Introduction to protective relaying and electromagnetic relays

1. Introduction to protective relaying and electromagnetic relays

Shital Patel, EE Department Switch Gear and Protection (2170908) 1

1.1. Electrical fault

Electrical fault is the deviation of voltages and currents from nominal values or states.

Under normal operating conditions, power system equipment or lines carry normal

voltages and currents which results in a safer operation of the system. But when fault

occurs, it causes excessively high currents to flow which causes the damage to

equipments and devices.

Causes of electrical faults are

o Weather conditions

Lightning strikes

Heavy rains/winds/snow

Salt deposition

o Equipment failures

Malfunctioning

Ageing

Insulation failure of cables and winding

o Human errors

Improper rating of equipment or devices

Forgetting metallic parts after servicing or maintenance

Switching the circuit while it is under servicing

o Smoke of fires

Ionization of air, due to smoke particles, surrounding the overhead lines results in

spark between the lines or between conductors to insulator

Electrical faults are classified as

o Short circuit faults: A short circuit fault is an abnormal connection of very low

impedance between two points of different potential. These are the most common and

severe kind of faults, resulting in the flow of abnormal high currents through the

equipment or transmission lines. Short circuit faults are also called as shunt faults.

Symmetrical faults: These are very severe faults and occur infrequently in the

power systems. It affects each of the three phases equally. In transmission line

faults, roughly 5% are symmetric.

Line to line to line to ground (L-L-L-G)

Line to line to line (L-L-L)

Unsymmetrical faults: These are very common and less severe than symmetrical

faults. It does not affect each of the three phases equally. In transmission line

faults, roughly 95% are unsymmetrical.

Line to ground (L-G)

Line to line (L-L)

Line to line to ground (L-L-G)



o Open circuit faults: These faults occur due to the failure of one or more conductors.

Open circuit faults are also called as series faults. These are unsymmetrical or

unbalanced type of faults except three phase open fault.

One conductor open

Two conductor open

o Transient faults: A transient fault is a fault that is no longer present if power is

disconnected for a short time and then restored. Basically, transients are momentary

changes in voltage or current that occur over a short period of time i.e. interval is

usually described as approximately one sixteenth of a voltage cycle.

Effects of electrical faults are

o Heavy short circuit current causes damage to equipment due to overheating and high

mechanical forces set up.

o Arc associated with short circuits causes risks of fire. There is also a possibility of the

fire spreading to other parts of the system if the fault is not isolated quickly.

o At the time of fault supply voltage of the healthy phases gets reduces, that results in

the loss of industrial loads.

o Short circuit causes the unbalancing of supply voltages and currents that increases

the heating of rotating machines.

o Reduction in supply voltage is sometimes so large that, a voltage coil of relay is

damaged.

o Fall of supply voltage and frequency due to fault, loads such as motors starts feeding

the faults i.e. acts as source of power to fault

o Due to fault, individual generators in a power station losses synchronism i.e.

possibility of complete shutdown of the system and in some cases loss of stability of

interconnected systems can also result.

o A very high fault current causes damage to the equipment of a power system that

results in interruption of supply. Hence utility companies loses the revenue due to

long interruption in service, as the repairs of the damaged equipment takes time. On

the other hand, industries are also in trouble because of loss of production.

Chances of occurrence of faults on the different elements of a power system are

Element Percentage of faults

Overhead transmission lines 50

Switchgears 12

Power transformers 10

Generators 07

Underground cables 10

CT & PT, control circuit equipments, relays etc. 11

It is marked that 50% of the total faults occur on overhead lines, hence overhead lines

requires more attention while planning and designing protective schemes for a power

system.



1.2. Major components of an electrical power system

CB CB

CB

CB

CB CB CB CB

CB

CB

CBCB CBCB

CB

CB

Transformer11/132 kV

Transformer132/66 kV

Transmission Line132 kV

132 kVSwitchyard

132 kVBus

66 kVBus

66 kVSwitchyard

Transmission Line66 kV

Transformer66/11 kV11 kV

Bus

Feeder11 kV

Transformer11/0.415 kV

Distributor415 V

Consumers

Electrical power is usually generated at voltages 11 kV and then this power is transmitted

at a voltage of 132 kV, 220 kV, 400 kV or higher value depending on line length and

amount of power.

