14EE704 Hall Ticket Number:

IV/IV B.Tech (Regular) DEGREE EXAMINATION

November, 2017 Electrical &Electronics EngineeringSeventh Semester Switchgear & ProtectionTime: Three Hours Maximum: 60 Marks

Answer Question No.1 compulsorily. (1X12 = 12 Marks)

Answer ONE question from each unit. (4X12=48 Marks)

1. Answer all questions (1X12=12 Marks) a) Define pick up current. b) Mention various types of over current relays. c) Where is negative phase sequence relay employed? d) What is comparator? e) What is numerical relay? f) Write the basic components of a static relay. g) What is soil resistivity? h) What is over fluxing in a transformer? i) Write any two abnormal condition of an alternator. j) What is the difference between air blast and air break circuit breakers? k) Define restriking voltage. l) What is breaking capacity of a circuit breaker?

UNIT I

2. a) Explain the operation of balanced beam type electromagnetic relay. 6M b) Explain with the help of neat sketch, construction and working of directional induction type

Over current relay. 6M

(OR) 3. a) Draw and explain construction and working of Buchholtz relay. 6M b) What is mho relay? Explain its operation with the characteristics. 6M

UNIT II 4. a) Explain the static inverse time over current relay with a block diagram 8M b) What are the merits and demerits of static relays over electromagnetic relays? 4M

(OR) 5. a) Explain the working principle of static relay with a block diagram. 6M b) Explain rectifier bridge type amplitude comparator. 6M

UNIT III6. a) Describe the working principle of restricted earth fault protection of alternator. 6M b) Explain the necessity of grounding in detail. 6M

(OR) 7. a) Explain the differential protection of large power transformer with a neat wiring diagram. 6M b) Describe the protective scheme employed against magnetizing inrush current of a

transformer 6M

UNIT IV8. a) Explain with the help of neat sketch, construction and working of minimum oil circuit

breaker. 6M

b) Explain the working principle of SF6 circuit breaker. 6M (OR)

9. a) Discuss energy balance theory of arc interruption in a circuit breaker. 6M b) Write short notes on current chopping phenomenon. 6M

IV/IV B.Tech (Regular) DEGREE EXAMINATION November, 2017 Electrical &Electronics Engineering

Seventh Semester Switchgear & Protection (14EE704)

KEY ‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐‐ 

1. a).Pick up current: The threshold value of the actuating current above which the relay operate. (OR)

It is the minimum value of an actuating quantity at which relay starts operating. b).Various types of over current relays: Definite time over current relay, Instantaneous over current rely, Inverse-time over current relay, Inverse definite time over current (IDMT) relay, Very inverse-time over current relay, Extremely Very inverse-time over current relay. c).Negative sequence relays are employed to protect electrical machines against overheating due to

unbalanced currents. d).Comparator: Which compares the two actuating quantities either in amplitude or phase is known

as the comparator. e). Numerical relay: Numerical relays are digital devices designated to carry out protection

functions of various electrical equipment such as generator, transformer, transmission lines, motor etc.

Numerical relays are based on the use of microprocessors. f).Basic components of static relays: 1.Input element: rectifier 2. Measuring element: Comparator 3. Output Element: Transistor as an amplifier 4. Feed element: DC Power supply 5. Tripping circuit g).Soil Resistivity: Soil resistivity is a measure of how much the soil resists the flow of electricity.

(OR) The measure of the resistance offered by the soil in the flow of electricity is called the soil resistivity.

h). The flux density in the windings is directly proportional to the induced voltage and inversely

proportional to the frequency and the number of turns in the winding. Over fluxing is usually encountered in Transformers which are directly connected to the

generator. It usually occurs when the generator is being started or stopped. As the rpm of the generator and consequently the frequency of the power falls, the same system voltage induces a higher magnetic flux.

i) 1. Overloading 2. Over speeding 3. Unbalanced loading 4. Overvoltage 5. Failure of prime mover 6. Loss of excitation (Field failure) 7. Cooling system failure. 8. Stator winding fault.

j). air break circuit breaker which operates in air at atmospheric pressure and air blast circuit breaker is operated a blast of compressed air.

k).Restriking voltage: The transient voltage which appears across the breaker contacts at the instant

of arc being extinguished is known as restriking voltage. l).Breaking capacity of circuit breaker: Circuit Breaker breaking capacity expresses the highest

rms value of the short circuit current that the circuit breaker is capable of breaking under specified conditions of transient recovery voltage and power frequency voltage.

(OR) Symmetrical breaking capacity: It is the rms value of the ac component of the fault current that

the circuit breaker is capable of breaking under specified conditions of recovery voltages. (OR)

Asymmetrical breaking capacity:It is the rms value of the total current comprising of both ac and dc components of the fault current that the circuit breaker can break under specified conditions of recovery voltage.

(OR) The breaking capacity of circuit breaker is generally expressed in MVA. Breaking capacity=√3xrated voltage in kV X rated current in kA.

UNIT-I 2.a). Diagram-2M Explanation-4M

2.b) Diagram-2M Construction and working Explanation-4M

(OR)

3.a) Diagram-2M Construction and working Explanation-4M

3.b) Mho relay-1M Operation-3M Characteristics-2M

UNIT-II 4.a) Block diagram-3M Explanation-5M

4.b) Merits-2M Demerits-2M Merits and Demerits of static relays over electromagnetic relays:

(OR)

5.a) Block diagram-2M Explanation-4M

5.b) Circuit diagram-2M Explanation-4M

UNIT-III

6.a) Circuit diagram-2M Explanation-4M

6.b)

.

. To save human life from danger of electrical shock or death by blowing a fuse i.e. to provide an

alternative path for the fault current to flow so that it will not endanger the user .

To protect buildings, machinery & appliances under fault conditions ie. to ensure that all exposed conductive parts do not reach a dangerous potential.

To provide safe path to dissipate lightning and short circuit currents.

To provide stable platform for operation of sensitive electronic equipments i.e. to maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment . To provide protection against static electricity from friction

(OR) 7.a) Diagram-3M Explanation-3M

NOTE:CONSIDER ANY ONE of the following (Merz-Price protection for star-delta transformer or star-star transformer)

7.b) Protection against magnetizing inrush current of a transformer:

UNIT-IV 8.a) Diagram-2M Construction and working principle Explanation-4M

8.b)

Working principle of SF6 Circuit breaker:

(OR)

9.a) waveform-2M

Explanation with derivation-4M

(OR)

9.b) Circuit, waveform-3M

Explanation with derivation-3M