Unit: 3 (Protective Relaying)

Zone of protection:

A power system consists of generator, transformer, bus-bar, transmission line, distribution line

etc. So it is necessary to provide the protection in each section of power system. Thus the

power system is divided into a number of zones for protection. These are

(i) Generator protection

(ii) Low voltage switch gear protection

(iii) Power transformer protection

(iv) High voltage switch gear protection

(v) Transmission line protection

When a fault occurs in a given zone, then only the circuit breaker within that zone will be

opened. And this will isolate only the faulty section from the rest of the system.

Essential qualities of protective relaying(Characteristics):

(i) Selectivity: It is the ability to select correctly the faulty part of the system and

disconnect it.

(ii) Speed: The relay system should disconnect the faulty section as fast as possible

(iii) Sensitivity: It is ability of the relay system to operate with low value of actuating

quantity.

(iv) Reliability: It is the ability to operate under pre-determined condition.

(v) Simplicity: The relaying system should be simple so that it can be easily maintained.

(vi) Economy: The protection system should be economical.

Classification of Relay system:

According to construction and principle:

A) Electromagnetic attraction relays (i) Plunger type

(ii) Hinged armature type

(iii) Balanced beam type

(iv) Moving iron type

B) Electromagnetic induction relays (i) Induction disc relay

(ii) Induction cup relay

(iii) Gas operated relay

(iv) Rectifier relay

(v) Static relay

According to application:

(i) Over current/over voltage relay

(ii) Under voltage/under current relay

(iii) Directional relay

(iv) Differential relay

(v) Distance relay

(vi) Negative relay

(vii) Under frequency relay

According to timing characteristic:

(i) Instantaneous relay

(ii) Definite time lag relay

(iii) Inverse time lag relay

(iv) Inverse definite minimum time lag relay(IDMT)

Basic relay terminology:

Protective relay: A protective relay is a device that detects the fault and initiates the

operation of the circuit breaker to isolate the faulty section from the rest of the system.

Trip circuit: It is a circuit that controls the circuit breakers for operating and it comprises of

trip coil, relay circuit, auxiliary switch, battery supply etc.

Operating force or torque: It is a force which tends to close the relay contacts.

Restraining force or torque: It is a force which opposes the operating force and tends

to prevent the enclosure of relay contacts.

Pick up current: It is the minimum current in the relay coil at which the relay starts to

operate.

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 tapping on the relay coil.

The current plug setting ranges from 50% to 200%.

Pick-up current = Rated secondary current of CT × current setting

Example: An over current relay having current setting of 125% is connected to supply

through a CT of 400/5 turns ratio. Then,

Pick-up current = 5×1.25

= 6.25Amp

Plug setting multiplier (P.S.M.): It is the ratio of fault current in the relay coil to the

pick-up current.

P.S.M. = 𝐹𝑎𝑢𝑙𝑡 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 𝑖𝑛 𝑟𝑒𝑙𝑎𝑦 𝑐𝑜𝑖𝑙

𝑃𝑖𝑐𝑘−𝑢𝑝 𝑐𝑢𝑟𝑟𝑒𝑛𝑡

Example: A relay is connected to 400/5 CT and set at 150% with primary fault current of

2400A. Then,

P.S.M. will be.

Pick-up current = 5×1.5

= 7.5

Fault current in relay coil = 2400×5

400

= 30Amp

P.S.M. = 30

7.5

= 4 (Ans)

Time setting multiplier: A relay is generally provided with control to adjust the time of

operation. This adjustment is known as time setting multiplier. The time setting dial is

calibrated from 0 to 1 in steps of 0.05sec. If the time setting is 0.1 and the time obtained from

the time/P.S.M. curve is 3second, then the actual relay operating time is 3×0.1 = 0.3sec.

Time/P.S.M.

From the Time/P.S.M. curve, if the P.S.M. is 10 then the time of operation is 3sec. The actual

time of operation is obtained by multiplying this time by the time setting multiplier. So in order

to calculate the actual relay operating time the following things must be known.

(i) Time/P.S.M. curve

(ii) Current setting

(iii) Time setting

(iv) Fault current

(v) CTs ratio

Problem1: A 10amp, 3sec o/c relay has current setting of 150% and T.S.M. of 0.5, the relay

is connected in the circuit through a CT having ratio of 500/5. Calculate the time of operation of

relay coil if the circuit carries of fault current of 6000amp. The relay has the following

characteristics

P.S.M. 3 4 5 6 7 8 9 10

Time in second 7 5.5 5 4 3.8 3.5 3.1 3

Soln: pick-up current = 5×1.5

= 7.5amp

Fault current = 6000×𝟓

𝟓𝟎𝟎 = 60amp

P.S.M. = 𝟔𝟎

𝟕.𝟓 = 8

From P.S.M./time curve for P.S.M. 8, the time in second is 3.5sec

Therefore actual operating time of relay coil = 3.5×8 = 1.75sec (Ans)

CT and PT used for protection: The current transformer (CT) and potential

transformer (PT) are also called instrument transformer. These are mainly used for measuring

and protection purpose. The current transformer is used to operate ammeter, the current coil

of wattmeter and relay coil from the high current source. Potential transformer is used to

operate voltmeter, the potential coil of wattmeter and relay coil from high voltage source. The

relay coil needs very small amount of current and voltage for operating purpose. But the

system to be protected carries a large amount of current with very high voltage. So, the current

and voltage should be stepped down by CT and PT respectively. In this case these are used as

protection purpose.

