SOPD Voltage Control & System Reliability 4 — Questions and Answers
Question 1: An operator notices that reactive power is flowing FROM the high-voltage transmission system INTO a generator's stator. What does this indicate?
- The generator is over-excited and supplying reactive power to the system
- The generator is under-excited and absorbing reactive power from the system (Correct answer)
- The generator has lost synchronism with the grid
- The power factor meter is malfunctioning
Correct answer: The generator is under-excited and absorbing reactive power from the system
When reactive power flows into a generator, the machine is operating under-excited (absorbing VARs), which can risk loss of synchronism at the generator's stability limit.
Question 2: Which condition is most likely to cause a cascading voltage collapse on an interconnected power system?
- A single generator tripping during light load
- Multiple contingencies depleting reactive reserves faster than they can be restored (Correct answer)
- A small frequency deviation from nominal
- A single high-resistance fault cleared within normal clearing times
Correct answer: Multiple contingencies depleting reactive reserves faster than they can be restored
Cascading voltage collapse typically results from successive contingencies that cumulatively exhaust reactive reserves, leaving no margin to sustain voltage.
Question 3: What is 'reactive power zoning' in the context of voltage control?
- Dividing the grid into areas where reactive power flow between zones is minimized to keep reactive support local (Correct answer)
- Scheduling reactive power imports from neighboring utilities
- A protection scheme that automatically inserts capacitors zone by zone
- A billing mechanism for reactive energy consumed by industrial customers
Correct answer: Dividing the grid into areas where reactive power flow between zones is minimized to keep reactive support local
Reactive power zoning partitions the system into areas to minimize long-distance reactive transfer, since reactive power is best controlled locally to avoid excessive losses.
Question 4: A system operator is directed to maintain a 5% reactive reserve margin. If total reactive capability is 2,000 MVAR, what is the minimum reactive reserve required?
- 50 MVAR
- 100 MVAR (Correct answer)
- 200 MVAR
- 500 MVAR
Correct answer: 100 MVAR
5% of 2,000 MVAR = 100 MVAR minimum reactive reserve required to maintain the specified margin.
Question 5: Which generator parameter directly limits the maximum reactive power (leading) that a synchronous generator can absorb?
- Stator current limit
- Rotor thermal limit
- Steady-state stability limit (armature reaction)
- Minimum excitation limit (under-excitation limiter) (Correct answer)
Correct answer: Minimum excitation limit (under-excitation limiter)
The under-excitation limiter (UEL) protects the generator from operating too far under-excited, which risks loss of synchronism due to insufficient rotor field current.
Question 6: During a voltage emergency, a Transmission Operator issues a Transmission Emergency instruction to reduce load. Under NERC EOP-011, this action is considered:
- A voluntary request with no compliance obligation
- A mandatory directive that the Balancing Authority must follow (Correct answer)
- An advisory notice only applicable to distribution providers
- A generator dispatch order that bypasses economic dispatch
Correct answer: A mandatory directive that the Balancing Authority must follow
NERC EOP-011 establishes that emergency operating instructions issued by the TOP must be followed by the Balancing Authority to mitigate reliability emergencies.
Question 7: What is the voltage effect of energizing a long, unloaded 345 kV transmission line?
- The receiving-end voltage drops significantly due to resistance losses
- The receiving-end voltage rises above sending-end voltage due to the Ferranti effect (Correct answer)
- Voltage remains the same as the sending end throughout the line
- The line acts as a large inductor and depresses voltage along its entire length
Correct answer: The receiving-end voltage rises above sending-end voltage due to the Ferranti effect
The Ferranti effect causes the receiving-end voltage of an open-circuited (unloaded) line to rise above the sending-end voltage due to distributed line capacitance.
SOPD Voltage Control & System Reliability 4
An operator notices that reactive power is flowing FROM the high-voltage transmission system INTO a generator's stator.
What does this indicate?