A UPS may operate normally from the utility supply, and the generator may pass its individual tests. However, when the Automatic Transfer Switch transfers the UPS to the generator, the UPS may remain on battery, repeatedly accept and reject the generator, or display alarms such as Input Out of Range or Bypass Not Available.
This does not always mean that the UPS or generator is faulty. The problem is often caused by the interaction between the UPS rectifier, battery charger, generator governor, AVR, ATS timing, and the sequence in which loads are connected.
What Happens During Transfer?
When the utility supply fails, the UPS immediately supports the critical load from its batteries.
The generator then starts, builds up its voltage and frequency, and the ATS transfers the emergency bus to the generator.
After the transfer, the UPS checks whether the generator voltage and frequency are within its permitted input limits. If the source is accepted, the rectifier begins transferring the load from the batteries to the generator.
At the same time, battery charging and other emergency loads may also start.
When these loads are applied too quickly, the generator voltage or frequency may fall. The UPS then rejects the generator and returns to battery operation.
Once the UPS removes its load, the generator recovers. The UPS detects a stable source and tries to reconnect again.
This accept-reject cycle may continue repeatedly.
1. Frequency Dip When the UPS Load Is Applied
Generator frequency is directly related to engine speed.
When the UPS rectifier reconnects, the generator engine must immediately produce additional mechanical torque. Before the governor increases the fuel supply, the engine speed may fall and cause a temporary frequency dip.
For example, the generator may initially operate at 50 Hz but fall briefly to 46 or 47 Hz. If the UPS input frequency range is 48 to 52 Hz, the UPS will reject the generator even if the frequency later returns to 50 Hz.
The important measurement is therefore not only the stabilized frequency. Technicians must record:
- Minimum frequency during transfer.
- Maximum frequency during recovery.
- Frequency rate of change.
- Duration of the frequency disturbance.
2. The Generator Supplies More Than the UPS Output Load
The generator does not only supply the critical load connected to the UPS output.
It may also supply:
- UPS conversion losses.
- Battery charging power.
- UPS cooling fans and auxiliaries.
- Emergency lighting.
- Ventilation systems.
- Pumps and other emergency loads.
A UPS supplying an 80 kW output load may therefore draw noticeably more than 80 kW from the generator.
The actual UPS input demand can be estimated as:
UPS Input Power = Critical Load ÷ UPS Efficiency + Battery Charging Power + Auxiliary Loads
The generator must also be evaluated for the largest load step, not only the final steady-state load.
3. Battery Charging Can Overload the Generator
After a long outage, the UPS batteries may be partially discharged.
When the generator supply becomes available, the battery charger may begin drawing a high current at the same time that the UPS rectifier accepts the critical load.
This additional demand may cause:
- Frequency dip.
- Voltage dip.
- Generator overload.
- UPS input rejection.
- Repeated transfer to battery operation.
Where supported by the UPS, the battery charging current should be limited during generator operation.
This setting may appear as:
- Charger Current Limit.
- Generator Charger Limit.
- Battery Recharge Limit.
- Maximum Charging Power.
4. Harmonics Can Distort the Generator Voltage
Some UPS rectifiers, especially older six-pulse or SCR-based designs, draw non-sinusoidal current.
This current contains harmonics. When harmonic current flows through the internal impedance of the generator, it produces voltage distortion.
The RMS voltage may appear normal on a standard multimeter, while the actual waveform is distorted.
This distortion may affect:
- The generator AVR.
- UPS synchronization.
- The static bypass.
- Electronic control equipment.
- Other loads connected to the generator.
The following values should be measured at the UPS input:
- Voltage THD.
- Current THD.
- Individual harmonic levels.
- Voltage and current waveforms.
- True power factor.
A Power Quality Analyzer should be used because a normal multimeter cannot capture short-duration events or waveform distortion.
5. Governor and AVR Response
The generator governor controls engine speed and frequency.
The Automatic Voltage Regulator controls alternator voltage.
If either control loop responds too slowly or is incorrectly adjusted, the generator may oscillate when the UPS load is applied.