This power is received by a receiving substation where it is stepped down to a voltage of

66 kV or 132 kV depending on distance of further transmission. The 66 kV transmission

line terminates at a distribution substation, where the voltage is stepped down to 11 kV.

11 kV feeders supplies some of HT consumers and pole mounted transformers in

different areas of cities and villages. Pole mounted transformers step down the voltage to

415 volts for use by the consumers.

Different components of the electrical power system are isolated by circuit breakers. In

case of fault, electrical quantities such as current, voltage, phase angle, power, frequency

etc. are sensed by relays with the help of current transformer (CT) and potential

transformer (PT).

The relays are operated as per their characteristics and on their operation, a signal is

transmitted to circuit breakers. On receiving the signal, circuit breaker opens its contacts

and isolates the faulty section from healthy section.

There is an economic limit to the amount that is spent on a protective system. The

protective system to be employed depends upon many factors such as probability of

occurrence of faults, probability of failure of equipment, importance of equipment, cost

of the system or plant, location of plant etc.

Element Cost of relaying (%)

Relays 0.55

Relay panels 0.25

Wiring 0.10

Current transformers 3.10

Potential transformers 1.08

Total 5.08



As a breaker manually makes or breaks the transmission line or electrical equipment, its

cost is not usually considered under protective gear.

1.3. Basic tripping circuit

CB

CT

Relay

Feeder

Bus

PT

Relay

Bus

CB

Feeder

CT

Auxiliary Switch

When any section of power system or equipment needs to be protected, a primary of

current transformer is connected in series with that.

While secondary of current transformer (CT) is connected to current coil of the relay.

If the relay is two quantity relay, secondary of potential transformer (PT) is also

connected to potential coil of the relay.

When fault occurs, a current flowing through that section of power system or equipment

increases to a very high value.

Fault current also flows through the CT primary and accordingly increases the CT

secondary current that further increases relay coil current.

Thus relay contacts get closed under the influence of such high fault current.

Consequently trip circuit of circuit breaker get closed and current starts flowing from

battery through trip coil.

Once trip coil of circuit breaker is energized, it activates the circuit breaker opening

mechanism and open the breaker contacts.

This is how faulty section of power system or equipment gets isolated from healthy one.

Another important device in trip circuit is an auxiliary switch (52-a) which is

mechanically coupled with operating mechanism of circuit breaker.

It is ON when the circuit breaker is ON and OFF when the circuit breaker is OFF.

As auxiliary switch is in trip circuit hence when it opens, it breaks the current in trip

circuit. Once the current in trip circuit interrupts, the relay contacts comes to normal

position.

The purpose of auxiliary switch is that breaking of trip circuit takes place at auxiliary



switch contacts and hence possible arcing due to current interruption across relay

contacts is eliminated.

Also a circuit breaker tripping takes a time ranging from 1 to 5 cycles. A trip coil is not

designed for energizing it continuously once the breaker trips.

It is possible that auxiliary relay contact gets locked due to some internal mechanism

failure, hence a continuous current flows through the trip coil if auxiliary switch (52-a) is

not provided in the trip circuit.

Many other functions such as annunciations, alarms, interlocks, are simultaneously

performed through multi contact auxiliary switch when the relay operates.

Auxiliary switch is generally placed in control cabinet of circuit breaker.

1.4. Zones of protection

CB

Generatorprotection

Transformerprotection

CB

CBCB

CBCB

CB CB

CB

CB

CB

CB

Transmission lineprotection

Switchgearprotection



A power system is combination of various equipments such as generators, transformers,

busbar, transmission lines and distribution lines.

Due to varied nature in operation of equipments or element, power system is divided into

a number of zones of protection, each covering one type of equipment. Specific circuit

breakers and relays are associated with each zone. Zones of protection are always

overlapped so that there is no blind spot i.e. no unprotected portion.

A protective zone covers one or at the most two elements of a power system. Adjacent



protective zones overlaps each other. If no overlapping is considered than fault on the

boundary of the zones does not lie in any of the zones, hence no circuit breaker trips.

