MCQs on Power Systems

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01․ If L and C are 4 mH and 0.0001 μF respectively a current chop of magnitude 50 Amp would induced a voltage
200 kV.
100 kV.
50 kV.
400 kV.


02․ If the percentage reactance of an element is 20 % and the full load current is 50 Amp, the short circuit current will be
250 Amp.
300 Amp.
200 Amp.
350 Amp.


03․ A 25 MVA, 11 kV alternator is supplying full load at a point of 0.75 lagging. Determine the percentage increase in earning capacity when it is operated at improved power factor of 0.96 lagging.
14 %.
28 %.
56 %.
None of above

Power supplied of any power factor = Rating x operating of increase in earning capacity = power supplied at improved power factor - power supplied at given power factor.

04․ In thermal power plants the size of coal after crushing
300 mm.
200 - 205 mm.
40 mm.
20 - 25 mm.

Any material is ground into small parts its surface area is increases make it to effectively utilize.

05․ Efficiency of thermal power plant can be taken as
product of efficiencies of all stages.
turbine efficiency.
efficiency of thermal cycle.
generator efficiency.

Efficiency of thermal power plant is given by product of 1. Boiler efficiency ( about 0.75 to 0.9) 2. Efficiency of thermal cycle ( about 0.35 to 0.5) 3. Internal efficiency of turbine (0.85 to 0.94) 4. Mechanical efficiency of turbine ( about 0.99 to 0.995) 5. Efficiency of generator ( about 0.98 to 0.985).

06․ Where do compounding done in thermal power plants
Coal handling plant.
Ash handling plant.

There will be several problems if steam energy is converted in one step i.e., if in a single row of nozzle blade combination all the heat is dropped the steam velocity becomes high and even supersonic. Velocity of steam is proportional to square root of heat dropped in nozzle. This makes rotor speed to a value of 30,000 rpm which is too high for practical use. So it is reduced to 3000 rpm by compounding i.e., connecting turbines in 3 stages - high pressure, intermediate pressure and low pressure.

07․ The only place where slip ring induction motors used in thermal power plants is
coal handling plant.
lubrication oil pumps.
none of above.

In coal handling plants the slip ring IM motors are used for tippling purpose i.e., to unload the coal. Because, to lift the wagon of weight of metric tons, the starting torque should be very high which can be maintained by slip ring IM.

08․ The amount of ash content in Indian coal
60 %.
45 %.
35 %.
20 %.

By standards the coal is graded by following statistics -
Moisture contained17.5 %22 %20.5 %
Ash23 %35 %30 %

09․ To improve the efficiency of thermal power plant ________________ is placed between boiler and turbine.
ash handling point.
air preheater.

Superheater is placed between boiler and turbine. A superheater is a device which raises the temperature of the steam much above the boiling point of water. In superheater, the heat is taken from flue gases and here radiant type of heat transfer method is used. It improves efficiency in thermal power plant.

10․ Which of the following is not base load power plant
Pump storage.
Run water.

Base load power plant is operating as low operating cost, capacity of working continuously. Pump storage plant can only supply power at peak loads not always.