SOPD Voltage Regulation 3 — Questions and Answers
Question 1: What is the primary cause of voltage collapse in a power system?
- Excess active power generation relative to load
- Insufficient reactive power supply to meet reactive demand (Correct answer)
- Frequency deviation beyond ±0.5 Hz
- Excessive transformer tap changes
Correct 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.
Question 2: 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?
- Voltage decreases further due to increased current draw
- Voltage increases toward nominal due to reactive power injection (Correct answer)
- Active power generation automatically decreases
- Frequency rises by approximately 0.1 Hz
Correct 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.
Question 3: 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?
- Reactive power hunting
- Circulating reactive current (Correct answer)
- VAR oscillation
- Cross-current compensation
Correct 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.
Question 4: What does the Q-V curve (reactive power vs. voltage curve) indicate at its 'nose point'?
- Maximum active power transfer capability of the line
- The voltage level at which reactive reserve is exhausted and voltage collapse is imminent (Correct answer)
- Optimal power factor for the bus
- The point where frequency equals 60 Hz under all conditions
Correct 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.
Question 5: How does a synchronous condenser differ from a static VAR compensator (SVC) in providing voltage support?
- A synchronous condenser cannot absorb reactive power, while an SVC can
- 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 (Correct answer)
- An SVC responds slower than a synchronous condenser
- A synchronous condenser injects only active power during voltage events
Correct 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.
Question 6: During off-peak hours, an operator notices transmission line voltages rising above the upper limit. Which action is MOST appropriate to lower voltage?
- Trip a shunt capacitor bank or energize a shunt reactor (Correct answer)
- Increase generator MW dispatch
- Reduce transformer LTC tap to a higher voltage position
- Open a parallel transmission line
Correct 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).
Question 7: What is the effect of a long, lightly loaded EHV (Extra High Voltage) line on system reactive power balance?
- It consumes large amounts of reactive power due to resistance losses
- It generates reactive power due to its shunt capacitance, potentially over-voltaging the receiving end (Correct answer)
- It has a neutral effect because EHV lines carry no charging current
- It absorbs reactive power at both sending and receiving ends equally
Correct 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.
SOPD Voltage Regulation 3
What is the primary cause of voltage collapse in a power system?