50 Most Important MCQ OF Power System | Objective Type Question OF Power system with Explanation

Ques 1. A shunt reactor at 100 MVAr is operated at 98% of its rated voltage and at 96% of its rated frequency. The reactive power absorbed by the reactor is

98 MVAr

104.02 MVAr

96.04 MVAr

100.04 MVAr

Answer.4. 100.04 MVAr

Explanation:

The reactive power Q is given as

Ques 2. A single-phase transformer with a 2-kVA rating has a 480-V primary and a 120-V secondary. Determine the primary and secondary full-load currents of the transformer.

4.17 A and 16.67 A

10 A and 13. 2 A

6 A and 16.67 A

Insufficient Data

Answer.1. 4.17 A and 16.67 A

Explanation:

Primary full-load current = VA rating / Primary Voltage

Primary full-load current = ( 2 kVA * 1 000 ) / 480 V = 4.17 A

Secondary full-load current = VA rating / Secondary Voltage

Secondary full-load current = ( 2 kVA * 1 000 ) / 120 V = 16.67 A

Ques 3. With the help of a reactive compensator, it is possible to have

Constant voltage operation only

Unity p.f. operation only

Both constant voltage and unity p.f.

Either constant voltage or unity p.f.

Answer.3.Both constant voltage and unity p.f.

Explanation:

Reactive power (VAR) compensation is defined as the management of reactive power to improve the performance of ac systems.The important features of reactive compensation are:

Load Compensation: The main objectives of load compensation are

Increase the power factor of the system

To balance the real power drawn from the system

Compensate voltage regulation

Eliminate current harmonics

Voltage Support: The main purpose of voltage support is to decrease the voltage fluctuation at a given terminal of the transmission line. Therefore the VAR compensation improves the stability of AC system by increasing the maximum active power that can be transmitted.

Methods of Reactive Power Compensation are

Shunt compensation

Series compensation

Synchronous condensers

Static VAR compensators

Static compensators

Ques 4. In case of 3 phase short circuit in a system, the power fed into the system is

Mostly active

Mostly Reactive

Both active and reactive power are equal

None of the above

Answer.2. Mostly Reactive

Explanation:

The fault current is mostly reactive since the fault bypasses the load ( the largest resistive part)

Now the entire transmission line is loaded with a high reactance because of the high frequency ( harmonics especially for unsymmetrical faults) and large transient current ( higher flux- larger the reactance) flowing.

Due to voltage drop in the transmission line, the Capacitance effect decreases.So the fault current is the current drawn by the transmission line which makes this current mostly reactive.Due to voltage drop in the transmission line, the Capacitance effect decreases.

Ques 5. The percentage resistance and percentage reactance of a 10 kVA, 400 V/200 V, 3- phase transformer are 2% and 10% respectively. If the constant losses in the machine are 1%, the maximum possible percentage efficiency of the transformer is

98.32%

96.85%

97.25%

96.12%

Answer.3. 97.25%

Explanation:

If x is the fraction of full load than transformer efficiency is given as

Ques 6. Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 kVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source.

420 Amp

520 Amp

530 Amp

480 Amp

Answer.2. 520 Amp

Explanation:

The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.

Ques 7. A star connected 440V, the 50Hz alternator has per phase synchronous reactance of 10Ω. It supplies a balanced load current of 20A, as shown in the per phase equivalent circuit diagram below. It is desirable to have zero voltage regulation. The load power factor should be

0.82

0.62

0.50

0.92

Answer.4. 0.92

Explanation:

Ques8. The synchronous motor connected to an infinite bus takes power at a lag p.f. If its excitation is increased

Terminal Voltage increase

Load angle increase

Power factor Increase

None of the above

Answer.3.Power factor Increase

Explanation:

Overexcited synchronous motor having leading power factor are widely used for improving power factor for those power system which employs a large number of an induction motor.

Ques 9. The synchronous generator connected to an infinite bus. If its excitation is increased

Supply reactive power

Absorb Reactive

Both 1 and 2

None of the above

Answer.1. Supply reactive power

Explanation:

If we increase the excitation the synchronous generator voltage V_{o} will increase and the synchronous Reactance Xs will experience a difference of potential Vx.

A current I will, therefore, circulate in the circuit I = (Vo – V)/X_{s} Where
V = system voltage

Since synchronous reactance is inductive in nature, therefore, the current lags 90^{o} behind the V_{o}

Hence when we over-excite the synchronous generator it supplies the reactive power to the system bus and the reactive power increase by increasing the DC excitation.

Ques 10. An alternator has a phase sequence of RYB for its phase voltage. In case the direction of rotation of alternator is reversed, the phase sequence will become

YRB

YBR

RYB

RBY

Answer.4. RBY

Explanation:

The phase sequence of an alternator can be reversed by interchanging any one pair of lines without causing any change in the supply sequence i.e changing RYB to RBY. Reversal of sequence results in reversal of the direction of rotation in case of an alternator.

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