SOPD Power System Operations 4 — Questions and Answers
Question 1: During a system restoration after a blackout, why must operators be careful about 'cold load pickup'?
- Cold equipment draws less current than normal during restoration
- De-energized loads lose diversity and simultaneously draw maximum demand when restored, causing overloads (Correct answer)
- Cold weather reduces line ratings significantly
- Transformers require preheating before energization
Correct answer: De-energized loads lose diversity and simultaneously draw maximum demand when restored, causing overloads
Cold load pickup occurs because thermostatically controlled loads all start simultaneously after a prolonged outage, producing current levels far above normal.
Question 2: What is the role of the 'black start' unit in power system restoration?
- A unit equipped with special protection relays for fault isolation
- A generator capable of starting and supplying initial energization without an external power source (Correct answer)
- A mobile substation used to bypass damaged equipment
- A dedicated reactive compensation bank used during restoration
Correct answer: A generator capable of starting and supplying initial energization without an external power source
Black start units (hydro, gas turbines, or diesel units) can start independently and energize the grid from scratch after a total blackout.
Question 3: What is 'transient stability' in power system operations?
- The ability of the system to maintain synchronism following a large sudden disturbance (Correct answer)
- The long-term voltage regulation capability under gradual load increase
- The rate at which frequency recovers after generator trip
- The ability of protection systems to clear faults within rated interrupting time
Correct answer: The ability of the system to maintain synchronism following a large sudden disturbance
Transient stability refers to whether synchronous generators remain in synchronism with each other following severe disturbances such as faults.
Question 4: A dispatcher receives a SCADA alarm showing a breaker is in 'split position.' What does this status most likely indicate?
- The breaker successfully interrupted fault current
- The breaker mechanism has failed and neither the open nor closed state is confirmed (Correct answer)
- The breaker is in the test position
- The breaker has been locked out by its protective relays
Correct answer: The breaker mechanism has failed and neither the open nor closed state is confirmed
A split or undefined position alarm indicates a discrepancy between the breaker's open and closed contacts, suggesting a partial or failed operation.
Question 5: What is the purpose of a 'transfer trip' scheme in transmission line protection?
- To manually transfer load between parallel lines under operator control
- To send a communication signal to trip the remote end breaker when a local relay detects a fault (Correct answer)
- To automatically reroute power flow around a faulted line
- To initiate automatic reclosing after a temporary fault clears
Correct answer: To send a communication signal to trip the remote end breaker when a local relay detects a fault
Transfer trip uses a communication channel (fiber, PLC, or radio) to command the remote breaker to open when the local end detects a line fault, enabling high-speed clearing.
Question 6: An operator notices that a 345 kV bus voltage has risen to 105% of nominal during light load conditions at night. What is the most likely cause?
- Increased resistive losses on lightly loaded lines
- Ferranti effect and capacitive charging of lightly loaded long transmission lines (Correct answer)
- Underexcitation of nearby generators
- Tripping of reactive compensation capacitor banks
Correct answer: Ferranti effect and capacitive charging of lightly loaded long transmission lines
The Ferranti effect causes receiving-end voltage to rise above sending-end voltage on lightly loaded or open-circuited long lines due to line charging capacitance.
Question 7: Under NERC standards, what is the maximum allowable time for a balancing authority to return ACE to zero after a disturbance under CPS2?
- 1 minute
- 10 minutes (Correct answer)
- 30 minutes
- 1 hour
Correct answer: 10 minutes
CPS2 requires that the 10-minute average of ACE be within a defined bound (L10), effectively requiring recovery within a 10-minute window.
SOPD Power System Operations 4
During a system restoration after a blackout, why must operators be careful about 'cold load pickup'?