1

DHANALAKSHMI COLLEGE OF ENGINEERING

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

EE2402 PROTECTION AND SWITCHGEAR

UNIT-I – INTRODUCTION

PART A

Importance of protective schemes for electrical apparatus and power system

1. What is meant by switchgear? (M-06)

2. What are the consequences of a short circuit?

3. Define – Protected Zone

4. What are unit system and non-unit system?

5. What is meant by primary protection?

6. What is meant by backup protection?

7. What are the protective zones of a power system?

8. Distinguish between primary protection scheme and secondary protection scheme.

Relay terminology - Definitions

9. Define – Energizing Quantity

10. Define – Operating Time of a relay

11. Define – Resetting Time of a relay

12. What is meant by time setting multiplier in protective relaying?

Essential qualities of protection.

13. What are the various essential qualities of protective relaying? (M/J-08, A/M-07)

14. What are the functions of protective relaying? (A-07)

2

Protection against over voltages due to lightning and switching

15. What are the causes of over voltage?

16. What are the protective measures taken against lightning over voltage?

17. Write the difference between the nominal system voltage and highest system voltage.

Ground wires

18. What is shielding angle of an overhead ground wire? What are the values as per American

and European practices?

Surge absorber and diverters

19. What is surge absorber? How do they differ from surge diverter? (N/D-11)

20. What is the significance of the coefficient of earthing in the selection of voltage rating of a

surge arrester?

Power system earthing

21. Define – Earthing

22. Define – Coefficient of Earthing

Neutral Earthing

23. What is the difference between equipment earthing and neutral earthing?

24. State the advantages of neutral grounding of an electrical system.

Basic ideas of insulation coordination.

25. Define – Insulation Coordination (N/D-11)

26. Define the following terms:

(a) Insulation Coordination

3

(b) Rated Voltage of Surge Arrester

PART-B

Importance of protective schemes for electrical apparatus and power system

1. Draw the protective zone diagram for a sample power system network and explain its

functions.(16) (N/D-06)

2. Explain in detail, the primary protection and backup protection.(16) (M/J-07)

3. (a) What is the necessity of protecting electrical equipments against travelling waves?

(6)

(b) Describe the protective devices used for the protection of electrical equipments

against travelling waves? (10)

Qualitative review of faults and fault currents

4. The positive, negative and zero-sequence reactances of a 20 MVA, 13.2 kV synchronous

generator are 0.3 p.u., 0.2 p.u., and 0.1. p.u., respectively. The generator is solidly

grounded and is not loaded. A line-to-ground fault occurs on phase a. Neglecting all the

resistances, determine the fault current.(16)

5. A 3 phase 11 kV, 25000 kVA alternator with Xgo = 0.05 p.u., X1 = 0.15 p.u., and

X2 = 0.15 p.u. is grounded through a reactance of 0.3 ohm. Calculate the line current for a

single line to ground fault.(16)

6. Explain how symmetrical components are used to analyse an unbalanced system.(16)

7. In a 3 phase, 4 wire system, the currents in R, Y and B lines under abnormal conditions

of loading are as under IR = 10030˚A, IY = 50300˚A, IB = 30180˚A. Calculate the

positive, negative and zero sequence currents in the R line and the return current in the

neutral wire.(16)

4

Essential qualities of protection.

8. Explain in detail, the essential qualities of a protective relay. (16) (M/J-07)

Protection against over voltages due to lightning and switching

9. (a) What are the causes of over voltage on a power system? (8)

(b) Why is it necessary to protect the transmission lines and other equipments of the

power system against over voltages? (8)

10. Explain in detail, the phenomenon of lightning. (16)

11. What are the protective measures taken against lightning over voltages? (16)

12. Explain the methods of protecting the power stations and sub-stations against direct

lightning stroke (16)

13. Describe the construction and working principle of

(a) Expulsion type lightning arrester (8)

(b) Valve type lightning arrester (8)

14. What are the various methods of overvoltage protection of overhead transmission lines?

(16)

15. How do earthing screen and ground wires provide protection against direct lightning

strokes? (16)

16. Explain the various types of lightning arresters used for protection against surge voltages.

(16)

17. Explain the construction and working of a thyrite lightning arrester. (16)

18. What are the basic requirements of a lightning arrester? Differentiate

(a) a lightning arrester from a lightning conductor

(b) a surge diverter from a surge absorber (16)

