Basics of Relays

1. EE3601 PROTECTION AND SWITCHGEAR
UNIT-2
Electromagnetic Relays
Operating principles of relays - the Universal relay
– Torque equation – R-X diagram Electromagnetic
Relays – Over current, Directional, Distance,
Differential, Negative sequence and Under
frequency relays.
1

2. Electromagnetic Relay
Electromagnetic relays are those relays which are operated
by electromagnetic action. Modern electrical protection relays
are mainly micro processor based, but still electromagnetic
relay holds its place. It will take much longer time to be
replaced the all electromagnetic relays
Practically all the relaying device are based on either one or
more of the following types of electromagnetic relays.
Magnitude Measurement,
Comparison,
Ratio measurement.
2

3. 3

4. • https://www.youtube.com/watch?v=cunddFi
Qzrk
4

5. Universal Relay Torque equation
The Universal Torque Equation is a equation which governs
the application of all types of relays. The equation has
variables and constants which can be ignored for specific
functions.
For example, to describe the over current relay, K2 and K3 can be
considered zero while K will be negative as it is used to describe the
restraining torque.
The Equation will then becomes
T=K1I2-K 5

6. Classification of Protective Relays
• All the relays consist of one or more elements
which get energized and actuated by the
electrical quantities of the circuit. Most of the
relays used now a days are electro-mechanical
type which work on the principles of
• Electromagnetic attraction
• Electromagnetic induction
6

7. Electromagnetic Attraction type relays
• Solenoid and plunger type
• Attracted armature type
– Hinged armature type
– Polarised moving iron type
• Balanced beam type
7

8. Induction type relays
• Induction disc type
– Shaded pole type
– Watt hour type
• Induction cup type
8

9. Directional type relays
• Non directional induction type overcurrent
relay
• Directional power relay
• directional induction type overcurrent relay
9

10. Relays based on Timing
• Instantaneous over current relays
• Inverse Definite minimum time relays
• Inverse definite time relays
• Very inverse relays
• Extremely inverse relays
10

11. Distance type relays
• Impedance type
• Reactance type
• Admittance or Mho type
Further classifications:
• Definite distance relays
• Distance time relays
11

12. Differential type relays
• Current differential type
• Biased beam relay or percentage differential
relay
• Voltage balance differential type
12

13. Other types of relays
• Frequency relay
• Negative sequence relay
• Under voltage, current, power relay
• Over voltage, current, power relay
• Thermal relay
• Rectifier relay
• Permanent magnet moving coil relay
• Static relay
• Gas operated relay
13

14. Electromagnetic Attraction Relays
• Attracted armature relay
1. Hinged armature type
2. Polarised moving iron type
• Solenoid and plunger type relay
14

15. Electromagnetic Attraction Relays
• Hinged armature type relay
15

16. Electromagnetic Attraction Relays
• Polarised moving iron type relay
16

17. Electromagnetic Attraction Relays
• Current-Time Characteristics
17

18. Electromagnetic Attraction Relays
• Solenoid and Plunger type relay
18

19. Electromagnetic Attraction Relays
• Operating Principle
• The electromagnetic force produced due to
operating quantity which is exerted on armature,
moving iron or plunger is proportional to the
square of the flux in the air gap. Thus neglecting
the saturation effect, the force is proportional to
the square of the operating current. Hence such
relays are useful for a.c. and d.c. both.
19

20. Electromagnetic Attraction Relays
• Operating Principle (for d.c. operation)
In d.c. operation, the electromagnetic force is
constant. When this force exceeds the restraining
force, the relay operates.
Now Fe = K1 I2
Where Fe = electromagnetic force
K1 = Constant
I = Operating current in a coil
Also Fr = K2
Where Fr = restraining force due to spring including
friction
20

21. Electromagnetic Attraction Relays
• Operating Principle (for d.c. operation)
On the verge of relay operating, electromagnetic
force is just equal to the restraining force.
• K1I 2 = K2
• I 2 = K2/ K1
This is the current at which relay operates in case
of d.c. operation.
21

22. Electromagnetic Attraction Relays
• Operating Principle (for a.c. operation)
In a.c. electromagnetic relays, the electromagnetic
force is proportional to square of the current but it
is not constant. It is given by,
Where Im = Maximum value of the operating
current
K = Constant
22

