# MCQs on Power Systems

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01․ Find the surge impedance of a line when inductance is 160 mH and capacitance is 1µF?
200 Ω
400 Ω
160 Ω
800 Ω

Characteristic impedance Zc = V(x)/I(x) = √(Z/Y) = √(R + jωL)/(G + jωC) For loss less line. R = G = 0 Surge impedance Zs = √(L/C) Zs = √((160 * 10^-3)/(1*10^-6)) Zs = 400 Ω

02․ A transmission line has a surge impedance of 400 Ω is connected with the cable having surge impedance of 40 Ω, a surge magnitude of 100 kV is travelling from the transmission line towards the cable. Find the transmitted voltage?
100 kV
30.6 Kv
18.18 kV
36. 36 kV

Transmitted voltage V" = V + V' Transmitted current I" = I + I' Where, V' = Reflected voltage V = Incident voltage I' = Reflected current I = Incident current Transmission or reflection coefficient of voltage Tv = V"/V Transmission or reflection coefficient of current Ti = I"/I Tv = V"/V = 2Zl/(Zl+Zs) Where, Zl = Surge impedance of load Zs = Surge impedance of line Therefore, V" = 100*10³*2*40/440 = 18.18 kV

03․ A transmission line has a surge impedance of 400 Ω is connected with the cable having surge impedance of 40 Ω, a surge magnitude of 100 kV is travelling from the transmission line towards the cable. Find the incident current?
100 A
200 A
250 A
2500 A

The voltage or current signals transmitting or travelling from source to load the load through the line are called incident waves. Incident current I = V/Zs I = 100*10³/400 I = 250 A

04․ Counter poise is used for
proper discharging of lighting surges
more surge impedance
less surge impedance
reduce losses

Counter poise is connected to the ground wire and it is having parallel paths to discharge faster rate. For proper discharging of lightning surges from the top of the tower to the ground, earth wire is used which has low resistance and this earth wire is terminated with parallel paths of copper conductors which has impedance equal to surge impedance . This is called counter poise. It is used in sub stations which are connected between one tower to the other tower.

05․ Surge impedance loading (SIL) is represented by
Vl²/Zs
Vl²*√(C/L)
Vph²/Zs
both 1 and 2

Surge impedance loading SIL = 3*Vph*Iph Where, Vph = Phase voltage Iph = Phase current Vl = Line voltage SIL = 3*(Vl/√3)²/Zs SIL = Vl²/Zs Surge impedance Zs = √(L/C) Therefore, SIL = Vl²*√(C/L)

06․ Surge impedance loading can be increased by
increasing of voltage level
increasing of capacitance value
both 1 and 2
increasing of inductance value

Surge impedance loading SIL = 3*Vph*Iph Where, Vph = Phase voltage Iph = Phase current Vl = Line voltage SIL = 3*(Vl/√3)²/Zs SIL = Vl²/Zs Surge impedance Zs = √(L/C) Therefore, SIL = Vl²*√(C/L) Surge impedance loading can be increased by, 1. Increasing of voltage level 2. Increasing of capacitance value 3. Decreasing of inductance value

07․ A transmission line is compensated with shunt capacitance Csh. If the uncompensated transmission line surge impedance is Zs, then find the new surge impedance Zn after shunt compensation?
Zs*√(1 + Kcsh)
Zs/√(1 + Kcsh)
√(1 + Kcsh)/Zs
all of the above

Surge impedance Zs = √(L/C) After shunt compensation new surge impedance Zn = √(L/C(1 + Kcsh)) Where Kcsh = Degree of shunt capacitance = Csh/C Therefore, new surge impedance Zn = Zs/√(1 + Kcsh)

08․ A transmission line is compensated with shunt capacitance Csh. After shunt compensation surge impedance loading will
increases
decreases
remains unaffected
none of the above

Surge impedance Zs = √(L/C) Surge impedance loading SIL = Vl² *√(C/L) After shunt compensation new surge impedance Zn = √(L/C(1 + Kcsh)) Where Kcsh = Degree of shunt capacitance = Csh/C Therefore, new surge impedance Zn = Zs/√(1 + Kcsh) New surge impedance loading SILn = SIL*√(1 + Kcsh) Therefore after compensation surge impedance loading will increase and surge impedance will decrease.

09․ Which of the following represents the transmission or reflection coefficient of voltage Tv? (Zl = Surge impedance of load, Zs = Surge impedance of line)
2Zs/(Zs + Zl)
2Zl/(Zs + Zl)
(Zs - Zl)/(Zs + Zl)
(Zl - Zs)/(Zs + Zl)

Transmitted voltage V" = V + V' Transmitted current I" = I + I' Where, V' = Reflected voltage V = Incident voltage I' = Reflected current I = Incident current I" = V"/Zl, I = V/Zs, I' = -V'/Zs V"/Zl = V/Zs - V'/Zs V' = V" - V Therefore, Tv = V"/V = 2Zl/(Zl + Zs)

10․ Reflection coefficient of voltage is Rv
2Zs/(Zs + Zl)
2Zl/(Zs + Zl)
(Zs - Zl)/(Zs + Zl)
(Zl - Zs)/(Zs + Zl)

Transmitted voltage V" = V + V' Transmitted current I" = I + I' Where, V' = Reflected voltage V = Incident voltage I' = Reflected current I = Incident current Transmission or reflection coefficient of voltage Tv = V"/V Transmission or reflection coefficient of current Ti = I"/I I" = V"/Zl, I = V/Zs, I' = -V'/Zs V"/Zl = V/Zs - V'/Zs V' = V" - V Therefore, Tv = V"/V = 2Zl/(Zl + Zs) Tv = V"/V = 2Zl/(Zl+Zs) Reflection coefficient of voltage is Rv = Tv - 1 Rv = (Zl - Zs)/(Zs + Zl)

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