current relay operates only when the value of current sensed is greater than the relay
set value. That is essentially what an overcurrent relay is. How does a Mechanism work for an
overcurrent relay? Well this relay consists of a current coil and a disc which is free to rotate between the poles of
two magnets. When normal current flows through this coil, the magnetic effect generated by the coil on the disc is not sufficient to move
the moving element of the relay, and that means the restraining force is greater than the deflecting force. But, when the current through
this coil increases, and after a certain level of current, the deflecting force becomes greater then then the restraining force. As a result, the
moving elements starts changing the contact position in the relay. The trip circuit operates the circuit breaker which isolates the fault.
Depending on the time and characteristic of operation of relays, they are placed in three categories:
The first category is the Instantaneous Over-Current Relay. The second category is the Definite time over current Relay. And the third
category is the Inverse definite minimum time (IDMT) over-current Relay So for the first one the Instantaneous
Over-Current Relay: is the one in which no intentional time delay is provided for the operation. It operates instantaneously when the
current rises to a very high value. This characteristic is usually used in case of short circuit faults when the relay has to trip
immediately without any delay. So that’s what an instantaneous over current relay is used for. The next characteristic is the Definite Time
Overcurrent Relays: In this type, two conditions must be satisfied for operation, the first condition is current must
exceed the setting value and the fault must persist for at least a time equal to the time-delay setting of the relay. The Definite Time
overcurrent relay characteristic is used for short circuit protection, where the relay has to trip immediately but a certain time delay is provided
to ensure coordination with other relays upstream and downstream.
The third category is called the Inverse definite minimum time (IDMT) over-current Relay:
Which is one in which the operating time is approximately inversely proportional to the fault current near pick-up value and then becomes
constant above the pick-up value of the relay. These relays are used for the protection against overcurrent faults.
The Inverse definite minimum or IDMT characteristic is further divided into three more categories that are known as degrees
of inverse curves and they are* Normal inverse (NI), Very inverse (VI) and Extremely inverse (EI). An overcurrent IDMT
relay may be programmed or set to work with any of these three degrees depending on the specialized type of protection that is required.
So Usually the overcurrent relays are used in: Motor Protection: Used against overloads and
short circuits in stator winding of motor. It’s also used in Sub transmission lines where the cost of distance relaying cannot be justified. And
Distribution system protection. The next type of relay is the Directional Relays.
This type of relay employs the principle of actuation of relay which means the action that causes the relay to operate, when the fault
current flows into the relay in any particular direction. So, If the power flows in opposite direction, the relay will not operate. That’s the
main difference between the directional versus the non-directional relays. As far as the MECHANISM is concerned,
Consider a feeder XY, that’s passing through a station A. A circuit breaker in feeder AY is provided with a directional relay R, which will
trip the breaker CB(y) if the power flow is in the direction of AY alone. So it will only trip when the power flow is in the direction of Ay and
beyond the circuit breaker. Therefore, for a fault in feeder AX, which is in the opposite direction the circuit breaker CB(y) will not and should not
does not trip unnecessarily because it is a directional relay. However, for faults in feeder AY, then CB(y) should trip and it’s because the
fault is in the direction of the relay, and it’s in the direction AY. So, these types of relays are Used in
transmission lines where the power flow is associated in a specific direction and in particularly in parallel feeders where if a fault
occurs on one feeder then the other feeder must not trip. So, also directional overcurrent relays are used in networked systems where
the systems are all interconnected with each other and if they are not radial or when power flow can occur in either directional line.
*Now in the next video we will be discussing Reverse Power flow relays and Distance or Impedance Relays used in Electrical
systems and their applications as well* *We hope that you have a continued interest in this topic and in this series as either a student
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