19. How is a lightning arrester selected for protecting a power transformer? (16)

5

20. Draw and explain a scheme for protecting rotating machine against over voltages. How

does this scheme differ from the protection of a power transformer? (16)

Arcing grounds

21. Explain the phenomena of arcing ground on an overhead transmission line. How does

neutral earthing oppose the arcing ground current? (16)

Peterson Coil

22. What is Peterson coil? How does this protect the equipment? (16)

23. Explain the different types of earthing the neutral point of a power system. Derive an

expression for the reactance of the Peterson coil in term of the capacitance of the

protected line. (16)

24. Write short notes on the following:

(a) Earthing of neutral

(b) Peterson coil

(c) Earthed transformer

(d) Neutral earthing arrangement in power stations (16)

Ground wires

25. What are the requirements of a ground wire for protecting power conductors against

direct lightning stroke? How can it be achieved in practice? (16)

Surge absorber and diverters

26. Explain the operation of various types of surge absorbers. (16)

27. Write a short note on switching surges and their causes. (16)

28. Explain the protective characteristic of a surge arrester against the withstand

characteristic of an equipment on a voltage/time curve. (16)

29. Explain the basic difference between the construction, operation and characteristics of a

SiC gapped surge arrester and ZnO surge arrester. (16)

6

30. Explain the principle of operation of Ferranti surge absorber. (16)

31. Write short notes on the following:

(a) Switching surges (4)

(b) Lightning arresters (4)

(c) Rod-gap arrester ` (4)

(d) Surge absorbers (4)

Power system earthing

32. Explain arc suppression coil earthing with a diagram. (16)

33. (a) What is tower-footing resistance? (8)

(b) Why should the tower-footing resistance be low and what are the methods adopted to

reduce it? (8)

34. What are the merits and the demerits of reactance earthing compared to solid earthing?

(16)

Neutral Earthing

35. Explain in detail, the advantages of neutral earthing. (16) (N/D-08)

36. (a) What are the various methods of earthing the neutral? (8)

(b) Compare them. (8)

37. What are the advantages of the following:

(a) Grounding the neutral of the system

(b) Keeping the neutral isolated (16)

38. What are ground rods and counterpoises? How can these be used to improve the

grounding conditions. Draw various arrangements of counterpoises. (16)

7

Basic ideas of insulation coordination.

39. Explain the need of insulation co-ordination in a large power system? What is meant by

Basic Impulse Level (BIL) of equipment? (16)

40. (a) Explain insulation coordination. (8)

(b) Describe the construction of volt time curve and also the terminologies used in

impulse testing. (8)

41. Explain the need of insulation co-ordination in a large power system? What is meant by

Basic Impulse Level (BIL) of equipment? (16)

42. Write short notes on the following:

(a) Pentograph and magnetic link (4)

(b) Rod gap (4)

(c) Arcing horns (4)

(d) Basic impulse insulation level (4)

8

UNIT-II OPERATING PRINCIPLES AND RELAY CHARACTERISTICS

PART-A

Electromagnetic relays

1. Write any two applications of electromagnetic relay. (M/J-08)

2. What are the fundamental requirements of a relay?

3. What is the need for relay coordination?

4. Explain the following:

(a) Sensitivity of a relay

(b) Stability of a protective system

5. List out the applications of electromagnetic relay.

Over current, directional and non-directional, distance, negative sequence, differential and

under frequency relays

6. What is the purpose of distance relay? (A/M-07)

7. A relay is connected to 400/5 ratio current transformer with a current setting of 150%.

Calculate the plug setting multiplier when the circuit carries a fault current of 4000 A.

(N-11)

8. Define – Pickup (M/J-06)

9. Define – Plug Setting Multiplier (M-06)

10. Define – Over Current Relay

11. Define – Undercurrent Relay

12. What are over and under current relays?

13. What are the different types of over current relays?

14. What is directional relay? Mention its use.

15. What are the features of a directional relay?

9

16. What is the use of distance relay?

17. Explain the significance of following terms in the distance protection

(a) Reach of a distance relay

(b) Under reach of a distance relay

18. Distinguish between circulating current differential protection and balanced voltage

differential protection with reference to the behaviour of CT.

19. State any two applications of differential relay.

20. What is biased differential bus zone protection?

21. What is an under frequency relay?

Introduction to static relays.

