Voltage Control & System Reliability Flashcards

Which device injects or absorbs reactive power dynamically to support transmission‑level voltage?

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.

Voltage stability margin refers to:

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.

An on‑load tap‑changing transformer regulates voltage by:

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.

Power factor correction improves reliability primarily by:

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 I²R losses and voltage drops, enhancing the overall efficiency and reliability of the transmission and distribution system.

Which fast‑acting device offers multilevel voltage support similar to an SVC but with IGBT technology?

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.

Loss of Load Probability (LOLP) is an index measuring:

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.

Reactive power reserves are scheduled so that after an N‑1 contingency the voltage remains within:

Answer: ±5 %

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.

A Ferranti effect is most pronounced on:

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.

Primary frequency response can indirectly aid voltage control because:

Answer: Stable frequency reduces real‑reactive 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.