CVT or CCVT:

A capacitive voltage transformer or coupling capacitive voltage transformer is used in power

system to step down the extra high voltage signals and provide a low voltage signal for

metering or operating a protective relay (above 100kV per phase). The CVT essentially consists

of a potential divider in conjunction with a conventional auxiliary transformer. The capacitive

potential divider step down the voltage and it is further stepped down by the auxiliary

transformer to the desired secondary voltage.

Application:

(i) To step down the high voltage to a measurable value.

(ii) To supply low voltage to the relay coil for protection.

(iii) To measure the power of the load through wattmeter supplying low voltage to its

potential coil.

(iv) The CVT is also useful in communication system.

Operating principle and construction of electromagnetic attraction

relays:

1.Plunger type: It consists of a solenoid and movable iron plunger. Under normal

operating condition, the spring holds the plunger in the position such that it cannot make

contact with the trip circuit. Under fault condition when current through the relay coil

increases, the solenoid draws the plunger upward direction. Due to this it makes contact with

the trip circuit, which results in opening of the circuit breaker.

Plunger type

2.Hinged armature type: It consists of a laminated electromagnet and a relay

armature. Under normal condition the current through coil such that the armature is balanced

by a counter weight or spring tension and it cannot make contact with the trip circuit.

Hinged armature

Under fault condition the current through relay coil increases sufficiently and the relay

armature is attracted upward. The upward movement of the relay armature closes the trip

circuit, thus opening the circuit breaker.

3.Balanced beam type:

It consists of an iron armature pivoted on balanced beam. Under normal condition the current

through the relay coil is such that the beam is held in the horizontal position by the spring.

However when fault occur the current through coil becomes very high and beam is attracted to

close the trip circuit, thus opening the circuit breaker.

Balanced beam type

Advantages of electromagnetic attraction relays:

(i) It can be used for both a.c. and d.c.

(ii) They have fast operation and fast reset.

(iii) They are almost instantaneous.

(iv) The pick-up is being high as 90% to 95% for a.c. and 60% to 90% for d.c.

(v) The modern relays are compact, reliable, simple and robust.

Disadvantages:

(i) Directional feature is absent.

(ii) Due to the fast operation the working can be affected by transient.

Application: It can be used for protection of both a.c. and d.c. equipments against

over/under current and over/under voltage.

Thermal relay: These relay operate on the principle of thermal effect of electric current. It

consists of bimetallic strip which is heated by heating coil supplied through a current

transformer. Under normal condition the strip remain straight but under fault condition the

strip is heated and bent and the tension of the spring is released. Thus the relay contacts are

closed which energies the trip circuit to operate the circuit breaker.

Thermal Relay

Application: These relay are mostly used for protection of low voltage squirrel cage

induction motor or d.c. motor of lower output rating.

Limitation: It is not suitable for operation on short-circuit faults.

Induction type over current relay: (non-directional)

Constructional details: It consists of a metallic (Aluminium) disc which is free to rotate

in between the two electromagnets. The upper electromagnet has a primary and secondary

winding. The primary is connected to C.T. through a plug setting bridge. The secondary is

energized by induction from primary and is connected in series with the winding of lower

magnet. The controlling torque is provided by the spiral spring.

Operation: These relay is used on a.c. circuit only and can operate for fault current flow in

either direction. During fault a driving torque on the disc is set up due to the induction

principle. This torque is opposed by the controlling (restraining) torque provided by the spring.

Under normal condition restraining torque is greater than the driving torque. Therefore the disc

remains stationary. However, if the current in the protected circuit exceeds the pre-set value,

the driving torque becomes greater than the restraining torque. And the disc rotates and the

moving contact bridges the fixed contact to trip the circuit breaker and isolate the faulty

section.

Induction type over current relay: (Directional)

Construction details: It consists of two relay elements

1. Directional element

2. Non-directional element

Directional element: It is essentially a power relay which operates when power flow in a

specific direction. The potential coil is connected through the P.T. to a system voltage. The

current coil is energized through a C.T. and is carried over the upper magnet of the non-

directional element. The trip contacts (1 and 2) of the directional elements are connected in

series with the secondary winding of the non-directional element. Therefore the non-

directional element cannot start to operate until its secondary is completed.

