BEE Power System 5 — Questions and Answers
Question 1: In a high-voltage DC (HVDC) transmission system, what type of converter is used at the rectifier station?
- Diode bridge rectifier only
- Thyristor-based line-commutated converter (LCC) or voltage-source converter (VSC) (Correct answer)
- Synchronous motor-generator set
- Autotransformer with tapped secondary
Correct 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.
Question 2: What does the X/R ratio of a power system bus indicate?
- The ratio of reactive power to active power consumed
- The relative magnitudes of system reactance to resistance, affecting fault current angle and protection (Correct answer)
- The transformer turns ratio at that bus
- The ratio of line voltage to phase voltage
Correct 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.
Question 3: What is the primary function of an underfrequency load shedding (UFLS) scheme?
- To disconnect faulty generators from the grid
- To automatically drop blocks of load to arrest frequency decline and prevent system collapse (Correct answer)
- To regulate voltage at the distribution level
- To disconnect capacitor banks during light load
Correct 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.
Question 4: The Gauss-Seidel load flow method converges slowly because:
- It requires the Jacobian matrix to be inverted at each iteration
- It updates bus voltages sequentially using the latest available values, leading to linear convergence (Correct answer)
- It cannot handle reactive power limits on generators
- It does not use admittance matrix formulation
Correct 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.
Question 5: A distance relay measures which quantity to determine if a fault is within its protective zone?
- Current magnitude alone
- Apparent impedance seen at the relay location (Correct answer)
- Real power flow through the line
- Bus voltage magnitude
Correct 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.
Question 6: Which phenomenon causes sub-synchronous resonance (SSR) in power systems?
- Harmonic injection from nonlinear loads
- Interaction between series-compensated transmission lines and the torsional modes of nearby turbine-generator shafts (Correct answer)
- Overvoltage on lightly loaded cables
- Resonance between transformer leakage inductance and bus capacitance
Correct 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.
Question 7: In power system stability, what is the critical clearing time (CCT)?
- The time required for a circuit breaker to fully open its contacts
- The maximum fault duration beyond which the generator will lose synchronism (Correct answer)
- The time constant of the excitation system's voltage regulator
- The settling time of governor response after a load change
Correct 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.
BEE Power System 5
In a high-voltage DC (HVDC) transmission system, what type of converter is used at the rectifier station?