Unit –I

1. What is over voltage?Voltage greater than the normal value is known as an over voltage. Over voltages are caused due to lightning , the opening of a circuit breaker , the grounding of the conductor etc.,

2. What are the sources of over voltage?The two sources of over voltage are,

a. External over voltages due to lightningb. Internal over voltages due to switching operation.

3. What is voltage surge?A sudden rise in voltage for a very short duration on the power system is known as voltage surge.

4. What are the causes of over voltages?The causes of over voltages are

Internal causes - Switching surges ,Insulation failure,Arcing ground ResonanceExternal causes - Lightning

5. Explain switching surges.The over voltages produced on the power system due to switching operations are known as switching surges

6. Explain insulation failureMost of the insulation failure in a power system is due to the grounding of conductors (i.e.) insulation failure between line and earth.

7. Explain about arcing groundThe phenomenon of intermittent arc taking place in line to ground fault of a 3Ф system with consequent production of transients is known as arcing ground.

8. How arcing ground are prevented?The transients produced due to arcing ground are cumulative and may cause series damage to the equipment in the power system by causing breakdown of insulation. arcing ground can be prevented by earthing the neutral.

9. What is lightning?It is an electrical discharge between clouds, between the separate charge centers in the same cloud, or between clouds and earth. It is a big spark. Large number of discharges occurs because of lightning.

10. What are the types of lightning strokes?The types of lightning strokes are: a. direct stroke b. indirect stroke

11. Explain about direct stroke.In the direct, the lightning discharge is directly subjected from the cloud to the equipment, e.g. an overhead line. The overvoltage set up due to the stroke may be large enough to flash over this path directly to the ground

12. What are the basic requirements of a surge diverter?The following are the basic requirements of a surge diverter

a) It should not pass any current at normal or abnormal (normally 5% more than the normal voltage) power frequency voltage.

b) It should breakdown as quickly as possible after abnormal high frequency arrives.

c) It should not only protect the equipment for which it is used but should discharge the surge current without damaging itself.

d) It should interrupt the power frequency follow current after the surge is discharged to ground.

13. What are the main types of surge diverters?

The three mainly types of surge diverters are,a) Rod gap b)Protector tube or expulsion type of lightning arrester b) Valve type of lightning arrester

14. What is protective angle?

Protective angle of the ground wire is defined as the angle between the vertical line passing through the ground wire and the line passing through the outermost power conductor.

15. What is protective zone?The protective zone is the zone which is a cone with apex at the location of the ground wire and the surface, generated by line passing through the outermost conductor.

16. What is a tower footing resistance?The tower footing resistance is the value if the footing resistance when measured at 50Hz.

17. What are the methods used for improving the grounds of transmission towers?The methods normally used for improving the grounds of transmission towers are the use of i. Ground rods ii. Counterpoises

18. What is the use of ground rods?Ground rods are used to reduce the tower footing resistance.

19. What is a surge absorber?A surge absorber is a device which absorbs energy contained in a travelling wave.

20. Gove one method of protecting the transmission lines against direct strokes.Shielding the lines by means of ground wires affords an effective method of protecting them from direct strokes. Alternatively the lines can be built without shielding but provided with auxiliary device, such as protected tubes for protection against strokes.

21. Classify valve type arresters.The valve type arresters are classified as

a) Station types b) Line types c) Arresters for the protection of rotating machines d) Distribution types arresters e) Secondary arresters

22. What is surge modifier or absorber?As damage depends not only on the amplitude of an incoming wave but also on the steepness of the wavefront, protection can be offered by any device which reduces this steepness. Such a device is known as surge absorber or modifier.

Unit.II

1. List the components of switch gear.The components of switch gear are ,

a) Bushings b) Circuit breaker contacsc) Instrument transformers d) Busbars and conductors

2. Discuss the arc phenomena on circuit breakers? When short circuit occurs, a heavy current flows through the contacts of the circuit breaker before opened by the protective system. At the instant when the contacts begin to separate the contact Area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between the contacts is sufficient to ionize the air Vapourise and ionize the oil. The ionized air or vapour acts as conductor and arc is struck between the contacts is quite small and is just sufficient to maintain the arc.