These zones are decided by locations of current transformers. For a fault in an overlapped

zone, the relays in both the concerned zones trips and isolates large portion of the power

system unnecessarily. But this is required to avoid the blind spot.

In practice all the zones are not as marked out, the start is defined but the reach depends

upon measurement of the system quantities i.e. subjected to changes in system conditions

and measurement errors.

1.5. Primary and backup protection

Power system is divided into various zones for its protection and there is a unique

protective scheme for each zone.

Two sets of relays, primary and back up, are usually provided for each zone of protection.

When fault occurs in a specific zone, primary relays of that zone are operated to isolate

the faulty element. The primary relay is the first line of defence.

Normally primary relays has a small zone of operation but operate instantaneously.

A second line of defence is always provided as back up protection which clears the faults

if relays or current transformers or potential transformers or circuit breakers of primary

protection fails to operate due to some reasons.

The reliability of protective scheme required is at least 95%, hence accurate design,

installation and maintenance of the relays, circuit breakers, trip mechanisms, AC and DC

wiring are carried out.

Back up relays has a large zone of operation and operate with a particular time delay

just to give the primary relay sufficient time to operate. They are made independent of

factors that causes primary relay to fail.

When a backup relay operates, a larger part of the power system is disconnected from

the power source. As far as possible, a backup relays are placed at different station.

Types of backup relaying are

(a) Relay backup

It is a local backup protection scheme in which an additional (duplicate) set of relays,

current transformers and potential transformers are provided that trips the same circuit

breaker.

Such backup protection scheme is very costly, hence it is recommended only if the

equipment to be protected is very costly and important.

Backup relays of this protection scheme has different principles of operation from those

of the primary protection.

(b) Breaker backup

It is also a local backup protection scheme used for busbar where a number of circuit

breakers are connected to it.

When a protective relay operates but feeder breaker fails to trip, fault becomes busbar

fault. In this condition all other circuit breaker of that busbar needs to trip. Hence a time



delay relay is operated by the main relay connected to all other circuit breaker.

(c) Remote backup

It is the backup protection scheme in which relays located at next (neighboring) station

provides the backup to entire primary protection to relays, current transformers,

potential transformers, circuit breakers and other elements of existing station when fails

to operate.

Remote backup protection is the most desirable as it does not fail due to the reasons

affecting the failure of the primary protection.

It is the cheapest and the simplest backup protection and is a widely used backup

protection of transmission lines.

1.6. Desirable qualities of protective relaying

Any protective system should satisfy the following requirements

(a) Reliability

It is the ability of the protective system to operate under the predetermined conditions

i.e. relay remain inoperative for a long time before a fault occurs, but if fault occurs relay

must operate instantly and correctly i.e. ability of protective system not failing ever. It is

achieved by redundancy i.e. duplicate the relaying system.

To achieve a high degree of reliability, greater attention is given to the design, installation,

maintenance and testing of the various elements of the protective system.

The contact pressure, the contact material of the relay, prevention of contact

contamination etc. are the important aspects for reliability.

Robustness and simplicity of the relaying equipment also contribute to reliability. A

typical value of reliability of a protective scheme is 95%.

(b) Selectivity or Discrimination

It is the ability of the protective system to isolate faulty section exclusively from the rest

of the healthy system.

A well designed and efficient relay system must be selective i.e. able to detect the point at

which the fault occurs and trip the circuit breaker closest to the fault with minimum or

no damage to the system.

Selectivity is absolute if the protection operates for internal faults in any element of the

power system and selectivity is said to be relative if coordinated settings of protective

relays of different zones used. Differential protection is absolutely selective, whereas

current time graded overcurrent protection and distance protection are relatively

selectivity.

Protective relay must be able to discriminate between a fault and loading conditions like

loss of synchronism of generator and power surges or magnetizing current inrush.

It must be also able to discriminate between conditions for which instantaneous tripping

is required or no operation or a time delay operation is required.



(c) Speed

Electrical equipments are short time rated for high fault currents, hence faster operation

of relays and breakers damages less to the equipment.

