POWER SYSTEM PROTECTION relays, types of relays, comparison, operation of relay switch gears. protection,

1. Dr.Latif Shaikh

2. Protective Relays
1.Introduction
2.Basic Current Relays Electromechanical Mechanism
3.Solid State Relays
4.Micrprocessor Controlled Relays
5.Over Current Relays
6.Under Voltage Relays
7.Differential relays
8.Distance Relays
9.Microcomputer Controlled Relays

3. Protective Relay
Protective relays are the devices that detect abnormal
conditions in electrical circuits by constantly measuring the
electrical quantities which are different under normal and
abnormal conditions .
Under abnormal condition current , voltage , phase angle
,frequency may change. The relay operates when it detects
the fault which completes the trip circuit thus opening the
circuit breaker.This disconnects the circuit.

4. Protective Relay
A typical relay is shown
.When short circuit occurs
at point F on transmission
line CT current
increases .This increases
current in CT coil which
closes trip coil of circuit
breaker and CB contacts
get opened. Thus faulty
section is isolated.

5. Requirements and Qualities of Relays
Selectivity and Discrimination
Speed
Sensitivity
Reliability
Simplicity
Economy

6. Important Terms Related to Relay Operation
1.Operating Force or torque- The force which tends to close the
contacts of the relay
2.Restraining Force- It is the torque which opposes the operating
torque and tends to prevent the closing of relay contacts.
3.Operating or Pick Up level – It is defined as the threshold value of
current ,voltage etc.above which the relay will close its contacts.
4.Drop out or Reset level- It is defined as the value of current,
voltage etc.below which the relay will open and its contacts will
return to normal position.
5.Flag or target – It is a device which indicates the operation of
relay.
6.Burden- It is power absorbed by the relay expressed in VA.

7. Important Terms Related to Relay Operation
7.Operating Time- It is defined as the time which elapses from
the moment when the actuating quantity attains a value
equal to pick up value until the relay operates its contacts.
8.Over –Reach –The relay is said to over reach when it
operates for a current which is lower than its setting.
9.Current Setting- Often it is desirable to adjust the pick up
current to any required value .This is known as current
setting and it is usually done by use of tapping on the relay
coil. The value assigned on each tap are expressed in terms
of percentage of full load rating of CT with which the relay
is associated and represents the value above which the relay
disc starts to rotate and finally closes the trip contacts

8. Attracted Armature Type Relays
These have a coil or
electromagnet energized by
a coil .The coil is energized
by operating quantity like V
or I.
Under normal conditions the
coil cannot attract the
plunger due to spring force.
Under fault condition the
fault current increases so
armature or plunger gets
attracted to close the
contacts .

9. Attracted Armature Type Relays
These have a coil or
electromagnet energized by
a coil .The coil is energized
by operating quantity like V
or I.
Under normal conditions the
coil cannot attract the
plunger due to spring force.
Under fault condition the
fault current increases so
armature or plunger gets
attracted to close the
contacts.

10. Attracted Armature Type Relays
Applications
1.For over current protection
2.Differential Protection
3.Auxiliary Relays
4.Definite time lag over
current and earth fault
protection

11. Static Relays
A static relay is the relay which uses solid state components like
transistors ,diodes etc.,for the measurement or comparison of
electrical quantities.
The static network is so designed that it gives an output signal in
the tripping direction whenever a threshold conditions reached.
The output signal in turn operates a tripping device which may be
electromagnetic or electronic.
Static relays are capable of performing the same function with the
use of electronic control circuit which an electromagnetic relay
performs with the use of moving parts or elements.

13. Static Relays
Figure shows the essential components of a static relay
1.The relaying quantity (the output of CT and PT or a
transducer)is first rectified and compared with the set
values.
2.The output is actuated when the dynamic
input(relaying quantity)reaches the threshold value.
This valued is amplified and given to the output
device which is usually an electromagnetic.This
energizes the trip coil and the relay operates.

