SOPD Voltage Regulation 4 — Questions and Answers
Question 1: A system operator needs to transfer reactive power support from a remote generator to a local source. Why might this approach be problematic?
- Reactive power transfers efficiently over long distances similar to active power
- Reactive power cannot be economically transmitted over long distances due to high reactive losses in line impedance (Correct answer)
- Remote generators cannot produce reactive power
- Reactive power transfer always causes voltage to rise above limits
Correct answer: Reactive power cannot be economically transmitted over long distances due to high reactive losses in line impedance
Reactive power is highly localized because transmission line impedance causes large reactive voltage drops over distance, making remote reactive sources much less effective than local ones.
Question 2: What is the relationship between power factor and reactive power consumption at an industrial load bus?
- Higher power factor means more reactive power is consumed
- Lower power factor means more reactive power is consumed for the same active power output (Correct answer)
- Power factor has no relationship to reactive power consumption
- Unity power factor means maximum reactive power consumption
Correct answer: Lower power factor means more reactive power is consumed for the same active power output
A lower power factor (more lagging) means a larger reactive current component for a given MW load, requiring more reactive power supply and causing greater voltage depression.
Question 3: Which protection relay function is specifically designed to prevent a generator from operating in an under-excited condition that could threaten voltage stability?
- Loss-of-field (40) relay (Correct answer)
- Distance (21) relay
- Overcurrent (51) relay
- Differential (87) relay
Correct answer: Loss-of-field (40) relay
The loss-of-field relay (ANSI 40) detects when a generator loses excitation, which would cause it to absorb reactive power (act as an induction generator) and potentially destabilize local voltage.
Question 4: An automatic voltage regulator (AVR) on a generator has a droop setting of 5%. What does this droop setting accomplish in a multi-machine system?
- It limits generator real power output to 95% of rated
- It causes terminal voltage to decrease slightly as reactive output increases, enabling stable reactive power sharing among parallel generators (Correct answer)
- It disconnects the AVR when voltage drops below 95% of nominal
- It increases excitation by 5% during voltage emergencies automatically
Correct answer: It causes terminal voltage to decrease slightly as reactive output increases, enabling stable reactive power sharing among parallel generators
Droop allows the voltage set point to decrease proportionally as reactive output increases, preventing reactive power hunting between parallel generators and enabling stable load sharing.
Question 5: When a heavily loaded 345 kV line trips, what is the IMMEDIATE effect on voltage at the sending-end bus?
- Voltage rises because the reactive load of the line is removed (Correct answer)
- Voltage drops because generation is lost
- Voltage oscillates between high and low for several cycles before stabilizing
- Voltage rises because charging power from the line disappears
Correct answer: Voltage rises because the reactive load of the line is removed
When a heavily loaded line trips, the inductive reactive losses associated with that load current disappear, and the reactive demand on the bus drops, causing voltage to rise transiently.
Question 6: What operational action should an operator take when a key 500/230 kV transformer's LTC reaches its maximum tap position but voltage is still below the lower limit?
- Trip the transformer to protect equipment
- Secure additional reactive power support from generators, capacitor banks, or SVCs (Correct answer)
- Reduce all load in the area immediately via load shedding
- Bypass the LTC and operate the transformer at a fixed ratio
Correct answer: Secure additional reactive power support from generators, capacitor banks, or SVCs
When the LTC is at its limit, voltage regulation must rely on reactive power sources; operators should bring generators out of VAR limits, switch in capacitor banks, or dispatch SVCs.
Question 7: What is meant by 'reactive capability curve' of a generator, and why is it operationally important?
- A graph showing how much reactive power the generator can produce or absorb at various MW outputs within thermal and stability limits (Correct answer)
- A curve showing the relationship between rotor speed and reactive generation
- A schedule of reactive power set points for each hour of the day
- A diagram of capacitor bank switching logic for the plant bus
Correct answer: A graph showing how much reactive power the generator can produce or absorb at various MW outputs within thermal and stability limits
The reactive capability curve defines the permissible operating region (P-Q envelope) bounded by armature current, field current, and stability limits, telling operators how much reactive flexibility is available at any MW level.
SOPD Voltage Regulation 4
A system operator needs to transfer reactive power support from a remote generator to a local source.
Why might this approach be problematic?