Voltage Regulation Flashcards

What is the primary cause of voltage collapse in a power system?

Answer: Insufficient reactive power supply to meet reactive demand

Voltage collapse occurs when reactive power demand exceeds available reactive supply, causing voltages to decay progressively until the system cannot sustain stable operation.

A bus voltage is at 0.92 pu under peak load. The operator closes a 100 MVAR shunt capacitor bank. Which outcome is MOST expected?

Answer: Voltage increases toward nominal due to reactive power injection

Shunt capacitor banks inject reactive power into the bus, offsetting inductive reactive losses and raising bus voltage toward nominal.

An operator observes that two adjacent generating units are fighting reactive power — one absorbing, the other supplying — while their terminals are at the same bus. What is this condition called?

Answer: Circulating reactive current

Circulating reactive current occurs when units share a bus but have mismatched voltage set points, causing reactive power to circulate without useful work.

What does the Q-V curve (reactive power vs. voltage curve) indicate at its 'nose point'?

Answer: The voltage level at which reactive reserve is exhausted and voltage collapse is imminent

The nose point of the Q-V curve represents the minimum reactive power margin; operating at or past it means the system cannot recover voltage without major topology changes.

How does a synchronous condenser differ from a static VAR compensator (SVC) in providing voltage support?

Answer: A synchronous condenser provides reactive power through rotating machinery and offers short-circuit strength; an SVC uses solid-state devices and adds no fault current

Synchronous condensers are rotating machines that add inertia and fault current capability; SVCs use thyristors and respond faster but add no physical short-circuit strength.

During off-peak hours, an operator notices transmission line voltages rising above the upper limit. Which action is MOST appropriate to lower voltage?

Answer: Trip a shunt capacitor bank or energize a shunt reactor

High voltages during light load are corrected by removing capacitive sources (switching off capacitor banks) or adding inductive absorption (energizing shunt reactors).

What is the effect of a long, lightly loaded EHV (Extra High Voltage) line on system reactive power balance?

Answer: It generates reactive power due to its shunt capacitance, potentially over-voltaging the receiving end

EHV lines have significant shunt capacitance; at light load, charging reactive power exceeds reactive losses, causing a net reactive generation that can raise receiving-end voltage.