PROTECTION AND SWITCHGEAR Page 1

MAHALAKSHMI

ENGINEERING COLLEGE

TIRUCHIRAPALLI – 621213

QUESTION BANK

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Sub. Code : EE2402 Semester : VII

Subject : PROTECTION AND SWITCHGEAR Unit : II

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PART A

1. Name the different kinds of over current relays. [AU/Apr/May/12/13]

Induction type non-directional over current relay, Induction type directional

over current relay & current differential relay.

2. Define energizing quantity. [AU/Nov/Dec/09/11]

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

3. Define operating time of a relay. [AU/Apr/May/11/13]

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.

4. Define resetting time of a relay. [AU/Nov/Dec/10/13]

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.

5. What are over and under current relays? [AU/Apr/May/12/11]

Over current relays are those that operate when the current in a line exceeds a

predetermined value. (e.g.: Induction type non-directional/directional over current

relay, differential over current relay) whereas undercurrent relays are those which

operate whenever the current in a circuit/line drops below a predetermined value.(e.g.:

differential over-voltage relay)

6. Mention any two applications of differential relay.

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

bus bars.

7. What is biased differential beam relay? [AU/Apr/May/12/13]

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.

8. What is the need of relay coordination?

The operation of a relay should be fast and selective, i.e., it should isolate the fault in

the shortest possible time causing minimum disturbance to the system. Also, if a relay

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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.

9. Give the limitations of Merz Price protection. [AU/Apr/May/12/13]

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 is an under frequency relay? [AU/Apr/May/12/13/10]

An under frequency relay is one which operates when the frequency of the system

(usually an alternator or transformer) falls below a certain value.

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

[AU/Apr/May/08/11]

Pilot wires refer 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.

12. Mention any two disadvantage of carrier current scheme for transmission line

only. [AU/Apr/May/08/12]

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.

13. What are the features of directional relay? [AU/Apr/May/12/13]

High speed operation; high sensitivity; ability to operate at low voltages; adequate

short-time thermal ratio; burden must not be excessive.

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

Fault between phase and ground; fault between phases and inter-turn fault involving

turns of the same phase winding.

15. What is static relay? [AU/Apr/May/10/11]

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.

16. What is a programmable relay? [AU/Apr/May/11/13]

A static relay may have one or more programmable units such as microprocessors or

microcomputers in its circuit.

17. What is CPMC? [AU/Nov/Dec/11/13]

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

system.

18. What are the advantages of static relay over electromagnetic relay? (nov/dec

2011)

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

Static relay is compact

Superior operating characteristics and accuracy

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

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

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

electrical in conjunction with transducers.

19. What is pick up value?

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

20. Define target. [AU/Apr/May/09/11]

It is the indicator used for showing the operation of the relay.

21. Define reach. [AU/April/May/10/12]

It is the distance up to which the relay will cover for protection.

22. Define blocking. [AU/Apr/May/12/13]

It means preventing the relay from tripping due to its own characteristics or due to

additional relays.

23. What are the different types of over current relays. [AU/April/May/12/13]

Instantaneous over current relays, Definite time over current relay, IDMT

24. What is the purpose of plug setting? [AU/Nov/May/12/13]

Plug setting is used to change the number of turns of the operating coil to get a

particular pick up value.

25. What is earth fault protection? [AU/Nov/May/12/13]

The protective scheme used for the protection of an element of a power system

against earth faults is called as earth fault protection

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

Calculate the PSM when the fault current is 4000A (nov/dec 2011)

4000/(7.5*80)=6.667

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PART B

1. Describe the construction and principle of operation of induction type directional

over-current relay. Explain IDMT characteristics and how they are obtained in

an induction type relay. [AU/Apr/May/12/13]

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2. Draw and explain the constructional details of non-directional induction type

relay.

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

characteristics on R-X diagram.

Since this type of relay involves impedance-type units, let us first become acquainted

with them. Generally speaking, the term “impedance” can be applied to resistance

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alone, reactance alone, or a combination of the two. In protective-relaying

terminology, however, an impedance relay has a characteristic that is different from

that of a relay responding to any component of impedance. And hence, the term

“impedance relay” is very specific. In an impedance relay, the torque produced by a

current element is balanced against the torque of a voltage element. The current

element produces positive (pickup) torque, whereas the voltage element produces

negative (reset) torque. In other words, an impedance relay is a voltage-restrained

overcurrent relay.

4. Explain the construction, working principle and characteristics of reactance

relay type distance relay.

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5. Explain the principle of current differential relay with diagrams.

6. What is static relay? List the advantages and disadvantages of static relays.

Static relays are those in which the designed response is developed by electronic or

magnetic means without mechanical motion. This means, that the designation ‘static

relay’ covers the electronic relays of both the analog and digital designs. The analog

relays refer to electronic circuits with discrete devices like transistors, diodes, etc.,

which were adopted in the initial stages. However, the digital designs incorporate

integrated chips, microprocessors, etc., which had been developed subsequently. In

recent years, very few relays of the analog type are being developed or introduced for

the first time. Most modern overcurrent relays are of the digital type.

There are many reasons for this, the main ones being associated with cost, accuracy,

flexibility, reliability, size, auxiliary power drain, etc. Many of these reasons will

become evident during the course of this chapter, which will concentrate on relays of

the digital type. Microprocessor relays are of the digital type. The main objective of

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using static relays is to improve the sensitivity, speed and reliability of a protection

system by removing the delicate mechanical parts that can be subject to wear due to

vibration, dust and corrosion.

During the early development of static relays, the use of static components were

particularly attractive for the more complicated relays such as impedance relays,

directional relays, voltage regulating relays, etc. On the other hand, the early static

IDMT overcurrent relays were expensive because it was difficult to match the inverse

time characteristic using analog protection circuits. The battery drain associated with

these static IDMT relays was too high and this discouraged the use of this type of

relay for medium-voltage applications.

The general developments in the field of electronics and the introduction of digital

circuits have overcome many of the above problems. Using modern microprocessor

relays, almost any characteristic is possible and economical, even for the simplest

applications such as, overcurrent relays and motor protection relays.

7. What are the complications of circulating current in a transformer? Write the

remedial measures also.

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8. List various building blocks that constitute the static relay and give brief

description about each of the blocks.

9. Briefly discuss about classification of relays. With neat diagrams explain the

different types of attracted armature type relays & induction type relays.

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10. Explain the principle of distance relays stating clearly the difference between

impedance relay, reactance relay and mho relay. Indicate the difference on R-X

diagrams and show where each type is suitable.(16)

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R-X Diagram of Impedance relay

R-X Diagram of Reactance relay - Characteristics

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R-X Diagram of MHO relay – Characteristics