Power System Flashcards
7 cards from real BEE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Power System flashcards as text
What is corona discharge in transmission lines, and what is its primary cause?
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.
The symmetrical components method decomposes an unbalanced three-phase phasor set into:
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.
A 345-kV transmission line uses bundled conductors primarily to:
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.
What is the purpose of a Buchholz relay in a power transformer?
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.
In power system economic dispatch, the criterion for optimal allocation of load among generators is:
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.
Which instrument transformer is always operated with its secondary short-circuited during service?
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.
What is the significance of the short-circuit ratio (SCR) of a synchronous generator?
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.