Different types of Protective relays used in Power Systems

2. UNIT-03
Protective Relays
CO-04- Understand protective relays

3. Protective Relay
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.

4. Required qualities of Protective Relay
i) Selectivity: It is the ability of protective system to
select correctly that part of system in trouble and
disconnect the faulty part without disturbing the
rest of the system.
ii) Speed: The relay system should disconnect the
faulty section as fast as possible to prevent the
electrical apparatus from damage and for system
stability.
iii) Sensitivity: It is the ability of the relay system to
operate with low value of actuating quantity.

5. iv) Reliability: It is the ability of the relay system to
operate under predetermined conditions.
v) Simplicity: The relay system should be simple so
that it can be easily maintained.
vi) Economy: The most important factor in the choice
of particular protection scheme is the economic
aspect. The protective gear should not cost more
than 5% of the total cost of equipment to be
protected.

6. Fundamental requirements of Protective Relay :
1) Detect abnormal conditions
2) Disconnect abnormally operating part so as to prevent
the subsequent fault.
3) Disconnect faulty part quickly so as to improve system
stability, service continuity and system performance
4) Improve Transient stability
Qualities of Protective Relay:
1) Selectivity.
2) Speed.
3) Sensitivity
4) Reliability / Trust worthiness.
5) Simplicity
6) Economical

7. Necessity for 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 very much necessary to protect the power systems,
equipments, motors, generators etc. from the dangerous
fault conditions in an electric supply.
• Hence it is necessary to have the arrangements with
which all these equipments can be switched on or off
under no load or load conditions or even fault
conditions. The collection of various equipments used
for the switching and protecting purpose in a power
system is called switchgear.
• The various components of switchgear are switches,
fuses, relays, circuit breakers etc. The switchgear
protects the system from fault and abnormal conditions
and assures continuity of an electric supply.

8. (i) Primary Protection.

9. • It is the protection scheme which is designed to
protect the component parts of the power system.
Thus referring to Fig.
• Each line has an over current relay that protects the
line. 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 defence.
• 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. Under
such conditions, back-up protection does the
required job

10. (ii) Back-up protection.
• It is the second line of defence in case of failure
of the primary protection. It is designed to
operate with sufficient time delay so that
primary relaying will be given enough time to
function if it is able to do.
• 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 will operate after a
definite time delay and clear the entire group of
lines.

11. Classification of Protective Relay
• Based on Characteristic the protection relay
can be categorized as-
1. Definite time relays
2. Inverse time relays with definite minimum
time(IDMT) Instantaneous relays.
3. IDMT with Instantaneous relays.
4. Stepped characteristic.
5. Programmed switches.
6. Voltage restraint over current relay.

12. • Based on of logic the protection relay can be
categorized as- Differential.
1. Unbalance.
2. Neutral displacement.
3. Directional.
4. Restricted earth fault.
5. Over fluxing.
6. Distance schemes.
7. Bus bar protection.
8. Reverse power relays.
9. Loss of excitation.
10.Negative phase sequence relays etc.

13. • Based on actuating parameter the protection
relay can be categorized as- Current relays.
1. Voltage relays.
2. Frequency relays.
3. Power relays etc.
• Based on application the protection relay can
be categorized as-
1. Primary relay.
2. Backup relay.

14. • IMPORTANT TERMS OF RELAY
i) Pickup current: It is the minimum current
in the relay coil at which the relay starts to
operate.
ii) Current setting : It is often desirable to
adjust the pick-up current to any required
value. This is known as current setting and is
usually achieved by the use of tappings on
the relay operating coil.

16. iii) Plug setting multiplier (PSM) : It is the ratio of fault
current in relay coil to pick-up current.
PSM =(Fault current in relay coil)/(Pickup current)
iv) Time Setting Multiplier (TSM) : The adjustment
arrangement provided for setting the operation time of
induction relay is known as Time Setting Multiplier
v) Reset current: The value of current below which the
relay resets and comes back to its original state is
called as reset current or dropout.

17. Plug setting multiplier (PSM) Curve

18. • Fig. shows the curve between time of operation and
plug setting multiplier of a typical relay. The horizontal
scale is marked in terms of plug-setting multiplier and
represents the number of times the relay current is in
excess of the current setting.
• The vertical scale is marked in terms of the time
required for relay operation. If the P.S.M. is 10, then
the time of operation (from the curve) is 3 seconds. The
actual time of operation is obtained by multiplying this
time by the time-setting multiplier.

