BEE Power Systems Engineering 2 — Questions and Answers
Question 1: In a power system, the per-unit impedance of a transformer is 0.05 pu on a 100 MVA base. If the base is changed to 200 MVA, what is the new per-unit impedance?
- 0.025 pu
- 0.05 pu
- 0.10 pu (Correct answer)
- 0.20 pu
Correct answer: 0.10 pu
Per-unit impedance scales linearly with the MVA base: Z_new = Z_old × (MVA_new / MVA_old) = 0.05 × (200/100) = 0.10 pu.
Question 2: Which relay type is most commonly used for primary protection of transmission lines because its operating time is inversely proportional to fault current magnitude?
- Differential relay
- Distance relay
- Inverse time overcurrent relay (Correct answer)
- Buchholz relay
Correct answer: Inverse time overcurrent relay
Inverse time overcurrent relays operate faster for larger fault currents, making them ideal for graded transmission line protection.
Question 3: A balanced three-phase system has line voltage of 415 V. What is the phase voltage?
- 415 V
- 239.6 V (Correct answer)
- 718 V
- 207.5 V
Correct answer: 239.6 V
Phase voltage = Line voltage / √3 = 415 / 1.732 ≈ 239.6 V in a balanced Y-connected system.
Question 4: What is the primary function of a shunt capacitor bank installed on a distribution feeder?
- Increase real power delivered
- Reduce system frequency deviation
- Improve power factor and reduce reactive power demand (Correct answer)
- Limit fault current magnitude
Correct answer: Improve power factor and reduce reactive power demand
Shunt capacitor banks supply reactive (VAR) power locally, improving power factor and reducing reactive current flow from the source.
Question 5: In the symmetrical component method, the positive-sequence, negative-sequence, and zero-sequence impedances of a generator are Z1, Z2, and Z0. For a single line-to-ground fault, the fault current is proportional to:
- 1 / Z1
- 1 / (Z1 + Z2)
- 3 / (Z1 + Z2 + Z0) (Correct answer)
- 1 / Z0
Correct answer: 3 / (Z1 + Z2 + Z0)
For a single line-to-ground fault, Ia = 3Ea / (Z1 + Z2 + Z0), so fault current is 3/(Z1+Z2+Z0) times the prefault voltage.
Question 6: What does the term 'load flow' or 'power flow' analysis determine in a power system?
- Transient stability margins after a fault
- Steady-state voltage magnitudes and power flows throughout the network (Correct answer)
- Short-circuit current levels at each bus
- Frequency response during a sudden load pickup
Correct answer: Steady-state voltage magnitudes and power flows throughout the network
Load flow analysis solves for the steady-state voltage magnitude, angle, and real/reactive power flows at every bus under specified loading conditions.
Question 7: Which of the following describes the Gauss-Seidel method as applied to power flow solutions?
- A Newton-Raphson variant using sparse Jacobian matrices
- An iterative method updating bus voltages one at a time using the most recent values (Correct answer)
- A direct solution method for linear power flow equations
- A decoupled method separating P-δ and Q-V equations
Correct answer: An iterative method updating bus voltages one at a time using the most recent values
The Gauss-Seidel method updates each bus voltage sequentially using already-updated values from the current iteration, iterating until convergence.
BEE Power Systems Engineering 2
In a power system, the per-unit impedance of a transformer is 0.05 pu on a 100 MVA base.
If the base is changed to 200 MVA, what is the new per-unit impedance?