Power Plant Engineering Objective Questions

1. The main objectives of load frequency control in a power system are:

1. to bring the steady state error to zero after load change.
2. to maintain the net tie-line flow.
3. to maintain voltages on all buses.
4. to economize the cost of generation.

(a) 1 and 2.
(b) 2 and 3.
(c) 3 and 4.
(d) 1, 2, 3 and 4.

2. In AGC, the voltage and frequency is controlled by

(a) excitation control.
(b) turbine control.
(c) turbine speed control and excitation control respectively.
(d) excitation control and turbine speed control respectively.

3. The main objective of load frequency controller is to apply control of:

(a) frequency alone.
(b) frequency and at the same time of real power exchange via the outgoing lines.
(c) frequency and at the same time of reactive power exchange via the outgoing lines.
(d) frequency and bus voltages.

4. Load frequency control uses

(a) proportional controllers alone.
(b) integral controllers alone.
(c) both proportional and integral controllers.
(d) either proportional or integral controllers.

5. Load frequency controls are carried out with

(a) P controllers only.
(b) I controllers only.
(c) D controllers only.
(d) PID controllers.

6. Consider the following statements regarding load frequency control:

1. Time constant of automatic load frequency control is about 15 seconds.
2. Integral control eliminates static frequency drop.
3. In tie-line load bias control, the control signal for each area is proportional to change in frequency as well as change in tie-line power.

Which of the statements given above are correct?
(a) 1, 2 and 3.
(b) 1 and 2.
(c) 1 and 3.
(d) 2 and 3.

7. An isolated 50 Hz synchronous generator is rated at 15 MW which is also the maximum continuous power limit of its prime mover. It is equipped with a speed governor with 5% droop. Initially, the generator is feeding three loads of 4 MW each at 50 Hz. One of these loads is programmed to trip permanently if the frequency falls below 48 Hz. If an additional load of 3.5 MW is connected then the frequency will settle down to

(a) 49.417 Hz
(b) 49.917 Hz
(c) 50.083 Hz
(d) 50.583 Hz [GATE E.E. 2007]

8. A power system has two synchronous generators. The Governor-turbine characteristics corresponding to the generators are

P₁ = 50 (50 – f),
P₂ = 100 (51 – f)
where f denotes the system frequency in Hz, and P₁ and P₂ are, respectively, the power outputs (in MW) of turbines 1 and 2. Assuming the generators and transmission network to be lossless, the system frequency for a total load of 400 MW is
(a) 47.5 Hz
(b) 48.0 Hz
(c) 48.5 Hz
(d) 49.0 Hz [GATE E.E. 2001]

9. For a synchronous generator connected to an infinite bus through a transmission line, how are the change of voltage (ΔV) and the change of frequency (Δf) related to the active power (P) and the reactive power (Q)?

(a) ΔV is proportional to P and Δf to Q.
(b) ΔV is proportional to Q and Δf to P.
(c) Both ΔV and Δf are proportional to P.
(d) Both ΔV and Δf are proportional to Q.

10. The voltage of a bus can be controlled by controlling the

(a) phase angle.
(b) reactive power of the bus.
(c) active power of the bus.
(d) phase angle and the reactive power.

11. The permissible variation of frequency in power system is

(a) ± 1%
(b) ± 3%
(c) ± 5%
(d) ± 10%

12. When there is a change in load in a power station having a number of generator units operating in parallel, the system frequency is controlled by

(a) adjusting the steam input to the units.
(b) adjusting the field excitation of the generators.
(a) changing the load divisions between the units.
(d) injecting reactive power at the station bus-bar.

13. Load frequency control is achieved by properly matching the individual machine’s

(a) reactive powers.
(b) generated voltages.
(c) turbine inputs.
(d) turbine and generator ratings.

14. In the load-frequency control system with free governor action, the increase in load-demand under steady conditions is met

(a) only by increased generation due to opening of steam valve.
(b) only by decrease of load-demand due to drop in system frequency.
(c) partly by increased generation and partly by decrease of load demand.
(d) partly by increase generation and partly by increased excitation

15. During load shedding

(a) system voltage is reduced.
(b) system frequency is reduced.
(c) system loads are switched off.
(d) system power factor is changed.

16. When the power system is not in a position to meet the load it will resort to

(a) power factor improvement at the generators.
(b) load shedding.
(c) efficient plant operation.
(d) penalizing high load consumers by increasing the charges.

17. Load shedding is done for

(a) reducing peak demand on the system.
(b) repairing of machines.
(c) power factor improvement.
(d) efficient operation of equipment.

18. When load shedding is resorted to, it can be concluded that

(a) plant is under repair.
(b) load on the system is more than the installed capacity.
(c) both of the above (a) and (b).
(d) none of the above.

19. Generators for base load plants are usually designed for maximum efficiency around

(a) 20% over-load.
(b) full-load.
(c) 75% full-load.

20. Generators for power plants to supply exclusively peak loads are usually designed for maximum efficiency to occur at

(a) full load.
(b) 50 – 75% full load.
(c) 25 – 50% full load.
(d) 10% full load.