For modern power system, the stability is very significant aspect, therefore operating

time of the protective system should not exceed the critical clearing time to avoid the loss

of synchronism.

The time setting of the relays is to be decided on the basis of its short time rating of

equipments to be protected. Operating time of a protective relay is usually one cycle or

half-cycle. For distribution systems the operating time is more than one cycle.

(d) Stability

It is the ability of the protective system to remain inoperative under specified conditions

when high values of fault current is flowing through its protective zone due to an external

fault, which does not lie in its zone.

It is a quality that only unit systems possess because they are required to remain

inoperative under all conditions associated with faults outside their own zones.

(e) Sensitivity

It is the minimum value of fault current at which protective system operates. Relay should

operate when the magnitude of the current exceeds the preset value i.e. pick-up current

and sensitive to operate when the operating current just exceeds pick-up value.

Sensitivity is usually expressed in operating quantity referred to the primary of a

transducer.

There is a difference between the sensitivity of a relay and the sensitivity of a protective

system. The sensitivity of a relay is expressed as the apparent power in VA required for

its operation i.e. 1VA relay is more sensitive than a 3 VA relay.

(f) Economy

It is one of the essential quality of protective system. As a rule, the protective system

should not cost more than 5% of total cost. But sometimes it is economically unjustified

to use an ideal scheme of protection.

(g) Simplicity

The protective system should be simple so that it can be easily maintained. Reliability is

closely related to simplicity. The simpler the protective system, the greater is its

reliability.

1.7. Terms of protective relaying

Operating force or torque: It is a force or torque that tends to close the contacts of the

relay.

Restraining force or torque: It is a force or torque that opposes the operating force or

torque.

Actuating quantity: It is an electrical quantity i.e. voltage, current, frequency, power

factor, phase sequence etc. to which relay responds.



Pick-up level: It is the threshold value of the actuating quantity above which the relay

operates.

Dropout or reset level: It is the threshold value of the actuating quantity below which

the relay is de-energized and returns to its normal position.

Relay time: It is the time delay from the instant of occurrence of fault to the instant of

closing of relay contacts.

Operating time: It is the time which delay from the instant at which the actuating

quantity exceeds the relay pick-up value to the instant at which the relay closes its

contacts.

Reset time: It is the time which delay from the moment the actuating quantity falls below

its reset value to the instant when the relay comes back to its normal position.

Breaker time: It is the time delay from the instant at which circuit breaker opens its

contact to the instant of complete extinguishing of arc.

Fault clearing time: It is the total time from the instant of occurrence of fault to the

instant of complete extinguishing of arc i.e. sum of relay time and breaker time.

Setting: It is the value of actuating quantity at which the relay is set to operate.

Plug setting: It is the pick-up value of current adjusted to the required level in relay.

Tappings are provided on relay coil and connections are carried out to a plug bridge. The

tap values are expressed in terms of percentage full load rating of current transformer

that is associated with relay.

Relay pick-up current = %Current setting × Rated secondary current of CT

If CT ratio is 1000/10 A and current setting is 200% then pick-up current is 20 A. So when

relay coil current is more than or equal to pick-up value, relay operates.

Plug setting multiplier (PSM): It is the ratio of fault current in the relay to relay pick-up

current.

CT primary currentCT secondary current Fault currentCT ratioPSM= =Relay current setting Relay current setting Relay current setting CT ratio

Time multiplier setting (TMS): It is the adjustment of travelling distance of an

electromechanical relay that can control relay time of operation. Its dial is calibrated from

0 to 1 in steps of 0.05.

Actual time of operation = TMS × Time in second corresponding to PSM

If TMS is selected as 0.2 and time corresponding PSM of 10 is 4 second (form relay

characteristic curve) then actual time of operation is 0.8 second.

Burden: It is the power consumed by the relay circuitry at the rated current.

Reach: It is the maximum length of transmission line up to which relay can protect.

Over reach: It is the terms used for relay when it operates when a fault point is beyond

its present reach i.e. its protected length.



Under reach: It is the terms used for relay when it fails to operates when a fault point is

within its present reach i.e. its protected length.

Seal-in relay: It is an auxiliary relay which is energized by the contacts of the main relay.