19. • It is evident from Fig. that for lower values of over
current, time of operation varies inversely with the
current but as the current approaches 20 times full-
load value, the operating time of relay tends to
become constant.
• This feature is necessary in order to ensure
discrimination on very heavy fault currents flowing
through sound feeders.

20. Induction type Non-directional over current relay

22. Here the upper electromagnet has a primary and
a secondary winding. The primary is connected to
the secondary of a CT in the line to be protected
and is tapped at intervals. The tapings are
connected to plug setting bridge by which the
number of active turns on the relay operating coil
can be varied there by giving the desired current
setting.

23. Principle
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 (C.T). These
relays are used on a.c. circuits only and can operate
for fault current flow in either direction.

24. Construction
• It consists of a metallic (aluminium) disc which is
free to rotate in between the poles of two
electromagnets. The upper electromagnet has a
primary and a secondary winding.
• The primary is connected to the secondary of a
C.T. in the line to be protected and is tapped at
intervals. The tappings are connected to a plug-
setting bridge by which the number of active
turns on the relay operating coil can be varied,
thereby giving the desired current setting. The
secondary winding is energized by induction
from primary and is connected in series with the
winding on the lower magnet

25. • The controlling torque is provided by a spiral
spring. The spindle of the disc carries a moving
contact which bridges two fixed contacts
(connected to trip circuit) when the disc rotates
through a pre-set angle. This angle can be
adjusted to any value between 0o and 360o. By
adjusting this angle, the travel of the moving
contact can be adjusted and hence the relay
can be given any desired time setting.

26. • Operation.
The driving torque on the aluminium disc is set
up due to the induction principle as This torque is
opposed by the restraining torque provided by the
spring.
Under normal operating conditions, restraining
torque is greater than the driving torque produced
by the relay coil current. Therefore, the
aluminium disc remains stationary.

27. • Under Fault conditions, the current in the protected
circuit exceeds the pre-set value, the driving torque
becomes greater than the restraining torque.
• Consequently, the disc rotates and the moving contact
bridges the fixed contacts when the disc has rotated
through a pre-set angle. The trip circuit operates the
circuit breaker which isolates the faulty section.

28. Introduction of Static relay
The relay which does not contain any moving
parts is known as the static relay. In such type of
relays, the output is obtained by the static
components like magnetic and electronic circuit
etc.
The relay which consists static and
electromagnetic relay is also called static relay
because the static units obtain the response and
the electromagnetic relay is only used for
switching operation.

29. Static Type over current relay
•The current derived from the main CT is feed to the
input transformer which gives a proportional output
voltage.
•The input transformer has an air gap in the iron core to
give linearity in the current voltage relationship up to the
highest value of current expected, and is provided with
taping on its secondary to obtain different current
settings.

30. • The output voltage is then rectified and then
filtered at a single stage to avoid undesirable
time delay in filtering so as to excurse high
speed of operation. A zener diode is also
incorporated in the circuit to limit the rectified
voltage to safe values even when the input
current is very high under fault conditions.
• A fixed portion of the rectified filtered voltage is
compared against a preset pick-up value by a
level detector and if exceeds the pick-up value,
a signal through an amplifier is given to the
output device which issues the trip signal.

31. Advantages of static over current relay:
1. Low power required hence less burden.
2. No motional parts hence bouncing, friction, erosion,
arcing etc. eliminated.
3. Not affected by gravity, may be used in any position.
4. Improved selectivity as resetting and overshoot
times is reduced.
5. Lower operating times.
6. One static relay can be used for multiple purposes.
7. Higher torque /weight ratio.
8. Compact in size.
9. Good discriminating characteristics and reliability.
10.Suitable for reliable remote operation with PLC.
11.Can be programmed as per requirement.

32. Disadvantages / Limitations of static over current relay:
1. Affected by voltage transients.
2. Affected by electrostatic discharges.
3. Sensitive to temperature.
4. Auxiliary power supply is needed.
5. Higher skilled manpower required to
handle/program/install.
6. Operating characteristics may be affected by
operation of output device.

