300MW+ Unit Generator Circuit Breaker Automatic Tripping: Causes, Phenomena & Handling Measures
Units with a capacity of 300MW and above are usually connected in a generator-transformer unit configuration and connected to the power system through the circuit breaker on the high-voltage side of the transformer. During the normal operation of the unit, the circuit breaker may trip automatically due to various reasons. Operators must make correct judgments and take timely measures to ensure the safe operation of the unit.
1. Causes of Automatic Circuit Breaker Tripping
Tripping due to relay protection action: For example, short-circuit faults inside or outside the unit trigger relay protection to trip; loss of excitation or water cutoff of the generator causes the loss-of-excitation protection and water cutoff protection to act and trip (note: the original text repeats "water cutoff protection" multiple times, which is retained as-is in translation).
Tripping caused by personnel's accidental contact, incorrect operation, or malfunction of relay protection.
2. Phenomena After Automatic Circuit Breaker Tripping
Tripping Due to Correct Protection Action:
An alarm horn sounds, and the position indicator lights of the unit's circuit breaker and field suppression switch flash. When a unit fault occurs, the generator main circuit breaker, field suppression switch, and high-voltage station service working branch circuit breaker trip automatically under the action of relay protection, and the green lights of each tripped circuit breaker flash. The high-voltage station service standby branch circuit breaker is automatically closed by interlocking, and the red light of the standby branch circuit breaker flashes.
The "accidental trip" indicator lights of the generator main circuit breaker, high-voltage station service working branch circuit breaker, and field suppression switch are activated, and the indicator lights related to the triggered protection actions are illuminated.
All relevant meters of the generator indicate zero. After an accidental trip of the generator, the readings of active power, reactive power, stator current and voltage, rotor current and voltage, and other meters drop to zero.
Simultaneously with the circuit breaker tripping, other units display abnormal signals, and their meters show corresponding abnormal indications. For example, when a generator trips due to a fault, other units may experience overloading, overcurrent, etc., with significant rises or fluctuations in meter readings.
3.Tripping Caused by Personnel's Accidental Contact or Protection Malfunction:
The circuit breaker position indicator light flashes, while the field suppression switch remains closed.
The generator stator voltage and unit speed increase.
Under the action of the automatic voltage regulator (AVR), the generator rotor voltage and current drop significantly.
Active power, reactive power, and other meters show corresponding indications. Since the station service branch circuit breaker does not trip, it still supplies power to the station service load.
Meters of other units show no fault indications, and there are no electrical system fault phenomena.
4. Handling Measures After Automatic Circuit Breaker Tripping
When the generator main circuit breaker trips automatically during operation, operators shall take timely handling measures based on meter readings, signals, and protection action status, following the scenarios below:
Handling for Correct Protection Action:
After the generator main circuit breaker trips automatically, check whether the field suppression switch has tripped. If 41SD and GSD (designated switch identifiers) have not tripped, disconnect them immediately.
After the generator main circuit breaker, field suppression switch, and high-voltage station service working branch circuit breaker trip, check whether the switching of the high-voltage station service working branch to the standby branch is successful. If unsuccessful, manually close the standby branch circuit breaker (if the working branch circuit breaker has not tripped, first open the working branch before closing the standby branch) to ensure the power supply for unit shutdown.
Reset the circuit breaker control switches and audio signals. Turn the control switches of the automatically tripped and closed circuit breakers to positions consistent with their actual status to stop the flashing signals. Press the reset button for the audio signal to stop the alarm.
Deactivate the generator's automatic voltage regulator (AVR).
Adjust and monitor the operating conditions of other fault-free units to maintain their normal operation.
Check the relay protection action status and take corresponding measures:
If the generator trips due to a system fault (e.g., bus differential protection, failure protection), maintain the steam turbine speed and inspect the primary system of the generator-transformer unit.
After the system fault is eliminated or isolated by switching the operating mode, contact the dispatching center to reconnect the unit to the system.
If the trip is caused by the action of internal protection of the generator-transformer unit, inspect the generator, main transformer, high-voltage station service transformer, and related equipment according to the protection scope, measure insulation, identify the cause and nature of the fault, and report to the dispatching center for shutdown and maintenance.
Restart and reconnect the unit to the system after the fault is eliminated. If the trip is caused by loss-of-excitation protection, identify the reason. For units with a backup excitation device that can be switched to, reconnect to the system; otherwise, shut down the unit for handling.
5.Handling for Incorrect Generator Tripping:
5.1Tripping Due to Generator Protection Malfunction:
When the circuit breaker trips, there shall be relay protection action signals, but no fault phenomena in the unit or system, and no abnormal signals from other electrical equipment. At this time, check which protection malfunction caused the trip.
If the trip is caused by backup protection malfunction, with the dispatching center's approval, deactivate the backup protection, reconnect the generator to the system first, and then eliminate the fault.
If the trip is caused by the malfunction of the unit's main protection, identify the reason for the protection malfunction and reconnect to the system only after eliminating the malfunction.
After the generator circuit breaker trips automatically, if no abnormalities are found in the primary system of the generator-transformer unit and the protection system, with the approval of the factory chief engineer and the dispatching center, manually perform a zero-voltage step-up for the generator. Before step-up, close the neutral point grounding isolating switch of the main transformer, and perform the step-up slowly.
During the step-up process, closely monitor the generator meter readings and the insulation status of the stator and rotor. When the voltage reaches 1.05 times the rated voltage, maintain it for 1 minute (i.e., withstand voltage test for 1 minute), then reduce it to the rated voltage and conduct a detailed inspection of the generator-transformer unit and related equipment. If no abnormalities are found, reconnect to the system. If abnormalities occur during the step-up process, shut down the unit immediately for handling.
5.2 Tripping Due to Personnel's Accidental Contact or Incorrect Operation:
Generally, the field suppression switch remains closed at this time, and each generator meter shows load rejection phenomena. At this time, manually trip the field suppression switch. After confirming that the trip is caused by human factors, reconnect the unit to the system as soon as possible.
