In short. A generator running in parallel can be driven by the system instead of driving it. Reverse power protection detects this “motoring” and trips before the engine or turbine is damaged. It sits within a wider set of generator protections.
Reverse power
If the engine loses fuel or its governor fails while the breaker is closed, the alternator draws real power and spins the engine as a motor. A diesel engine can tolerate this only briefly. The reverse power relay measures active power flow into the generator and trips after a short delay. Settings depend on the engine’s motoring power, usually a few percent of rating, so the relay needs sensitive, accurate measurement.
Other generator protections
| Function | Detects |
|---|---|
| Loss of excitation (40) | Field loss, which draws large reactive power from the system |
| Overload / overcurrent (51) | Thermal or short-circuit conditions. See overcurrent |
| Negative sequence (46) | Unbalanced loading. See negative sequence |
| Stator earth fault (51G/REF) | Winding earth faults |
| Differential (87G) | Phase faults in the stator |
| Over/under voltage and frequency | Abnormal operation. See voltage and frequency |
| Overspeed, low oil pressure, high temperature | Engine protections handled by the controller |
Setting reverse power
- Find the engine’s motoring power from the manufacturer.
- Set the pickup below this, with allowance for measurement error.
- Choose a time delay long enough to ride through synchronising swings and short disturbances, short enough to protect the engine.
- Verify with a controlled test when the engine is at low load.
Where it matters
- Parallel with the utility, where the grid can drive the generator.
- Multiple generators in parallel, where one can be driven by the others.
Illustrative example
Two generators share a bus. One loses its fuel supply and the governor output falls to zero, while its breaker stays closed. The healthy unit begins to drive the failed one. The reverse power relay sees power flowing in and trips the breaker after a few seconds, and the healthy unit continues to carry the load.
What reverse power means
When a generator running in parallel loses its fuel supply or its prime mover fails, it stops delivering power and starts absorbing real power from the system, motoring its engine. This reverse power can damage the prime mover and may be hazardous.
Typical reverse power levels
| Prime mover | Motoring power (order of magnitude) | Consequence |
|---|---|---|
| Diesel engine | Several percent up to around 15 percent of rating, depending on engine | Mechanical stress, fire risk from unburnt fuel |
| Gas turbine | Higher percentages | Gearbox and shaft stress |
Use the engine maker’s motoring data for settings.
Setting approach
- Set pickup below the engine’s motoring power but above measurement error and transient swings.
- Add a time delay to avoid tripping during synchronising or load transfer transients.
- Trip the generator breaker and shut down the engine.
Other generator protections in the same panel
- Loss of excitation (40) for field failure.
- Overcurrent with voltage restraint for faults near the machine.
- Stator earth fault and differential.
- Negative-sequence for unbalance. See negative sequence.
- Over and under voltage and frequency. See voltage and frequency protection.
Utility parallel operation
When a generator runs in parallel with the grid, reverse power can also mean export to the utility. Many utilities forbid export or require specific protection. Check agreements and use the correct relay function and settings.
Testing
Inject a reverse power signal and confirm the trip, time and shutdown action. Record results. Test again after control changes.
Common mistakes
- Setting too insensitive, leaving the engine unprotected.
- Delays too short, causing trips at synchronising.
- Reverse power not enabled on the generator breaker relay.
What to send us
Send the generator and relay data. Send your application details. See also relay architecture.
Questions engineers ask
Is reverse power needed for a single standby set?
No, since nothing can drive it. It is for parallel operation.
What happens without it?
A motoring engine can overheat or suffer damage and fuel-less running problems.
How is it tested?
By secondary injection and a controlled on-site check.
Does it replace loss-of-excitation protection?
No. They detect different problems.
This guide gives general principles. Settings need engine and relay data.