22. What is meant by a static relay?

23. What are the merits and the demerits of a static relay over electromagnetic relay?

24. List out the applications of static relay.

PART-B

Electromagnetic relays

1. What are the different types of electromagnetic relays? Explain their field of applications.

(16)

2. Describe the construction of an induction disc relay. State its principle of operation. What

are the advantages of induction relay? How current setting and time setting are selected?

(16)

3. Describe the construction, working principle and applications of an induction disc and

induction cup relays. What is the ratio of reset to pick up value in case of these relays?

(16)

10

4. Describe the various types of construction of attracted armature type relay. Whether can

they operate with AC and DC? State their salient features. (16)

Over current, directional and non-directional, distance, negative sequence, differential and

under frequency relays

5. Draw the construction of a non-directional induction relay. (16) (N/D-06)

6. Draw and explain the schematic of an impedance relay and its operating characteristics

on R-X diagram. (16) (N/D-06)

7. Describe the construction and principle of operation of an induction type directional over

current relay. (16)

8. Explain in detail, the construction, working principle and characteristics of a reactance

type distance relay. (16)

9. Explain in detail, the working of a current differential relay with a neat sketch. State the

disadvantages of this relay. (16) (N/D-08)

10. Explain in detail, the principle of a current differential relay with a neat diagram. (16)

(N-06)

11. Explain in detail, the construction and working of a reactance relay. (16)

12. (a) Explain the applications of microprocessor in power system protection. (8)

(b) Explain microprocessor based inverse time over current relay. (8)

13. Explain the realization of a directional over current relay using microprocessor. (16)

14. Draw a neat diagram and explain the working principle of the following:

(a) Directional over current protection (8)

(b) Earth fault protection by residual connection. (8)

15. Explain the working principle of distance relay. (16)

16. Derive a generalized mathematical model of a distance relay for a digital protection.

(16)

11

17. How does arc resistance introduce an error in the distance measurement? (16)

18. Explain the working principle of impedance type distance relay and explain its

characteristics on V-I and R-X planes. (16)

19. Explain in detail, the operating principle, constructional features and area of applications

of reverse power relay. (16)

20. Explain the working principle of negative sequence and under frequency relay. (16)

21. Explain R-X diagrams of the following distance relays:

(a) Impedance relay (5)

(b) Mho relay (5)

(c) Reactance relay (6)

Introduction to static relays.

22. Explain in detail, the advantages and the disadvantages of static relay. (16) (N-06)

23. Explain the operation of a static relay. (16)

24. Explain in detail, the different types of static relays. (16)

12

UNIT-III APPARATUS PROTECTION

PART-A

Main considerations in apparatus protection

1. What is meant by time-graded system of protection? (N/D-11)

Transformer, generator and motor protection

2. What are the various types of transformer faults? (M/J-08)

3. Why is the protection of a generator complex? (M/J-08)

4. What are the limitations of Buchholz relay?

5. What are the problems that arise in differential protection of a power transformer and

how are they overcome?

6. What is an over fluxing protection in a transformer?

7. State the short comings of Merz Price protection scheme applied to a power transformer.

8. List out the limitations of Merz Price protection.

9. What are the uses of Buchholz relay?

10. What are the various faults to which a turbo alternator is likely to be subjected?

11. What is an under frequency relay?

12. What are the causes of over speed in an alternator and how is it prevented?

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

14. What are the various faults that affect an alternator?

15. What is the purpose of adding a neutral resistor between neutral and earth of an

alternator?

16. Why does the backup protection used for an alternator?

13

Protection of busbars.

17. What is the importance of bus bar protection? (M/J-06)

18. What are the two types of protections used for bus-bars? (A/M-07)

19. Why is bus bar protection needed?

Transmission line protection

20. What are the common methods used for line protection?

21. Define – Power Line Protection

Zones of protection.

22. What are the causes of bus zone faults?

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

CTs and PTs and their applications in protection schemes.