Its contacts are placed in parallel with the contacts of the main relay and is designed to

relieve the contacts of the main relay from its current carrying duty. It remains in the

circuit until the circuit breaker trips. The seal-in contacts are usually heavier than the

contacts of the main relay.

Reinforced relay: It is an auxiliary relay which energized from the contacts of the main

relay. Its contacts are placed in parallel with the contacts of the main relay and it is also

designed to relieve the contacts of the main relay from its current carrying duty.

The difference between reinforced relay and seal-in relay is that seal-in relay is designed

to remain in the circuit till the circuit breaker operates while reinforced relay is designed

to hold a signal from the main relay for a longer period.

1.8. Classification of protective relays

Protective relays are classified depending on the technology used for construction, speed

of operation, development trends, function etc.

(a) Classification of relays based on technology

Depending on the technology used for construction and operation, relays are classified as

Electromechanical relays

o Electromagnetic relay works on the principle of either electromagnetic attraction or

electromagnetic induction.

o Attracted armature type electromagnetic relay operates through an armature which

is attracted to an electromagnet or through a plunger drawn into a solenoid. It is the

simplest type in construction and responds to AC and DC instantaneous over current

or over voltage.

o Induction type electromagnetic relays contains an electromagnet or a permanent

magnet and a moving part. When the actuating quantity exceeds a certain

predetermined value, an operating torque is developed which is applied on the

moving part. This causes the moving part to travel and to finally close a contact to

energize the trip coil of the circuit breaker. It responds to AC instantaneous over

current or over voltage.

Static relays

o Static relays contain electronic circuitry and electronic components i.e. a comparator

which compares two or more currents or voltages and generates output signal. This

signal is applied to slave relay, an electromagnetic relay which finally closes the

contact.

o Static relays has advantages of having low burden on the CT and PT, fast operation,

absence of mechanical inertia and contact trouble, long life and less maintenance.

o Only disadvantages of static relay is high cost and skilled personnel required to carry

out maintenance and repair.



Numerical relays

o Numerical relay collects sequential samples of the AC quantities in digital data form

through the data acquisition system, and process the data numerically using an

algorithm to calculate the fault discriminants and make trip decisions.

o Key component of this relay is numerical devices i.e. microprocessors,

microcontrollers, digital signal processors (DSPs) etc.

o Main benefits of numerical relay are their economy, compactness, flexibility

reliability, self-monitoring and self-checking capability, multiple functions and low

burden on CT/PT.

(b) Classification of relays based on speed of operation

Depending on the speed of operation, relays are classified as

t

i

Instantaneous relaysOperating zone

t

i

Time delay relaysOperating zone

t

i

Inverse time relaysOperating zone

Instantaneous relays

o Instantaneous relay operates as soon as actuating quantity crosses the pickup value

i.e. no intentional time delay is operation.

o Used as overcurrent protection of distribution feeder.

Time delay relays

o Time delay relay operates after definite time however actuating quantity has crossed

the pickup value.

o Used as a backup protection of distance relay of transmission line, differential relay

of transformer

Inverse time relays

o Inverse time relay operates more quickly when actuating quantity is very high i.e.

time of operation is inversely proportional to the current passing through the relay

coil.

o Used as overcurrent protection of cable, transformer, generator and transmission

line.

(c) Classification of relays based on functionality

Depending on the functionality, relays are classified as

Overcurrent relays

o Overcurrent relay operates when the current exceeds a certain limit

Over/under voltage relays

o Over/under voltage relay operates when the voltage reaches to certain over/under



voltage preset limit

Over/under frequency relays

o Over/under frequency relay operates when the frequency reaches to certain

over/under frequency preset limit

Impedance relays

o Impedance relay measures the line impedance between the relay location and the

point of fault and operates if the point of fault lies within the protected section.

Directional relays

o Directional relays check whether the point of fault lies in the forward or reverse

direction

(d) Classification of relays based on number of inputs

This type of relays are basically comparators that carries out addition, subtraction,

multiplication or division of some scalar or some phasor quantities and make

comparisons of the input quantities as desired. Depending on the number of input, relays

are classified as

Single input relays

o Single input relay has only one input signal i.e. known as level detectors. It

continuously monitor one electrical quantity and compare it with a reference

quantity.

o An over current relay is an example of single input type that measures the current of

a circuit and compares it with a certain preset value.

o These relays are the simplest in construction and operation, but they fails to attain the

desired reliability as their action depends upon a single quantity only.

