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The function of protective relays:
The function of protective relaying is to detect any abnormal condition in the system
and to isolate the faulty section or element from the system as quick as possible. This helps to
limit the damage of the faulty section or element and to keep the rest of the system undisturbed.
The relaying equipment is aided in this task by circuit breaker that can capable of disconnecting
the faulty element when they are called to do so by relaying equipment.
Although the principal function of protective relaying is to avoid the damage caused by short
circuits, protective relaying should also take care of any abnormal operating conditions,
particularly in generators or motors.
A secondary function of protective relaying is to provide an indication of location of fault and
type of failure which is helpful for maintenance people.
For the protection of a part of a power system against short circuit, the protective relaying may
be classified into two groups:
1) Primary relaying: First line of defense
2) Backup relaying: This functions only when the primary relaying fails.
What is the advantage of overlapping zone?
This can be explained by taking the following system of the system.
Let us consider the fault shows
just above breaker C which is
within zone A but outside zone B.
Hence zone A protection scheme
detects the fault and tips breakers
C and S. But fault current will
continue to feed through the link
This type of relaying is used only for the protection of short circuits, because short circuits are
the predominant type of power system failure. Experience has shown that backup relaying for
other than short circuits is not economically justifiable.
A clear knowledge of the possible cause of primary relaying failure is essential in the design of
backup relaying. Primary relaying fails due to the failure of the following:
1) Current or voltage supply to the relays.
2) D-C tripping voltage supply.
3) Protective relays.
4) Tripping circuit or breakers mechanism.
5) Circuit breaker
In order to provide satisfactory backup protection the backup relaying should be completely
independent from primary relaying equipment i,e, nothing should be in common . So far as
possible, the practice is to locate the backup relays at different station.
To give an example
For fault at K, only R3 should operate to clear the fault, if however, R3 does not operate for any
reason, R2 should clear the fault. So R2 is backing up the R3, similarly R1 will back up R2 for
Describe the necessity of having more protective zones for the protection of the power
system to achieve better service.
In order to service continuity to as many areas as possible under fault condition, the power
system should be divided as many zones as possible so that the protective scheme of only the
affected zone can isolate the fault. (This is called selectivity).
To give an example the following two causes can be considered:
For fault at P1, A, B and C should be tripped to isolate the fault. On the other hand for fault at P2
the breakers A, B, C, D & E should be tripped to isolate the fault.
If we increase the protective zone of case-1, we get the following:
In this case for fault at P1, only breakers A and F will be tripped to isolate the fault. And for fault
at P2 breakers C D E & G should be tripped to isolate the fault.
By comparing case-1 and case-2, we can see that less number of consumers will be affected i,e, a
batter service continuity can be achieved if more protective zones are used .
General requirements for protective relaying:
a) Selectivity or discrimination
b) Speed of operation
a) Selectivity or Discrimination:
It is the ability of the protection to isolate only the faulty section of the circuit after the
occurrence of a short circuit. It is an important requirement for providing a reliable supply to
For fault at F3, the breakers A2 and B2 should be tripped
For fault at F2, the breakers C1 and D1 should be tripped
b) Speed of operation:
A short circuit must be cleared as quickly as possible to reduce the damage to the equipment.
High speed relaying is essential to reduce the voltage dropping time and to maintain system
The requirement of high speed in a number of cases is a decisive factor for providing the stability
of the operation of power station and power systems.
The sensitivity of a protection must be such that it can operate in the case of a short circuit under
minimum operating condition of the system. Under these conditions the variation of the actuating
quantity (current) will be minimum. For example, if one generator is disconnected from station A
during any under loading condition, as shown in the following fig, the short circuit current at the
relay point near the fault will be decreased.
The sensitivity of the protection must be high enough to act under this minimum condition. The
sensitivity of a protection can be expressed by sensitivity factor Ks.
Where, I short circuit minimum = Minimum short circuit current.
Iop = the smallest current at which the protection starts operating.
Dependability implies that the protection must faultlessly operate in the case of a short circuit
within the prescribed zone and must not operate incorrectly under conditions which do not
include provision for its operation. The dependability is of primary importance. Missing or
unnecessary operating of a protective scheme already involves extra isolations.
Classification of Relays
1) Electro mechanical relays.
2) Static relays.
3) Microprocessor based relays.