A typical sequence may be:
- The UPS applies its load.
- Frequency or voltage falls.
- The governor or AVR overcorrects.
- Frequency or voltage rises above the acceptable limit.
- The UPS rejects the generator.
- The load is removed and the generator recovers.
- The UPS attempts to reconnect.
Governor and AVR settings should not be adjusted randomly. Their response must be recorded under load and reviewed by a qualified generator technician.
What Is Generator Mode in a UPS?
Generator Mode is not a single standardized function, and it may operate differently depending on the UPS manufacturer and model.
It is generally used to make the UPS less demanding on the generator.
Depending on the UPS design, Generator Mode may:
- Limit the UPS input current.
- Reduce battery charging power.
- Apply the rectifier load gradually.
- Adjust the response to generator frequency variations.
- Change the operation of input filters.
- Receive an external signal from the ATS indicating that the generator is operating.
The same functions may appear under different names, including:
- Generator Mode.
- Genset Mode.
- Input Current Limit.
- Charger Limit.
- Rectifier Walk-In.
- Generator Input Contact.
The UPS manual must always be checked before changing these settings.
Rectifier Walk-In
Rectifier Walk-In controls how quickly the UPS transfers its load from the batteries to the generator.
Without walk-in, the rectifier may apply most of the UPS load in one sudden step.
With walk-in enabled, the rectifier increases its input power gradually over several seconds.
This gives the generator governor and AVR enough time to respond and reduces the risk of voltage or frequency collapse.
Practical Site Inspection
The investigation should start with the UPS event log.
Look for alarms such as:
- Input Frequency Out of Range.
- Input Voltage Out of Range.
- Rectifier Off.
- Mains Not Available.
- Bypass Not Available.
- Generator Input Alarm.
Compare the time of each alarm with the ATS transfer time and generator controller records.
Measurements should be taken at the UPS input terminals, not only at the generator output.
Record:
- Phase-to-phase voltage.
- Phase-to-neutral voltage.
- Frequency.
- Minimum and maximum values during transfer.
- Voltage THD.
- Current THD.
- kW and kVA.
- True power factor.
- Battery charging current.
- UPS operating status.
- Rectifier and bypass status.
Recommended Test Sequence
Test the system in a controlled sequence:
- Start the generator without load.
- Confirm stable voltage and frequency.
- Transfer the ATS to the generator.
- Connect the UPS with battery charging limited.
- Monitor voltage, frequency, and UPS status.
- Increase battery charging gradually.
- Add the remaining emergency loads one at a time.
- Repeat the test with partially discharged batteries.
- Confirm that the UPS static bypass remains available.
If the system works correctly when loads are introduced gradually, the main problem is likely load sequencing or excessive transient demand rather than insufficient generator capacity alone.
Practical Solutions
The following actions may improve UPS-generator compatibility:
- Enable Rectifier Walk-In.
- Limit UPS input current during generator operation.
- Reduce battery charging current.
- Use an ATS auxiliary contact to activate Generator Mode.
- Delay the starting of motors, pumps, and ventilation loads.
- Check the UPS input voltage and frequency limits.
- Measure voltage and current harmonics.
- Review governor and AVR performance.
- Check generator cable sizing and voltage drop.
- Test the full system under real operating conditions.
Conclusion
UPS-generator compatibility problems usually occur during the first few seconds after the load is transferred.
It is not enough to confirm that the generator produces 400 V and 50 Hz after stabilization. Engineers and technicians must record what happens at the exact moment when the UPS rectifier, battery charger, and other emergency loads are connected.
The investigation should focus on three points:
- The minimum voltage and frequency during load application.
- The combined demand of the UPS rectifier and battery charger.
- The sequence in which emergency loads are connected.
Installing a larger generator may help in some cases, but it will not correct improper UPS settings, excessive battery charging, poor load sequencing, or unstable control response.
The UPS and generator may both be healthy. The real problem may be how they operate together.
Prepared by Eng. Mohamed Badr