3. State the different methods of arc-extinction?The methods used for arc extinction are,a. High resistance method. b.Low resistance or current zero method.

4. What are the two theories that are used to describe the arc extinction phenomena?

The two theories which are used to explain the phenomenon of current zero interruption are,

a. Energy balance theory (Cassie’s theory)b.Voltage race theory (or) recovery rate theory (Slepian’s theory)

5.Define recovery voltage in circuit breakers? Recovery voltage is defined as the normal frequency (50HZ) rms voltage that appears

across the contacts of the circuit breaker after final arc extinction. It is approximately equal to system voltage.

6.Define re-striking voltage in circuit breakers? Restriking voltage is defined as the transient voltage that appears across the

contacts at or near current zero during arcing period.

7.List the factors affecting the Transient Recovery Voltage?

The factors on which the active recovery voltage depends are,a.The power factor, b.The armature reaction, c.The circuit conditions.

8. What is meant by the term ‘current chopping’?

When interrupting a low inductive current, e.g., that of a shunt reactor or the magnetizingcurrent of a transformer, the rapid de-ionizing and blast effect may cause the current to fall abruptly to zero well before the natural current zero. This phenomenon is termed as current chopping. Current chopping is otherwise known as breaking of low inductive currents.

9.. Mention the different methods of increasing arc resistance.

a) Arc lengthening b) Arc splitting c) Arc cooling d) Arc constraining10. How is self compensated explosion pot used in oil circuit breakers? self compensated explosion pot is the combination of both plain and cross jet explosion pot.11. Classify oil circuit breakers.

The different types of oil circuit breakers are a) Bulk oil circuit breakers b) Low oil circuit breakers

12. Define breaking capacity of circuit breaker. The breaking capacity of a breaker is the product of the breaking current and the

recovery voltage 13. Define making capacity.

Making capacity is the product of the making current it can make and carry instantaneously at the rated service voltage. 14. What are the different types of circuit breakers. The types of circuit breakers are a. oil circuit breakers – i. plain – break oil circuit breaker

i. the controlled – break oil circuit breakerii. minimum oil circuit breaker

b. air circuit breakeri. Arc- chute air circuit breaker

c. Air blast circuit breakeri. Axial blast circuit breakerii. Cross blast circuit breaker

d. sulphur hexafluoride circuit breaker (SF6) e. vaccum circuit breaker 15. Write any two advantages of SF6 circuit breakers. The advantages of SF6 circuit breakers zre

a. The current chopping tendency is minimized by using the gas SF6

at low pressure and low velocityb. The circuit breaker performance is not affected due to the

atmospheric conditionc. Electrical clearences are drastically reduced to high dielectric

strength of SF6 .d. No carbon particles is formed during arcing and therefore, there is

no reduction in the dielectric strength of the gas.

16. suggest the types of circuit breaker for the following 3.3-33KV - vacuum circuit breaker 132-220KV - Air blast circuit breaker 44-760 KV - sulphur hexafluoride (SF6)circuit breaker

17. What is a circuit breaker? A circuit breaker can make or break a circuit either manually or automatically under all

conditions viz. noload, full load and short circuit.18. Define RRRV and give the expression for it.

RRRV is defined as the slope of the steepest tangent to the restriking voltage curve. RRRV/dt = dvc = Em ω sin ωt

19. What are the factors affecting restriking voltage characteristics?

The factors affecting restriking voltage characteristics area) Configuration of the network b) Its natural frequencyc)Relative position of the resistances

20. What are the types of testing of circuit breaker? The types of testing of circuit breaker are,

a) Routine test b)Type test

21. What are the indirect testing methods of testing of circuit breaker? The indirect testing methods of testing of circuit breaker are

a. Unit testing b. Synthetic testing 22. How does a circuit breaker differ from a switch?

Circuit breaker can be switched automatically but the switch has to be operated manually. 23. A 3 phase oil circuit breaker is rated at 1200A, 1300MVA, 11KV. Find the rated symmetrical breaking current.Rated breaking current = 1300

√3 x 11=68 KA

UNIT 3 HRC FUSES AND SWITCHES

1.Define fuse? It I a short piece of metal inserted in the circuit,which melts when excessive current flows through it and thus it breaker the circuit.

