EE2402 – PROTECTION AND SWITCHGEAR

SHORT QUESTIONS AND ANSWERS

UNIT I

INTRODUCTION

1. What are the functions of protective relays? (Dec-06, May-07,13)

To detect the fault and initiate the operation of the circuit breaker to isolate the defective element
from the rest of the system, thereby protecting the system from damages consequent to the fault.

2. Define protected zone. (May-2011)

Are those which are directly protected by a protective system such as relays, fuses or
switchgears.If a fault occurring in a zone can be immediately detected and or isolated by a
protection scheme dedicated to that particular zone.

3. What is the need for protection zones in the system? (May-08)

Any fault occurring within the given zone will provide necessary tripping of relays or
disconnecting or opening of circuit breakers and thus the healthy section is safe guarded.

If a fault occurs in the overlapping zone in a proper protected scheme, more circuit breakers
than the minimum necessary to isolate the faulty part of the system would trip.

4. What is back up protection? (May-06,Dec-12)

Is the second line of defence , which operates if the primary protection fails to activate within a
definite time delay.

5. Define operating time of a relay. (Dec-12)

It is defined as the time period extending from the occurrence of the fault through the relay
detecting the fault to the operation of the relay.

6. What is necessity of earthing ? (Dec-10,14)

Earthing is necessary to ensure the safety of operating personal against electrical shocks and
avoid accidents. It is also essential to protect the equipment against lightning voltage surges and
stress.

7. What is pick up value? (May-07,Dec-10)

It is the minimum current in the relay coil at which the relay starts to operate.

8. State the different types of faults. (Dec-07, May-12, 14)

Symmetrical faults and unsymmetrical faults and open conductor faults.

Protection & Switchgear

9. Define the term “insulation coordination” (nov/dec 2011)

Grading of withstand level of apparatus/equipment with the protective levels of surge arresters
and co-ordination at entire voltage level and various other voltage levels.

10. What is surge absorber? How do they differ from surge diverter? (nov/dec 2011)

Surge absorber is a device designed to protect electrical equipment from transient high voltage to
limit the duration and amplitude of the following current. Surge diverter discharge the impulse
surge to the earth and dissipates energy in the form of heat.

11. What is meant by switchgear? (May-07)

The apparatus used for switching, controlling and protecting the electrical
circuits and equipment is known as switchgear.

12 What are the desirable qualities of protective relaying? Or Mention the essential
features of the power system protection. Or List the essential features of switchgear.
(May -04, Dec-06,08)

1. Selectivity 2. Speed & time 3. Sensitivity

4. Reliability 5. Simplicity 6. Economy

Part - B

1. Describe the essential qualities of a protective relaying. (Dec-11, 12, May-12, 14, Dec-14)

2. Define pickup value, plug setting multiplier and auto reclosing. (May-12)

3. Differentiate C.T. and P.T. (May -12)

4. Discuss the role of protective relaying and protective zones in a modern power system.
(Dec-12)

5. What do you understand by a zone of protection? Discuss various types of Zones of protection.
(Dec-12)

6. Explain the operation of various types of surge absorbers (May-12)

7. Describe the protection of stations and sub-stations against direct lightning stroke. (May-13)

8.Explain the operation of various types of surge Diverter. (May-12)

9. With a neat block diagram, explain the operating principle of Peterson coil. (May-12, Dec –
06, 07, 12, 14)

10. Explain the term insulation coordination. (May – 13, Dec- 13)

11. Write Short notes on power system earthing. (May -13)

Protection & Switchgear

 UNIT II

OPERATING PRINCIPLES AND RELAY CHARACTERISTICS

1. What are the advantages of static relay over electromagnetic relay? ( may-08, nov/dec
2011)
o Low power consumption as low as 1mW

o No moving contacts; hence associated problems of arcing, contact bounce, erosion,

replacement of contacts

o No gravity effect on operation of static relays. Hence can be used in vessels ie, ships,
aircrafts etc.

o A single relay can perform several functions like over current, under voltage, single
phasing protection by incorporating respective functional blocks. This is not possible
in electromagnetic relays

o Static relay is compact

o Superior operating characteristics and accuracy

o Static relay can think , programmable operation is possible with static relay

o Effect of vibration is nil, hence can be used in earthquake-prone areas

o Simplified testing and servicing. Can convert even non-electrical quantities to

electrical in conjunction with transducers.