23. Electromagnetic Attraction Relays
• Operating Principle (for a.c. operation)
It shows that the electromagnetic force consists of
two components,
1. Constant, independent of time
2. Pulsating at double the frequency of applied
voltage
The total force thus pulsates at double the
frequency.
If the restraining force Fr which is produced by the
spring is constant then the armature of relay will
be picked up at time t1 and it drops off at time t2 as
shown in the figure.
23

24. Electromagnetic Attraction Relays
• Electromagnetic force and current curves
24

25. Advantages of electromagnetic relays
• Can be used for both a.c. and d.c.
• They have fast operation and fast reset
• There are almost instantaneous. Though
instantaneous, the operating time varies with
current. With extra arrangements like dashpot,
copper ring etc. low operating and resetting times
can be obtained.
• High operating speed with opearting time in few
milliseconds also can be achieved
• The pickup can be as high as 90-95% for d.c.
operation and 60-90% for a.c. operation
• Modern relays are compact, simple, reliable and
robust.
25

26. Disadvantages of electromagnetic relays
• The directional feature is absent
• The working can be affected by the transients. As
transients contain d.c. as well as pulsating
component, under steady state value less than set
value, the relay can operate during transients.
• Due to the presence of moving parts, the response is
not very quick due to inertia of the parts, compared
to modern static relays.
• Due to moving parts, frequent maintenance is
required. The bearing friction and contact troubles
may exist
• Due to high burden on current and potential
transformers, the size is large and cost is more.
26

27. Applications of electromagnetic relays
• The protection of various a.c. and d.c. equipments
• The over/under current and over/under voltage
protection of various a.c. and d.c. equipments
• In the definite time lag over current and earth
fault protection along with definite time lag over
current relay.
• For the differential protection
• Used as auxiliary relays in the contact systems of
protective relaying schemes.
27

28. Induction Type Relays
• It is also called magnitude relays
• Works on the principle of the induction motor or
an energy meter.
• Metallic disc is allowed to rotate between the two
electromagnets.
• Coils of the electromagnets are energized by a.c.
• Torque produced in these relays due to the
interaction of one alternating flux with eddy
currents induced in the rotor by another
alternating flux.
28

29. Induction Type Relays
• Two fluxes have same frequency but are displaced
in time and space.
• Not used for the d.c. quantities. Only for a.c.
quantities.
• Based on the construction, classification are
1. Shaded pole type
2. Watt hour meter type
3. Induction type
29

30. Torque equation for Induction Type Relays
30

31. Torque equation for Induction Type Relays
31

32. Torque equation for Induction Type Relays
32

33. Shaded Pole Type Induction Relay
33

34. Shaded Pole Type Induction Relay
34

35. Shaded Pole Type Induction Relay
35

36. Watt-hour Meter Type Induction Relay
36

37. Induction Cup Type Relay
37

38. Induction Cup Type Relay
38

39. Universal Relay Torque Equation
• In general torque produced by current winding is
proportional to square of the current
• Torque produced by voltage winding is
proportional to square of the voltage
• Torque produced by the both the winding is
product of voltage and the current.
39

40. Universal Relay Torque Equation
40

41. Use of Universal Torque Equation
41

42. Overcurrent Relay
• The over current relays are classified depending
upon the time of operation. these relays are
classified as.
1. Instantaneous overcurrent relays (low 0.01 sec.)
2. Inverse definite time relays
3. Inverse definite minimum time relays (IDMT)
4. Very inverse relays
5. Extremely inverse relays
42

43. Characteristics of various Overcurrent Relays
43

44. Non directional Induction Type Overcurrent Relay
(or) Earth leakage induction type relay
44

45. Time Current Characteristics of Non directional
Induction Type Overcurrent Relay
45

46. Calculation of relay operating time
• Practically, it is necessary to calculate the actual
operating time of the relay, under the specific
fault current levels. For these calculations, the
following parameters related to the relay must be
known.
1. Time/P.S.M curve or tabular data
2. Current setting
3. Time setting multiplier
4. Level of fault current
5. Corresponding C.T. ratio
46

47. Calculation of relay operating time
The relay operating time can be obtained as,
1. Using C.T. ratio, convert the fault current level to
relay coil current level
2. Calculate the plug setting multiplier from the
relay coil current and current setting
3. From the Time/P.S.M. curve on data, obtain the
time corresponding to the plug setting multiplier
calculated above
4. Multiplying the time obtained by time multiplier
setting, the actual relay time can be obtained.
47

48. Directional Power Relay
48

49. Directional Power Relay
• This relay is used for providing the reverse
power protection to synchronous machines.
The relay can be single phase or three phase.
49

