MCQs on Power Systems

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01․ A 3 - Φ, 33 kV oil circuit breaker is rated 1200 A, 2000 MVA, 3 s. The symmetrical breaking current is
1200 A.
3600 A.
35 kA.
104.8 kA.

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02․ What are the tests performed for oil-filled and gas-filled cables ?
Load cycle tests.
Thermal stability test.
Impulse tests.
All of these.

Other tests include dielectric thermal resistance tests, cold power factor test, mechanical test and binding tests.
03․ What are the methods of laying underground cables?
Direct laying.
Draw - in - system / duct - pipe laying.
Both A and B.
None of these.

There is another method i.e solid system / through laying.
04․ A 3 core cable gives on test a capacitance of 3.7 µF between two cores. Find the line charging current of the cable when it is connected to 11 kV, 50 Hz system?
14.76 A.
1.476 A.
14.7 mA.
1 A.

C2 = 3.7 µF and capacitance between core to neutral CN = 2 X 3.7 = 7.4 µF. \
05․ String efficiency depends upon
voltage rating of whole string.
no. of SCR in the string.
voltage rating of one SCR.
all of these.

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06․ A thermal protection switch is used to provide protection against
over voltage.
short circuit.
temperature.
overload.

In a thermal protection switch, the metallic link (or) thermal link is connected with the load in series. When the current flows through this thermal link, it gets hot and deformed (Bend). Once it reaches the thermal cutoff level due to the heavy over load, then it disconnects the load from the power supply due to bending effect. So, the load is protected against over load.
07․ In a 400 KV network, 360 KV is recorded at a 400 KV bus. The reactive power absorbed by shunt rated for 50 MVAr, 400 KV connected at the bus is
40.5 MVAr.
45 MVAr.
55.56 MVAr.
61.73 MVAr.

The capacity of reactor = 50 MVAr when it is operated at 400 KV. ∴ Impedance of reactor = V2/MVAr = (400)2/500 = 3200 Ω Recorded KV = 360 KV ∴ Reactive power absorbed = V2 2 = (360 X 103)2/500 = 40.5 MVAr.
08․ When a transformer is loaded, the secondary terminal voltage gains. That means power factor is
lagging.
zero.
unity.
leading.

Approximate relation of no load and full load voltages in a transformer is
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Where E2, V2, R, X and cosθ  are no load voltage, full load voltage, secondary resistance of transformer, secondary leakage reactance of transformer and power factor respectively. If power factor is leading, θ  is negative which means { Rcos( - θ ) + Xsin( - θ ) } or, { Rcosθ  - Xsinθ  } may be less than zero if Rcosθ  < Xsinθ . In that case full load secondary voltage V2 may be greater than no load secondary voltage E2.
09․ Pelton wheel turbine is a
high head turbine.
medium head turbine.
low head turbine.
none of above.

In hydro power plants, Pelton wheel turbines are employed at high head. For Pelton wheel turbine the head is > 300 m. The Pelton wheel is a water impulse turbine. The Pelton wheel extracts energy from the impulse of moving water as opposed to its weight like traditional, overshot water level. For high speed, maximum power and efficiency, the turbine system is deigned such that the water-jet velocity is twice the velocity of the bucket. Pelton wheels are the preferred turbine for hydro power. When the available water source has relatively high hydraulic head at low flow rates, where Pelton wheel is most efficient.
10․ Which of following is a medium and low head turbines
Pelton and Kaplan turbines.
Kaplan and Francis turbines.
Francis and Kaplan turbines.
None of above.

Francis turbines is employed at where head is 60 - 250 m. Kaplan turbines are employed at where head is < 60 m.
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