2. Mention any two applications of differential relay. (May -2011)

It is comparing miniature to heavy loads.
Fault current can be easily detected.
Used for protecting generator, generator unit transformer unit, large motors and bus bars.

3. Give the application of electromagnetic relays. (Dec-08,10)

Protection of a.c. and d.c. equipments.
The over/ under voltage and over and under current protection.
For the differential protection.

5. What is an under frequency relay? (Dec-13,14)

An under frequency relay is one which operates when the frequency of the system (usually an
alternator or transformer) falls below a certain value.

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6. Draw the block diagram of a static differential relay. (Dec-08)

7. Name the important distance relay.

1. Impedance relay 2. Reactance Relay 3. Mho Relay

8. Compare static and electromagnetic relay. (Dec-06,11)

Electromagnetic relays are1st generation relays they use principle of
electromagnetic principle. They depend upon gravitation also and the value changes
to the surrounding magnetic fields also.
Static relays are 2nd generation relays. In this relays transistors and IC's r been
used. There value may vary with respect to temperature also.
Numerical relays are present generation relays. They use Microprocessor. Within
built software and predefined values if any values extend the predefined values then
relay gets on by this relay we can find distance of fault also.

9. What is the need of relay coordination?

The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest
possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there
should be sufficiently quick backup protection so that the rest of the system is protected. By
coordinating relays, faults can always be isolated quickly without serious disturbance to the rest
of the system.

10. Define the term pilot with reference to power line protection.

Pilot wires refers to the wires that connect the CT’s placed at the ends of a power transmission
line as part of its protection scheme. The resistance of the pilot wires is usually less than 500
ohms.

11.What are the features of directional relay?

High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time
thermal ratio; burden must not be excessive.

Protection & Switchgear

 16 Marks Questions

1.What are the different types of electromagnetic relays? Discuss their field of applications.
(Dec-13)

2.Describe the construction and principle of operation of an induction type directional over
current relay. (Dec-11, May - 12)

3.Explain the operation of negative sequence relay. (Dec 13,14, May 14)

4.Describe the operating principle, constructional features and area of applications of directional
Relay (May-13)

5. Explain the operation of negative sequence relay (May -14)

6.Explain with sketches and their R-X diagrams for the following distance relays (Dec-11,14
May-12, )

(i) Impedance relay (5)

(ii) Mho relay (5)
(iii) Reactance relay(6)

6.Write a detailed note on differential relays.

UNIT III

APPARATUS PROTECTION

1. What are faults associated with a transformer? (Dec-08)

(a) External fault or through fault
(b) Internal fault
1, Short circuit in transformer winding and connection
2, Incipient or slow developing faults

2. What are the various faults that would affect an alternator? (Dec-10,May -13)

(a) Stator faults

1, Phase to phase faults 2, Phase to earth faults 3, Inter turn faults

(b)1, Earth faults

2, Fault between turns

3, Loss of excitation due to fuel failure

(c)1, Over speed 2, Loss of drive

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3, Vacuum failure resulting in condenser pressure rise, resulting in shattering of the turbine low
pressure casing

(d)1, Fault on lines

2, Fault on busbars

3. Give the limitations of Merz Price protection. (May -11)

Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it
becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs
near the neutral point, the voltage may be insufficient to operate the relay. Also it is extremely
difficult to find two identical CT’s. In addition to this, there always an inherent phase difference
between the primary and the secondary quantities and a possibility of current through the relay
even when there is no fault.

4. Why busbar protection is needed? (May -07,08,13)

(a) Fault level at busbar is high
b) The stability of the system is affected by the faults in the bus zone.
(c) A fault in the bus bar causes interruption of supply to a large portion of the system
network.

5. What are the main safety devices available with transformer? (May-12)
Oil level guage, sudden pressure delay, oil temperature indicator, winding temperature
indicator .

6. What is meant by time graded system protection? (Dec-11)

To ensure selectivity of operation the operating time of the protection is increased from
the far end of protected circuit towards the generating source.

7. What are the problems in bus zone differential protection? (May-12)

_ Large number of circuits, different current levels for different circuits for external
faults.
_ Saturation of CT cores due to dc component and ac component in short circuit
currents. The saturation introduces ratio error.
_ Sectionalizing of the bus makes circuit complicated.
_ Setting of relays need a change with large load changes.

8.What are faults associated with an alternator?