Dual input relays

o Dual input relay has two input signals. Such relays measure one quantity and compare

it with another quantity.

o Distance relays and differential relays are the examples of dual input type. The

distance relay measures the current entering the circuit and compares it in magnitude

or in phase angle with the local bus voltage and differential relay measures the current

entering the circuit and compares it with the current leaving the circuit at the other

end.

o Dual input relays are of two type, amplitude comparator type and phase comparator

type. The amplitude comparator compares only the amplitude of the two input signals

irrespective of phase angle between them, whereas the phase comparator compares

only the phase angle between the two input signals irrespective of their magnitudes.

Multi input relays

o Multi input relay has more than two input signals and are used for the realization of

special characteristics other than straight lines or circle.

o Two types of multi input relays are available, amplitude comparator type and phase

comparator type. The amplitude comparator is used for realization of conic



characteristics such as elliptical or hyperbolic, whereas phase comparator is used for

realization of quadrilateral characteristic.

1.9. Classification of electromagnetic relays

Electromagnetic relays are operated by the mechanical force produced by the input

quantity. This force results in movement of the moving part that closes the relay contacts.

Principal types of electromagnetic relays are

o Attracted armature relays

Hinged armature type relay

Plunger type relay

Balanced beam type relay

Moving coil type relay

Polarized moving iron type relay

Reed type relay

o Induction relays

Induction disc relay

Induction cup relay

Attracted armature type electromagnetic relay operates through an armature which is

attracted to an electromagnet or through a plunger drawn into a solenoid. It is the

simplest type in construction and responds to AC and DC instantaneous over current or

over voltage.

Induction type electromagnetic relays contains an electromagnet or a permanent magnet

and a moving part. When the actuating quantity exceeds a certain predetermined value,

an operating torque is developed which is applied on the moving part. This causes the

moving part to travel and to finally close a contact to energize the trip coil of the circuit

breaker. It responds to AC instantaneous over current or over voltage.

1.10. Attracted armature type electromagnetic relays

Attracted armature relay responds to AC and DC quantity. These relays are operate by

the virtue of armature being attracted to the pole of electro magnet or plunger drawn into

a solenoid.

The electromagnetic force exerted on the moving element i.e. armature or plunger is

proportional to the square of the flux in the air gap or the square of the current. It is single

input type of relay that responds to AC and DC quantity both. The motion of the moving

element is controlled by an opposing force generally due to gravity or a spring.

For DC relays, the total electromagnetic force remains constant.

2

DCF KI

For AC relays, the total electromagnetic force pulsates at double frequency.

22 2 2 21 2 1 12

2 2 2m m m m

Cos tF Ki K I Sin t KI KI KI Cos t



o Above equation has two components, first component independent of time and

second components dependent of time.

o Hence restraining force produced by spring is constant and electromagnetic force

developed is pulsating.

o Because of pulsation, armature vibrates at double the power frequency. This leads

relay to produce hum, noise, sparking and undesirable make and break of relay

contacts.

(a) Hinged armature type relay

SpringControl

BackStop

Coil

MovingArmature

MovingContact

To TripCircuit

Input

SpringControl

BackStop

Coil

MovingArmature

MovingContact

To TripCircuit

Input

ShadedRing

In hinged armature type of relay, coil is energized by an operating quantity i.e. system

voltage or current that produces a magnetic flux which in turn produces an

electromagnetic force.

An attractive electromagnetic force is proportional to the square of the flux in the air gap

and its magnitude tends to increase as the armature approaches the pole of the

electromagnet.

This type of a relay is used as main relay for the small rating machines and as auxiliary

relays as indicating flags, slave relays, alarm relays, annunciators, semaphores, etc.

Hinged armature type relay works for AC and DC actuating quantity. For DC relays, the

total electromagnetic force remains constant while for AC relays, the total

electromagnetic force pulsates at double frequency. It leads armature to vibrate at double

the frequency and consequently produces a hum and noise.

