POWER SYSTEMS MCQS SET 09
PER UNIT QUANTITIES
1. At slack bus, which one of the following combinations of variables is not specified?
a. P and V c. P and Q c. V and δ d. Q and V
Explanation:
Load bus:
Known quantities - P and Q
Unknown quantities - V and δ
Generator bus:
Known quantities - P and V
Unknown quantities - Q and δ
Slack or reference bus:
Known quantities - V and δ
Unknown quantities - P and Q
a. P and V c. P and Q c. V and δ d. Q and V
Explanation:
Load bus:
Known quantities - P and Q
Unknown quantities - V and δ
Generator bus:
Known quantities - P and V
Unknown quantities - Q and δ
Slack or reference bus:
Known quantities - V and δ
Unknown quantities - P and Q
2․ Which of the following statements are true?
a. Bus admittance matrix is a sparse matrix
b. Bus impedance matrix is a full matrix
c. both 1 and 2
d. nether 1 nor 2
Explanation:
Y bus matrix is a sparse matrix, containing more number of zero elements. So that faster calculation is possible. The Y bus matrix is used for the load flow studies.
Z bus algorithm or matrix is used for the fault analysis. Inverse of Y bus matrix gives the Z bus matrix, But Z matrix is a full matrix even though Y bus is a sparse matrix. More time is required for inverse of Z bus matrix if the size of the matrix is more than three. But a practical system network has large number of buses and hence Z bus matrix is not uses for the load flow analysis, Y bus is preferred.
a. Bus admittance matrix is a sparse matrix
b. Bus impedance matrix is a full matrix
c. both 1 and 2
d. nether 1 nor 2
Explanation:
Y bus matrix is a sparse matrix, containing more number of zero elements. So that faster calculation is possible. The Y bus matrix is used for the load flow studies.
Z bus algorithm or matrix is used for the fault analysis. Inverse of Y bus matrix gives the Z bus matrix, But Z matrix is a full matrix even though Y bus is a sparse matrix. More time is required for inverse of Z bus matrix if the size of the matrix is more than three. But a practical system network has large number of buses and hence Z bus matrix is not uses for the load flow analysis, Y bus is preferred.
3․ Which of the following statements is/are true? The elements of each row of a Y bus matrix for load flow studies in power system add up to zero
a. always b. if the shunt admittances at the buses are ignored
c. if the mutual couplings between transmission lines are absent
d. if both 2 and 3 are satisfied
Explanation:
Properties of Y bus matrix:
1. Y bus is a square matrix.
2. For n bus power system size of Y bus matrix is n*n
3. Value of diagonal element corresponding to bus i, then Yii = Sum of the admittances connected to bus i.
4. The value of off diagonal elements Yij = Yji , which is connected between bus i and bus j and which is represented with negative sign.
5. Sum of the elements of row i equal to shunt admittances connected to bus i. If this summation is zero, indicates there is no shunt admittance and mutual coupling between the transmission lines.
a. always b. if the shunt admittances at the buses are ignored
c. if the mutual couplings between transmission lines are absent
d. if both 2 and 3 are satisfied
Explanation:
Properties of Y bus matrix:
1. Y bus is a square matrix.
2. For n bus power system size of Y bus matrix is n*n
3. Value of diagonal element corresponding to bus i, then Yii = Sum of the admittances connected to bus i.
4. The value of off diagonal elements Yij = Yji , which is connected between bus i and bus j and which is represented with negative sign.
5. Sum of the elements of row i equal to shunt admittances connected to bus i. If this summation is zero, indicates there is no shunt admittance and mutual coupling between the transmission lines.
4․ In a Gauss Seidel load flow method, the number of iterations may be reduced if the correction in voltage at each bus is multiplied by
a. Gauss constant b. Acceleration factor
c. Blocking factor d. Lagrange multiplier
Explanation:
Advantages of gauss siedel method:
1. It is a simple algebraical equation, so that calculation time for each iteration is less.
2. More suitable for small size networks.
Disadvantages of gauss siedel method:
1. More number of iterations are required, so that it has slow convergence.
2. Initial approximate guessing value is required for convergence.
3. It required accelerating factor for convergence.
4. The choice of slack bus affects the convergence.
5. It is not applicable for the large power system networks. Accelerating factor is used for reducing number of iterations using Gauss Seidel method. The value of accelerating factor is around 1.6 to 1.8 .
a. Gauss constant b. Acceleration factor
c. Blocking factor d. Lagrange multiplier
Explanation:
Advantages of gauss siedel method:
1. It is a simple algebraical equation, so that calculation time for each iteration is less.
2. More suitable for small size networks.
Disadvantages of gauss siedel method:
1. More number of iterations are required, so that it has slow convergence.
2. Initial approximate guessing value is required for convergence.
3. It required accelerating factor for convergence.
4. The choice of slack bus affects the convergence.
5. It is not applicable for the large power system networks. Accelerating factor is used for reducing number of iterations using Gauss Seidel method. The value of accelerating factor is around 1.6 to 1.8 .
5. Pin insulators are used up to
a. 11 kV b. 33 kV c. 120 kV d. any voltage level
Explanation:
Overhead line insulators:
1. Pin insulators used up to 33 kV.
2. Suspension type insulators all voltages 33 kV to 765 kV.
3. Strain insulators are used in river/ road/ railway crossings and dead end tower to reduce sag.
Pin type insulators are used only up to 33 kV. Beyond this voltage more insulation material is required, which increases cost of the insulation. At higher voltage suspension or string insulators are used.
a. 11 kV b. 33 kV c. 120 kV d. any voltage level
Explanation:
Overhead line insulators:
1. Pin insulators used up to 33 kV.
2. Suspension type insulators all voltages 33 kV to 765 kV.
3. Strain insulators are used in river/ road/ railway crossings and dead end tower to reduce sag.
Pin type insulators are used only up to 33 kV. Beyond this voltage more insulation material is required, which increases cost of the insulation. At higher voltage suspension or string insulators are used.
6․ Find the number of insulator discs required in string insulator, if operating voltage is 220 kV and maximum voltage of each disc is 11 kV?
a. 20 b. 10 c. 13 d. 21
Explanation:
Number of insulator discs n = ( Operating voltage/phase)/ Maximum voltage of each disc
Operating voltage/phase = 220*10³/√3 = 127 kV
Maximum voltage of each disc = 11 kV
Number of insulator discs n = 127/11 =11.547 ≈ 12
For safety operation one extra disc is added to the string.
Therefore, total number of insulator discs = 13
a. 20 b. 10 c. 13 d. 21
Explanation:
Number of insulator discs n = ( Operating voltage/phase)/ Maximum voltage of each disc
Operating voltage/phase = 220*10³/√3 = 127 kV
Maximum voltage of each disc = 11 kV
Number of insulator discs n = 127/11 =11.547 ≈ 12
For safety operation one extra disc is added to the string.
Therefore, total number of insulator discs = 13
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