(a)External fault or through fault

(b)Internal fault

1, Short circuit in transformer winding and connection 2, Incipient or slow developing faults

9. What are the main safety devices available with transformer?

Protection & Switchgear

Oil level guage, sudden pressure delay, oil temperature indicator, winding temperature indicator .

10.What are the limitations of Buchholz relay?

(a)Only fault below the oil level are detected.

(b)Mercury switch setting should be very accurate, otherwise even for vibration, there can be a
false operation.

(c)The relay is of slow operating type, which is unsatisfactory.

11.What are the problems arising in differential protection in power transformer and how
are they overcome?

1.Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting
adjustable resistors to pilot wires to get equipotential points on the pilot wires.

2.Difference in CT ratio error difference at high values of short circuit currents that makes the
relay to operate even for external or through faults. This is overcome by introducing bias coil.

3.Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay
will sense this and act. Bias coil will solve this.

4.Magnetizing inrush current appears wherever a transformer is energized on its primary side
producing harmonics. No current will be seen by the secondary.

CT’s as there is no load in the circuit. This difference in current will actuate the differential
relay. A harmonic restraining unit is added to the relay which will block it when the transformer
is energized.

12. What is over fluxing protection in transformer?

If the turns ratio of the transformer is more than 1:1, there will be higher core loss and the
capability of the transformer to withstand this is limited to a few minutes only. This phenomenon
is called over fluxing.

13. What are the causes of bus zone faults?

 Failure of support insulator resulting in earth fault

 Flashover across support insulator during over voltage _ Heavily polluted insulator
causing flashover
 Earthquake, mechanical damage etc.

14. What are the problems in bus zone differential protection?

 Large number of circuits, different current levels for different circuits for external faults.
_ Saturation of CT cores due to dc component and ac component in short circuit currents.
 The saturation introduces ratio error.
 Sectionalizing of the bus makes circuit complicated.

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  Setting of relays need a change with large load changes.

16 Marks questions

1. Explain a protection scheme for stator interturn fault of a transformer. (May-12, Dec-12)

2. Discuss the protection employed against loss of excitation of an alternator. (Dec-12)

3. What are the various faults that may occur in an alternator. Give the diagram for circulating
current protection in alternator. (May -13)

4.Write a notes on generator protection. (May -14)

5. Explain the factor causing difficulty in applying Merz-price circulating principle to a transformer
and how are they overcome. (Dec-11)

6.Explain the protection scheme used in star/star and star/delta transformer. (Dec-12)

7.Describe the constructional and working of Buchholz relay. (May 13)

8. Describe the differential protective scheme for transformer. (Deec13, May 14)

9. Explain the protective schemes used for protection of bus bar. (Dec-13)

10. With neat sketchs explain the different types of protective schemes for transmission lines.
(Dec 13)

UNIT IV

THEORY OF CIRCUIT INTERRUPTION

1. What is resistance switching? (Dec-13)

It is the method of connecting a resistance in parallel with the contact space(arc). The resistance
reduces the restriking voltage frequency and it diverts part of the arc current. It assists the circuit
breaker in interrupting the magnetizing current and capacity current.

2. What do you mean by current chopping?

When interrupting low inductive currents such as magnetizing currents of the transformer, shunt
reactor, the rapid deionization of the contact space and blast effect may cause the current to be
interrupted before the natural current zero. This phenomenon of interruption of the current before
its natural zero is called current chopping.

3. What are the methods of capacitive switching?

•Opening of single capacitor bank

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•Closing of one capacitor bank against another

4. What is an arc?

Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy
load or fault or short circuit condition.

5. Give the two methods of arc interruption? (May-14)

High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so
that the arc is fully extinguished _ Current zero method:-The arc is interrupted at current zero
position that occurs100 times a second in case of 50Hz power system frequency in ac.

6. What is restriking voltage? (nov/dec 2011)

It is the transient voltage appearing across the breaker contacts at the instant of arc being
extinguished.

7. What is meant by recovery voltage? (nov/dec 2011)

The power frequency rms voltage appearing across the breaker contacts after the arc is
extinguished and transient oscillations die out is called recovery voltage.

8. What is RRRV?

It is the rate of rise of restriking voltage, expressed in volts per microsecond. It is closely
associated with natural frequency of oscillation.