This difficulty is overcome by using shaded pole electromagnetic construction. 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 i.e. 0.08 W at pick up for the relay with one contact and 0.2 W for

the relay with four contacts.

This relay is an instantaneous relay that operates at very high speed i.e. less than 5 ms.



(b) Plunger type relay

Input

Coil

FixedContact

MovingContact

To Trip Circuit

SpringControl

Plunger type relay, has a solenoid and an iron plunger that moves in and out of the

solenoid to make and break the contact. The movement of the plunger is controlled by a

spring.

This relay works for AC and DC actuating quantity. For DC relays, the total

electromagnetic force remains constant while for AC relays, the total electromagnetic

force pulsates at double frequency.

Now a day’s plunger type relay construction becomes obsolete as it draws more current.

(c) Balanced beam type relay

MovingContact

To TripCircuit

SpringControl

BalancedBeam

RestrainingCoil

OperatingCoil

Balanced beam type relay consists of a beam carrying two electromagnets at its ends. One

is designed to produce operating torque and second is designed to produce 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.

Now a day’s balanced beam type relay construction becomes obsolete. In old days it was

used as impedance relay and differential relays because of its toughness and fast in

operation i.e. within 1 cycle.

(d) Moving coil type relay

Permanent Magnet

MovingContact

FixedContactTo Trip

Circuit

Input

MovingCoil

N s

Input

MovingCoil

Permanent Magnet

MovingContact

FixedContact

Permanent magnet moving coil relay is called polarized DC moving coil relay because it

responds to only DC actuating quantities. Sometimes it can be used for AC actuating

quantities in conjunction with rectifiers.

Rotary moving coil type and axially moving coil type of relay constructions are used now

a days.

The rotary moving coil type construction has a coil wound on nonmagnetic former which

is placed between poles of permanent magnet. The moving coil assembly carries an arm

that closes the contact.

The operating torque is produced due to the interaction between field of the permanent

magnet and that of the coil and it is proportional to the current carried by the coil.

A phosphor bronze spiral spring is provided for resetting torque. The torque exerted by

the spring is proportional to deflection.

Damping is provided by an aluminum former. A copper former is used for heavier

damping.

An axially moving coil type construction has coils wounded on a cylindrical former which

is suspended horizontally. The coil has only axial movement.

This type of relay has only one air gap hence it is more sensitive than the rotary moving

coil relay.

As contact gap is small, axially moving coil relay is a delicate and it has to be handled

carefully.

Moving coil relay has inverse operating time characteristic i.e. operating time of about 1

to 2 cycle. They are most sensitivity type electromagnetic relays i.e. sensitivity of 0.1 mW.



(e) Polarized moving iron type relay

To TripCircuit

MovingContact

FixedContact

Coil

N S

Polarized moving iron relay uses permanent magnet for polarization i.e. permanent

magnet produces flux in addition to the main flux.

As its current carrying coil is stationary, it is more robust in operation. Usually the

operating time of relay is 2 to 15 ms.

Polarization increases the sensitivity of the relay i.e. 0.03 to 1 mW.

(f) Reed type relay

Coil

To TripCircuit

Input

Glass capsule Reed

Seal

Reed type relay consists of a coil and nickel-iron strips sealed in a closed glass capsule.

The coil surrounds the reed contact.

When the coil is energized, a magnetic field is produced that causes the reeds to come

together and close the contact.

Reed relays are very reliable and maintenance free. A 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.

The sensitivity of the relay is 1to 3 W and operating time is about 1 to 2 ms.

1.11. Induction type electromagnetic relays

Induction relay responds to AC quantity only.

Induction type electromagnetic relays contains an electromagnet or a permanent magnet



and a moving part. When the actuating quantity exceeds a certain predetermined value,

an operating torque is developed which is applied on the moving part. This causes the

moving part to travel and to finally close a contact to energize the trip coil of the circuit

breaker.

Two alternating magnetic flux are required to turn the moving element in induction type

relay and to produce an operating torque, these two fluxes must have a phase difference

between them.

