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1. Explain unit & non-unit protection of transmission line.
The graded overcurrent systems described earlier do not meet the protection requirements
of a power system. The grading is not possible to be achieved in long and thin networks
and also it can be noticed that grading of settings may lead to longer tripping times closer
to the sources, which are not always desired. These problems have given way to the
concept of ‘unit protection’ where the circuits are divided into discrete sections without
reference to the other sections. Ideally, to realize complete selectivity of protection, the
power system is divided into discrete zones. Each zone is provided with relays and circuit
breakers to allow for the detection and isolation of its own internal faults. The protection
used in this manner – essentially for internal faults in a particular zone – is referred to as
main or unit protection.
2. Explain with block diagram carrier-current protection of transmission line.
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3. How are three phase motors protected from unbalance & single Phasing Protection?
Unbalance protection
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4. Explain the different protective schemes used for protecting a bus zone
Differential protection
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Fault bus protection
5. (a) A 3Φ transformer of 220/11000 line volts is connected in star delta. The protective
transformers on 220 V side have a current ratio of 600/5.What should be the CT ratio on
11000 V side? (8 Marks)
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(b) A 11 kV, 3Ф alternator has full load rated current of 200 A. Reactance of armature
winding is 15 %. The differential protection system is set to operate on earth faults of
more than 200 A. Find the neutral earthing resistance, which gives earth fault protection
to 90% of stator winding. (8 Marks)
Refer principles of power systems - V.K.Metha& Rohit metha page no 529.
6. Explain the factors which cause difficulty in applying Merz-Price circulating current
principle to a power transformers.(8) (nov/dec 2011)
For protection of generators, Merz-price circulating current system is unquestionably the
most satisfactory. Though this is largely true of transformer protection, there are cases
where circulating current system offers no particular advantage over other systems or
impracticable on account of the troublesome conditions imposed by the wide variety of
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voltages, current and earthing conditions invariably associated with power transformers.
Under such circumstances, alternative protective systems are used which in many cases
are as effective as the circulating current system.
7. A three phase transformer of 220/11000 line volts is connected in star/delts. The
protective transformers on 220 v side have a current ratio of 600/5. What should be the
current transformer ratio on 11000 v side?(8) (nov/dec 2011)
Refer answer in Part B Unit III question no 10 (a).
8. Describe the differential pilot wire method of protection of feeder(16) (nov/dec 2011)
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Unit IV
1. Explain the phenomenon of arc and the various methods of arc extinction.
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2. Define the terms recovery voltage and RRRV. Explain the effect of above quantities on
the operation of circuit breaker under fault conditions.
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3. Explain successive restrikes and current chopping as applied to the interruption of
capacitive and low inductive currents.
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4. Show schematic arrangements of a breaker with a resistor connected across the contacts
and derive its Laplace equivalent.
5. A 50 Hz, 11KV, three-phase alternator with earthed neutral has a reactance of 5 ohms per
phase, and is connected to bus bar through a circuit breaker. The capacitance to earth
between the alternator and the circuit breaker is 0.02 micro farads per phase. Assuming
the resistance of the generator to be negligible calculate the following (a) Maximum
voltage across the contacts of the circuit breaker. (b) Frequency of oscillation (c) The
average rate of rise of restriking voltage up to the first peak.
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Refer principles of power systems - V.K.Metha& Rohit metha page no 484.
6. What causes the initiation of electric arc at the instant of contact separation? Which of
these is chiefly responsible for the creation of arc in circuit breakers and why?
Refer answer in Part B Unit IV question no 1.
7.
In a short circuit test on a 130 kV ,3Φ system , the breaker gave the following results :
p.f of fault 0.45 ,recovery voltage 0.95 times full line voltage , breaker current
symmetrical , and restriking transient had a natural frequency 16 kHz .determine average
RRRV .Assume fault is grounded.( 6 Marks ) In a 220 kV system , the reactance and
capacitance up to the location of circuit breaker is 8Ω and 0.025 μF respectively. A
resistance of 600 Ωis connected across the contacts of the circuit breaker. Determine the
following: Natural frequency of oscillation Damped frequency of oscillation. Critical
value of resistance which will give no transient oscillation. The value of resistance which
will give damped frequency of oscillation, one fourth of the natural frequency of
oscillation. (10 Marks )
Refer principles of power systems - V.K.Metha& Rohit metha page no 484.
8. Explain how arc is initiated and sustained when the circuit breaker contacts break(6)
(nov/dec 2011)
Refer answer in Part B Unit IV question no 1.
9. Explain in detail various methods of arc extinction in a circuit breaker.(10) (nov/dec
2011)
Refer answer in Part B Unit IV question no 1.
10. What is current chopping? Explain how can the effect of current chopping be
minimized?(8) (nov/dec 2011)
Refer answer in Part B Unit IV question no 4
11. Derive an expression for the rate of rise of restriking voltage in a circuit breaker(8)
(nov/dec 2011)
Refer answer in Part B Unit IV question no 3.
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Unit V
1. How is SF6 gas is used as an arc quenching medium? Describe the construction and
working of a SF6 circuit breaker. State its advantages and disadvantages and its
applications.
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2. Explain the role of oil in arc quenching in oil circuit breakers. Show sectional view of
working portion of a typical low –oil circuit breaker.
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3. What are the requirements of a contact material for a vacuum circuit breaker? Why
current chopping is not a serious problem with this circuit breaker?
4. Describe with sketches one form of arc control device for an oil circuit breaker and
explain its action. Why are such devices necessary in oil circuit breaker?
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5. Discuss testing of circuit breakers.
6. With neat sketch explain the principles of axial blast air circuit breaker. Enumerate the
advantages and disadvantages of air blast circuit breakers.
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7. Explain with neat diagram the working principle of air blast circuit breakers
Refer answer in Part B Unit V question no 7.
8. Explain with neat diagram the working principle of minimum oil circuit breakers.
Explain the advantages of Minimum oil circuit breakers.
Refer answer in Part B Unit V question no 2.
9. Explain with neat sketch, the construction and working of minimum oil circuit breaker.
Also give its merits and demerits. (16) (nov/dec 2011)
Refer answer in Part B Unit V question no 2.
10. Describe the various types of rating of circuit breaker (8) (nov/dec 2011)
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11. Describe the various methods of testing of circuit breaker (8) (nov/dec 2011)
Refer answer in Part B Unit V question no 6.
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