In short. Reactive power is shared through the AVRs. If voltage settings differ, one set drives reactive current into another, which shows as circulating current and heat. Droop or cross-current compensation makes sets share kVAr in proportion to their rating.
Physics
Real power follows the engine, reactive power follows the excitation. A set with a slightly higher voltage setpoint supplies kVAr to the bus, and one with a lower setpoint absorbs it. Because the alternator reactance is small, even small setpoint differences cause large kVAr flows.
Sharing methods
| Method | How it works | Note |
|---|---|---|
| Voltage droop | Voltage falls slightly with reactive load | Simple; voltage varies with load |
| Cross-current compensation | CTs in a loop let sets balance without droop | Holds voltage; needs correct polarity and burden |
| Digital sharing | Controllers exchange kVAr data | Flexible; check fail-safe behaviour |
| Power-factor control | Holds a power factor when paralleled with the utility | Voltage then follows the grid |
With the utility
When paralleled to a stiff grid, the generator cannot set voltage. Voltage control is replaced by power-factor or kVAr control. Switching between voltage mode in island and power-factor mode in parallel is a key control step. See interconnection.
Common problems
- Circulating current: setpoint mismatch or poor droop. Shows as high ampere with low kW.
- Wrong CT polarity: sharing worsens instead of improving.
- Hunting: AVR gain too high.
- Loss of excitation protection trips: from low field. See generator protection.
Testing
- Check CT polarity and wiring before paralleling.
- Parallel at no load and record kVAr and current per set.
- Load in steps and watch kVAr sharing.
- Test the power factor mode against a known utility condition.
See kW and kVAr sharing.
Illustrative example
Two sets run at no load and show 60 A each, with no kW. This is circulating current from a voltage setpoint difference of about 1 percent. After matching the setpoints and enabling droop, the circulating current falls to a few amperes.
Reactive power is shared by the AVRs
Real power sharing is set by governors. Reactive power (kVAr) sharing is set by the voltage regulators. If two AVRs hold slightly different voltage setpoints, one machine over-excites and pushes reactive current into the other, a circulating current that heats both and uses capacity without serving the load.
| Method | Principle |
|---|---|
| Voltage droop (reactive droop) | Voltage falls as reactive load rises, forcing sharing |
| Cross-current compensation | CTs in a loop cancel the in-phase part and share the reactive part |
| Digital kVAr or power factor control | Controllers share by measured values |
Droop effect
A 4 percent reactive droop lets bus voltage fall by 4 percent between no load and rated reactive load at zero power factor. A voltage-sensitive load may need a correction by secondary control.
Grid-parallel operation
When the utility fixes the voltage, the generator’s reactive output is set by its excitation. Power factor control mode holds a set power factor, while voltage mode would swing reactive power wildly. Select the mode to match the situation.
Troubleshooting
- Check for circulating current by comparing kVAr readings: one positive and one negative suggests mismatch.
- Verify CT polarity and phase for cross-current wiring.
- Compare AVR setpoints and droop.
See kW and kVAr sharing and kW load-sharing control.
The master engineering question
With sets in parallel, is any machine supplying or absorbing reactive power beyond its capability curve, and would a setpoint change or AVR failure push it there?
What to send us
Send AVR and alternator data. Send your application details.
Questions engineers ask
What does circulating current look like?
Current with little or no kW, and sometimes warm alternators at no load.
Do I need droop on every set?
Usually, unless cross-current compensation is used on all sets.
Why is power-factor control used on the grid?
The grid sets voltage, so the generator controls its own kVAr.
What if an AVR fails?
Protection should trip the affected set; plan for a fall-back.
Values are illustrative. Follow manufacturer guidance.