Symptom
On an outage the generator runs but the loads stay dead; the loads transfer but the controllers, HMIs, and radios reset; drives fault on generator power; pumps run backward on generator; the return to utility causes a second outage; the generator starts during a brief utility blink and transfers when it did not need to.
The short answer
Generator Transfer Problems
A generator transfer involves a chain: utility fails, the transfer switch senses it, the generator is signaled to start, it reaches voltage and frequency, the switch transfers after its delay, the loads restart on generator power, and on utility return the switch retransfers after a stabilization delay and cools down the generator. Each link fails in its own way: the switch sensing set wrong or its controller faulted, the generator not starting or not reaching stable output, the transfer delay long enough to drop controllers without a UPS, an open-transition switch causing a second gap on retransfer, a neutral bonded on both the generator and the service so that ground current flows, phase rotation different on the generator so pumps run backward, and drives faulting on generator voltage or frequency. The switch log, the generator log, and a monthly transfer test under real load with someone watching the controls find each.
Key points
- Read the transfer switch and generator logs; they record every step of the sequence with times.
- The gap between utility loss and transfer is seconds to tens of seconds; the controls need a UPS to cross it.
- Open-transition retransfer is a second gap; the timers and the UPS have to allow for it.
- Neutral bonding and phase rotation are the two wiring questions that a transfer test answers.
- Test monthly under load with someone at the controls; a generator that only exercises unloaded has never been tested.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Transfer switch did not sense the outage or did not signal the generator | The switch controller log and its voltage sensing settings; the start contact to the generator. A switch controller faulted or its sensing set too wide never calls the generator. |
| Generator failed to start or to reach voltage and frequency | The generator controller log: crank failure, low fuel, battery, coolant, overspeed, regulator. A generator that starts and never stabilizes never gets a transfer. |
| Transfer delay longer than the control ride-through | The transfer delay setting against the UPS and the controller hold-up; controllers reset during the gap. A UPS on the controls sized for the delay. |
| Open-transition retransfer causing a second gap | The retransfer event in the log and a second reset of the controls. The UPS covers it, or a closed-transition switch where the utility permits. |
| Neutral bonded at both the generator and the service | Ground current on the generator neutral when running; neutral-to-ground voltage changes on transfer. A three-pole switch with a generator neutral bond is two bonds; the design decides which bond and whether the neutral is switched. |
| Phase rotation different on generator | Three-phase motors run backward on generator; a rotation meter at the switch load side on utility and on generator. Correct the generator connection. |
| Drives faulting on generator voltage or frequency | Drive fault codes on generator: undervoltage, overvoltage, or frequency out of range. Generator regulation, drive input tolerance settings, and a soft restart sequence. |
| Generator overloaded on transfer | Every load returns at once; the generator sags and trips. The controller staggers the pump restarts on generator; the generator sized for the start. |
| Nuisance transfer on a brief utility dip | The switch transfers on a momentary sag; the time delay to engine start set too short. Set the delay per the utility characteristics and the process tolerance. |
| Retransfer timer or utility sensing set wrong | The switch stays on generator after utility returns, or retransfers before the utility is stable. Review the return-to-utility delay and sensing. |
The sequence and its timers
| Step | Typical timer | What goes wrong |
|---|---|---|
| Utility fails; switch senses undervoltage | Immediate | Sensing set too wide misses a partial failure; a faulted controller misses everything |
| Time delay to engine start | One to ten seconds | Too short: starts on every blink. Too long: adds to the gap |
| Generator cranks, starts, stabilizes | Five to fifteen seconds | Crank failure, fuel, battery, regulator |
| Time delay to transfer | A few seconds after the generator is stable | Adds to the gap; the controls must ride through the total |
| Transfer; loads restart | Immediate | Overload if everything restarts at once; drives fault; pumps backward if rotation is wrong |
| Utility returns; time delay to retransfer | Minutes, so the utility is stable | Too short: bounces back and forth on an unstable utility |
| Retransfer | Open transition: a brief second gap | Controls reset again without a UPS |
| Cooldown and stop | Minutes | Generator stops before cooldown; or never stops because the switch still calls it |
Diagnosing after an event
- 1
Collect the logs
The transfer switch controller events, the generator controller events, the controller power-fail log, the UPS log, and the SCADA alarm journal, aligned by time.