14. Static Relays
Advantages:
1.More accurate and fast in operation
2. No moving parts.
3.VA ratings of CT and PT is comparatively lower.
4.Low power consumption.
5.Resetting time and overshoot can be reduced.
6.Very compact.
7.Superior characteristics and accuracy.
8.Several functions can be accommodated in a static
relay
9.Simplified testing and servicing is possible.

15. Static Relays
Disadvantages
1.Reliability cannot be predicted
2.Construction is not very robust.
3.Easily affected by surrounding interference.
4.Auxiliarty DC supply is required
5,.Affected by voltage transients.
Applications
1.Ultra high speed protection of EHV AC transmission lines
utilizing distance protection.
2.In over current and earth fault protection schemes
3.As main element in differential relay

16. Microprocessor Controlled Relays
In microprocessor based relays the digital processing unit is
the microprocessor. Microprocessor is now used for
protection, fault recording ,fault locating ,data monitoring
and several other functions . Compared to other analog and
digital relays ,the programmable microprocessor based
relays have following superior features
1.Ability to combine a large number of protective and
monitoring functions.
2.Measured values of variables are processed digitally by
microprocessor.
3.High level of flexibility – Various protective functions can be
freely selected and allocated to various relays by the
software.
4.The memory of relay enables the relay to retain values
responsible for tripping ,time taken by relay etc.

17. 5.Speed- High speed relays are available for line protection.
They use microprocessors.
6.User friendly yet highly capable- Microprocessor based
relays are easy to apply, operate and use. They are highly
capable.
7.Increased reliability due to self checking-Microprocessor
based relay checks itself the circuits, power supply
memories. This improves the reliability of the relay.

18. Microprocessor Based Static Relays

19. Microprocessor Based Static Relays

20. Under Voltage Relays
Under voltage protection is provided for AC circuits ,bus –
bars ,motors ,rectifiers, transformer set. Such protection is
given by means of Under Voltage relays. Under voltage
relays are necessary for voltage control and reactive power
control of network buses and load buses.
Under voltage relays have instantaneous characteristics or
inverse characteristics depending upon the construction
and design. Inverse time under voltage relays have inverse
characteristics, their operating time reduces with the
reduction in voltage.
Induction disc type of construction is used for under voltage
relays. The relay coil is energized by the voltage to be
measured either directly or through a PT.

21. Over Current Protection
Over current protection is that protection in which the relay
picks up when the magnitude of the current exceeds the pick
up value.
The over current relays are connected to the system usually
through a CT..

22. Over Current Protection
Protective Devices used for over current protection are
Fuses
Miniature circuit breakers
Circuit breakers fitted with overloaded coils or tripped
by over current relays
Over current relays along with current transformers

23. Applications of Over Current Protection
Motor protection
Transformer protection
Line protection
Protection of utility equipment

24. Characteristics of Relay Units for Over Current Protection
Definite Characteristics
Inverse Characteristics
Extremely Inverse
Very inverse
Inverse

27. Connection scheme with three over current relays

28. Connection scheme with three over current relays
The figure shows three current transformers and relay coils
connected in star point is earthed. When short circuit
occurs in the protected zone the protected zone the
secondary current of CTs increases. The current flows
through the coil and the relay picks up.
The relay closes and thereby the trip circuit is closed and the
circuit breaker operates.
The over current protection scheme with three over current
relays responds to phase faults including single phase to
earth fault. Therefore such schemes are used with solidly
earthed systems where phase to phase and phase to earth
faults are likely to occur.

29. Over current relay with time delay relay
In this case an additional relay is used.An additional
auxiliary relay and a definite time relay is used.
When a fault occurs the over current relay operated.
Definite time relay is used in the trip circuit.
The over current relay operates and closes the circuit of
definite time relay.
The definite time relay operates after a defined minimum
time.
So the circuit breaker opens only after a over current fault
and after a definite time.