24. What is meant by burden on C.T.? (A-07)

25. Why should not the secondary of a current transformer be open? (N/D-11)

26. What are the merits of carrier current protection?

27. What are the various errors of CT?

28. What is meant by a power swing?

29. What is meant by programmable relay?

30. What is meant by REF relay?

31. What is meant by field suppression?

14

PART-B

Transformer, generator and motor protection

1. Draw a diagram of connections of the Merz Price circulating current system for the

protection of a 1000 kVA, 1000/400 V, delta/star, 3 phase transformer with the star point

connected to ground. Indicate on the diagram the turns ratios of the CTs for a nominal

secondary current of 5 A. (16) (N-06)

2. Explain in detail, the protection scheme of a 3 phase AC induction motor. (16)

3. (a) Explain the differential system of protection applied to star delta connected

transformers with a neat diagram.(8)

(b) What are the difficulties experienced in the above method? (8)

4. What are the various faults that may occur in an alternator? Draw the diagram for the

circulating current protection in an alternator. (16)

5. A 5000 kVA, 6000 V star-connected alternator has a synchronous reactance of 2 ohm per

phase and 0.5 ohm resistance. It is protected by a Merz Price balanced current system

which operates when the out of balance current exceeds 30% of the load current.

Determine what proportion of the alternator winding is unprotected if the star point is

earthed through a resistor of 6.5 Ω? (16)

6. Explain in detail, the faults which may occur in an alternator. (16)

7. A star connected 3 phase 10 MVA, 66 kV alternator is protected by Merz Price

circulating current principle using 1000/5 ratio current transformer. The star point of the

alternator is earthed through a resistance of 7.5 ohm. If the minimum operating current of

the relay is 0.5 A, calculate the percentage of each phase of the stator winding which is

unprotected against earth faults when the machine is operating at normal voltage. (16)

8. Draw and explain the Merz Price protection of an alternator stator winding. (16)

15

9. A generator is protected by restricted earth fault protection. The generator ratings are

10 MVA, 13.2 kV, the percentage of winding protected against phase to ground fault is

85%. The relay setting is such that it trips for 20% out of balance current. Calculate the

resistance to be added in the neutral to ground connection. (16)

10. Explain in detail, the relay schemes that are employed for the protection of a modern

alternator. (16)

11. Explain in detail, the types of protective device used for the protection of an alternator

against overheating of its stator and rotor? (16)

12. Explain in detail, the protective scheme employed against loss of excitation of an

alternator. (16)

Transmission line protection

13. Explain in detail, the types of pilot protection schemes which are used for EHV and UHV

transmission lines. (16)

CTs and PTs and their applications in protection schemes.

14. Explain in detail, the carrier current protection scheme. (8)

16

UNIT-IV THEORY OF CIRCUIT INTERRUPTION

PART-A

Physics of arc phenomena and arc interruption

1. What is meant by arc?

2. What is meant by arcing ground?

3. List out the methods used for arc interruption.

Restriking voltage and recovery voltage

4. Define – Recovery Voltage (A/M-07)

5. Distinguish recovery voltage and restriking voltage. (N-11)

6. What is meant by restriking voltage?

Rate of rise of recovery voltage

7. Define – Rate of Rise of Restriking Voltage

Resistance switching

8. What is meant by resistance switching?

Current chopping

9. What is meant by current chopping? (M-06)

Interruption of capacitive current

10. Which are the methods used for capacitive switching?

17

PART-B

Physics of arc phenomena and arc interruption

1. Explain the following phenomena:

(a) Initiation of an arc (6)

(b) Maintenance of an arc (6)

(c) Methods for interrupting it (4) (N-06)

2. Explain in detail, the phenomena of an arc and the various methods of arc extinction.

(16)

3. What are the different methods of arc interruption? Explain any one of them in

detail. (16) (N/D-06)

DC and AC circuit breaking

4. Explain in detail, the method of breaking a DC circuit. (16) (M/J-07)

Restriking voltage and recovery voltage

5. A 50 Hz, 11 kV, 3 phase alternator with earthed neutral has a resistance of 5 ohm per

phase and is connected to a bus bar through a circuit breaker. The distributed

capacitance up to circuit breaker between phase and neutral is 0.001 μF. Determine

(a) Peak restriking voltage across the contacts of the breaker

(b) Frequency of oscillation

(c) Average rate of rise of restriking voltage up to the first peak. (16)

6. Describe in detail, the recovery rate and energy balance theories for an arc interruption

in a circuit breaker. (16)

18

Rate of rise of recovery voltage

7. Derive an expression for the rate of rise of restriking voltage.

8. Explain the following phenomena:

(a) Restriking voltage (3)

(b) Recovery voltage (3)

(c) RRRV (3)

(d) Derive an expression for restriking voltage and RRRV (3)

(e) What are the measures taken for their reduction? (4)

9. A three phase alternator has the line voltage of 11 kV. The generator is connected to a

circuit breaker. The inductive reactance and the distributed capacitance up to circuit

breaker between phase and neutral are 5 ohm per phase and 0.01µF respectively.

