Load-Sharing Controls

In short. Once generators are paralleled, load-sharing controls decide how real power in kW and reactive power in kVAr are divided between them. Real power is shared by the governors through speed droop or isochronous load-share signalling; reactive power is shared by the AVRs through voltage droop, cross-current compensation or a kVAr-share bus. Sets should carry load in proportion to their ratings, proven by reading each set’s kW and kVAr under load.

Why paralleling is only half the job

Synchronising answers the question of whether a set can safely connect to the bus. Load sharing answers what happens in the hours afterwards. Two sets can close together perfectly and still divide the work badly, with one engine at its limit and the other idling, or with reactive current circulating between the alternators that heats both without useful power. The connection step is covered under synchronising controls; the question of which sets should be running at all belongs to the master generator controller; what follows starts at the moment the breaker is closed.

Real power is shared by the governors

In a paralleled plant all sets run at exactly the same frequency, so frequency can no longer tell an individual governor how much fuel to inject. Something else must allocate kW.

Droop

Under speed droop each governor is set so that its speed reference falls slightly as load rises, typically by 3 to 5 percent from no load to full load. When sets share a bus, each settles at the load where its droop line crosses the common frequency, so a set with the same droop percentage carries the same percentage of its rating. Droop is simple, needs no communication between sets, and is tolerant of controller failure. Its drawback is that bus frequency falls as plant load rises, by perhaps 1 to 2 Hz at full load, which some loads will not accept.

Isochronous load sharing

Isochronous control holds frequency constant at 50 Hz regardless of load. Several isochronous governors on one bus will fight, with one taking all the load and the others shedding it, unless they exchange a load-share signal. The signal is carried on dedicated analogue load-share lines between the controllers or on a digital bus such as CAN, and each controller biases its governor until its percentage load matches the plant average. The result is constant frequency and proportional sharing, at the cost of a communication dependency that must have a defined fallback, normally a reversion to droop if the share signal is lost.

Proportional sharing with mixed ratings

Sets of different sizes should share in proportion to their capacity.

P_i = P_total × (S_i / ΣS)

Here P_i is the real power that set i should carry, P_total is the plant load, S_i is the rated capacity of set i and ΣS is the sum of the rated capacities of all sets online. Worked example (illustrative). A 500 kVA set and a 250 kVA set, both at 0.8 power factor, carry a plant load of 480 kW. The larger set should carry 480 × 500/750 = 320 kW and the smaller 160 kW, each at 80 percent of rating. Split equally at 240 kW each, the smaller set would be at 120 percent and trip on overload. The arithmetic for your own plant can be checked with the kW load-sharing calculator.

Reactive power is shared by the AVRs

Voltage, like frequency, becomes common to all paralleled sets, so the AVRs need their own sharing mechanism. If one AVR holds a slightly higher voltage setpoint than the others, it over-excites its alternator, exports reactive power, and the others absorb it. The circulating kVAr adds current in every alternator without adding kW. The symptoms are one set at a lagging power factor well below 0.8 and another near unity or leading, and alternators running hotter than their kW suggests.

Whatever method is used, the AVRs must have matched characteristics, the droop CTs must be on the same phase and polarity on every set, and the AVR gain must be stable at the plant level, not merely for a set alone. Excitation systems and their adjustment are covered under alternator AVR, and the whole kW and kVAr picture is explained in kW and kVAr load sharing.

Transient sharing and load steps

Steady-state proportional sharing does not mean a sudden load step divides proportionally. When a large motor starts, the first few hundred milliseconds of its demand are met according to the sets’ inertia and sub-transient reactance, and only afterwards do the governors redistribute. A plant of one large and one small set can see the small set take a disproportionate share of the step. Transient sharing is checked by applying a step with a load bank and recording each set’s kW trace, and the governor dynamics are tuned together rather than one set at a time.

Reverse power as a load-sharing symptom

If one set’s governor drifts low or its fuel supply falters, the other sets push it until it absorbs power and motors. Reverse-power protection, usually set at 5 to 10 percent of rating with a short delay, trips the set before the engine is harmed. A reverse-power trip is often reported as a protection nuisance when it is really a load-sharing or fuel fault that the relay has correctly detected.

Verifying sharing at commissioning and after changes

Load the plant in steps to at least 50 percent, and record kW, kVAr, power factor, voltage and frequency for each set at each step. Sharing is acceptable when every set is within a few percent of its proportional share in both kW and kVAr. Then remove one set under load and confirm the remaining sets pick up its share smoothly without a frequency excursion that trips anything. Save each controller’s configuration, note the droop or share settings, and repeat the test whenever a controller, AVR, governor or engine is replaced. Load banks for this work are described under load-bank testing.

Common mistakes

• Judging sharing from total plant kW. The total looks fine while one set is at 110 percent and another at 40 percent, and the overloaded set trips on the next load step.
• Configuring several isochronous governors without a load-share connection. The sets fight, one takes all the load and the others swing towards reverse power.
• Ignoring kVAr while tuning kW. Circulating reactive current overheats the alternators even though real power is perfectly divided.
• Fitting droop CTs on different phases or with reversed polarity on one set. That set’s AVR sees the wrong signal and drives the plant voltage sharing apart.

What to send us

Send the rating and nameplate of each set, the make and model of each genset controller, governor and AVR, and a single-line diagram of the bus. If sharing is poor, a screenshot or log of each set’s kW, kVAr and power factor at the same moment under a known plant load tells us more than a description. Use the application engineering enquiry for new paralleling plants or for a plant that does not share correctly.

Questions engineers ask

Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.