50. Directional Induction Type Overcurrent Relay
50

51. Directional Induction Type Overcurrent Relay
The following conditions must be satisfied to have
the operation of the entire relay.
1. The direction of current in the circuit must
reverse to operate of the entire relay
2. The current value in the reverse direction must
be greater than the current setting
3. The high value f current must persist for a time
period which is greater than the time setting of
the relay.
51

52. Distance Relay
• In the relays discussed upto now, the operation of
the relay is dependent on the magnitude of the
current or voltage of the circuit to be protected.
• In distance relays, the operation is dependent on
the ratio of the voltage and current, which is
expressed interms of an impedance. Hence
basically distance relays are called impedance
relays.
• The relay operates when the ratio V/I (i.e.)
impedance is less than a predetermined value. As
the ratio V/I affects the performance of these
relays, the relay are also called ratio relay.
52

53. Distance Relay
• Dependent on the ratio of V and I there are three
types of distance relays which are,
1. Impedance relay which is based on measurement of
impedance Z.
2. Reactance relay which is based on measurement of
reactance X.
3. Admittance or Mho relay which is based on
measurement of component of admittance Y.
• In short, a distance relay is one whose performance is
based on the measurement of impedance, reactance
or admittance of line between the location of relay
and the point where fault occurs. 53

54. Impedance Relay
• Current element – operating torque, pickup
torque, positive torque
• Voltage element – restraining torque, reset
torque, negative torque
54

55. Impedance Relay
• Torque equation:
55

56. Impedance Relay
• Torque equation:
56

57. Impedance Relay
• Operating Characteristics:
57

58. Impedance Relay
• Operating Characteristics on R-X Diagram:
58

59. Impedance Relay
• Operating Characteristics on R-X Diagram:
59

60. Impedance Relay
• Operating Characteristics on R-X Diagram:
60

61. Impedance Relay
• Disadvantages of Plain Impedance Relay:
1. It is nondirectional and can operate for faults on
both sides of a point where relay is connected.
Hence it fails to discriminate between internal and
external faults
2. When faults occurs, an arc exists. The arc resistance
of line faults affects the performance of this relay
3. As a large area is covered by the circle on each side
on R-X plane, the power swings also can affect the
performance of this relay.
The nondirectional performance can be made
directional by adding a directional element in the plain
impedance relay.
61

62. Directional Impedance relay
62

63. Directional Impedance relay
63

64. Directional Impedance relay
64

65. Directional Impedance relay
65

66. Reactance Relay
• In this relay the operating torque is obtained by
current while the restraining torque due to a
current-voltage directional relay. The overcurrent
element develop the positive torque and
directional unit produces negative torque.
• Thus the reactance relay is an overcurrent relay
with directional restraint.
• The directional element is so designed that the
maximum torque angle is 90 degrees.
66

67. Reactance Relay
67

68. Reactance Relay
• Torque Equation
68

69. Reactance Relay
Torque Equation
• Thus the relay operates on the reactance
only. The constant X means a straight line
parallel to X –axis and R-X diagram. For the
operation of the relay, the reactance seen by
the relay should be smaller than the
reactance for which the relay is designed.
69

70. Reactance Relay
• Operating characteristics:
70

71. Reactance Relay
Disadvantages:
• This relay as can be seen from the characteristics is a
nondirectional relay. This will not be able discriminate
when used on transmission line, whether the fault
has taken place in the section where relay is located
or it has taken place in the adjoining section.
• It is not possible to use a directional relay of the type
used with basic impedance relay because in that case
the relay will operate even under normal load
condition if the system is operating at or near unity
power factor conditions.
• The reactance relay with directional feature is called
mho relay or admittance relay.
71

72. Admittance Relay (or) Mho Relay
• In the impedance relay a separate unit is required
to make it directional while the same unit cannot
be used to make a reactance relay with directional
feature.
• The mho relay is made inherently directional by
adding a voltage winding called polarizing
winding.
• This relay works on the measurement of
admittance Y ∠θ. This relay is also called angle
impedance relay.
72

73. Admittance Relay (or) Mho Relay
73

74. Admittance Relay (or) Mho Relay
• Torque Equation:
74

75. Admittance Relay (or) Mho Relay
• Operating Characteristics:
75

76. Admittance Relay (or) Mho Relay
• Operating Characteristics:
76

77. Classification of Distance Relay
Definite distance relay:
• These can be of impedance type, reactance type
or mho type. This operates instantaneously for
the faults upto certain predetermined distance
from the relay.
Distance time relay:
• These can be also of impedance type, reactance
type or mho type. In these relays the time of
operation is proportional to the distance of the
fault from the point where relay is installed. The
fault nearer to the relay operates it faster than the
faults farter away from the relay. 77