- 2
Find the step that failed
Which link in the sequence did not happen or happened late, from the logs and the table.
- 3
Check the settings
Sensing thresholds, the start delay, the transfer delay, the retransfer delay, against the design and the utility characteristics.
- 4
Check the wiring questions
Phase rotation on both sources with a rotation meter; the neutral bonding and whether the neutral is switched; the ground current on the generator neutral while running.
- 5
Check the ride-through
The UPS runtime against the measured gap; which loads are on it; the drive settings for generator power.
- 6
Correct and test
The setting, the wiring, or the UPS, and a transfer test under load with the controls watched.
The controls through the gap
From the utility failure to the transfer is the sum of the start delay, the generator start and stabilization, and the transfer delay: commonly ten to thirty seconds, sometimes more. The controller, the HMI, the network switches, the radio, and the alarm dialer are on a UPS sized for that gap with margin, and for the retransfer gap. Without it the controllers reset twice per outage, the retentive data may be lost, and the site reports nothing while it is dark. The UPS input is on the generator-backed circuit so that it recharges once the generator is on.
The monthly test
- Under real load: the switch is transferred with the plant running, not the generator exercised unloaded on a timer.
- Someone at the controls: watching the controllers, the HMI, the drives, and the radio through the transfer and the retransfer.
- The times recorded: utility loss to transfer, retransfer gap, and the UPS runtime consumed.
- The pumps watched for rotation and the drives for faults on generator.
- The logs read afterward, and the findings on a work order.
- The fuel, the battery, the coolant, and the block heater checked as part of the same visit.
Frequently asked questions
- The generator exercises every week and runs fine. Why did it fail in the outage?
- A weekly exercise without transfer proves the engine starts; it does not prove the switch transfers, the loads restart, or the controls ride through. Exercise under load, with the transfer, on a schedule.
- The pumps ran backward on generator. How?
- The generator was connected with a different phase sequence from the utility, and the transfer switch passes whatever rotation its source has. A rotation meter at the switch load side on each source shows it; the fix is at the generator connection, and the first transfer test after any generator work checks it.
- The controls have a UPS and still reset on transfer. Why?
- The gap was longer than the UPS runtime, the UPS battery has aged, the UPS was in bypass, or the controls are not on it. Measure the gap from the switch log, and test the UPS under load.
- Should the transfer switch be closed transition?
- A closed-transition switch parallels the generator with the utility for a moment and transfers without a gap, which is better for the process and requires utility approval and a generator that can synchronize. Many utilities use open transition with a UPS on the controls, which is simpler. The decision is in the design; the controls need a UPS either way.
Related topics
- Lift Station Generator OperationStandby power at a lift station: what the transfer switch does, how the controls ride through the transfer, staggered pump restarts, what SCADA should see, load testing, and the portable generator connection.
- UPS Not Carrying LoadThe power failed and the controls went down anyway: a battery aged out or never connected, a UPS sitting in bypass since a fault, an overload, loads on the wrong receptacles, or a failed self-test nobody read. Finding out which, and testing properly.
- Voltage Sag on Motor StartLights dim, contactors drop, and controllers reset every time the big pump starts: the starting current of an across-the-line motor pulling down a weak source. Measuring the sag, checking the source, and fixing it with a soft starter or ride-through.
- High Neutral-to-Ground VoltageWhat a neutral-to-ground voltage reading means, the values that are normal, and the causes of a high one: a long or heavily loaded neutral, harmonic current, a loose or open neutral, a missing service bond, or an extra bond downstream, and telling them apart.
- Retentive Data LostThe plant came back from a power cycle with zero setpoints, zero totals, and the wrong pump as lead. Which backup failed, which restore overwrote what, and how to get the data back and keep it.
- Drive Faults on OvervoltageA DC bus overvoltage trip on a drive: where the bus voltage comes from, why deceleration and a driving load raise it, the utility-side causes from a high supply and capacitor switching, the deceleration ramp and overvoltage controller settings, braking resistors, and how to tell a regeneration trip from a supply transient.
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