BEE Power System 4 — Questions and Answers
Question 1: What is corona discharge in transmission lines, and what is its primary cause?
- Arcing between conductors caused by insulator failure
- Ionization of air around conductors when electric field stress exceeds air's dielectric strength (Correct answer)
- Electromagnetic interference from nearby communication lines
- Insulation breakdown due to moisture
Correct answer: Ionization of air around conductors when electric field stress exceeds air's dielectric strength
When the electric field gradient at the conductor surface exceeds approximately 30 kV/cm (peak), air molecules ionize producing visible glow, hissing, and power loss.
Question 2: The symmetrical components method decomposes an unbalanced three-phase phasor set into:
- Positive, negative, and zero sequence components (Correct answer)
- Fundamental, harmonic, and DC components
- Active, reactive, and apparent components
- Line, phase, and neutral components
Correct answer: Positive, negative, and zero sequence components
Fortescue's theorem shows any unbalanced three-phase system equals the superposition of positive-sequence, negative-sequence, and zero-sequence balanced phasor sets.
Question 3: A 345-kV transmission line uses bundled conductors primarily to:
- Reduce line inductance and increase current-carrying capacity while reducing corona (Correct answer)
- Lower the cost of tower construction
- Eliminate the need for ground wires
- Increase the line's series resistance
Correct answer: Reduce line inductance and increase current-carrying capacity while reducing corona
Bundling reduces the conductor's geometric mean radius effect on inductance, lowers surface electric field gradient to reduce corona, and increases thermal rating.
Question 4: What is the purpose of a Buchholz relay in a power transformer?
- To protect against overvoltage on the secondary side
- To detect internal faults by sensing gas and oil surge inside the conservator tank (Correct answer)
- To control tap-changer operation automatically
- To measure differential current between primary and secondary
Correct answer: To detect internal faults by sensing gas and oil surge inside the conservator tank
A Buchholz relay detects gas generated by incipient internal faults (arcing or overheating) and oil surges from sudden severe faults, triggering alarms or trips.
Question 5: In power system economic dispatch, the criterion for optimal allocation of load among generators is:
- Each generator operates at maximum output
- Incremental cost of generation is equal across all committed units (Correct answer)
- Unit with lowest rated capacity serves base load
- Generators are loaded inversely proportional to their fuel cost
Correct answer: Incremental cost of generation is equal across all committed units
The equal incremental cost criterion (lambda dispatch) minimizes total fuel cost by equating the marginal (incremental) cost λ across all generating units.
Question 6: Which instrument transformer is always operated with its secondary short-circuited during service?
- Potential transformer (PT)
- Current transformer (CT) (Correct answer)
- Both PT and CT
- Neither; both operate open-circuited
Correct answer: Current transformer (CT)
Opening a CT secondary while primary current flows causes dangerous high voltages due to the large turns ratio and unsaturated core flux, so the secondary must remain closed.
Question 7: What is the significance of the short-circuit ratio (SCR) of a synchronous generator?
- It defines the ratio of rated voltage to rated current
- It indicates the machine's voltage regulation capability and synchronizing power (Correct answer)
- It measures the generator's efficiency at full load
- It specifies the maximum allowable rotor speed
Correct answer: It indicates the machine's voltage regulation capability and synchronizing power
SCR = 1/Xd (in per unit); a higher SCR means lower synchronous reactance, better voltage regulation, stronger synchronizing torque, but higher cost due to more iron.
BEE Power System 4
What is corona discharge in transmission lines, and what is its primary cause?