78. Classification of Distance Relay
Definite distance type impedance relay:
78

79. 79

80. Classification of Distance Relay
Definite distance type impedance relay:
80

81. Classification of Distance Relay
Definite distance type impedance relay:
Its advantages are:
1. Superior to the time graded overcurrent relay
2. Number of feeders in series which can be
protected is unlimited as the relay time is
constant.
The one limitation of these relays is the absence of
backup protection. 81

82. Classification of Distance Relay
Definite time impedance relay:
82

83. Classification of Distance Relay
Definite time impedance relay:
83

84. Applications and Advantages of Distance Relay
Advantages:
1. Gives faster operation
2. Simpler to co-ordinate
3. Less effect of fault levels and fault current magnitudes
4. Permits high line loading
5. With the need at readjustment, permanent settings can
be done.
Thus the distance relays are used for providing the primary
(i.e) main protection and backup protection for a.c.
transmission and distribution lines against the following
faults,
1. Three phase faults
2. Phase to phase faults
3. Phase to earth faults
84

85. Comparison between various distance
relays
Parameters Impedance
Relay
Reactance Relay Mho Relay
Operating
quantity
Current Current Directional element
MTR = τ
Restraining
quantity
Voltage Directional element
MTA=90
Voltage
Directional
Property
No No Yes
Area occupied on
R-X Plane
Moderate Very Large Smallest
Effect of fault
resistance
Under reaches Reach unaffected Reach unaffected
Performance on
load
Restrains Trips Restrains
85

86. Differential Relay
A differential relay is defined as the relay that
operates when the phasor difference of two or
more similar electrical quantities exceeds a
predetermined value.
Types of Differential Relay:
1. Current differential relay
2. Biased beam relay or percentage differential
relay
3. Voltage balance differential relay
86

87. Current Differential Relay
87

88. Current Differential Relay
Disadvantages:
1. The current transformers are connected through
cables called pilot cables. The impedance of such
pilot cables generally causes a slight difference
between the currents at the ends of the section to
be protected. A sensitive relay can operate to a very
small difference in the two currents, though there is
no fault existing.
2. The relay is likely to operate inaccurately with heavy
through current flows. This is because the assumed
identical current transformers may not have
identical secondary currents due to the
constructional errors and pilot cable impedances.88

89. Current Differential Relay
Disadvantages:
3. Under severe through fault conditions, the
current transformers may saturate and cause
unequal secondary currents. The difference
between the currents may approach the pick value
to cause the inaccurate operation for the relay.
4. Under heavy current flows, pilot cable
capacitances may cause inaccurate operation of the
relay.
89

90. Biased Beam Relay (or) Percentage Differential Relay
90

91. Biased Beam Relay (or) Percentage Differential Relay
91

92. Voltage Balance Differential Relay
92

93. Voltage Balance Differential Relay
1. For achieving the perfect balance between C.T.
pairs, a special multi gap transformer
construction is required.
2. Under heavy current flows, capacitances of pilot
cable wires may cause inaccurate operation of
the relay.
3. The system is suitable for protection of lines
having relatively short lengths.
All these disadvantages are eliminated in a translay
relay.
93

94. Frequency Relay
1. The frequency of the induced e.m.f. is related to
the speed of the synchronous generators by the
relation,
2. The frequency relays can be electromagnetic or
static relays. These can be under frequency or
over frequency relays.
94

95. Frequency Relay
95

96. Negative Sequence Relays (or) Unbalance Relays
• Provide protection against negative sequence
component of unbalanced currents existing due
to unbalanced loads or phase-phase faults.
• Unbalanced currents overheating motors and
generators.
• Negative sequence relay has a filter circuit which
is operative only for negative sequence
components.
• Low order of overcurrent also can cause
dangerous situations hence a negative sequence
relay has low current settings.
96

97. Negative Sequence Relays (or) Unbalance Relays
97

98. Negative Sequence Relays (or) Unbalance Relays
98

99. Negative Sequence Relays (or) Unbalance Relays
99

100. Negative Sequence Relays (or) Unbalance Relays
100

101. Negative Sequence Relays (or) Unbalance Relays
101

102. Thermal Relays
• Used in protection of low voltage a.c. and d.c. motors. In
case of large motors, the bimetallic strip is connected
through current transformer.
102

103. Thermal relay
103