BEE Power Systems and Machines 3 β Questions and Answers
Question 1: A 50 MVA, 11 kV synchronous generator has a subtransient reactance of 0.15 pu. What is the subtransient fault current (in pu) for a three-phase bolted fault at its terminals?
- 6.67 pu (Correct answer)
- 0.15 pu
- 15 pu
- 3.33 pu
Correct answer: 6.67 pu
Subtransient fault current = 1.0 / X''d = 1.0 / 0.15 = 6.67 pu.
Question 2: Which of the following best describes the 'equal area criterion' in power system stability?
- A graphical method to assess transient stability by comparing accelerating and decelerating energy areas on a P-Ξ΄ curve (Correct answer)
- A method for sizing protective relays
- A criterion for load flow convergence
- A technique for balancing three-phase loads
Correct answer: A graphical method to assess transient stability by comparing accelerating and decelerating energy areas on a P-Ξ΄ curve
The equal area criterion states that a system is transiently stable if the decelerating area on the P-Ξ΄ curve equals or exceeds the accelerating area.
Question 3: What is the slip of a three-phase induction motor running at 1440 RPM with a synchronous speed of 1500 RPM?
- 0.04 (Correct answer)
- 0.96
- 0.06
- 0.40
Correct answer: 0.04
Slip s = (Ns β Nr) / Ns = (1500 β 1440) / 1500 = 60/1500 = 0.04.
Question 4: In per-unit system analysis, which quantity remains the same on both sides of a transformer?
- Per-unit impedance (on the same MVA base) (Correct answer)
- Actual voltage in kV
- Actual current in amperes
- Turns ratio
Correct answer: Per-unit impedance (on the same MVA base)
When converting to a common MVA and kV base, per-unit impedances are the same on both sides, simplifying transformer analysis.
Question 5: A DC series motor is preferred for traction applications because it provides:
- High starting torque at low speed (Correct answer)
- Constant speed under variable loads
- Low starting current
- Constant torque at all speeds
Correct answer: High starting torque at low speed
In a series motor, torque β IaΒ², giving very high torque at startup when current is high, ideal for traction.
Question 6: The Ferranti effect in a long transmission line refers to:
- The receiving-end voltage exceeding the sending-end voltage under no-load or light-load conditions (Correct answer)
- Voltage drop due to line resistance
- Increase in line current at resonance
- Harmonic distortion at the receiving end
Correct answer: The receiving-end voltage exceeding the sending-end voltage under no-load or light-load conditions
Under no-load, the capacitive charging current causes a voltage rise along the line, making receiving-end voltage higher than sending-end.
Question 7: Which winding configuration in a three-phase transformer eliminates the triple-harmonic (zero-sequence) currents from flowing into the external system?
- Delta (Ξ) winding (Correct answer)
- Grounded star (YG) winding
- Ungrounded star (Y) winding
- Zig-zag winding
Correct answer: Delta (Ξ) winding
A delta winding provides a closed path for triple-harmonic (zero-sequence) currents to circulate internally, preventing them from entering the network.
BEE Power Systems and Machines 3
A 50 MVA, 11 kV synchronous generator has a subtransient reactance of 0.15 pu.
What is the subtransient fault current (in pu) for a three-phase bolted fault at its terminals?