EE MCQ

MGVCL Exam Paper (30-07-2021 Shift 1)
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Two-wattmeter method is used to measure the power taken by a 3-phase induction motor on no load. The wattmeter readings are 375 W and –50 W. Calculate the reactive power taken by the load.

Reactive power taken, Q = √3*(W1 - W2)

Q = √3*(375 + 50)

Q = √3*425 kVAR

Related Question

MGVCL Exam Paper (30-07-2021 Shift 1)
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The expression for voltage regulation of a short transmission line for lagging power factor is given by: ___, where, 'I' is the line current, 'V_r' is the receiving end voltage, 'R' is the line resistance and 'X' is the line reactance.

For lagging power factor = (IR*cosφ + Ixsinφ)/Vr

For leading power factor = (IR*cosφ - Ixsinφ)/Vr

Zero voltage regulation occurs on leading power factor only.

MGVCL Exam Paper (30-07-2021 Shift 1)
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A single phase motor connected to 400 V, 50 Hz supply takes 30 A at a power factor of 0.7 lagging. Calculate the capacitance required in parallel with the motor to raise the power factor to 0.9 lagging.

For single phase induction motor,

P = VIcosφ

P = 400*30*0.7

P = 8400 W

Q1 = P*tan(φ) = 8400*1.020

Q = 8568.82 VAR

Now power factor change to 0.9

So, φ' = 25.84 degree

Q = 8400*tan(25.84)

Q = 4067.95

Qnet = Q - Q'

Qnet = 8568.82 - 4067.95

Qnet = 4500.8

C = V²/(2ᴨf)

C = (400*400)/(2ᴨ*50)

C = 8.95*10⁻⁵

C = 89.5 μF

MGVCL Exam Paper (30-07-2021 Shift 1)
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Determine the ohmic value of the current limiting reactor per phase external to a 30 MVA, 11 kV, 50 Hz, three phase synchronous generator which can limit the current on short circuit of 5 times the full load current. The reactance of the synchronous generator is 0.05 pu.

The ratio of full load current to short circuit current = 1/5

Xsc = j/(1/5)

External reactance required = j*((1/5) - 0.05))

= j*0.15 pu

Full load current = (30*1000)/(√3*11)

= 1574.6 A

Per unit reactance = j*0.15 = (I*Xr)/V

j*0.15 = (1574.6*Xb)/((11/√3)*1000))

= 0.60 ohm

MGVCL Exam Paper (30-07-2021 Shift 1)
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Stability of power system can be improved by

1. Reducing voltage of transmission

2. Using parallel transmission lines

3. Using series compensators

Which of the above statements are correct?

1. Reducing voltage of transmission

2. Using parallel transmission lines

3. Using series compensators

Which of the above statements are correct?

The methods often employed in practice to improve system stability are:

1. Increasing System Voltage

2. Reduction in Transfer Reactance

3. Using High Speed Circuit Breaker

4. Automatic Reclosing

5. Transient Stability

6. Turbine Fast Valving

7. Application of Braking Resistors

8. Single Pole Switching: