A generator 32 trips when the machine takes active power from the system instead of delivering it, that is, when it motors. That happens if the prime mover loses power: fuel, governor, valves or a badly sequenced shutdown. It also appears when synchronizing with the machine slower than the grid, when load sharing between paralleled generators fails and, without real motoring, from reversed polarity or CT angle error with low thresholds.
Symptom
The generator of an industrial plant, a cogeneration unit or a power station trips on reverse power (32). It can happen at the moment of synchronizing, in steady operation with no earlier warning, when unloading the unit to stop it or when several generators share the load. Sometimes the machine synchronizes again without problem and the trip repeats days later.
Reverse power protection 32 measures the active power at the generator terminals and trips when that power flows from the system into the machine above a threshold for a set time. That means the generator is running as a synchronous motor: the grid keeps it spinning and the prime mover no longer contributes power. The damage is not in the generator but in what drives it: depending on the type of machine, motoring can overheat the blades of a steam turbine, cause mechanical problems in a diesel or gas engine, or cavitation in a hydraulic turbine. The threshold needed depends on the motoring power of each machine, which the manufacturer declares.
That threshold can be small compared with rated power, especially in turbines, and there lies the difficulty of this function: accurately measuring low active power when the machine delivers a lot of reactive power demands instrument transformers that are precise in angle. That is why a 32 trip does not always indicate real motoring, and the diagnosis must confirm with another measurement what the machine was doing.
Possible causes, from most to least likely
| Cause | Why it happens | What gives it away |
|---|---|---|
| Loss of the prime mover | Lack of fuel, clogged filter, valve that closed, governor failure | Power that falls gradually or suddenly to negative values |
| Badly sequenced shutdown | The prime mover stops before the breaker opens | Trip during a normal shutdown |
| Synchronizing while the machine runs slow | The synchronizer closes with generator frequency below grid frequency | Brief negative power right after closing |
| Load sharing failure | Governors with different droops, lost sharing link, one unit in isochronous mode | One unit takes the load and another motors |
| Reversed polarity or direction reference | Error at commissioning or after replacing CTs or VTs | The 32 sees negative power while the machine is generating |
| Angle error with low threshold | Very low power factor and phase error of the metering CTs | Trips with the machine unloaded and a lot of reactive power |
| Power swings | System disturbances or unstable control | Power oscillating around zero |
| Loss of potential | VT fuse altering the power calculation | Coincident fuse failure alarm |
What to check first
- Active and reactive power record in the minutes before: whether it fell gradually (fuel, governor), suddenly (valve, prime mover trip) or was oscillating.
- Time of the trip: synchronizing, shutdown, steady load or load change.
- Prime mover and governor events: fuel, pressure and speed alarms, and control mode. See synchronous generator and its protection.
- Load sharing configuration if several units are in parallel: droop, isochronous mode and communication between controllers. See generator in parallel with the grid.
- Comparison of the relay power with that of another meter at the same point, with the machine generating a known load.
- Does another meter confirm the machine was taking power from the system?
- No: Reversed polarity, direction reference, angle error or loss of potential
- Yes: When did it happen?
- On synchronizing: Closing with the machine slower than the grid, or short time delay
- During a shutdown: Prime mover stopped before the breaker opened
- In operation with several units: Load sharing failure between governors
- In operation with a single unit: Loss of the prime mover: fuel, valves or governor
How to confirm
- Direction test under load: with the machine delivering a known power, the relay must show positive power in the generating direction.
- Review of the synchronizer and its permitted frequency difference setting. See synchronism check 25.
- Governor and genset controller records at the moment of the event, showing whether the prime mover lost power.
- Function test with secondary injection, including the compensation angle if the relay has one and the threshold at low power factor. See secondary injection.
How to keep it from coming back
- 32 threshold and time based on the motoring power declared by the prime mover manufacturer, with margin for measurement errors, per IEEE C37.102.
- Synchronizing with the generator slightly faster than the grid, so it takes load on closing, following the synchronizer manufacturer’s criterion.
- Sequential shutdown: unload the machine and open the breaker on the 32 or on low forward power before stopping the prime mover.
- Review of load sharing every time a unit is added or a controller is changed.
- CTs of adequate accuracy when the threshold is low, or angle compensation if the relay offers it.
What not to do
- Do not raise the threshold or the time of the 32 to avoid trips on synchronizing: the problem is in the synchronizer.
- Do not disable the 32 on sets that operate in parallel with the grid.
- Do not resynchronize repeatedly a set that motored without checking its fuel and governor.
- Do not confuse the generator 32 with the power function at the grid connection point.
How the problem changes with the prime mover
The power a machine absorbs when motoring is not the same for all prime movers, and the setting criterion comes from that. The application notes we consulted agree on two points. A reciprocating combustion engine (diesel or gas) dragged by the grid takes a much larger fraction of its rated power than a condensing steam turbine spinning in vacuum with no steam. Also, a non-condensing turbine sits above the condensing one. The published figures for steam turbines vary between authors, so they cannot be taken from a generic table: the valid figure is the one declared by the manufacturer of each machine.
That difference has two practical consequences:
- The 32 pickup is set as a fraction of the machine’s own motoring power, so the relay does not operate on measurement noise but does operate on real motoring. In turbines that fraction can sit near the accuracy limit of the CT, VT and relay chain; that is why the guide on reverse power protection 32 insists on angle accuracy.
- Time and action depend on the damage to be avoided. In a steam turbine the risk is blade heating from friction without steam. That is why it is usual to combine the 32 with closing the steam valve before opening the breaker. In a combustion engine the risk is mechanical and the tolerance for time while motoring is defined by the manufacturer.
In a plant with several generators, each unit should have its own setting, documented together with the prime mover manufacturer’s data. That file is the first thing a serious trip analysis asks for; in our case we cross it with the logic of generator synchronization and with the generator protection scheme.
When to call us
If your generator trips on reverse power and it is not clear why, send us the events from the relay and the genset controller: we quote the trip analysis, a remote service available worldwide. Our team reviews the 32 settings, synchronization and shutdown logic through remote relay configuration; if direction and threshold must be verified on site, we do it with protection testing, in Colombia or elsewhere on request. Write to us.
Related guides
- Reverse power protection 32
- Synchronous generator and its protection
- Generator protection scheme
- Generator in parallel with the grid
- Synchronism check 25
- Emergency diesel generator set
References
- IEEE C37.102-2023, Guide for AC Generator Protection.
- Blackburn and Domin, Protective Relaying: Principles and Applications.
Frequently asked questions
Why does the 32 trip right when the generator is synchronized?
Because when the breaker closed the machine was spinning slightly slower than the grid. At that instant the grid accelerates it and the generator takes active power for a few moments. If the 32 time delay is short or the synchronizer allows closing with the generator frequency below the grid frequency, the 32 trips.
Why does a generator in parallel with others start motoring?
Because load sharing failed: one governor takes more load than its share, the sharing signal between controllers was lost or the droops do not match. The unit left without load can go to negative power and its 32 trips.
Is the generator 32 the same as the one at the grid connection point?
No. The generator one protects the machine and its prime mover from motoring. At the connection point of a self-generator, the power function monitors the flow between the plant and the grid as agreed with the utility, and its direction and setting are different.