EE3601 PROTECTION AND SWITCHGEAR

2 Marks

UNIT I
1. What are the functions of protective relays?
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. Give the consequences of short circuit.

Whenever a short-circuit occurs, the current flowing through the coil increases to an enormous value.
If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by
closing its contacts.The trip circuit is then closed , thecircuit breaker opens and the fault is isolated from
the rest of the system. Also, a low voltage may be created which may damage systems connected to the
supply.

3. Define protected zone.

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.

4. What are unit system and non unit system?

A unit protective system is one in which only faults occurring within its protected zone are
isolated.Faults occurring elsewhere in the system have no influence on the operation of a unit system.A
non unit system is a protective system which is activated even when the faults are external to its protected
zone.

5. What is primary protection?

Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit
breaker.It serves as the first line of defence.

6. What is back up protection?

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

7. Name the different kinds of over current relays.

Induction type non-directional over current relay,Induction type directional over current relay &
current differential relay.

8. Define energizing quantity.

It refers to the current or voltage which is used to activate the relay into operation.

9. Define operating time of a relay.

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

10. Define resetting time of a relay.

It is defined as the time taken by the relay from the instant of isolating the fault to the moment when
the fault is removed and the relay can be reset.
11. What are over and under current relays?
Overcurrent relays are those that operate when the current in a line exceeds a predetermined value. (eg:
Induction type non-directional/directional overcurrent relay, differential overcurrent relay)whereas
undercurrent relays are those which operate whenever the current in a circuit/line drops below a
predetermined value.(eg: differential over-voltage relay)

12. Mention any two applications of differential relay.

Protection of generator & generator transformer unit; protection of large motors and busbars

13. What is biased differential bus zone reduction?

The biased beam relay is designed to respond to the differential current in terms of its fractional
relation to the current flowing through the protected zone. It is essentially an over-current balanced beam
relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite
direction to the operating force.

UNIT II
1. 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.

2. Mention the short comings of Merz Price scheme of protection applied to a power transformer.
In a power transformer, currents in the primary and secondary are to be compared. As these two
currents are usually different, the use of identical transformers will give differential current, and operate
the relay under no-load condition. Also, there is usually a phase difference between the primary and
secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential
current may flow through the relay under normal condition.
3. What are the various faults to which a turbo alternator is likely to be subjected?
Failure of steam supply; failure of speed; overcurrent; over voltage; unbalanced loading; stator
winding fault .

4. What is an under frequency relay?

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

5. 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.

6. Mention any two disadvantage of carrier current scheme for transmission line only.
The program time (ie, the time taken by the carrier to reach the other end-upto .1% mile); the response
time of band pass filter; capacitance phase-shift of the transmission line.

7. 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.

8. What are the causes of over speed and how alternators are protected from it?
Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are
provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the
prime mover when a dangerous over-speed occurs.

9. Give the limitations of Merz Price protection.

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.

10. What are the uses of Buchholz’s relay?

Bucholz relay is used to give an alarm in case of incipient( slow-developing) faults in the transformer
and to connect the transformer from the supply in the event of severe internal faults. It is usually used in
oil immersion transformers with a rating over 750KVA.

UNIT III

1. What are the types of graded used in line of radial relay feeder?
Definite time relay and inverse-definite time relay.

2. What are the various faults that would affect an alternator?

(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 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. Why neutral resistor is added between neutral and earth of an alternator?

In order to limit the flow of current through neutral and earth a resistor is introduced between them.

4. What is the backup protection available for an alternator?

Overcurrent and earth fault protection is the backup protections.

5. 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

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

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

7. 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.

8. 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.

9. What is REF relay?

It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer,
the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the
relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the
transformer is restricted to cover only around 85%. Hence the relay is called REF relay.
10. 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.
11. Why busbar protection is needed?
(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.

12. What are the merits of carrier current protection?

Fast operation, auto re-closing possible, easy discrimination of simultaneous faults .

13. What are the errors in CT?

(a) Ratio error
Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100
The value of transformation ratio is not equal to the turns ratio.
(b) Phase angle error:
Phase angle _ =180/_[(ImCos _-I1Sin _)/nIs]

14. What is field suppression?

When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the
fault continues to fed because EMF is induced in the generator itself.Hence the field circuit breaker is
opened and stored energy in the field winding is discharged through another resistor.

15. 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.

16. 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. _ Setting of relays need a change with
large load changes.

17. What is static relay?

It is a relay in which measurement or comparison of electrical quantities is made in a static network
which is designed to give an output signal when a threshold condition is passed which operates a tripping
device.

18. What is power swing?

During switching of lines or wrong synchronization surges of real and reactive power flowing in
transmission line causes severe oscillations in the voltage and current vectors. It is represented by curves
originating in load regions and traveling towards relay characteristics.
19. What is a programmable relay?
A static relay may have one or more programmable units such as microprocessors or microcomputers
in its circuit.

20. What is CPMC?

It is combined protection, monitoring and control system incorporated in the static system.

21. What are the advantages of static relay over electromagnetic relay?
Low power consumption as low as 1mW
No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of
contacts
No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc.
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

UNIT V
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?
_ Air break circuit breaker
_ Oil circuit breaker
_ Minimum oil circuit breaker
_ Air blast circuit breaker
_ SF6 circuit breaker
_ Vacuum circuit breaker

3. 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.

4. What are demerits of MOCB?

_ Short contact life
_ Frequent maintenance
_ Possibility of explosion
_ Larger arcing time for small currents
_ Prone to restricts

5. 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
6. What are the hazards imposed by oil when it is used as an arc quenching medium?
There is a risk of fire since it is inflammable. It may form an explosive mixture with arc. So oil is
preferred as an arc quenching medium.

7. 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
8. 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.

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

Axial-blast type _ Cross blast _ Radial-blast

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

11. What are the demerits of using oil as an arc quenching medium?
• The air has relatively inferior arc quenching properties
• 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

12. What is meant by electro negativity of SF6 gas?

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.

13. What are the characteristic of SF6 gas?

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.
14. Write the classifications of test conducted on circuit breakers.
_ Type test
_ Routine test
_ Reliability test
_ Commissioning test

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

o Unit test
o Synthetic test
o Substitution testing
o Compensation testing

16. What are the advantages of synthetic testing methods?

• The breaker can be tested for desired transient recovery voltage and RRRV.
• Both test current and test voltage can be independently varied. This gives flexibility to the test
• The method is simple
• With this method a breaker capacity (MVA) of five time of that of the capacity of the test plant can be
tested.

17. How does the over voltage surge affect the power system?
The over voltage of the power system leads to insulation breakdown of the equipment’s. It causes the
line insulation to flash over and may also damage the nearby transformer, generators and the other
equipment connected to the line.