In short. Once generators are paralleled, two separate control loops decide how the load divides between them. The governors share real power in kW by controlling engine fuel, and the AVRs share reactive power in kVAr by controlling excitation. Either droop settings or an isochronous load-sharing line can be used for each. Poor kW sharing overloads one engine; poor kVAr sharing overheats one alternator and circulates current between machines. Both must be checked across the operating range.
Two problems, two control systems
Real power is what the engines deliver. If one engine is given more fuel than its partner at the same speed, it takes more of the kW. Reactive power is what the alternators exchange with the load and with each other. If one alternator is excited more strongly than its partner at the same terminal voltage, it supplies more of the kVAr, and in the extreme it pushes reactive current into the other machine. The governor cannot influence kVAr and the AVR cannot influence kW to any useful degree, so a site that reports uneven loading has to identify which quantity is uneven before touching any setting. A controller display showing kW, kVAr and power factor for each set is the first diagnostic tool; a panel that shows only amperes hides the distinction, because current rises with either imbalance.
The power triangle and what it means for sharing
kVA² = kW² + kVAr²
Here kW is the real power, kVAr the reactive power and kVA the apparent power, all measured at the same point and instant for a sinusoidal supply. The alternator is rated in kVA and heats according to current, so the kVA share is what limits it; the engine is rated in kW and is loaded only by the real-power share. Take two identical 500 kVA sets on a 640 kW, 480 kVAr load (illustrative). Shared perfectly, each carries 320 kW and 240 kVAr, which is 400 kVA at 0.8 power factor, or 80 percent of rating. If the kW share drifts to 380 and 260 while kVAr stays equal, the first engine is at 95 percent of its 400 kW rating. If instead kVAr drifts to 360 and 120 while kW stays equal, the first alternator carries 482 kVA, 96 percent of rating, at a power factor of 0.66, while the second runs at 0.94. Both drifts look like a small difference in amperes and both put one machine near its limit. The kW load sharing calculator and the kVAr and power factor sharing calculator work through other combinations.
Droop and isochronous control
| Aspect | Droop | Isochronous with load-sharing lines |
|---|---|---|
| Frequency or voltage at full load | Falls by the droop setting, typically 3 to 5 percent | Held constant at the set point |
| How sharing is achieved | Each machine finds its load from its own speed or voltage curve; no communication needed | Controllers exchange a load signal and trim each governor or AVR to equalise percentage load |
| Sharing accuracy | Depends on how closely the droop curves match; typically within 5 to 10 percent | Typically within 2 to 3 percent |
| Sensitivity to wiring faults | None | A broken or noisy load-sharing line upsets every set |
| Typical use | Mixed-make plants, paralleling with the grid, fallback mode | Island plants with identical controllers |
In droop, each governor is set so that speed falls slightly as load rises, and each AVR so that voltage falls slightly as reactive load rises. Two machines with identical droop curves and identical no-load set points settle at the same percentage load automatically, because any machine carrying more than its share runs its curve down to a lower frequency or voltage and sheds load until the pair balance. The penalty is that frequency and voltage drift with load. Isochronous load sharing removes the drift by letting the controllers communicate and bias each governor and AVR to the average load, holding 50 Hz and 415 V regardless. Almost every modern paralleling plant in India runs isochronous kW sharing between its own sets and switches to droop, or to a baseload mode, when paralleled with the DISCOM supply, because the grid fixes the frequency and the governor must then control kW directly. The kW load sharing control and kVAr and power factor sharing control pages go into the control architecture.
Cross-current compensation
Where AVRs are to share kVAr without voltage droop, a cross-current compensation loop connects the current transformers of each set in a loop so that each AVR sees only the difference between its own reactive current and the average. Any machine supplying more than its share has its excitation trimmed down. The CT polarity and the loop wiring are a frequent source of trouble at commissioning, and a single open CT secondary in the loop is both a sharing fault and a safety hazard.
Unequal machines
When a 750 kVA set is paralleled with a 500 kVA set, equal kW sharing in absolute terms would overload the smaller machine. The correct target is equal percentage loading, so the larger set carries 60 percent of the total and the smaller 40 percent. Load-sharing controllers handle this through a rating entry for each set; droop systems handle it naturally, provided the droop percentages are identical, because each machine’s curve is expressed in percent of its own rating. Response speed also differs between a mechanically governed older engine and an electronic common-rail engine, and the slower machine will momentarily carry more or less of every load step until the controls catch up. The load sharing systems page covers what is needed to pair machines of different ages and makes.
Diagnosing poor sharing on site
- Record kW, kVAr, voltage and frequency for each set at three load levels and compare the percentage loading, not the amperes.
- If kW is uneven and frequency is steady, suspect the governor bias, the load-sharing line or a difference in droop setting.
- If kVAr is uneven and voltage is steady, suspect the AVR voltage set points, the cross-current CT wiring or a mismatched voltage droop.
- If kVAr oscillates between sets, suspect AVR gain set too high or a reversed cross-current CT.
- If one set trips on reverse power at light load, check that the kW share is not drifting negative as the plant load falls below the minimum for the number of sets online.
Common mistakes
- Judging load sharing from the ammeters alone, which cannot distinguish a kW imbalance from a kVAr imbalance and leads to the wrong control being adjusted.
- Adjusting the voltage set point of one AVR to correct a kVAr imbalance, which shifts the bus voltage and moves the imbalance rather than removing it.
- Paralleling a new electronic set with an older mechanically governed set without matching droop settings, so the new set takes every load step and the old one idles.
- Leaving a cross-current compensation loop open or with one CT reversed after maintenance, producing reactive circulation and overheating.
What to send us
Send the controller make and model for each set, a table or photographs of kW, kVAr, voltage and frequency readings per set at two or three load levels, the AVR type and whether cross-current compensation is wired, and the governor type. A photograph of the load-sharing wiring between panels and any event log entries help. Use the application engineering enquiry and we will identify which loop is at fault and what to adjust.
Questions engineers ask
Why does one generator carry more load than the other when they are identical?
Identical nameplates do not mean identical settings. Small differences in governor droop, no-load speed, AVR set point or sensing calibration are enough to shift load. Measure kW and kVAr separately for each set, decide which is uneven, and adjust the corresponding loop rather than both at once.
What is a normal sharing tolerance between paralleled sets?
With isochronous load-sharing controllers, kW within 2 to 3 percent of rating and kVAr within about 5 percent is achievable across the range. With pure droop, 5 to 10 percent is typical. Anything beyond 10 percent on either quantity should be investigated before the plant is loaded heavily.
Can a generator share load with the DISCOM grid in the same way?
No. The grid fixes frequency and voltage, so droop-based sharing would push the set to full load or no load depending on tiny frequency variations. When paralleled with the grid, the controller must switch to a kW set-point or baseload mode and a power-factor set-point mode, and the arrangement needs DISCOM approval and protection.
Does unequal kVAr sharing damage anything if the total load is low?
It can. Reactive current circulates between the alternators even when the load is small, heating both stators and adding losses without doing work. A set running at a leading power factor because another set is over-excited can also lose stability. The imbalance should be corrected regardless of the total load.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.