POWER SYSTEMS MCQS SET 08

POWER SYSTEMS MCQS SET 08

PER UNIT QUANTITIES

1. 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
Answer: a

2. 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
Answer: a

3. 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
Answer: a

4. 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
Answer: a
 

5․ 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 reflected voltage?
a. 81.82 kV           b. 18.18 kV         c. -81.82 kV            d. -18.18 kV
Explanation:
Transmitted voltage V" = V + V'
Transmitted current I" = I + I'
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)
Reflection coefficient of voltage is Rv= V'/V = Tv - 1
Rv = V'/V = (Zl - Zs)/(Zs + Zl)
 

6․ Surge impedance loading of a line is increased by which of the following compensation methods?
a. shunt capacitor         b. series capacitor         c. shunt reactor
d. both 1 and 2
Explanation:
Surge impedance Zs = √(L/C)
Surge impedance loading SIL = Vl² *√(C/L)
Shunt capacitor compensation:
After shunt capacitor 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)
Series capacitor compensation:
New surge impedance Zn = Zs*√(1 - Kse)
New surge impedance loading SILn = SIL/√(1 - Kse)
Shunt reactor compensation:
New surge impedance Zn = Zs/√(1 - Kcsh)
New surge impedance loading SILn = SIL*√(1 - Kcsh)
Therefore, surge impedance loading of a line increases by shunt capacitor compensation and series capacitor compensation and surge impedance loading decreases with shunt reactor compensation.
 

7․ A transmission line has surge impedance of 400 Ω, operating at Vs = Vr = 400 kV line is compensated with a shunt capacitance of 30%. Find the surge impedance loading with compensation?
a. 400 MW          b. 478 MW        c. 456 MW          d. 386 MW
Explanation:
Surge impedance loading with out compensation SIL = Vl²/Zs = (400*10³)²/400 = 400 MW
Surge impedance loading with shunt capacitance compensation
Shunt capacitor compensation:
After shunt capacitor 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) SILn = 400*√(1+0.3) =456 MW
 

9․ If the load is short circuited, then reflection coefficient of voltage is
a. -1          b. 1           c. 0               d. -2
Explanation:
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)
Reflection coefficient of voltage is Rv = Tv - 1
Rv = (Zl - Zs)/(Zs + Zl)
When load is short circuited, V" = 0
Therefore, Tv = 0 and Rv = -1
 

10․ The Y bus matrix of a 100 bus interconnected system is 90% sparse. Then find the number of non zero elements in the Y bus matrix?
a. 9000         b.  90            c. 1000            d. 10
Explanation:
Sparsity of Y bus matrix = Number of zero elements/ Total number of elements
Total number of elements for 100 bus = 100*100 = 10000
Number of zero elements = 0.9 * 10000 =9000
Therefore, number of non zero elements = 1000