Determine the following:

(a) Peak restriking voltage across the C.B.

(b) Frequency of restriking voltage transients

(c) Average rate of restriking voltage up to peak restriking voltage

(d) Maximum R.R.R.V (16)

10. (a) The RMS voltage is 19.1 kV, L is 10mH and C is 0.02 mF in a power system.

Determine the average rate of rise of restriking voltage when the circuit breaker opens.

(8)

(b) In a short circuit test on a circuit-breaker, the following readings are obtained.

(1) Time to reach the peak restriking voltage 70µsec.

(2) The peak restriking voltage 100kV.

Calculate the average rate of rise of restriking voltage and the natural frequency. (8)

Resistance switching

11. Draw the schematic diagram of a breaker with a resistor connected across the contacts

and its Laplace equivalent. (16)

19

12. (a) Explain resistance switching in detail. (4)

(b) Derive an expression for the resistance to be provided across circuit breaker

contacts for critical damping and its usefulness. (12)

Current chopping

13. (a) Explain the phenomena of current chopping in a circuit breaker. (12)

(b) What are the measures taken to reduce current chopping? (4)

Interruption of capacitive current

14. Describe the problems associated with the interruption of the following:

(a) Low inductive current (5)

(b) Capacitive current (5)

(c) Fault current if the fault is very near to the substation (6)

20

UNIT-V CIRCUIT BREAKERS

PART-A

Types of circuit breaker

1. What are the basic requirements of a circuit breaker?

(M/J-08)

2. What is meant by circuit breaker?

3. List out the classification of circuit breakers based on the medium used for arc extinction.

4. What are the main problems of a circuit breaker?

5. What are the different types of air blast circuit breaker?

6. Why is single line to ground fault frequent in a transmission line?

7. What are the symmetrical components?

8. Define – Positive Sequence Component

9. Define – Zero Sequence Component

10. Define – Negative Sequence Component

11. List the different types of faults.

12. List the various types of unsymmetrical faults.

13. Define – Per Unit Value

14. Define – Single Line Diagram

Air blast, air brek, oil, SF6 and Vacuum circuit breakers

15. What are the special features of a vacuum circuit breaker?

21

16. What is meant by electro-negativity of SF6 gas?

Comparative merits of different circuit breakers

17. Write any two merits of a vacuum circuit breaker.

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

19. What are the demerits of MOCB?

20. What is the advantage of oil as an arc quenching medium?

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

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

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

Testing of circuit breakers.

24. List the routine tests conducted on a circuit breaker. (A/M-07)

25. What is meant by a self-compensated explosion pot?

PART-B

Types of circuit breaker

1. (a) Enumerate the various types of ratings of a circuit breaker. (4)

(b) What are symmetrical and asymmetrical breaking capacities of a circuit breaker?

(4)

(c) What is making capacity of a circuit breaker (4)

(d) What is short-time current capacity of a circuit breaker (4)

2. What is the difficulty in the development of HVDC circuit breaker? Describe its

construction and operating principle. (16)

22

Air blast, air brek, oil, SF6 and Vacuum circuit breakers

3. Draw a sectional view and explain the working principle of a low oil circuit breaker. (16)

(N/D-06)

4. Explain in detail, the construction, working principle, merits and demerits of minimum

oil circuit breaker with a neat diagram. (16) (N/D-08)

5. Draw the construction of SF6 breaker and write its advantages and disadvantages. (16)

6. (a) What are the physical, chemical and dielectric properties of SF6 a gas? (8)

(b) Define – Switchgear (4)

(c) What are the essential features of the switchgear? (4)

Comparative merits of different circuit breakers

7. What are the advantages and disadvantages of an air blast circuit breaker? (16) (M/J-07)

Testing of circuit breakers.

8. Explain synthetic testing of a circuit breaker. (16) (M/J-07)

9. Explain in detail, the different classifications of testing of a circuit breaker. (16)

10. Explain in detail, the different methods of testing of a circuit breaker. (16)