Ship Generator Reverse Power Testing: Comprehensive Analysis, Fault Diagnosis, and Operational Practices

Ship's Electrical and Electronics Officer (ETO) Technical Training Core Material

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:

(1) The prime mover is a diesel engine
8% to 15% Rated power of generator

(2) The prime mover is a turbine
2% to 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.

🔍 Classic case:During the voyage of a bulk carrier, the fuel filter of its diesel engine was seriously clogged, causing insufficient fuel supply and gradual loss of generator output, which eventually led to the operation of the inverse power relay and the generator was cut off. After replacing the filter, the generator resumed normal operation.

(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.

🔍 Classic case:An oil tanker had two generators connected to the grid. One of the generators experienced an AVR fault, resulting in a lower than normal output voltage. As the load was not properly distributed, the generator gradually entered a reverse power state, which eventually led to a relay trip and the generator was removed.

(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.

🔍 Classic case:The governor feedback signal of a container ship was abnormal, which caused the diesel engine speed to drop, the generator power to gradually decrease, and eventually enter the reverse power state, leading to relay tripping. After adjusting the governor parameters, normal operation was restored.

(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.

🔍 Classic case:A cruise ship reverse power relay was not maintained for a long period of time and the aging of the relay coil failed to act in time, resulting in the generator reverse power lasting too long and the diesel engine stopped due to overspeed alarm. After replacing the relay, the system returned to normal.

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:

1. Monitoring of active power trends:
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.

2. Observe changes in diesel engine speed and sound:
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.

3. Monitoring of reactive power changes in generators:
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.

4. Monitor inverse power relay operation:
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.

5. Analyze the load distribution of the grid-connected system:
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.

Operational details:Calibration can be accomplished in stand-alone operation by swapping the voltage or current connections on the relay. At this point the relay measures positive power as reverse power. The load is applied from no load and timed when the set value is reached, recording the time of operation.

(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.

Marine Electrical and Electronic Officer (ETO) Technical Training Practices | 2026 Revision | Full Text Version