LENDI INSTITUTE OF ENGINEERING AND TECHNOLOGY
Jonnada, Andhra Pradesh- 535005
UNIT -II
Electromagnetic Protection (PART I)
Presented by,
Dr. Rohit Babu, Associate Professor
Department of Electrical and Electronics Engineering

Syllabus
Relay connection – Balanced beam type attracted armature relay - induction
disc and induction cup relays–Torque equation - Relays classification–
Instantaneous– DMT and IDMT types– Applications of relays: Over current
and under voltage relays– Directional relays– Differential relays and
percentage differential relays– Universal torque equation– Distance relays:
Impedance– Reactance– Mho and offset mho relays– Characteristics of
distance relays and comparison.
Department of Electrical and Electronics Engineering

Relays
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Relays were invented in 1835 by American
electromagnetism pioneer Joseph Henry, in
a demonstration at the College of New
Jersey.

Relays
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The purpose of the protective relays and protective relaying systems is to operate the correct
circuit breakers so as to disconnect only the faulty equipment from the system as quickly as
possible, thus minimizing the trouble and damage caused by faults when they do occur.
There are two groups of relaying equipment for protecting any equipment:
1. Primary relaying equipment.
2. Back-up relaying equipment.
Primary relaying is the first line of defence for protecting the equipments whereas the back-up
protection relaying works only when the primary relaying equipment fails

Video- 1
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Relays
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Primary relaying may fail because of failure of any of the following:
(i) Protective relays (moving mechanism etc.).
(ii) Circuit breaker.
(iii) D.C. tripping voltage supply.
(iv) Current or voltage supply to the relays.

Relays: Some Important Terms
Department of Electrical and Electronics Engineering
Relay: A relay is an automatic device which senses an abnormal condition in an electric circuit and
closes its contacts. These contacts in turn close the circuit breaker trip coil circuit, thereby it opens the
circuit breaker and the faulty part of the electric circuit is disconnected from the rest of the healthy
circuit.
Pick up Level: The value of the actuating quantity (current or voltage) which is on the threshold
(border) above which the relay operates.
Reset Level: The value of current or voltage below which a relay opens its contacts and comes to
original position.
Operating Time: The time which elapses between the instant when the actuating quantity exceeds the
pick-up value to the instant when the relay contacts close.
Reset Time: The time which elapses between the instant when the actuating quantity becomes less than
the reset value to the instant when the relay contact returns to its normal position.

Relays: Some Important Terms Contd.
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Primary Relays: The relays which are connected directly in the circuit to be protected.
Secondary Relays: The relays which are connected in the circuit to be protected through current and
potential transformers.
Auxiliary Relays: Relays which operate in response to the opening or closing of its operating circuit to
assist another relay in the performance of its function. This relay may be instantaneous or may have a
time delay.
Reach: A distance relay operates whenever the impedance seen by the relay is less than a prespecified
value. This impedance or the corresponding distance is known as the reach of the realy.
Underreach: The tendency of the relay to restrain at the set value or the impedance or impedance
lower than the set value is known as underreach.
Overreach: The tendency of the relay to operate at impedances larger than its setting is known as
overreach.

Functional Characteristics of Protective Relays
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A protective relay is required to satisfy four basic functional characteristics:
(i) reliability,
(ii) selectivity,
(iii) speed, and
(iv) sensitivity.
(a) Reliability: The relay should be reliable is a basic requirement. It must operate when it is
required. There are various components which go into operation before a relay operates. Therefore,
every component and circuit which is involved in the operation of the relay plays an important role;
for example, lack of suitable current and voltage transformers may result in unreliable operation.

Functional Characteristics of Protective Relays
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Inherent reliability is a matter of design based on long experience. This can be achieved partly by:
(i) simplicity and robustness in construction,
(ii) high contact pressure,
(iii) dust free enclosures,
(iv) good contact material,
(v) good workmanship, and
(vi) careful maintenance.
(b) Selectivity: It is the basic requirement of the relay in which it should be possible to select which part
of the system is faulty and which is not and should isolate the faulty part of the system from the healthy
one.
Selectivity is achieved in two ways: (i) unit system of protection, and (ii) non-unit system of protection.