Non-directional element: The spindle of the disc of this element carries a moving

contact which closes the fixed contact after the operation of directional element. It may be

noted that plug-setting bridge is also provided in the relay for current setting but has omitted in

figure for clarity and simplicity.

Operation: Under normal condition power flows in the normal direction in circuit protected

by the relay. Therefore, directional power relay does not operate, thereby keeping the over

current element un-energized. However when short circuit occurs, directional element is

energized and bridge the fixed contact 1 and 2. This complete the over current element and

disc start to move and close the trip circuit.

Induction type directional power relay:

Construction: It consists of an aluminium disc which is free to rotate in between two

electromagnets. The upper electromagnet is connected to P.T. and lower electromagnet is

connected to C.T. through a plug setting bridge. The controlling torque of the disc is provided

by the spiral spring. The spindle of the disc carries a moving contact which bridges the fixed

contact when the disc rotates through a pre-set angle.

Operation: The driving torque on the disc is produced by the interaction of the field

obtained from both voltage and current source of the circuit. It is clear that the direction of

driving torque on the disc depends upon the direction of power flow in the circuit. When the

power in the circuit flows in the normal direction, the disc remains stationary. However, the

reversal of current in the circuit reverses the direction of driving torque on the disc. When the

driving torque is large enough the disc rotates and the moving contact closes the trip circuit.

Distance relay protection: Distance protection is the name given to the protection,

whose action depends upon the distance of the feeding points to the fault point. It is double

actuating quantity relay with one coil energized by voltage and the other by current. It can be

used as primary as well as back up protection.

Classification of distance relays: Distance relays are classified into two types

(i) Definite-distance relay: These relays operate when the impedance falls below the

specified value. These may be of impedance type, mho type or reactance type.

(ii) Distance-time impedance relay: In this relay the time of operation depends upon

the value of impedance (admittance or reactance) i.e. depends upon the distance of

fault from the relay point. These may be of impedance type, mho type or reactance

type.

Area of application: Distance protection is used for the protection of transmission or

sub-transmission lines usually 33kV, 66kV, 110kV and 132kV lines.

Definite-distance relay:

Construction: It consists of a pivoted beam F and two electromagnet energized by C.T. and

P.T. respectively. The beam is provided with a bridging piece for the trip circuit. The relay is so

designed that the torque produced by the two magnets are in opposite direction.

Operation: Under normal condition the torque due to the voltage element is greater than

that of the current element. Therefore, the relay contact remains open. However, when a fault

occurs in the protected zone, the applied voltage to the relay coil decreases whereas the

current increases. The ratio of the voltage to current falls below the pre-determined value.

Therefore, the torque of the current element will exceed the voltage element and this causes

the beam to close the trip contact.

Distance-time impedance relay:

Construction: It consists of a current driven induction element in which the spindle is

connected by means of a spiral coupling to a second spindle which carries the bridging piece of

the moving contact. This bridge is normally kept in open position by an armature held against

the pole face of an electromagnet energized by the potential coil.

Operation: At normal condition the pull of the armature is more than that of the induction

element and hence the trip contact remains open. When fault occurs the disc of the induction

element starts to rotate at a speed depending upon the operating current. The disc continues

to rotate till the tension of the spring is sufficient to overcome the pull of the armature and as

soon as this armature is released the trip contacts are closed to open the circuit breaker.

Protection Scheme: A protection scheme is used to protect an equipment or section of

the line. In modern power system the protection are classified as:

(i) Over current protection: This scheme is used for protection of distribution lines,

large motors, equipments etc.

(ii) Distance protection: This scheme is used for protection of transmission lines or sub-

transmission lines, usually 33kV, 66kV, 110kV and 132kV lines.

(iii) Carrier current protection: This scheme is used for protection of EHV and UHV lines,

generally 132kV and above.

(iv) Differential protection: This scheme is used for the protection of generator,

transformer, motor of very large size and bus-zone etc.

Differential relay: A differential relay is defined as the relay that operates when the

phase difference of two or more electrical quantities exceeds a pre-determined value. Most of

the differential relays are of the current differential type.

In this system two CTs are connected on both sides of the equipment which is being protected.

The secondary of the CTs are connected with operating coil of the relay. Under normal

condition there is no phase difference in secondary side. When fault occurs the entering

current is not same with leaving current. So, a difference is occurred in the secondary side

which operates the relay.

Voltage differential relay: In this system two identical current transformer (CTs) are

connected at either end of the element to be protected. The secondary of the CTs are

connected in series with a relay in such a way that under normal condition their induced e.m.f.

is in opposite direction. Under healthy condition, equal current (I1=I2) flows 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 coil. When fault occurs in the protected zone

the current in the two primaries will differ from one another and their secondary voltage also

are in difference. This voltage difference will cause a current to flow through the relay coil

which closes the trip circuit.

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