30. Over current relay with time delay relay

31. Differential relays
Differential relay is that type of relay which operates when
the phase difference of two or more similar electrical
quantities exceeds a predetermined value.
Almost any type of relay ,when connected in a certain way
can be made to operate as a differential relay.
The two fundamental systems of differential or balanced
protection are
1.Current Balance protection
2.Voltage balance protection

32. Differential Relays (Current Differential Relay(

33. Current differential relay
Figure shows the arrangement of an over current relay
connected to work on the current differential protection.
In this arrangement a pair of current transformers are
fitted either ends of the element to be protected and
secondary windings of CTs are connected in series so that
they carry induced currents in the same direction.
Under normal conditions, where there is no fault or there is
external fault ,the currents in the two CTs secondary are
equal and relay operating does not carry any current.
Whenever there is an internal fault ,currents in the two
secondaries of CTs are different ,the relay operating coils
gets energized by the current equal to their difference and
the trip circuit is completed to operate the circuit breaker

34. Current Differential Relay
This system is employed for the protection of feeders
,alternators and transformers.
The CTs of equal ratio are employed when used either at the
two ends of an alternator winding or at the lower ends of a
feeder with no tapping.
When this system is to be used for protection of
transformers, correction must be made for different
currents determined approximately by the transformers
turns ratio.

35. Percentage Differential relay

36. Percentage Differential Relay
This relay is called percentage differential relay because the
current required to trip can be expressed as a percentage
of load current. The percentage differential relay is shown
in figure.
The current in the operating coil is (i1-i2) while the current
in the restraining coil is (i1+i2)/2 .
Greater the current flowing through the restraining coil, the
higher the value of current required in the operating coil
to trip the relay. Thus when the load is heavy, a greater
differential current through operating coil is required for
operation than under light load.

37. Percentage Differential Relay
Since the relay has a rising characteristic that is the pick
up value increases as the magnitude of through current
increases, the relay is restrained or biased against
operating inaccurately.
Due to this the relay is called a biased relay.
The advantage of this relay is that it is less likely to operate
inaccurately than a differentially connected over current
relay when a short circuit occurs external to the
protected zone.

38. Voltage Balance Differential Relay

39. Voltage Balance Differential Relay
Under normal healthy condition, the two voltages e1 and e2
are same ,so the difference between them is zero. So the
relay will not operate under normal condition.
In case of fault, the two voltages will be different either in
magnitude or in phase or on magnitude and phase both.
So under faulty condition, there will be current in the
operating coil and the relay operates.

40. Distance Relays
The relays in which the operation is governed by the ratio of
the applied voltage to current in the protected circuit are
called Distance or Impedance relays(‘impedance’ is an
electrical measure of distance along the transmission line).A
distance or impedance relay is essentially an ohmmeter and
operates when the impedance of the protected zone falls
below a predetermined value.
In impedance relay, the torque is produced by a current
element is opposed by the torque produced by the voltage
element,. The relay operates when the ration V/I is less than
a predetermined value.

41. Basic Principle of Operation of Impedance Relay

42. Basic Principle of Operation of Impedance Relay
The principle of operation of an impedance relay is shown in
figure.
The voltage element of the relay is excited through a P.T. from
the line to be protected .The current element of the relay is
excited from a C.T. in series with the line. The portion LM of
the line is the protected zone.
Under normal operating conditions, the impedance of the
protected zone is ZL .The relay is so designed that it closes
its contacts whenever the impedance of the protected zone
falls below the predetermined value.(ZL in this case).
On the occurrence of fault at point F1 in the protected zone
,the impedance Z(V/I,V and I being substation voltage and
fault current respectively) is less than ZL ,the relay operates.

43. Basic Principle of Operation of Impedance Relay
However the relay will not operate if the fault occurs beyond
the protected zone( point F2),since Z is greater than ZL at
this point.