Generator Voltage and Reactive Power During Grid Disturbance
A 200 MW steam turbine generator is running at 180 MW and 0.95 power factor lagging when a transmission line outage causes system voltage to fall from 1.0 pu to 0.93 pu. The automatic voltage regulator is in automatic mode with a setpoint of 1.0 pu. The operator sees stator current rise, generator terminal voltage fall to 0.94 pu, and reactive power output increase from 60 MVAR to 95 MVAR. The generator rated stator current is being approached. The step-up transformer tap is fixed. The dispatcher asks for maximum reactive support.
1. Explain the generator's response and the limiting factors.
Read the worked answer
When system voltage falls, the AVR increases excitation to hold terminal voltage, which raises reactive power output. The generator moves toward a more lagging power factor. The limiting factors are stator current, rotor field current, and the generator capability curve. At 180 MW and 95 MVAR the apparent power is about 203 MVA, close to or above the machine's rating depending on its MVA base, and stator current is approaching its limit.
The step-up transformer tap being fixed means the generator terminal voltage and the high-side voltage move together, so the machine cannot fully restore system voltage on its own. The AVR setpoint of 1.0 pu may not be achievable at 0.93 pu system voltage without exceeding field limits. The operator must balance reactive support against stator and rotor heating.
2. What would you do to provide maximum support without exceeding limits, and what would you monitor?
Read the worked answer
I would keep the AVR in automatic and allow it to raise excitation up to the machine's capability curve, while monitoring stator current, field current, stator winding temperature, and rotor temperature. I would not exceed the continuous stator current rating or the field current limit. If the dispatcher wants more reactive support, I would explain the capability limit and ask whether other sources, such as capacitors or nearby units, can contribute.
I would also monitor generator terminal voltage, power factor, and the step-up transformer high-side voltage, and watch for overexcitation limiter action. If the overexcitation limiter picks up, I would coordinate with the dispatcher rather than fight the limiter. I would reduce active power if necessary to free up reactive capability, but only with dispatcher agreement.
3. What are the risks if voltage remains low for an extended period, and what would you report?
Read the worked answer
Extended low voltage with high reactive output means sustained high stator and rotor currents, which raises winding temperatures and thermal aging. If voltage falls further, the generator may lose synchronism or the auxiliaries may be affected, since motors draw more current at low voltage. There is also a risk of voltage collapse if the system does not recover.
I would report the terminal voltage, reactive output, stator current, field current, and winding temperatures, and state the capability limit clearly. I would document the AVR and limiter behavior and any dispatcher instructions. After the event, I would review the capability curve and the transformer tap setting to see whether the machine can provide more support within limits, and I would recommend a voltage-support study if low-voltage events are recurring.