Why won't two generators share load equally when running in parallel?

When paralleled generators refuse to share load, the first diagnostic step is to establish which kind of load is unbalanced, because active power (kW) and reactive power (kVAr) are controlled by entirely different equipment. Read the kW meters, the ammeters and the power factors together: machines with unequal kW have a governor-side problem; machines with similar kW but very different currents and power factors have an excitation-side problem, with reactive current circulating between them.

Active power sharing is set by the engine governors. Paralleled machines are locked to one frequency, so the kW each takes depends on its governor speed setting and its droop characteristic, typically a few percent of speed from no load to full load. If one machine hogs the load or sheds toward reverse power, the speed settings are mismatched, the droop settings differ, a fuel linkage is sticking, or a governor is faulty. The cure is to trim the governor speed setters, raising the lightly loaded machine and lowering the heavy one to keep frequency constant, and, if the imbalance drifts back, to check droop settings, linkages and the governors themselves. Watch for one engine whose output wanders: hunting suggests a governor or fuel supply fault on that engine, not a sharing adjustment.

Reactive power sharing is set by the automatic voltage regulators. Since paralleled machines share one busbar voltage, the split of kVAr follows each machine's excitation and its quadrature droop compensation. Unequal kVAr with circulating current points to mismatched AVR voltage settings, a faulty droop current transformer or its wiring, or a failing regulator, and is corrected by trimming the excitation and verifying the droop circuits. High current at normal kW is the classic sign; left alone it heats the windings and wastes generating capacity.

Instrumentation deserves suspicion too: a drifting watt transducer or a faulty meter can make good sharing look bad, so cross-check the switchboard meters against the power management readouts before adjusting anything.

Do not live with poor sharing. A machine pushed toward reverse power will trip on its reverse power relay, and the remaining machine then takes the whole load and may trip on overload, giving a blackout that started as a trimming problem. If sharing cannot be balanced, shed non-essential load, take the suspect machine off the board for investigation, and confirm both machines have matched settings and characteristics before running them in parallel for any length of time.