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
In a high-voltage DC (HVDC) transmission system, what type of converter is used at the rectifier station?
Answer: Thyristor-based line-commutated converter (LCC) or voltage-source converter (VSC)
HVDC systems use LCC (thyristor bridge) for classical HVDC or VSC (IGBT-based) for modern HVDC, both converting AC to controlled DC for long-distance transmission.
What does the X/R ratio of a power system bus indicate?
Answer: The relative magnitudes of system reactance to resistance, affecting fault current angle and protection
A high X/R ratio means fault currents are nearly purely reactive, which affects DC offset in fault current and the design of protective relay settings.
What is the primary function of an underfrequency load shedding (UFLS) scheme?
Answer: To automatically drop blocks of load to arrest frequency decline and prevent system collapse
UFLS automatically disconnects prescheduled load blocks when frequency falls below set thresholds, restoring generation-load balance before frequency collapses.
The Gauss-Seidel load flow method converges slowly because:
Answer: It updates bus voltages sequentially using the latest available values, leading to linear convergence
Sequential updating of bus voltages one at a time gives only linear (first-order) convergence, requiring many iterations compared to Newton-Raphson's quadratic convergence.
A distance relay measures which quantity to determine if a fault is within its protective zone?
Answer: Apparent impedance seen at the relay location
A distance relay computes Z = V/I at its terminal; if the calculated impedance falls within the relay's impedance zone on the R-X diagram, it operates.
Which phenomenon causes sub-synchronous resonance (SSR) in power systems?
Answer: Interaction between series-compensated transmission lines and the torsional modes of nearby turbine-generator shafts
Series capacitor compensation creates an LC resonant frequency below synchronous frequency; if this matches a shaft torsional mode, the shaft can experience growing oscillations and fatigue damage.
In power system stability, what is the critical clearing time (CCT)?
Answer: The maximum fault duration beyond which the generator will lose synchronism
CCT is the longest fault duration the system can sustain before the generator's rotor angle exceeds the critical clearing angle, after which synchronism cannot be recovered.