2.Define fuse element? It is a part of fuse which melts,when the current flowing in the circuit exceeds predetermined value.

3.what are the materials commonly used for fuse element?

Tin,lead,zinc,silver,copper,aluminium etc.,and practically zinc,lead,tin are the suitable material of low melting point.

4.Define minimum fusing current?what are the factors at which it depends?

Minimum fusing current at which the fuse element melt and thus disconnects the circuit protected by it. It depend on length,shape and area of cross section,size and location of the terminal,type of enclosure and number of strands in the fuse wire.

5.How the minimum fusing current does differs for standed wires? It depends on the cors sectional area of the wires.

Sl.no SWG Rated

current (A)

Minimum fusing

current (A)

1 20 34 70

2 25 15 30

3 30 8.5 13

4 35 5 8

5 40 1.5 3

6.Define fusing factor?

Ratio of minimum fusing current to current rating of the fuse.

7.what are the type of fuses?1.Rewirable type

Open or semienclosed type

2.totally enclosed type HRC fuse D-type catridge fuse

8.what are the main advantages of HRC fuses?

Capability of clearing high values of fault currents. Fast operation Non deterioration for long periods. Inverse time current characteristics.

9. what are the main disadvantages of HRC fuses?

It requires replacement after each operation. Interlocking Is possible. It produces overheating of the adjacent contacts.

10.Mention the points in selection of fuses.

Able to withstand momentry over current due to starting a motor and transient current surges due to switching on transformers,capacitors and fluorescent lightning etc.,

Operation must be ensured when sustained overload or short circuit occurs.

11.Define isolator and explain is requirement.

Isolator (disconnecting switch)operates under no load condition.Isolator are necessary on the supply side of circuit breaker in order to isolation of circuit breaker from live parts for the purpose of maintenance.

12. Define earthing switch? It is connected between the line conductor and earth.Normally it is open.when the line is disconnected,the earthing witch is closed so as to discharge the voltage trapped on the line.

13.what is the necessacity of earthing switch?

Though the line is disconnected,there is some voltage on the line to which the capacitance between line and earth is charged.This voltage is significant in high voltage system.Before starting the maintenance work these voltages are discharged to each by closing the earthing switch.

14.what are the types of isolators? Vertical break type Horizontal break type Vertical pantograph

15.what are the main advantages of pantograph isolator?- cover less floor area , - each pole can be located at suitable point - three poles cannot be in on line, can be located in a time at desirable angle with bus axis

16.what is a gas insulated substation? SF6 gas insulated substation are preferred for voltage ratings of 72.5kv,145kv,300kv and 420kv and above.

Unit IV1. What is biased differential bus zone reduction?

The biased beam relay is designed to respond to the differential current in termsof 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.

2. 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 .

3. 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.

4. 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 .

5. 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.

6. What are the main types of stator winding faults?

Fault between phase and ground; fault between phases and inter-turn faultinvolving turns of the same phase winding.

7. 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.

8. 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.

9. What are the types of graded used in line of radial relay feeder?

Definite time relay and inverse-definite time relay.

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

(a) Stator faults1, Phase to phase faults2, Phase to earth faults3, Inter turn faults(b)1, Earth faults2, Fault between turns3, Loss of excitation due to fuel failure(c) 1, Over speed

2, Loss of drive3, Vacuum failure resulting in condenser pressure rise, resulting inshattering of the turbine low pressure casing(d) 1, Fault on lines2, Fault on busbars

11. 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 isintroduced between them.

12. What are faults associated with an alternator?(a) External fault or through fault(b) Internal fault1, Short circuit in transformer winding and connection2, Incipient or slow developing faults

13. What is REF relay?It is restricted earth fault relay. When the fault occurs very near to the neutralpoint 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.

14. 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 thesystem network.

15. What are the merits of carrier current protection?Fast operation, auto re-closing possible, easy discrimination of simultaneousfaults

16. 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.

17. What are the problems in bus zone differential protection? Large number of circuits, different current levels for different circuits forexternal 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.

