Paralleling standby generators — synchronisation and droop control

The ATS for standby power guide covers the automated switchover between the grid and a single standby generator. This article covers a different situation: what is required to run two or more generators simultaneously on the same busbars — parallel operation — and why this imposes fundamentally different requirements than a single generator with an automatic transfer switch.

The synchronisation conditions before paralleling

Before a generator is connected to a busbar on which another voltage source (the grid, or an already-running generator) is already active, the following quantities must be nearly equal at the moment the breaker closes:

• Frequency: typically within about ±0.05 Hz of the busbar being synchronised to.
• Voltage: typically within about ±5% of the busbar voltage.
• Phase sequence and phase angle: the phase sequence (rotation, for example L1-L2-L3) must be identical, and the phase angle difference at the moment of closing must be small (typically less than about 10°).

A synchronisation system (automatic or manual, with a synchroscope) continuously monitors these quantities and closes the generator breaker only once all conditions are met simultaneously. If a generator is paralleled outside these limits, a sudden current surge and mechanical shock on the generator shaft occurs, risking damage to the winding and the drive train.

Note: this is a different kind of compatibility requirement than vector group compatibility for transformers (see the transformer vector groups guide) — with generators it concerns a momentary synchronisation condition before the breaker closes, whereas with transformers it concerns a fixed, structural property (the vector group) that does not change per switching event.

After synchronisation: droop control instead of isochronous control

Once two generators are paralleled, the load must be shared proportionally between them. This is achieved with droop control (typically a speed/frequency droop of 4–5%): the governor of each generator allows the speed (and therefore the frequency) to drop slightly as the load increases, following a fixed, proportional characteristic. Because all generators operating in parallel must maintain the same frequency (it is a single electrical network), this shared drop results in a load distribution proportional to each generator's capacity.

For reactive power (kVAr), a similar principle applies via voltage droop: the excitation control allows the voltage to drop slightly as the delivered reactive power increases, so that generators operating in parallel also share reactive power proportionally.

Note: isochronous control (where a generator tries to keep its frequency constant regardless of load) works excellently for a single generator, but leads to instability once multiple generators simultaneously try to regulate isochronously — both governors then "fight" to hold the frequency. For parallel operation of multiple generators, droop control (or one generator running isochronous as "master" with the rest in droop) is the standard solution.

Practical relevance

When designing or assessing an installation with multiple generators operating in parallel, it must be established whether the synchronisation system actually monitors all required quantities (frequency, voltage, phase sequence, phase angle) before closing the generator breaker, and whether the control strategy after synchronisation (droop on all generators, or one isochronous master with the rest in droop) delivers a stable, proportional load distribution.

Common mistakes

1. Running multiple generators isochronously at the same time — this causes an unstable, oscillating load distribution once more than one generator actively tries to hold the frequency.
2. Not checking the phase sequence when connecting a replacement or additional generator — a reversed phase sequence is not always detected by frequency and voltage monitoring alone, and can cause a severe, short-circuit-like current surge when the breaker closes.
3. Confusing droop control with a fixed, static load distribution — droop control is a dynamic, continuous regulation based on actual load, not a one-time fixed ratio between generators.

Related

• ATS for standby power
• Transformer vector groups & parallel operation
• Standby generator load testing
• Standby generators — §551