33. Applications of static relays.
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.

34. Block diagram and working of Microprocessor based
Over Current Relay

35. • The ac voltage proportional to the load current is
converted in to dc through a precision rectifier.
Thus the microprocessor accepts d. c. voltage
proportional to the load current. The schematic
diagram is shown in the figure.
• The output of rectifier is fed to the multiplexer.
The output of multiplexer is fed to the A/D
converter to obtain the signal in digital form. The
A/D converter ADC 0800 has been used for this
purpose. The microprocessor sends signals to the
ADC for starting the conversion.

36. • The microprocessor reads the end of
conversion signal to examine whether the
conversion is over or not. As soon as
conversion is over, the microprocessor reads
the current signal in digital form and then
compares it with the pickup values.
• The microprocessor first determines the
magnitude of the fault current and then selects
the corresponding time of operation from the
look up table. Then it goes in delay subroutine
and sends a trip signal to the circuit breaker
after the predetermined time delay.

37. Applications of Microprocessor-based relays
1. These relays are used for modified existing protection
systems.
2. These relays are used for new substations or
transmission lines & They are very cost-effective.
3. microprocessor-based relays used for include
communications scheme logic.
4. These relays are used for relays record the system
conditions & performance analysis.
5. These relays are used where programmable logic
allows the user to define the operation of the relay

38. Comparison of Static Relays with Electro-Magnetic Relays and
microprocessor based relays
Particular
Electro-Magnetic
Relay
Static Relay
Numerical Relay or
microprocessor based
Relay
Technology
Standard
1st generation
relays.
2nd generation
relays.
Present generation
relays.
Operating
Principle
They use
principle of
electromagnetic
principle.
In this relays
transistors and
IC’s been used
They use
microprocessor.
Within built software
with predefined
values
Measuring
elements/
Hardware
Induction disc,
electromagnets,
induction cup,
balance beam
R, L, C,
transistors,
analogue ICs
comparators
Microprocessors,
digital ICs, digital
signal processors
Relay Size Bulky Small Compact
Speed of
Response
Slow Fast Very fast

39. Timing function
Mechanical
clock works,
dashpot
Static timers Counter
Time of Accuracy
Temp.
dependant
Temp.
dependant
Stable
Reliability High Low High
Vibration Proof No Yes Yes
Characteristics Limited Wide Wide
Requirement of
Draw Out
Required Required Not required
CT Burden High Low Low
CT Burden 8 to 10 VA 1 VA < 0.5 VA
Reset Time Very High Less Less
Auxiliary supply Required Required Required

40. Differential relay
A differential relay is one that operates when
the phasor difference of two or more similar
electrical quantities exceeds a pre-determined
value.
There are two fundamental systems of
differential or balanced protection viz.
(i) Current balance protection
(ii) Voltage balance protection

41. Current Differential Relay

42. • Fig. shows an arrangement of an over current
relay connected to operate as a differential relay.
• A pair of identical current transformers are fitted
on either end of the section to be protected
(alternator winding in this case).
• The secondaries of CT’s are connected in series in
such a way that they carry the induced currents in
the same direction.
• The operating coil of the over current relay is
connected across the CT secondary circuit. This
differential relay compares the current at the two
ends of the alternator winding.

43. • Under normal operating conditions, suppose the
alternator winding carries a normal current of 1000 A.
Then the currents in the two secondaries of CT’s are
equal These currents will merely circulate between
the two CT’s and no current will flow through the
differential relay.
• Therefore, the relay remains inoperative. If a ground
fault occurs on the alternator winding as shown in
Fig. the two secondary currents will not be equal and
the current flows through the operating coil of the
relay, causing the relay to operate. The amount of
current flow through the relay will depend upon the
way the fault is being fed.

44. • Voltage Balance Differential Relay

45. • Fig. shows the arrangement of voltage balance
protection. In this scheme of protection, two
similar current transformers are connected at
either end of the element to be protected (e.g. an
alternator winding) by means of pilot wires.
• The secondaries of current transformers are
connected in series with a relay in such a way that
under normal conditions, their induced e.m.f.s’
are in opposition

46. • Under healthy conditions, equal currents (I1 = I2)
flow in both primary windings. Therefore, the
secondary voltages of the two transformers are
balanced against each other and no current will
flow through the relay operating coil.
• When a fault occurs in the protected zone, the
currents in the two primaries will differ from one
another (i.e. I1 ≠ I2) and their secondary voltages
will no longer be in balance. This voltage
difference will cause a current to flow through the
operating coil of the relay which closes the trip
circuit.

