BEE Power Systems Engineering 4 — Questions and Answers
Question 1: Which bus type in a power flow study has both voltage magnitude and angle specified as the slack (reference) bus?
- PQ bus (load bus)
- PV bus (generator bus)
- Vδ bus (swing bus) (Correct answer)
- Shunt bus
Correct answer: Vδ bus (swing bus)
The slack (swing or Vδ) bus has fixed voltage magnitude and angle (usually 1.0∠0°) and absorbs the system's power imbalance.
Question 2: What is the main advantage of high-voltage DC (HVDC) transmission over AC transmission for very long distances?
- Easier transformation of voltage levels
- No reactive power losses and no skin effect over long distances (Correct answer)
- Lower cost of converter stations compared to AC switchgear
- Simpler protection using conventional circuit breakers
Correct answer: No reactive power losses and no skin effect over long distances
HVDC has no reactive power losses, no charging current, no skin effect, and no stability limit over distance, making it efficient for bulk long-distance transmission.
Question 3: An underground cable has significantly higher capacitance per unit length than an overhead line. What operational problem does this create for long AC cables?
- Higher I²R losses due to increased resistance
- Excessive charging current that can exceed the cable's thermal rating (Correct answer)
- Reduced short-circuit level at the receiving end
- Increased harmonic distortion of the voltage waveform
Correct answer: Excessive charging current that can exceed the cable's thermal rating
The high capacitance generates large charging currents that can occupy most of the cable's thermal capacity before any load current is even delivered.
Question 4: In a power system protection scheme, what is the purpose of a 'zone of protection'?
- To define the geographic boundary of a utility's service territory
- To identify the section of the system that a specific relay is responsible for isolating during faults (Correct answer)
- To specify the voltage level at which a relay operates
- To limit the number of circuit breakers that must operate for a given fault
Correct answer: To identify the section of the system that a specific relay is responsible for isolating during faults
A protection zone defines the electrical boundary within which a relay detects faults and initiates breaker tripping to isolate only the faulted section.
Question 5: What is the purpose of a power system stabilizer (PSS) installed on a synchronous generator excitation system?
- Maintain terminal voltage at the scheduled setpoint
- Damp low-frequency electromechanical oscillations to improve small-signal stability (Correct answer)
- Limit fault current contribution from the generator
- Control real power output according to the dispatch schedule
Correct answer: Damp low-frequency electromechanical oscillations to improve small-signal stability
A PSS adds a supplementary signal to the voltage regulator to provide damping torque, suppressing rotor oscillations (typically 0.1–2 Hz) that can destabilize interconnected systems.
Question 6: A distribution transformer is rated 25 kVA, 11 kV / 240 V. What is the full-load secondary current?
- 2.27 A
- 104.2 A (Correct answer)
- 22.7 A
- 1042 A
Correct answer: 104.2 A
I_secondary = kVA / V_secondary = 25,000 / 240 ≈ 104.2 A.
Question 7: What does the term 'spinning reserve' mean in power system operation?
- Generating capacity from units already online and synchronized, available to respond immediately to load increases (Correct answer)
- Standby generation that can be brought online within 10 minutes
- Excess transmission capacity held for emergency re-routing
- Battery storage systems pre-charged for frequency support
Correct answer: Generating capacity from units already online and synchronized, available to respond immediately to load increases
Spinning reserve is synchronized online capacity running at part load that can instantly increase output to cover sudden generation loss or load pickup.
BEE Power Systems Engineering 4
Which bus type in a power flow study has both voltage magnitude and angle specified as the slack (reference) bus?