Generator parallel operation connects two or more sets to a common bus so they share the load. The closing conditions are strict — voltage magnitude, frequency, phase sequence and phase angle must all match at the instant the breaker closes — and once closed, active load in kW is shared through the governors while reactive load in kVAr is shared through the automatic voltage regulators. Parallel systems are chosen for redundancy, fuel efficiency at part load, and maintainability without shutdown, not simply for extra capacity.
Why Parallel Instead of One Large Set
A single large machine is simpler, so there has to be a reason to accept the extra controlgear. The reasons that justify it are usually one of these:
- Redundancy. An N+1 arrangement keeps the critical load supplied when one set is out of service.
- Fuel efficiency at part load. Three sets each running near their efficient load point often burn less fuel than one large set running lightly.
- Maintenance without outage. Individual sets can be taken offline while the bus stays energised.
- Phased expansion. Capacity is added as the site grows, rather than purchased upfront.
- Transport and space limits. Two smaller sets may be the only option where access or floor loading restricts a single large machine.
The Four Conditions for a Safe Close
Closing a breaker onto a live bus with mismatched conditions produces a large circulating current and a mechanical shock to both machines. All four of the following must be satisfied:
- Voltage magnitude. The incoming machine's voltage must be close to bus voltage; a difference drives reactive circulating current.
- Frequency. The incoming machine must be at very nearly the same frequency, with a small deliberate offset so load picks up smoothly rather than motoring.
- Phase sequence. The phase order must be identical. This is checked once at commissioning, but it must be checked.
- Phase angle. At the instant of closing, the phase displacement must be close to zero, which is why synchronisers close slightly ahead of alignment to account for breaker operating time.
How synchronisation is achieved
Three approaches are in common use. Manual synchronisation with synchronising lamps or a synchroscope puts the decision in the operator's hands. An automatic synchroniser adjusts speed and voltage, then issues the close command when conditions are met. A check-synchronising relay adds a supervisory layer that blocks the close if any condition is outside limits, which is good practice even on automatic systems.
How Load Is Shared
Sharing splits into two independent problems, and confusing them is the most common source of trouble on parallel installations.
Active load sharing — kW
The table below summarises the two control modes used for generator parallel operation.
| Mode | Behaviour | Where it is used |
|---|---|---|
| Droop | Frequency reference falls slightly as load rises, so machines with matched droop share load in proportion to rating | Two or more sets on a common bus; self-correcting and stable |
| Isochronous | Frequency held constant regardless of load | A single set, or one lead machine in a group |
Active power follows the engine. Each machine's governor controls fuel, and therefore kW. Two control modes exist. In droop mode, a machine's frequency reference falls slightly as its load rises, so machines with identical droop settings share load in proportion to their ratings in a stable, self-correcting way. In isochronous mode, a machine holds frequency constant regardless of load, which is used for a single set or for one lead machine in a group.
Reactive load sharing — kVAr
Reactive power follows excitation. If two machines have different AVR settings, they will circulate reactive current between each other even while delivering the correct total, heating both alternators and wasting capacity. The standard correction is reactive droop compensation, sometimes called cross-current compensation, which biases each AVR in proportion to its own reactive current so that kVAr divides according to rating.
Circulating current that persists after correction usually indicates either mismatched droop settings, an incorrect current transformer polarity, or machines with different voltage setpoints.
Paralleling With the Utility
Running in parallel with the mains is a different proposition from paralleling generators with each other, because the utility is effectively an infinite bus. Typical applications are peak shaving, where the generators reduce imported power during high-tariff periods, and soft loading, where the set is loaded before transfer to avoid a step change on the plant.
The protection requirements are correspondingly stricter. Anti-islanding protection is mandatory, and the package normally includes reverse power protection, over and under voltage and frequency relays, rate-of-change-of-frequency protection, vector shift detection, and a synchronising check on the tie breaker. Interconnection rules are set by the local utility and by the applicable interconnection standard, so the protection specification must be agreed with the network operator before design is frozen.
Control, Protection and What to Specify
A parallel system needs more than a breaker and a controller. The items below should be on the specification:
- Governors with adjustable droop, or electronic governors with load-sharing lines
- AVRs with reactive droop or cross-current compensation
- Automatic synchroniser plus a check-synchronising relay
- Reverse power and reverse reactive protection on each machine
- Tie and machine breakers with electrical and mechanical interlocking
- Current transformers with correct polarity for sharing and protection
- A load management controller capable of start/stop by demand and load shedding
- Metering that shows kW and kVAr per machine, not only bus totals
Load management is what turns a parallel system into an efficient one. A controller that starts and stops sets according to measured demand keeps each machine in its efficient band, which is where the fuel savings come from.
Problems Seen on Site
| Symptom | Likely cause | What to check |
|---|---|---|
| Breaker will not close | Synchronising window too narrow, or a condition out of limit | Voltage and frequency match, phase angle window, check-sync relay settings |
| kW hunts between machines | Droop settings mismatched, or isochronous mode on more than one machine | Governor mode and droop percentage on each set |
| kVAr circulating between machines | AVR voltage setpoints differ, or reactive droop not enabled | Cross-current compensation wiring and CT polarity |
| One machine takes most of the load | Different droop percentages relative to rating | Normalise droop to the same percentage of each rating |
| Neutral or earth circulating current | Multiple neutral-earth bonds on the bus | Only one neutral-earth bond should exist |
| Bus voltage drifts with load | AVR in droop without compensation, or sensing error | AVR mode, sensing wiring, voltage setpoint |
State the operating philosophy as well as the ratings, because it determines the control architecture:
- Number of sets, individual ratings, and whether the configuration is N, N+1 or N+2
- Whether the system parallels generators only, or also the utility
- Required load-sharing accuracy in kW and kVAr
- Preferred synchronisation method and whether manual backup is required
- Load management logic, including start/stop thresholds and shedding priority
- Protection list agreed with the utility where the mains is involved
- Site conditions — altitude, ambient temperature, enclosure, and whether sets are co-located
Large-frame machines such as the Cummins 1000 kW diesel generator are frequently deployed in parallel configurations, and gas generator sets are often paralleled in continuous-duty CHP applications where load following matters. The transfer arrangement should be designed together with the automatic transfer switch logic, since the two determine how load is applied after a mains failure.
FAQ
Can generators of different sizes run in parallel?
Yes, provided load sharing is proportional to rating. That requires matched droop settings expressed as a percentage of each machine's rating, and compatible governor and AVR control modes.
Do the generators need to be the same make?
Not necessarily, but it is easier if they are. Different excitation and governor designs can be made to share load, but the settings work is greater and the result is more sensitive to drift.
What is droop, in plain terms?
It is a deliberate, small drop in frequency reference as load increases. Because every machine in the group follows the same rule, they settle into a stable share of the load without communicating with each other.
Why is reverse power protection needed?
If a machine's engine loses output while its breaker stays closed, the bus drives the alternator as a motor. That is damaging to the engine and it hides a failure, so the protection trips the breaker.
Is paralleling always more fuel-efficient?
No. The saving comes from keeping each machine near its efficient load point. If the load profile is flat and close to a single machine's rating, one set is simpler and may be equally efficient.
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