# Power Systems Questions and Answers – Per Unit (PU) System

This set of Power Systems Multiple Choice Questions & Answers (MCQs) focuses on “Per Unit (PU) System”.

1. A power system network is connected as shown in the figure. Sd1=15+j5 pu
Sd2=25+j15 pu
Zcable = j0.05pu
|V1|=|V2|=1 pu.
The torue angle for the system will be __________
a) 14.4
b) 22.1
c) 16.2
d) 18.2

2. A single phase distributor of 1 km long has resistance and reactance per conductor of 0.1Ω and 0.15 Ωrespectively. If the far end voltage Vb=200V and current is at 100A at 0.8 lag. At the midpoint a current of 100A is tapped at a pf of 0.6 pf with ref to voltage Vm at mid point. The voltage magnitude at M is ________
a) 218V
b) 200V
c) 232V
d) 220V

Explanation: Drop in MB= (100ʟ-36.67)(0.1+j0.15)= 18.027ʟ19.44
VM= 200+18.027 = 218.027 V.

3. A single phase motor is connected to 400V, 50Hz supply. The motor draws a current of 31.7A at a power factor 0.7 lag. The capacitance required in parallel with motor to raise the power factor of 0.9 lag (in micro farads) is __________
a) 94.62
b) 282.81
c) 108.24
d) 46.87

Explanation: Active power drawn by the motor=VIcosФ = 400*31.7*0.7 = 8876 W
Reactive power = VIsinФ=400*31.7*sin(45.57.29) = 9055.3 VAR
New power factor=cosθ2 = 0.9
θ2=cos-1(0.9)
Q2=8876*tan(25.84) = 4298.855 VAR
Change in reactive power=9055.3-4298.855 = 4756.4 VAR
Qc = V2/Xc = V2*2πfC
C=4756.4/(4002*2π*50) = 94.62μF.

4. A single phase motor is connected to 400V, 50Hz supply. The motor draws a current of 31.7A at a power factor 0.7 lag. The additional reactive power (in VAR) to be supplied by the capacitor bank will be ___________
a) 4756
b) 4873
c) 4299
d) 9055.3

Explanation: Active power drawn by the motor=VIcosФ = 400*31.7*0.7 = 8876 W
Reactive power = VIsinФ=400*31.7*sin(45.57.29) = 9055.3 VAR
New power factor=cosθ2 = 0.9
θ2=cos-1(0.9)
Q2=8876*tan(25.84) = 4298.855 VAR
Change in reactive power=9055.3-4298.855 = 4756.4 VAR.

5. A 275 kV TL has following line constants A=0.85ʟ5o, B=200ʟ75o. The active power received if the voltage to be maintained is 275kV will be __________
a) 117.63
b) 220
c) 120
d) 115.25

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6. A 275 kV TL has following line constants A=0.85ʟ5o, B=200ʟ75o. The active power angle such that the voltage to be maintained at the other end will be 275 kV ____________
a) 22
b) 16
c) 18
d) 24

7. A power system has a maximum load of 15 MW. Annual load factor is 50%. The reserve capacity of plant is _____ if the plant capacity factor is 40%.
a) 3.75 MW
b) 4.75 MW
c) 18.75 MW
d) 5.75 MW

Explanation: LF = (Average Demand)/(Maximum Demand)=0.5
Plant capacity factor =(Average Load)/(Plant Capacity)= 0.5/0.4
Plant capacity= (0.5/0.4)*15 = 18.75 MW
Reserve Capacity = 18.75-15 = 3.75 MW.

8. A 100 MVA synchronous generator operates on full load at a frequency of 50 Hz. The load is suddenly reduced to 50 MW. Due to time lag in governor system, the steam valve begins to close after 0.4s. The change in the frequency is ________(H=5 kW-s/KVA).
a) 1
b) 0.5
c) -1.5
d) 0.8

9. A 50 Hz four pole turbo-generator rated 100 MVA, 11 kV has an inertia constant of 8 MJ/MVA. If the mechanical input is suddenly raised to 80 MW for an electrical load of 50MW, then the rotor acceleration is ____________
a) 337.5
b) 3.375
c) 457.5
d) 4.57

10. A single phase TL has copper conductor of 0.775 cm2 cross section through which 200 kW at UPF at 330 V is to be maintained. If the efficiency of transmission line is 90%, then the minimum length of TL is ___________(in km and take specific resistance to be 1.785 μΩ/cm).
a) 13.6 km
b) 14 km
c) 136 km
d) 16.4 km

Explanation: Pr=200 kW, efficiency=0.9
Ps= 200/0.9=222.22 kW
Losses=22.22 kW
Current, I=200000/3300 = 60.60 A
Line losses=2I2R (for a 2 wire line)
R=22.22/(2*60.602)=3.02 Ω
R=ρl/a
Length, l = (3.025*0.775)/(1.785*10-6) = 13.6 km.

11. A three phase transformer has a nameplate rating of 30 MVA, 230Y/69Y kV with a leakage -reactance of 10% and the transformer connection is wye-wye. Choosing a base of 30MVA and 230 kV on high voltage side, the reactance of transformer in per units is __________
a) 0.1
b) 0.3
c) 0.03
d) 1.5

Explanation: The pu value of a transformer does not change.

12. A three phase transformer has a nameplate rating of 30 MVA, 230Y/69Y kV with a leakage -reactance of 10% and the transformer connection is wye-wye. Choosing a base of 30MVA and 230 kV on high voltage side, the high voltage side impedance ____________
a) 1763.3 Ω
b) 158.7 Ω
c) 15.87 Ω
d) 176.3 Ω

Explanation: On the high voltage side, Zb=kVb2/MVAb(3-ph) = 2302/30 = 1763.33 Ω.

13. A three phase transformer has a nameplate rating of 30 MVA, 230Y/69Y kV with a leakage -reactance of 10% and the transformer connection is wye-wye. Choosing a base of 30MVA and 230 kV on high voltage side, the low voltage side impedance is ___________
a) 158.7 Ω
b) 176.3 Ω
c) 1763.3 Ω
d) 15.87 Ω

Explanation: On the low voltage side, Zb=kVb2/MVAb(3-ph) =692/30 = 158.7 Ω.

14. A three phase transformer has a nameplate rating of 30 MVA, 230Y/69Y kV with a leakage -reactance of 10% and the transformer connection is wye-wye. Choosing a base of 30MVA and 230 kV on high voltage side, the transformer reactance referred to the high voltage side will be _________(in ohms).
a) 176.33 Ω
b) 17.67 Ω
c) 158.7 Ω
d) 15.87 Ω

Explanation: Zb=kVb2/MVAb(3-ph) = 2302/30 = 1763.33 Ω
X Ω = Xpu*Xb(HV) = 0.1*1763.33 = 176.33 Ω.

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