SOPD Voltage Control & System Reliability â Questions and Answers
Question 1: Which device injects or absorbs reactive power dynamically to support transmissionâlevel voltage?
- Onâload tap changer
- Static VAR Compensator (SVC) (Correct answer)
- Frequency converter
- Phaseâshift transformer
Correct answer: Static VAR Compensator (SVC)
A Static VAR Compensator (SVC) is a flexible AC transmission system (FACTS) device that uses power electronics to rapidly inject or absorb reactive power into the transmission system. This dynamic reactive power compensation helps to maintain stable voltage levels, improve power transfer capability, and enhance system stability.
Question 2: Voltage stability margin refers to:
- Frequency deviation
- Reserve capacity
- Distance to voltage collapse (Correct answer)
- System inertia
Correct answer: Distance to voltage collapse
Voltage stability margin quantifies how close a power system is to a state of voltage collapse, where the system can no longer maintain acceptable voltage levels. It represents the maximum additional load that can be served before the system experiences an uncontrollable and rapid decline in voltage, leading to a blackout.
Question 3: An onâload tapâchanging transformer regulates voltage by:
- Varying phase angle
- Switching series capacitors
- Adjusting tap position to alter turns ratio (Correct answer)
- Changing line impedance
Correct answer: Adjusting tap position to alter turns ratio
An on-load tap-changing (OLTC) transformer is equipped with a mechanism that allows its turns ratio to be adjusted while it is energized and carrying load. By changing the tap position, the transformer effectively alters the voltage transformation ratio, thereby regulating the voltage on the secondary side to maintain desired levels.
Question 4: Power factor correction improves reliability primarily by:
- Increasing real power losses
- Reducing line current and voltage drop (Correct answer)
- Raising system frequency
- Providing inertial response
Correct answer: Reducing line current and voltage drop
A low power factor means that a larger apparent power (kVA) is required to deliver the same amount of real power (kW). Improving the power factor reduces the reactive power flowing through the lines, which in turn decreases the total current. This reduction in current minimizes IÂČR losses and voltage drops, enhancing the overall efficiency and reliability of the transmission and distribution system.
Question 5: Which fastâacting device offers multilevel voltage support similar to an SVC but with IGBT technology?
- Synchronous condenser
- STATCOM (Correct answer)
- Shunt reactor
- Series capacitor
Correct answer: STATCOM
A STATCOM (Static Synchronous Compensator) is a modern FACTS device that uses voltage-source converters (typically IGBT-based) to dynamically inject or absorb reactive power. Similar to an SVC, it provides fast and precise voltage support, but with superior dynamic performance and a wider operating range due to its active power electronics.
Question 6: Loss of Load Probability (LOLP) is an index measuring:
- Voltage drop magnitude
- Chance of generation shortfall (Correct answer)
- System oscillations
- Transformer overheating
Correct answer: Chance of generation shortfall
Loss of Load Probability (LOLP) is a reliability index that measures the probability or expected number of days/hours per year when the system's available generation capacity is insufficient to meet the forecasted demand. It quantifies the risk of generation shortfall, indicating the likelihood of not being able to supply all customer load.
Question 7: Reactive power reserves are scheduled so that after an Nâ1 contingency the voltage remains within:
- ±1âŻ%
- ±3âŻ%
- ±5âŻ% (Correct answer)
- ±10âŻ%
Correct answer: ±5âŻ%
N-1 contingency refers to the ability of the power system to withstand the loss of any single critical component (e.g., a generator, transmission line, or transformer) without violating operational limits. For reactive power reserves, this typically means ensuring that even after such an event, system voltages remain within a ±5% tolerance band to prevent instability or collapse.
Question 8: A Ferranti effect is most pronounced on:
- Heavily loaded short feeders
- Lightly loaded long transmission lines (Correct answer)
- Distribution transformers
- Generator stators
Correct answer: Lightly loaded long transmission lines
The Ferranti effect describes the phenomenon where the receiving-end voltage of a long, lightly loaded, or open-circuited transmission line can be higher than the sending-end voltage. This occurs due to the line's shunt capacitance, which generates reactive power that charges the line and boosts the voltage, especially over long distances with minimal load.
Question 9: Primary frequency response can indirectly aid voltage control because:
- It injects capacitive VARs directly
- Stable frequency reduces realâreactive coupling stress on AVR systems (Correct answer)
- It switches capacitor banks automatically
- It adjusts tap changers
Correct answer: Stable frequency reduces realâreactive coupling stress on AVR systems
Primary frequency response acts to stabilize system frequency by adjusting generator output in response to immediate load-generation imbalances. While not directly controlling voltage, maintaining a stable frequency reduces the stress on automatic voltage regulators (AVRs) and prevents large frequency deviations that can exacerbate voltage issues due to the coupling between real and reactive power flows.
SOPD Voltage Control & System Reliability
Which device injects or absorbs reactive power dynamically to support transmissionâlevel voltage?