Functional Characteristics of Protective Relays
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(c) Speed: A protective relay must operate at the
required speed. It should neither be too slow which
may result in damage to the equipment, nor should it
be too fast which may result in undesired operation
during transient faults.
(d) Sensitivity: A relay should be sufficiently sensitive
so that it operates reliably when required under the
actual conditions in the system which produce the
least tendency for operation.

Relay Connection
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Relay Connection
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• The proper operation of the power
system requires an efficient, reliable and
fast acting protection scheme
• A protective relay, acting as a brain
behind the whole system.
• It detects abnormal conditions on a
power system by constantly monitoring
the electrical quantities of the system.
• The basic electrical quantities which are
likely to change during abnormal
conditions are current, voltage, phase
angle (direction) and frequency.
Relay connection

Relay Connection Contd.
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Protective relays are broadly classified into the following three categories depending on the
technologies they use for their construction and operation.
(i) Electromechanical relays
(ii) Static relays
(iii) Numerical relays

ELECTROMECHANICAL RELAYS
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• 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 replace all electromagnetic relays by micro processor based static
relays.
Most of the present day electromechanical relays are of either induction disc type or induction cup
type.
The following are the principal types of electromechanical relays:
1. Electromagnetic relays
(i) Attracted armature relays, and
(ii) Induction relays
2. Thermal relays

Attracted Armature Relays
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• Attracted armature relays are the simplest type which respond to ac as well as dc.
• These relays operate through an armature which is attracted to an electromagnet or through a
plunger which is drawn into a solenoid.
• The electromagnetic force exerted on the moving element, i.e., the armature or plunger, is
proportional to the square of the flux in the air gap or the square of the current.
• In dc relays this force is constant.
• In case of ac relays, the total electromagnetic force pulsates at double the frequency.
• The motion of the moving element is controlled by an opposing force generally due to gravity or a
spring.

Attracted Armature Relays
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The following are the different types of construction of attracted armature relays.
i. Hinged armature type
ii. Plunger type
iii. Balanced beam type
iv. Moving-coil type
v. Polarised moving-iron type
vi. Reed type

Attracted Armature Relays: Hinged armature type
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Hinged armature and plunger type constructions are most commonly used for these types of
electromagnetic relays.
Among the two constructional design, hinged armature type is more commonly used.
Hinged armature-type relay
• The coil is energised by an operating quantity proportional to
the system current or voltage.
• The operating quantity produces a magnetic flux which in turn
produces an electromagnetic force.
• The electromagnetic force is proportional to the square of the
flux in the air gap or the square of the current.
• The attractive force increases as the armature approaches the
pole of the electromagnet.

Attracted Armature Relays: Hinged armature type
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In the case of ac relays, sinusoidal current flows through the coil and hence the force of attraction is
given by
Modified hinged armature-type relay
The restraining force is provided by a spring. The reset to pick-up
ratio for attracted armature type relays is 0.5 to 0.9. The VA
burden is low, which is 0.08 W at pick-up for the relay with one
contact, 0.2 W for the relay with four contacts. The relay is an
instantaneous relay. For a modern relay, the operation time is
about 5 ms.

Attracted Armature Relays: Plunger-Type Relays
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Plunger-type relay
In this type of a relay, there is a solenoid and
an iron plunger which moves in and out of
the solenoid to make and break the contact.
The movement of the plunger is controlled by
a spring.

Attracted Armature Relays: Balanced Beam Relays
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Balanced beam relay
• It consists of a beam carrying
two electromagnets at its ends.
• One gives operating torque
while the other retraining
torque.
• The beam is supported at the
middle and it remains
horizontal under normal
conditions.
When the operating torque exceeds the restraining torque, an armature fitted at one end of the
beam is pulled and its contacts are closed.

