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

  1. 98 MVAr
  2. 104.02 MVAr
  3. 96.04 MVAr
  4. 100.04 MVAr
Answer.4. 100.04 MVAr

Explanation:

The reactive power Q is given as

 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

 

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.

  1. 4.17 A and 16.67 A
  2. 10 A and 13. 2 A
  3. 6 A and 16.67 A
  4. 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

  1. Constant voltage operation only
  2. Unity p.f. operation only
  3. Both constant voltage and unity p.f.
  4. 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:
    1. Load Compensation: The main objectives of load compensation are
      1. Increase the power factor of the system
      2. To balance the real power drawn from the system
      3.  Compensate voltage regulation
      4. Eliminate current harmonics
    2. 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
    1. Shunt compensation
    2. Series compensation
    3. Synchronous condensers
    4. Static VAR compensators
    5. Static compensators

 

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

  1. Mostly active
  2. Mostly Reactive
  3. Both active and reactive power are equal
  4. 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

  1. 98.32%
  2. 96.85%
  3. 97.25%
  4. 96.12%
Answer.3. 97.25%

Explanation:

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

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

 

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.

  1. 420 Amp
  2. 520 Amp
  3. 530 Amp
  4. 480 Amp
Answer.2. 520 Amp

Explanation:

 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.

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
A star connected 440V, 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

  1. 0.82
  2. 0.62
  3. 0.50
  4. 0.92
Answer.4. 0.92

Explanation:

A star connected 440V, 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

 

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

  1. Terminal Voltage increase
  2. Load angle increase
  3. Power factor Increase
  4. 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

  1. Supply reactive power
  2. Absorb Reactive
  3. Both 1 and 2
  4. None of the above

Answer.1. Supply reactive power

Explanation:

  • If we increase the excitation the synchronous generator voltage Vo 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)/Xs Where
    V = system voltage
  • Since synchronous reactance is inductive in nature, therefore, the current lags 90o behind the Vo
  • 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

  1. YRB
  2. YBR
  3. RYB
  4. 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.