Ship Generator Reverse Power Testing: Comprehensive Analysis, Fault Diagnosis, and Operational Practices
I. Overview of generator reverse power
When a generator and other units running in parallel, due to some special reasons, the generator may enter the motor running state, to the grid to draw active power (called reverse power). When the generator is in reverse power state, it not only can not share the load of the grid, but will increase the burden on the grid, should take protective measures to run the generator in reverse power from the grid. Generator reverse power protection is the protection of generators running in parallel.
Generator just put into parallel, due to the existence of frequency difference and phase angle difference, in the process of pulling into the synchronization will also cause the generator to be parallel inverse power, but as long as the frequency difference and the phase angle difference in the permissible range, short-term reverse power is allowed. The reverse power protection should have a certain time limit to avoid the short-time reverse power shock that occurs during the synchronization process.
? Steel Seagoing Vessel Classification Code mandatory provisions
The specification is clear: alternators operating in parallel shall have a time-delayed 3~10 s Reverse power protection for action. The value of the reverse power can be set according to the type of prime mover:
8% ～ 15% Rated power of generator
2% ～ 8% Rated power of generator
II. Analysis of the causes of ship generator reverse power failure
Ship generator reverse power failures are caused by the following four core conditions:
(i) Insufficient power or stalling of diesel engine
Key scenarios:Interrupted or unstable fuel supply (fuel pump failure, clogged fuel filter); Mechanical failure of diesel engine (e.g. supercharger failure, insufficient air intake into cylinders); Governor abnormality, resulting in insufficient fuel supply and the generator is unable to maintain output power.
Preventive measures:Regularly check the fuel system and clean the fuel filter to ensure a stable fuel supply; monitor the operating parameters of the diesel engine to detect anomalies in time; calibrate the governor regularly to ensure normal fuel supply control.
(ii) Generator grid connection faults
Probable Cause:Failure to synchronize correctly before grid connection (e.g. voltage, frequency, phase mismatch); uneven load distribution, resulting in a generator being underloaded or even sending power in reverse; excessive setting of the inverse power relay, which fails to trip in time.
Preventive measures:Strictly follow the requirements of voltage, frequency and phase sequence synchronization for grid-connected operation; set the appropriate reverse power protection value (usually 2%~5% of the rated power); monitor the load distribution situation to ensure that multiple generators reasonably bear the load.
(iii) Governor or AVR failure
Probable Cause:Governor failure, the diesel engine can not maintain normal speed, power drop; AVR abnormalities, generator voltage drop, can not provide sufficient active power; load changes too quickly, the governor and AVR response is not timely, resulting in the generator into the reverse power state.
Preventive measures:Regularly check the operating status of the governor and AVR for calibration; monitor the output power of the generator and make timely adjustments when abnormal power is detected.
(iv) Improper setting or failure of the inverse power relay
Probable Cause:A relay setpoint that is too high, resulting in a reverse power condition that lasts too long and fails to trip in a timely manner, or a damaged or faulty relay wiring that fails to properly remove the generator when a reverse power fault occurs.
Preventive measures:Set reasonable reverse power protection values; periodically check relay contacts, coils, and action response times to ensure reliability; conduct reverse power protection tests to verify that relays can operate correctly.
III. Judgment of ship generator reverse power failure
(i) Preliminary judgment
Generator reverse power protection tripping mainly occurs in parallel operation, in addition, the closing moment to grasp the improper, or parallel operation of the load distribution operation regulation direction reversed, or parallel one of the diesel engine governor damage or fuel interruption and other occasions will occur inverse power protection tripping.
(ii) Detailed judgment process
Deep fault localization is performed through the following five core dimensions:
Normal condition: The active power P of the generator should always be positive, i.e. the generator supplies power to the load. Inverse power state: If P becomes negative, the generator is no longer outputting power, but absorbing power from the grid.
During normal operation, the speed is stable and the sound is even. After entering the reverse power state, the generator becomes an electric motor, and the diesel engine will be driven by the busbar, resulting in an increase or fluctuation in speed. If you hear abnormal roaring or rapid acceleration sound, it may be that the diesel engine is driven to accelerate by entering the reverse power state.
During normal grid-connected operation, the generator provides reactive power to maintain voltage stability. When a reverse power fault occurs, the reactive power may drop sharply or even reverse.
Check the relay indicator. As soon as an active power back-feed from the bus is detected and its duration exceeds the set value (usually 3~10s), the relay operates and removes the faulty machine.
Normally the load should be distributed in a balanced manner. If the load of a generator gradually decreases or even is sent backwards, then the load distribution is abnormal. This may be caused by a fault in the AVR, governor or grid control.
IV. Inverse power relay operation correctness calibration
(i) Stand-alone test (forward power calibration method)
Alternator inverse power protection is realized by the inverse power relay, whose setting value is generally 8% ~ 15% (diesel) of the rated power, with a delay of 3 ~ 10 s action.
(ii) Parallel testing (manual oil reduction method - recommended)
The method of manually reducing the diesel engine fuel supply to produce reverse power during parallel operation of generator sets is the closest to the real-world scenario.
- Step 1:Start the grid connection, make sure it is running properly, and observe the active power P (kW).
- Step 2:Slowly reduce diesel fuel supply (adjust fuel regulator lever or reduce governor setting).
- Step 3:Observe that P gradually decreases and becomes negative.
- Step 4:Verify that the inverse power relay trips within 3~10s.
- Step 5:Reset the relay and verify that normal operation is restored.