18. What are the merits of carrier current protection?Fast operation, auto re-closing possible, easy discrimination of simultaneousfaults .

19. 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 generatoritself. Hence the field circuit breaker is opened and stored energy in the field winding isdischarged through another resistor. This method is known as field suppression.

20.. 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.

UNIT V

1. What are the various zone of protection for a modern power system?The various zone of protection for a modern power system are,a) Generator protection b) Transfer protectionc) Transmission line protection d) Bas bar protection

2. What are the fundamental requirements of relay?The fundamental requirements of relay are,a) Selectivity b) Speed c) Sensitivity d) Reliability e) Simplicity f) Economy

3. What meant by selectivivity or discrimination?Selective or discrimination is the ability of the relay to discriminate between faulty conditions and normal conditions ( or between a fault within the protected section and outside the protected section )

4. What are the classifications of protective schemes?a) Over current protection b) Distance protectionc) Carrier current protection d)Differential protection

5. What is meant by primary protection?If a fault occurs, it is the duty of the primary protective scheme to clear the fault. It acts as a first line of defence. If its fails, the back up protection clear the fault.

6. What is meant by protective system?Protective system is combination of protective gear equipment to secure isolation of the faulty element under predetermined conditions, usually abnormal or to give an alarm signal or both.

7. Define protective relay?

A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system.

8. Differentiate fuse and relay.A fuse performs both detection and interruption functions automatically but it is limited for the protection of low voltage circuits only. But a relay and circuit breaker combination is used as a protective gear for high voltage applications.

9. What is the function of relay in an electrical system?The relay detects the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. Though the changes in one or more of these quantities, it detects this faulty section and close the contacts of circuit breaker. This result the CB contacts open and disconnect the faulty section from the rest of the system.

10.What is the basic principle of induction relay?Torque produced in these when one alternating flux reacts with the current induced in the rotor by another alternating flux displaced in time and space but having the same frequency.Induction relay are widely used for protective relaying purpose involving AC quantities.

11.What is meant by instantaneous relay?An instantaneous relay has no intentional time delay in its operation. It operates in 0.1 sec. Sometimes the terms high set or high speed relays are also used for the relays which have the operating times listen 0.1sec.

12.What is meant by inverse time relay?Inverse time relay is the relay in which the operating time is inversely proportional to the magnitude of the operating current.

13.What is meant by IDMT relay?The relay which gives an inverse time characteristics at lower values of the operating current and definite time characteristics at higher values of the operating current. This is called inverse definite minimum time relay.

14.What is meant by distance relay?The relay which measure impedance or a component of the impedance at the relay location is kwon as a distance relay. It is used for the protection of the transmission line.

15.What is meant by MHO relay?MHO relay is used to measure the particular values of impedance i.e., Z/ cos (F-q ), where ‘F’ is the power factor angle and ‘q ‘ is the design angle to shift MHO characteristics on the R-X diagram. Its characteristics on the R-X diagram are a circle passing through the origin. It is a directional relay. It is also kwon as an admittance or angle admittance relay.

16.What meant by differential relay?The relay which operates in response to the difference of two actuating quantities are called differential relay.

17.What are the components of switchgear?a) Bushing b) Circuit breaker contacts c) Instrument transformers d) Bus bars & conductors.

18. What are the basic needs of protection? The basic needs of protection are,

a) If the fault is not cleared quickly, it may cause unnecessary interruption of service to the customers.b) Rapid disconnection of faulted apparatus limits the amount of damage to it and prevents the effect of fault from spreading into the system.

19. Classify relays based upon the time of operation. a) Instantaneous relay b) Definite time – lag relay c) Inverse time – lag relay

d) Inverse definite minimum time lag, IDMT relays.

20. What is meant by static relay? A static relay is defined as one which there is no armature or other moving elements, the designed response is being developed by the electromagnetic solid state, magnetic or other components without mechanical motion.

21. Mention the advantages of the static relays.a) A high degree of accuracy b) A high speed of operationc) Low power of consumption d) Simplified testing and servicing is possible.

22. what are the advantages of microprocessor based relay over other relays?

Attractive flexibility,low cost , faster, more accurate and compactness

23. what is meant by under frequency relays?

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