Attracted Armature Relay: Moving Coil Relays
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Rotating moving coil relays
• Polarised dc moving coil relay.
• It can be used with ac actuating quantities in
conjunction with rectifiers.
• Modern relays have a sensitivity of 0.1 mW.
• Costlier than induction cup or moving iron type relays.
• These are used as slave relays with rectifier bridge
comparators.
Two types:
1. Rotary moving coil, and
2. Axially moving coil

Attracted Armature Relay: Moving Coil Relays
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Rotating moving coil relays
1. Rotary moving coil
• The rotary moving coil type is similar to a moving coil
indicating instrument.
• The components are: a permanent magnet, a coil wound
on a non-magnetic former, an iron core, a phosphor
bronze spiral spring to provide resetting torque, jeweled
bearing, spindle, etc.
• The operating torque is produced owing to the interaction
between the field of the permanent magnet and that of the
coil.
• The operating torque is proportional to the current carried
by the coil.

Attracted Armature Relay: Moving Coil Relays
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Axial moving coil relay
• It is more sensitive than the rotary moving coil relay.
• It is faster than the rotary moving coil relay because of
light parts.
• Its coils are wound on a cylindrical former which is
suspended horizontally.
• The relay has an inverse operating time/current
characteristic.
• The axially moving coil relay is a delicate relay and
since the contact gap is small, it has to be handled
carefully.
2. Axially moving coil

Attracted Armature Relay: Polarised Moving Iron Relays
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Polarised moving iron relay
• The permanent magnet produces flux in addition to the
main flux.
• Using transistor amplifiers, a relay’s sensitivity can be
increased to 1 mW for pick-up.
• As its current carrying coil is stationary, it is more robust
than the moving coil type dc polarised relay.
• Its operating time is 2 msec to 15 msec depending upon the
type of construction.
• An ordinary attracted armature type relay is not sensitive to
the polarity of the actuating quantity whereas a dc polarised
relay will only operate when the input is of the correct
polarity.

Attracted Armature Relay: Reed Relays
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Reed relay
• The coil surrounds the reed contact.
• When the coil is energised, a magnetic field is produced which
causes the reeds to come together and close the contact.
• Used for control and other purposes, protective relays, slave relays,
bounce free and are more suitable for normally-closed applications.
• Their input requirement is 1 W to 3 W and they have speed of 1 or
2 msec.
• Heavy duty reed relays can close contacts carrying 2 kW at 30 A
maximum current or at a maximum of 300 V dc supply.
• The voltage withstand capacity for the insulation between the coil
and contacts is about 2 kV. The open contacts can withstand 500 V
to 1 kV.

Induction Relays
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• Induction relays use electromagnetic induction principle for their operation.
• Two types of construction:
1. Induction disc
2. induction cup
• In both types of relays, the moving element (disc or cup) is equivalent to the rotor of the
induction motor.
• There is one contrast from the induction motor, i.e., the iron associated with the rotor in the relay
is stationary.
• The moving element acts as a carrier of rotor currents, whereas the magnetic circuit is completed
through stationary magnetic elements.

Induction Disc Relay
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Types of construction of induction disc relays:
1. Shaded pole type
2. Watt hour meter type
1. Shaded pole type
(a) Simple construction
(b) Construction in practice
In the shaded pole type construction,
--a C-shaped electromagnet is used.
--One half of each pole of the electromagnet is surrounded by a
copper band know as the shading ring.
--The shaded portion of the pole produces a flux which is displaced
in space and time with respect to the flux produced by the unshaded
portion of the pole.
--Torques are produced by the interaction of each flux with the eddy
current produced by the other flux.
--The resultant torque causes the disc to rotate.

Induction Disc Relay
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2. Watt hour meter type
Fig. Wattmetric type induction-disc relay
In wattmeter type of construction,
--two electromagnets are used: upper and lower one.
--Each magnet produces an alternating flux which
cuts the disc.
--To obtain a phase displacement between two fluxes
produced by upper and lower electromagnets, their
coils may be energised by two different sources.
--If they are energised by the same source, the
resistances and reactances of the two circuits are
made different so that there will be sufficient phase
difference between the two fluxes.

