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. Most of the relays in
service on power system today operate on principle of electromagnetic attraction or
electromagnetic induction. Regardless of the principle involved, relays are generally
classified according to the function they are called upon to perform in the protection of
electric circuits.

Type of Protection
When a fault occurs on any part of electric power system, it must be cleared quickly in
order to avoid damage and /or interference with the rest of the system. It is usual practice
to divide the protection scheme into two classes those are primary protection and back-up

Primary Protection
It is the protection scheme which is designed to protect the component parts of the power
system. If a fault occurs on any line, it will be cleared by its relay and circuit breaker. This
forms the primary or main protection and serves \as the first line of defense. The service
record of primary relaying is very high with well over ninety percent of all operations
being correct. However, sometimes faults are not cleared by primary relay system because
of trouble within the relay, wiring system or breaker.

Back-up Protection
It is designed to operate with sufficient time delay so that primary relaying is given enough
time to function if it is able to. Thus referring to fig…, relay A provides back-up protection
for each of the four lines. If a line fault is not cleared by its relay and breaker, the relay A
on the group breaker operate after a definite time delay and clear and entire group of lines.
It is evident that when back-up relaying functions, a larger part is disconnected than when
primary relaying functions correctly. Therefore, greater emphasis should be placed on the
better maintenance of primary relaying.

Over-current Relay
This type of relay works on the induction principle and initiates corrective measures when
current in the circuit exceeds the predetermined value. The actuating source is a current in
the circuit supplied to the relay from a current transformer. These relays are used on a.c.
circuits only and can operate for fault current flow in either direction.

Directional Power Relay
This type of relay operates when power in the circuit flows in a specific direction. Unlike a
non-directional over-current relay is so designed that it obtains its operating torque by the
interaction of magnetic fields derived from both voltage and current source of the circuit it
protects. Thus this type of relay is essentially a wattmeter and the direction of the torque
set up in the relay depends upon the direction of the current relative to the voltage with
which it is associated.

Distance or Impedance Relay
The operation of the relay is governed by the ratio applied voltage to current in the
protected circuit. Such relays are called distance or impedance relays. In impedance relay,
the torque produced by ac current element is opposed by the torque produced by a voltage
element. The relay operates when the ratio V/I is less than a predetermined value.

Differential relay
Differential relay one that operates when the phasor difference of two or more similar
electrical quantities exceeds a pre-determined value. It has advantages over other relays. It
is more sensitive than other relays and makes correct distinction between heavy load
conditions and minor fault conditions. Thus a current differential relay is one that
compares the current entering a section of the system with the current leaving the section.
Under normal operating conditions, the two currents are equal but as soon as a fault occurs,
this condition no longer applies. The difference between the incoming and outgoing
currents is arranged to flow through the operating coil of the relay. If this differential
current is equal to or greater than the pickup value, the relay operates and opens the circuit
breaker to isolate the faulty section.