47. • Distance or Impedance Relays

48. • The operation of the depended upon the
magnitude of current or power in the
protected circuit. However, there is another
group of relays in which the operation is
governed by the ratio of applied voltage to
current in the protected circuit. Such relays are
called distance or impedance relays.
• In an impedance relay, the torque produced by
a current element is opposed by the torque
produced by a voltage element. The relay will
operate when the ratio V/I is less than a
predetermined value. Fig. illustrates the basic
principle of operation of an impedance relay.

49. • The voltage element of the relay is excited
through a potential transformer (P.T.) from the
line to be protected. The current element of the
relay is excited from a current transformer (C.T.)
in series with the line.
• The portion AB 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 impedance of the protected section
falls below the pre-determined value i.e. ZL in
this case

50. Now suppose a fault occurs at point F1 in the
protected zone. The impedance Z (= V/I)
between the point where the relay is installed
and the point of fault will be less than ZL and
hence the relay operates. Should the fault occur
beyond the protected zone (say point F2), the
impedance Z will be greater than ZL and the
relay does not operate.

51. • Types of Distance or Impedance Relays
A distance or impedance relay is essentially an
ohmmeter and operates whenever the impedance of
the protected zone falls below a pre-determined
value. There are two types of distance relays in use
for the protection of power supply, namely ;
(i)Definite-distance relay which operates
instantaneously for fault upto a pre-determined
distance from the relay.
(ii)Time-distance relay in which the time of operation is
proportional to the distance of fault from the relay
point. A fault nearer to the relay will operate it
earlier than a fault farther away from the relay.

52. • Definite – Distance Type Impedance Relay

53. • Construction :
• Fig. shows the schematic arrangement of a
definite-distance type impedance relay. It consists
of a pivoted beam F and two electromagnets
energized respectively by a current and voltage
transformer in the protected circuit.
• The armatures of the two electromagnets are
mechanically coupled to the beam on the opposite
sides of the fulcrum. The beam is provided with a
bridging piece for the trip contacts. The relay is so
designed that the torques produced by the two
electromagnets are in the opposite direction.

54. • Operation:
Under normal operating conditions, the pull due
to the voltage element is greater than that of the
current element. Therefore, the relay contacts
remain open. However, when a fault occurs in the
protected zone, the applied voltage to the relay
decreases whereas the current increases. The
ratio of voltage to current (i.e. impedance) falls
below the pre-determined value. Therefore, the
pull of the current element will exceed that due to
the voltage element and this causes the beam to
tilt in a direction to close the trip contacts.

56. Time-distance Impedance Relay

57. A time-distance impedance relay is one
which automatically adjusts its operating time
according to the distance of the relay from the
fault point i.e.
Operating time, T ∝ V/I ∝ Z ∝ distance

58. Construction.
Fig. shows the schematic arrangement of a typical
induction type time distance impedance relay. It
consists of a current driven induction element
similar to the double winding type induction over-
current relay. The spindle carrying the disc of this
element is connected by means of a spiral spring
coupling to a second spindle which carries the
bridging piece of the relay trip contacts. The bridge
is normally held in the open position by an
armature held against the pole face of an
electromagnet excited by the voltage of the circuit
to be protected.

59. • Operation.
Under normal load conditions, the pull of the
armature is more than that of the induction
element and hence the trip circuit contacts
remain open.
However, on the occurrence of a short-circuit, the
disc of the induction current element starts to
rotate at a speed depending upon the operating
current. As the rotation of the disc proceeds, the
spiral spring coupling is wound up till the tension
of the spring is sufficient to pull the armature
away from the pole face of the voltage-excited
magnet

60. • Immediately this occurs, the spindle carrying the
armature and bridging piece moves rapidly in
response to the tension of the spring and trip
contacts are closed. This opens the circuit breaker
to isolate the faulty section.
• The speed of rotation of the disc is approximately
proportional to the operating current, neglecting
the effect of control spring.
• Also the time of operation of the relay is directly
proportional to the pull of the voltage-excited
magnet and hence to the line voltage V at the
point where the relay is connected. Therefore, the
time of operation of relay would vary as V/I i.e. as
Z or distance.

61. Numerical relays
These relays are controlled by a smart controlling
unit that continually monitors the grid parameters
(such as voltages, currents, temperature, etc) and
switches the appropriate relays in case fault
conditions occur. Most of the data processing
happens in the digital domain thus, these relays are
often called Numerical Protection Relays.