Induction Disc Relay: Wattmeter
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-- It is robust and reliable.
-- It is used for overcurrent protection.
-- It gives an inverse time current characteristic and are slow compared to the induction cup and
attracted armature type relays.
-- It is used for slow-speed relays.
-- Its operating time is adjustable and is employed where a time-delay is required.
-- Its reset/pick-up ratio is high, above 95% because its operation does not involve any change in the
air gap.
-- The VA burden depends on its application, and is generally of the order of 2.5 VA.
-- The torque is proportional to the square of the actuating current if single actuating quantity is
used.

Induction Disc Relay: Wattmeter
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Current Setting
In disc type units, there are a number of tapping provided on coil to select the desired pick-up value
of the current.
Time Setting
The distance which the disc travels before it closes the relay contact can be adjusted by adjusting the
position of the backstop.

Induction Disc Relay: Printed Disc Relay
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Printed Disc Relay
• Its operating principle is the same as
that of a dynamometer type
instrument.
Fig. Printed disc inverse time relay
• There is a permanent magnet to
produce a magnetic field.
• The current from the CT is fed to the
printed disc through a rectifier.

Induction Disc Relay: Printed Disc Relay
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Fig. Printed disc extremely inverse time relay
The construction of a printed disc
extremely inverse time relay (sqr(I)*t = K
relay).
--To obtain sqr(I)*t = K characteristic, an
electromagnet and a printed disc are used.
--The electromagnet is energised from the
CT through a rectifier.
--Printed disc relays give a much more
accurate time characteristic.
--They are also very efficient.

Induction Cup Relay
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• The basis construction of this relay is just like four poles or eight pole induction motor.
• The number of poles in the protective relay depends upon the number of winding to be
accommodated.
• The figure shows a four pole induction cup relay.
Fig. Induction cup relay
• In four pole unit, the eddy current produced in the cup
due to one pair of poles, directly appears under other
pair of poles.
• If magnetic saturation of the poles can be avoided by
designing, the operating characteristics of the relay can
be made linear and accurate for a wide range of
operation.

Induction Cup Relay: Working Principle
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• As per working principle of induction motor, the cup starts rotating in the direction of rotating
magnetic field, with a speed slightly less than the speed of rotating magnetic field.
• The aluminum cup is attached with a hair spring : In normal condition the restoring torque of
the spring is higher than deflecting torque of the cup.
• But during faulty condition of system, the current through the coil is quite high, hence,
deflecting torque produced in the cup is much higher than restoring torque of spring, hence the
cup start rotating as rotor of induction motor.
• The contacts attached to the moving of the cup to specific angle of rotation.

Induction Cup Relay: Construction
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• The magnetic system of the relay is constructed by
attaching numbers of circular cut steel sheets.
• The field coils are wound on these laminated poles.
• The field coil of two opposite facing poles are
connected in series.
• The aluminum cup or drum, fitted on a laminated iron
core is carried by a spindle whose ends fit in jeweled
cups or bearings.
• The laminated magnetic field is provided on inside the
cup or drum to strengthen the magnetic field cutting
the cup.

Induction Cup Relay: Theory of Induction Relay Torque
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Figure shows how force is produced in a rotor which is cut by phi 1and phi 2. These fluxes are
alternating quantities and can be expressed as follows.
Fig. Torque produced in an induction relay
where, theta is the phase difference between phi 1 and phi
2. The flux phi 2 leads phi 1 by theta.
Voltages induced in the rotor are:

Induction Cup Relay: Theory of Induction Relay Torque
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As the path of eddy currents in the rotor has negligible self-inductance, with negligible error it may
be assumed that the induced eddy currents in the rotor are in phase with their voltages.
The current produced by the flux interacts with other flux and vice versa. The forces produced are:
As these forces are in opposition, the resultant force is

Induction Cup Relay: Theory of Induction Relay Torque
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The suffix m is usually dropped and the expression is written in the form of F = K*phi 1*phi 2*sin
theta.
In this expression, phi 1and phi 2 are rms values.
If the same current produces phi 1 and phi 2 the force produced is given by
where, theta is the angle between phi 1 and phi 2. If two actuating currents M and N produce phi 1
and phi 2, the force produced is