Cause Analysis of Automatic Tripping of Generator Circuit Breaker

300MW and above units are usually connected to generator transformer unit wiring and connected to the system through the breaker at the high-voltage side of the transformer. During the normal operation of the unit, the circuit breaker trips automatically due to various reasons. The operator should correctly judge and deal with it in time to ensure the safe operation of the unit.

1. Reasons for automatic tripping of circuit breaker:

1) Relay protection trips. For example, relay protection trips due to internal or external short circuit fault of the unit; The excitation loss protection and water break protection of the generator trip due to loss of excitation or water break; In case of failure of the thermal engine system, the operator on duty shall make an emergency trip locally, or in case of failure of the thermal system, the thermal engine protection shall act and the circuit breaker shall be interlocked to trip;

2) The circuit breaker trips due to the accidental touch or operation of the staff and the misoperation of the relay protection.

2. Phenomenon after automatic tripping of circuit breaker:

Tripping caused by correct protection action:

1) The horn sounds, and the position indicator of the unit circuit breaker and the field suppression switch flashes. When the unit fails, the generator main circuit breaker, deexcitation switch and high-voltage auxiliary branch circuit breaker automatically trip under the action of relay protection, and the green light of each tripping circuit breaker flashes. The high-voltage auxiliary standby branch circuit breaker is interlocked and automatically closed, and the red light of the standby branch circuit breaker flashes.

2) The main circuit breaker of the generator, the branch circuit breaker of the high-voltage plant service, and the light sign of the field suppression switch "accident trip" give an alarm, and the light sign of the relevant protection action is on.

3) The indication of relevant meters of generator is zero. After the emergency tripping of the generator, the active power, reactive power, stator current and voltage, rotor current and voltage and other meter indications are all zero.

4) When the circuit breaker trips, other units have abnormal signals, and meters have corresponding abnormal indications. If the generator trips due to fault, other units shall have overload, overcurrent and other phenomena, and the meter indication shall rise or swing significantly.

Tripping caused by accidental touch and protection:

1) The circuit breaker position indicator flashes, and the field suppression switch is still in the closing position.

2) The generator stator voltage increases and the unit speed increases.

3) Under the action of automatic excitation regulator, the voltage and current of generator rotor decrease greatly.

4) Active power, reactive power and other meters have corresponding indications. Since the auxiliary branch circuit breaker does not trip, it still carries the auxiliary power load.

5) The meter of other units has no fault indication and no electrical system fault.

3. Handling of automatic tripping of circuit breaker:

When the main circuit breaker of the generator in operation trips automatically, the operator shall make timely treatment according to the meter, signal and protection action, and the following situations are divided.

Handling of correct protection action:

1) After the generator main circuit breaker automatically trips, check whether the field suppression switch has tripped. If 41SD and GSD have not tripped, disconnect them immediately.

2) After the main circuit breaker, deexcitation switch and working branch circuit breaker of high-voltage auxiliary power supply of the generator trip, check whether the working branch of high-voltage auxiliary power supply is successfully switched to the standby branch. If it is not successful, the standby branch circuit breaker shall be closed manually (if the working branch circuit breaker does not trip, the working branch shall be pulled first and then the standby branch shall be closed) to ensure the power demand for unit shutdown.

3) Reset the circuit breaker control switch and audio signal. Turn the control switch of the automatic tripping and automatic closing circuit breaker to the position consistent with the actual position of the circuit breaker to stop the flashing signal. Press the reset button of the audio signal to stop the audio.

4) Deactivate the automatic excitation regulator (AvR) of the generator.

5) Adjust and monitor the operating conditions of other trouble free units to maintain their normal operation.

6) Check the relay protection action and make corresponding treatment. If the generator trips due to system fault (such as bus differential protection and failure protection), maintain the turbine speed and check the primary system of the generator transformer unit. After the system fault is eliminated or the fault is isolated through switching operation, contact the dispatcher to re integrate the unit into the system; In case of tripping due to internal protection action of generator transformer unit, the generator, main transformer, high-voltage auxiliary transformer and relevant equipment shall be inspected according to the protection scope, and the insulation shall be measured to find out the cause of tripping, determine the fault point and nature, and report to the dispatcher for shutdown maintenance. After troubleshooting, restart the grid connection. In case of tripping due to loss of excitation protection action, the cause shall be found out, and the unit that can be switched to the standby excitation device for operation can be connected to the grid again, otherwise, it can only be shut down for handling.

Handling of generator false tripping:

1) Generator protection maloperation trips. When the circuit breaker trips, there should be a relay protection action signal, but the unit and system have no faults, and other electrical equipment have no abnormal signals. At this time, check what protection misoperation causes tripping. If it is a backup protection malfunction, it can be stopped after obtaining the consent of the dispatcher. First, connect the generator to the grid, and then eliminate the fault; If the tripping is caused by the misoperation of the main protection of the unit, the cause of the misoperation of the protection shall be found out. The grid connection can be resumed only after the malfunction is eliminated. After the generator circuit breaker automatically trips, check that the primary system of the generator transformer unit is normal and the protection is normal. With the consent of the chief engineer of the plant and the dispatcher, the generator can be boosted manually from zero. Before boosting, close the neutral grounding disconnector of the main transformer, and the boosting shall be slow. In the process of voltage rise, the generator meter indication and stator and rotor insulation shall be closely monitored. When the voltage rises to 1.05 times of the rated voltage, it shall remain for 1min (i.e. withstand voltage for 1min), and then it shall be reduced to the rated voltage. The generator transformer unit body and related equipment shall be carefully inspected. If there is no abnormality, re connect to the grid for operation. If there is any abnormality in the pressure rise process, stop the machine immediately for handling.

2) Manual false touch and false operation trip. Generally, the de excitation switch is still in the closing position at this time, and all meters of the generator indicate load rejection. At this time, the de excitation switch shall be tripped manually, and it shall be checked that the tripping is caused by human factors