Let, two alternation flux Φ1 and Φ2 induces an emf in the moving part i.e. disc or cup that

further circulate eddy current i1 and i2 in moving part. The interaction of two eddy current

i1 and i2 produces force which rotates the moving part.

1 1

2 2

m

m

Sin t

Sin t

As induced voltages are proportional to rate of change of flux hence eddy currents are

also proportional to rate of change of flux.

11 1 1

22 2 2

m m

m m

d di Sin t Cos t

dt dt

d di Sin t Cos t

dt dt

Net force acting on moving part is the difference of force produced due to eddy current i1

and i2.

2 1

2 1 1 2

2 1 1 2

1 2

1 2

1 2

1 2

m m m m

m m

m m

m m

m m

F F F

i i

Sin t Cos t Sin t Cos t

Sin t Cos t Cos t Sin t

Sin t t

Sin

F K Sin

Above equation suggest that net force action on moving part at every instant is same.

Hence relay operation is free from vibration.

Direction of rotation of moving element depends upon which flux is leading another flux.

As net force on moving element depends on α i.e. torque is zero when α=0 and torque is

maximum when α=90, always there must exists a phase difference between two fluxes.

A various constructions are used to produce the phase difference between two fluxes.



(a) Induction disc type relay

A shaded pole type and watt-hour meter type induction disc relay constructions are

widely used.

Coil

DiscShadedRing

InputRelay Coil

SecondaryCoil

DiscTo TripCircuit

Lower Magnet

Upper Magnet

In the shaded pole type construction, a C-shaped electromagnet with one half of each pole

is surrounded by a copper band i.e. shading ring is used.

Shaded portion of the pole produces a flux that is displaced in space and time with respect

to the flux produced by the unshaded portion of the pole.

These two alternating fluxes displaced in space and time cut the aluminium disc and

produce eddy currents.

Torque produced by the interaction of each flux with the eddy current produced by the

other flux causes the disc to rotate.

In watt-hour meter type construction, two electromagnets with one at upper side and

second at lower side are used.

Each magnet produces an alternating flux that is displaced in space and time cuts the

aluminium disc.

Phase displacement between two fluxes produced by upper and lower electromagnets

are obtained by either energizing coils of each magnet by two different sources or

resistances and reactances of each coil is designed different.

Permanent magnet is employed to produce eddy current braking to the disc. The braking

torque is proportional to the speed of the disc. When the operating current exceeds pick-

up value, driving torque is produced and the disc accelerates to a speed where the braking

torque balances the driving torque.

A spring is used to supply the resetting torque. At a current below pick-up value, the disc

remains stationary by the tension of the control spring acting against the normal

direction of disc rotation.



The disc rests against a backstop. The position of the backstop is adjustable i.e. distance

by which the moving contact of the relay travels before it closes contacts. This distance of

travel is adjusted for the time setting of the relay.

The disc carries an arm which is attached to its spindle. The spindle is supported by

jewelled bearings. The arm bridges the relay contacts.

Induction disc type relay has inverse operating time/current characteristic and are slow

in operation compared to attracted armature type relays.

The reset to pick up ratio for induction disc type relays is 0.95 because its operation does

not involve any change in the air gap.

VA burden of relay depends on its application i.e. usually 2.5 VA.

(b) Induction cup type relay

Coil

Cup

IronCore

In induction cup type construction, a stationary iron core is placed inside the rotating cup

and spindle of cup carries an arm that closes contacts.

Two pairs of coils produce a rotating field which induces current in the rotor and

interaction between the rotating flux and the induced current produces the torque.

A spring is employed to provide a resetting torque. Brake magnets are not used in this

type of relays.

The inertia of the cup is much less than that of a disc.

The magnetic system is more efficient due to minimum magnetic leakage and low

resistance of the induced current path in the rotor.

Due to the low weight of the rotor and efficient magnetic system its torque per VA is about

three times that of an induction disc type construction i.e. VA burden is greatly reduced.

High torque/inertia ratio makes this relay quite suitable for higher speeds of operation

i.e. operating time of 0.01 second.

Introduction to protective relaying and electromagnetic relays

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Transcript of Introduction to protective relaying and electromagnetic relays