16 Marks Questions

1. Explain in details various methods of arc extinction.(Dec-11)

2.Drive the expression for restriking and rate of rice of recovery voltage(Dec-11, May -12)

3. Write short notes on resistance switching. (Dec-12, May-13)

4. Discuss the phenomenon of current chopping. ( May-12,13, Dec-12,13)

5. Write brief note on interruption of capacitive current. (Dec-13, May-12)

6. With necessary diagrams describe the recovery rate theory and energy balance theory of arc
interruption in a circuit breaker. (Dec-12,14)

UNIT V

Protection & Switchgear

 CIRCUIT BREAKERS

1. What is circuit breaker?

It is a piece of equipment used to break a circuit automatically under fault conditions. It breaks a
circuit either manually or by remote control under normal conditions and under fault conditions.

2. Write the classification of circuit breakers based on the medium used for arc extinction?
(Dec-14)

_ Air break circuit breaker _ Oil circuit breaker

_ Minimum oil circuit breaker _ Air blast circuit breaker

_ SF6 circuit breaker

_ Vacuum circuit breaker

3. Give the advantage of SF6 circuit breaker over air blast circuit breaker (nov/dec 2011,
May 13)

High electro negativity, compactness, reduced switching over voltages, reduced insulation time,
superior arc interruption and increased safety

4. What is meant by electro negativity of SF6 gas? (May 13)

SF6 has high affinity for electrons. When a free electron comes and collides with a neutral gas
molecule, the electron is absorbed by the neutral gas molecule and negative ion is formed. This is
called as electro negativity of SF6 gas.

4. What are the characteristic of SF6 gas? (May 13)

It has good dielectric strength and excellent arc quenching property. It is inert, non- toxic,
noninflammable and heavy. At atmospheric pressure, its dielectric strength is 2.5 times that of
air. At three times atmospheric pressure, its dielectric strength is equal to that of the transformer
oil.

5. What is the main problem of the circuit breaker?

When the contacts of the breaker are separated, an arc is struck between them. This arc delays
the current interruption process and also generates enormous heat which may cause damage to
the system or to the breaker itself. This is the main problem.

6. What are demerits of MOCB?

_ Short contact life

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_ Frequent maintenance _ Possibility of explosion

_ Larger arcing time for small currents _ Prone to restricts

7. What are the advantages of oil as arc quenching medium?

•It absorbs the arc energy to decompose the oil into gases, which have excellent cooling
properties

•It acts as an insulator and permits smaller clearance between line conductors and earthed
components

8. What are the advantages of MOCB over a bulk oil circuit breaker?

• It requires lesser quantity of oil

• It requires smaller space

• There is a reduced risk of fire

• Maintenance problem are reduced

9. What are the disadvantages of MOCB over a bulk oil circuit breaker?

The degree of carbonization is increased due to smaller quantity of oil o There is difficulty of
removing the gases from the contact space in time o The dielectric strength of the oil deteriorates
rapidly due to high degree of carbonization.

10. What are the types of air blast circuit breaker?

_ Arial-blast type _ Cross blast

_ Radial-blast

11. What are the advantages of air blast circuit breaker over oil circuit breaker?

o The risk of fire is diminished

o The arcing time is very small due to rapid buildup of dielectric strength between contacts o The
arcing products are completely removed by the blast whereas oil deteriorates with successive
operations

12. What are the demerits of using oil as an arc quenching medium?

• The air has relatively inferior arc quenching properties

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• The air blast circuit breakers are very sensitive to variations in the rate of rise of restriking
voltage

• Maintenance is required for the compression plant which supplies the air blast

13. Write the classifications of test conducted on circuit breakers.

_ Type test

_ Routine test

_ Reliability test

_ Commissioning test

14. What are the indirect methods of circuit breaker testing?

o Unit test

o Synthetic test

o Substitution testing

o Compensation testing o Capacitance testing

16 Marks Questions

1. With a neat figure explain the construction and working of minimum oil circuit breaker. What
are its advantages and disadvantages? (Dec-11,May-13)

2. With a neat figure explain the construction and working of SF6. What are its advantages and
disadvantages? (May-12,14,Dec-12,)

3. Explain construction and working of air circuit breaker. (Dec-13)

4. Briefly describe the different types of testing schemes of circuit breakers. ( May -14)

5. Explain the following with reference to circuit breakers (May-12)

i) Breaking capacity ii) Making Capacity iii) Show time capacity iv) Rated Current

Protection & Switchgear