62. Block Diagram of Numerical Relays / Digital Relay

64. • Digital relay/ Numerical relay consists of functional block diagram
• Analog input subsystem.
• Digital input subsystem.
• Digital output subsystem.
• A processor along with RAM (data scratch pad),
main memory (historical data file) and power
supply.

65. • The 3-Ø voltage and current signals are analog in nature.
Since, a computer works with digital data , analog signals
have to be sampled and discretized.
• Additionally, signal scaling and isolation to protect the
low voltage computer system and scale the voltage and
current signals to proportionate voltage signal (e.g.,
within ± 5V ) is necessary.
• This functionality is provided by the analog input
subsystem. Typically, it consists of sample and hold
circuit, Analog to Digital Converter (ADC) and multiplexer
interfaced to the processor.
• The digital input data consists of Circuit Breaker (CB)
status (open or close). The digital output is relay's
operate / do not operate decision. Once, the data is
acquired within RAM, it is filtered by a digital filter and
processed by the relay logic & algorithms.

66. Advantages of Numerical relays
1. Economy and Good performance.
2. Dependability (ವಿಶ್ವಾಸವರ್ಹತೆ) and security.
3. Less Complexity and simplicity.
4. High Speed and accuracy.
5. Multifunction capability.
6. User-definable logic.
7. Power system measurements available.

67. Types of Numerical relays.
• POWER SYSTEM PROTECTION
1. Distance Protection relays
2. Line Differential Protection relays
3. Line Differential Protection (Pilot Wire Protection) relays
4. Transformer Protection relays
5. Low-Impedance Bus-bar Protection relays
6. High-Impedance Differential relays
7. Frequency, Voltage Protection relays Circuit Breaker Failure Protection relays
8. Auto-reclosing and Synchronism Check relays
• DISTRIBUTION SYSTEM PROTECTION
1. Over-current Protection relays
2. Under/Overvoltage Protection relays
3. Directional Over-current Protection relays
4. Feeder Manager Relay

68. Testing Methods for Relays
1. Acceptance tests.
2. Installation test (0r Commissioning test ).
3. Manufacturers tests.
4. Maintenance tests.
5. Repair test.
1. Acceptance tests :
Acceptance tests are generally performed in the laboratory.
Acceptance tests fall into two categories:
A. On new relays which are to be used for the first time,
intensive testing is done to prove its characteristics and to
gain information about it.
B. On Relays which have been earlier. only minimum necessary
checks should be done. Acceptance tests are done in the
presence of the customer or by the customer.

69. 2.Installation test (0r Commissioning test ).
Installation tests are field tests to determine that
the relay operates correctly in actual service. These
tests are not done unless incorrect operation
occurs. Most of the times these tests are performed
by means of portable test sets.

70. 3.Maintenance tests.
Maintenance testing is done in the field
periodically like Continuously,Daily,Monthly,Yearly .
The performance of a protective relay is affected
by maintenance. The basic requirements such as
sensitivity ,selectivity , reliability and stability can
be satisfied only if the maintenance is excellent.
4.Repair tests.
Repair tests involve recalibration and are
performed after major repairs. These are usually
done in the laboratory. Minor repairs done in the
field need not follow complete recalibration.

71. MODEL QUESTION BANK
Cognitive Level: Remember
1. Define Relay
2. List different requirements of Protective Relays
3. List different types of protective Relaying
4. List the applications of Static Relays
5. List the applications of Microprocessor based
Relays
6. Explain the Necessity for Protection
7. List the different types of Numerical relays
8. List the Advantages of Numerical relays
9. List the testing methods on Relays

72. Cognitive Level: Understanding
10. Explain Primary and Back up protection
11. Explain terms: Pickup current, current setting, P S M, T S M, Time P S
M Curve
12. Explain construction and working of Induction type Non-directional
over current relay
13. Explain construction and working of Static Type Over Current Relay
14. Compare Static Relays with Electro-Magnetic Relays
15. Explain with block diagram Microprocessor based Over Current Relay
16. Explain with a neat sketch the working of Voltage balance differential
Relay
17. Explain with a neat sketch the working of Current differential Relay
18. Explain with a neat sketch the working of Definite Distance Type
Impedance Relays
19. Explain with a neat sketch the working of Time-distance Impedance
Relay
20. Draw block diagram of Numerical relays
21. Compare Static relays and Microprocessor based relays