Electro mechanical relays can be classified into following two groups:
a) Electromagnetic attraction.
b) Electromagnetic induction.
The first one operates by virtue of a plunger being drawn into a solenoid or an armature being
attracted to the poles of an electromagnet. Such relays may be actuated by d.c or by a.c
Electromagnetic induction type relay use the principle of an induction motor where torque is
developed by induction in the rotor. This operating principle applies only to actuate by
alternating current. They are better known as induction relays.
Mechanical movement of the operating mechanism is imported to a contact structure to close or
to open contacts.
a) N/O Contact
b) N/C contact
The smallest value of the actuating quantity that causes the contact "a" to just close and contact
"b" to just open is called pick up value.
Reset Value or Dropout:
The largest value of the actuating quantity at which the relay resets and comes back to its
Adjustment of Pickup or Reset:
Adjustment of pickup or reset is provided electrically by tapped current coils or by tapped
auxiliary potential transfers or rectors; or adjustment is provided mechanically by adjustable
spring tension or by varying the initial air gap of the operating element with respect to its
solenoid or electromagnet.
Instantaneous and Time Delay Relay:
In instantaneous or high speed relay there is no intentional time delay and then operating time
approximately .05 sec or less . Occasionally a supplementary auxiliary relay having fixed time
delay may be used when a certain delay is required that is entirely independent of the magnitude
of the actuating quantity in the protective relay.
Time delay is obtained in induction type relays by a drag Magnet "Which is a permanent magnet
arranged so that the relay rotor cuts the flux between the poles of the magnet. This produces a
retarding effect on the motion of the rotor in either direction.
Single Quantity Relay:
Electromagnetic attraction type:
This is the simplest type of relay. This has an electromagnet energized by coil. The coil is
energized by the operating quantity which may be proportional circuit current or voltage.
There are two types of Electromagnetic attraction type relay:
a) Hinged attracted armature type
b) Plunger type attraction relay
Both these relays work on both ac and dc. The actuating quantity is a single current or voltage.
Hinged attracted armature type
The electromagnetic force exerted on the moving element is proportional to the square of the
flux in air gap. If saturation is neglected then exerted force proportional to square of operating
Where, F=net force
k1=force conversion constant
I= Magnitude (r.m.s) of the actuating current
k2=the restraining force including friction
When the relay is on the verge of picking up, the attractive force becomes equal to the
restraining force. So, the force becomes zero, i.e. F=0
k1I2 = k2
The pick current of the relay should be just greater than I
Ratio of reset to pick up:
One characteristic that effect the application of these relays is the relatively large difference
between their pick up and reset values. This is due to the fact that once the relay has picked up.
The air gap is shortened. This permits a smaller magnitude of coil current to keep the relay
picked up than was required to pick it up. This effect is less pronounced in ac than that in dc
relays. By special design the reset can be made as high as 90-95% of pick up in ac. relay and 6090% in dc relays.
Pick up value can be adjusted by adjusting the initial air gap. A higher pick up calibration will
have a lower ratio of reset to pick up.
Tendency towards vibration:
Unless the pole pieces of such relay have " shading rings" to split the air gap flux into two out
phase components, such relay are not suitable for continuous operation on alternating current in
the picked up position . This is because there would be excessive vibration that would produce
objectionable noise and cause excessive wear. This tendency to vibrate is related to the fact that
an ac relay without shading ring has a tendency to reset every half cycle when flux passes
Special feature of attracted armature type relay:
1. Attracted armature type relays respond to both ac. and dc.
2. These are fast relays. They have fast operation and fast reset because of small length of travel
and light moving parts.
3. They are described as instantaneous relays. But their operating time does vary with current.
Slow operating time and reset time can be obtained by delaying in field or delay of flux in the
magnetic circuit by fitting copper ring around the magnet and by means of bellows, dash pot etc.
Operating time as slow as .1 sec and resetting time as .5 sec can be obtained by such means.
4) These relays do not have any directional feature unless they are provided with additional
5) As they are fast and operate on both ac and dc, they are affected by transients. The transients
contain dc component as well as other harmonies. Therefore, though the steady state current is
less than pick up value, the relay may pick up during transients state.
6) VA burden depends on its construction.
0.2 - 0.6 VA for current range 0.1-0.4 amp.
7) Modern attraction armature relays are compact, robust and reliable.