SOPD Power System Operations 3 — Questions and Answers
Question 1: A 500 kV line trips unexpectedly and operators observe voltage dropping on adjacent buses. What is the most likely cause of the voltage depression?
- Increased real power loading on remaining lines
- Loss of reactive power support that the tripped line was providing through its charging current (Correct answer)
- Reduced system inertia following the trip
- Overexcitation of nearby generators
Correct answer: Loss of reactive power support that the tripped line was providing through its charging current
Transmission lines generate reactive power through their shunt capacitance; losing a line removes this reactive support and depresses voltage.
Question 2: Under what condition is a power system said to be operating in a 'voltage collapse' scenario?
- When system frequency drops below 59.5 Hz
- When voltage instability leads to uncontrollable reduction in voltage despite increased reactive power injection (Correct answer)
- When more than two transmission lines trip simultaneously
- When all generators reach their MVA rating simultaneously
Correct answer: When voltage instability leads to uncontrollable reduction in voltage despite increased reactive power injection
Voltage collapse occurs when the system can no longer supply adequate reactive power and voltage spirals downward beyond the point of recovery.
Question 3: What is the primary benefit of operating generators in 'droop control' mode rather than isochronous control?
- It allows faster fault clearing times
- It enables stable load sharing among multiple generators without hunting or instability (Correct answer)
- It eliminates the need for automatic voltage regulators
- It maximizes individual generator efficiency
Correct answer: It enables stable load sharing among multiple generators without hunting or instability
Droop control provides inherent load sharing by causing each generator's output to increase proportionally as frequency falls, preventing instability from competing governors.
Question 4: What does the term 'spinning reserve' mean in power system operations?
- Generation capacity held offline but able to start in 10 minutes
- Unloaded generation capacity synchronized to the grid that can respond immediately to load changes (Correct answer)
- Excess transmission capacity on standby
- Emergency diesel generators at critical load centers
Correct answer: Unloaded generation capacity synchronized to the grid that can respond immediately to load changes
Spinning reserve is synchronized, partially loaded capacity that can increase output immediately without start-up delay.
Question 5: Which protective relay function is specifically designed to detect loss of excitation in a synchronous generator?
- Distance relay (Zone 1)
- Directional overcurrent relay
- Loss-of-field relay (impedance-based, ANSI 40) (Correct answer)
- Differential relay (ANSI 87G)
Correct answer: Loss-of-field relay (impedance-based, ANSI 40)
The ANSI 40 (loss-of-field) relay uses impedance measurement to detect when a generator has lost its excitation and begins absorbing reactive power.
Question 6: A power system operator must shed 1,000 MW of load to prevent cascading outages. Which approach is preferred to minimize customer impact?
- Tripping large industrial feeders only
- Rotating or controlled load shedding distributed across multiple areas (Correct answer)
- Dropping an entire distribution substation
- Manually opening transmission tie lines to reduce import
Correct answer: Rotating or controlled load shedding distributed across multiple areas
Rotating load shedding distributes the interruption impact equitably and avoids prolonged outages in any single area.
Question 7: What is the significance of the 'short-circuit level' (fault MVA) at a bus in system planning?
- It determines the maximum real power that can be imported at that bus
- It indicates the magnitude of current during a bolted three-phase fault and drives equipment ratings (Correct answer)
- It measures the steady-state voltage sensitivity to load changes
- It defines the maximum allowable reactive power compensation at the bus
Correct answer: It indicates the magnitude of current during a bolted three-phase fault and drives equipment ratings
Fault MVA determines the interrupting duty required of breakers and the withstand ratings of equipment at that bus.
SOPD Power System Operations 3
A 500 kV line trips unexpectedly and operators observe voltage dropping on adjacent buses.
What is the most likely cause of the voltage depression?