In short. Generator paralleling connects two or more diesel sets to a common bus so they behave as one power plant: capacity is added in modules, sets can be serviced while others carry the load, and sets start only when demand needs them. It requires synchronising before each breaker closes, kW and kVAr sharing afterwards, reverse-power and fault protection, and a bus rated for the combined fault current. Paralleling gives redundancy only when there is spare capacity beyond normal demand.
Why sites parallel generators
A single large generator is the simplest standby plant and, up to a point, the cheapest. Beyond that point, and for reasons other than cost, multiple sets on a common bus become the better architecture.
- Modular capacity. A plant that needs 2,000 kVA can be built as two 1,000 kVA or four 500 kVA sets, which are easier to transport, install in a Hyderabad basement and service, and allow the plant to grow in steps.
- Redundancy. With one more set than the load requires, any single set can fail or be serviced without loss of supply. This is the N+1 architecture that data centres, hospitals and pharmaceutical plants specify.
- Efficiency at part load. Diesel engines burn disproportionately more fuel per kWh below about 30 percent load and suffer wet stacking. A plant that runs only as many sets as the load needs keeps each one in its efficient band.
- Staged operation. Large motor starts or transformer energisation can be scheduled when enough sets are online to absorb the transient.
The conditions for closing a breaker onto a live bus
The first set to start closes onto a dead bus and establishes voltage and frequency. Every subsequent set must be synchronised to that bus before its breaker closes: same phase sequence, voltage within a few percent, frequency within a fraction of a hertz, and phase angle within a few degrees. The generator controller trims the engine speed and the AVR setpoint to bring the set into that window, and a synchronism-check relay supervises the close independently. The procedure is the same one used for grid connection and is described under generator synchronisation. When several sets are ready at the same moment with the bus dead, the control system must arbitrate so that only one closes first and the rest synchronise to it; two sets closing onto a dead bus simultaneously is one of the classic ways a new paralleling plant damages itself during its first test.
What must be coordinated after connection
Once connected, each set is locked to the bus frequency and voltage and can no longer decide them alone. What it can decide is how much of the load it carries, and that decision is split between two independent control loops.
| Quantity | Controlled by | What goes wrong without coordination |
|---|---|---|
| Real power (kW) | Engine governor, through speed setpoint or load-sharing signal | One engine takes the whole load while another motors; reverse-power trips |
| Reactive power (kVAr) | Alternator excitation, through the AVR setpoint, droop CT or cross-current compensation | Circulating current between alternators, overheating, voltage instability |
| Frequency of the bus | Isochronous load sharing or droop among all governors | Frequency drifts with load, or governors fight each other |
| Voltage of the bus | AVR droop or shared voltage reference | Voltage drifts with reactive load |
The practical rule is that a plant which shares kW well may still share kVAr badly, because the two are controlled by different hardware. Both must be set up and tested, as described under load sharing.
Paralleling does not automatically create redundancy
If a site’s normal demand is 1,600 kVA and the plant is two 1,000 kVA sets in parallel, the loss of one set leaves 1,000 kVA for 1,600 kVA of load, and the survivor either trips on overload or the load management system sheds 600 kVA. The plant is parallel but not redundant. Redundancy requires that the sets remaining after a failure can still carry the load that must stay on: three 1,000 kVA sets (N+1), or two sets with 600 kVA declared non-essential and shed automatically. Which is right is a decision about the cost of capacity against the cost of shedding, and it belongs in the sizing study, not the switchgear specification.
Worked example (illustrative): a pharmaceutical plant with 900 kW of essential load at 0.8 power factor needs 1,125 kVA. Three 500 kVA sets leave 1,000 kVA after one failure, short by 125 kVA. Three 630 kVA sets leave 1,260 kVA and cover the load with 12 percent margin. The second arrangement is N+1; the first is not, although both are paralleling plants.
The common bus and its switchgear
The bus is where the sets combine and it is the part of the plant most often under-specified. Its current rating is the sum of the connected generator currents, and its short-circuit rating is the sum of their fault contributions plus any motor contribution from the load. Three 630 kVA sets, each able to deliver several times rated current into a fault, produce a bus fault level far above what any one would, and the generator breakers, bus coupler and feeder breakers must all be rated for it. The sub-transient reactance of each alternator sets its contribution; the method is under fault current.
Each generator breaker must be motorised or have a closing release, must have reliable auxiliary contacts for position feedback, and must trip from protection on a supply that does not depend on the bus being live. Alternator neutrals need a deliberate earthing scheme, because several solidly earthed alternators in parallel create paths for third-harmonic circulating current; the usual answer is to earth one neutral at a time through a neutral contactor, as covered under earthing and neutral architecture.
Protection for a paralleled plant
A single standby set needs little more than overcurrent, earth fault and its engine protections. A paralleled set needs reverse power, so that an engine which loses fuel is tripped before the others motor it; loss of excitation; over and under voltage and frequency on the bus; and a protection scheme that is selective, so that a fault on one outgoing feeder trips that feeder and not every generator breaker. Differential protection on larger alternators and the bus becomes worthwhile above a few megawatts. The settings depend on the number of sets online, which changes the fault level, and good relays allow setting groups to follow the plant state. See protection coordination.
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Common mistakes
- Calling a two-set plant redundant when both sets are needed to carry normal demand; the loss of one set still drops load.
- Rating the bus and feeder breakers for a single generator’s fault contribution; with all sets online a fault exceeds the breaker rating.
- Commissioning kW sharing and declaring the plant ready without testing kVAr sharing; alternators overheat from circulating current at full load.
- Solidly earthing every alternator neutral; third-harmonic current circulates through the neutrals and trips earth-fault protection.
What to send us
Send the load list or a demand profile with the loads you consider essential marked, the nameplates and controller types of any existing sets you want to include, the single-line diagram, the space available for the plant and switchgear, and the redundancy you are aiming for in plain words, for example “any one set can be out and the plant still runs”. We will propose the number and size of sets, the bus and breaker ratings, and the control and protection scheme. Start with the application engineering enquiry.
Questions engineers ask
Can generators of different sizes be paralleled?
Yes, if the load-sharing controls allocate load in proportion to each set’s rating and all governors and AVRs accept the sharing signals. A 500 kVA and a 1,000 kVA set sharing proportionally will each run at the same percentage load. Equal sharing between unequal sets would overload the smaller one.
Can two different makes of generator be paralleled?
Usually, provided both have electronic governors and AVRs with external bias inputs, and the controllers either speak to each other over a load-sharing line or a common master controller drives both. Alternator winding pitch and reactance should be compared, because mismatched machines circulate harmonic current.
Does paralleling save fuel?
It can. Running two sets at 30 percent load each burns more fuel per kWh than running one at 60 percent, and the lightly loaded sets suffer carbon build-up. A plant that starts and stops sets to follow demand keeps each engine in its efficient range, which is one of the main operating benefits of a paralleling system.
What is a dead bus start?
When all sets are off and the bus is de-energised, the first set to be ready closes its breaker without synchronising, because there is nothing to synchronise to. The control system must guarantee that only one set does this and that the others wait and synchronise to the live bus.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.