In short. Reactive power (kVAr) sharing is controlled by each alternator’s voltage regulator. If two AVRs are not matched, one can push reactive current into the other, a circulating current that heats the windings without doing useful work. Droop or cross-current compensation makes units share kVAr in proportion.
Why kVAr sharing matters
- Unequal kVAr loads one alternator more than another, even when kW is shared well.
- Circulating current wastes capacity and heats windings.
- Voltage on the bus becomes unstable if regulators fight.
Methods
| Method | How it works | Notes |
|---|---|---|
| Voltage droop | AVR setpoint falls with reactive current, so units share naturally | Simple; slight voltage drop with load |
| Cross-current compensation | Current transformers in a loop let AVRs see each other’s reactive current without voltage droop | Better voltage regulation; wiring must be correct |
| Power-factor control | AVR targets a set power factor, often when paralleled to the grid | Not for islanded sharing alone |
| Digital load-share (communication) | Controllers exchange kVAr data | Common in modern plants |
Droop in numbers
A typical droop of 2 to 5 per cent at rated kVAr is common. Two identical units with equal droop share kVAr evenly, and the bus voltage falls slightly as reactive load rises, which a voltage trim can correct.
Common faults
- CT polarity or phase reversed in a cross-current loop, causing units to push kVAr against each other instead of sharing.
- Different AVR settings, gains or droop.
- A faulty AVR or sensing connection.
- Different alternators with different reactances.
- Mixed 3-phase and single-phase sensing.
Checks
- Record each unit’s kW, kVAr and current at several loads.
- Compare kVAr as a share of each unit’s rating.
- Verify droop settings and CT wiring.
- Step the voltage setpoint slightly on one unit and watch the kVAr change, to confirm sensitivity.
- Check the power factor of each unit stays within the alternator’s limits.
Worked example (illustrative)
Two 1,000 kVA units serve a load of 1,400 kW at 0.85 power factor lagging, so about 870 kVAr total. Equal sharing is 435 kVAr each. Measured: 650 and 220 kVAr. Unit one carries 65 per cent of its kVAr rating share more than unit two; its current is higher even though its kW matches. A cross-current CT found reversed on unit two is corrected, and sharing improves to 440 and 430 kVAr. The fault would not have shown in the kW readings.
Grid-connected operation
When paralleled with the grid, the grid sets voltage, and each generator’s AVR controls kVAr or power factor against it. Use power-factor or kVAr control mode, not droop alone, to avoid large reactive swings. See kVAr and power-factor sharing control.
Reactive sharing in numbers
Total reactive load divides among sets according to the voltage regulators. With reactive droop, a set’s share of kVAr follows its rating and its droop setting.
kVAr = √(kVA² − kW²); power factor = kW ÷ kVA
Worked example (illustrative)
Two 500 kVA sets share 600 kW and 300 kVAr. Total kVA = 671. Each carries 300 kW and 150 kVAr, 335 kVA each (67 percent), power factor 0.89. If one AVR setpoint is 1 percent higher, it may take 250 kVAr and the other 50 kVAr: kVA for the first = √(300² + 250²) = 390 (78 percent), and for the second = √(300² + 50²) = 304 (61 percent). The imbalance arises from a small voltage setpoint difference, which shows why reactive sharing needs droop or cross-current compensation.
Limits to watch
- Alternator capability: reactive output is limited by field heating.
- Circulating kVAr heats both machines without serving load.
- Power factor lower than the nameplate value reduces available kW.
Fixes
- Set equal reactive droop.
- Check CT polarity and wiring for cross-current compensation.
- Match AVR setpoints and verify at several load levels.
See kVAr sharing control and kW and kVAr sharing.
Common mistakes
- Checking only kW sharing.
- Wiring errors in cross-current loops.
- Running AVRs in different modes on the same bus.
What to send us
Send alternator and AVR models and any commissioning records. Send your application details. See also kW load sharing.
Questions engineers ask
What is circulating current?
Reactive current flowing from one alternator into another because of voltage differences. It heats windings without serving the load.
Droop or cross-current compensation?
Droop is simple; cross-current compensation keeps better voltage regulation but needs correct wiring.
Can kW sharing be good while kVAr sharing is poor?
Yes, because they are controlled by different loops: governor and AVR.
Does the grid change sharing?
Yes. The grid sets voltage and frequency; generators then control output by mode.
Figures are illustrative. Use manufacturer settings and commissioning data.