Generator paralleling

Paralleling means connecting two or more generators to a common bus so they act as one larger source. Each generator is synchronized to the others before its breaker closes, and the controls share the load between them. Sites parallel generators for more capacity, for redundancy so one failed set does not drop the load, and to service one set while the others stay available.

Why parallel instead of one big generator

A single generator is simpler, and for many buildings it is the right answer. Paralleling becomes attractive when:

Redundancy matters. With an extra set beyond what the load needs, often called N+1, a set can fail to start or trip without dropping the critical load. See data centre redundancy: N, N+1 and 2N.
The load is large. Very large single generators can be harder to source, transport and install than several smaller ones.
The load changes over time. Sets can be added as a campus or data centre grows.
Maintenance cannot wait for an outage window. One set can be taken out of service while the rest remain ready.
Light-load running is a concern. Running only as many sets as the load needs keeps each one reasonably loaded, which diesel engines prefer.

The trade-off is complexity: paralleling switchgear, controls and protection add cost and become critical equipment in their own right.

How a paralleling system is arranged

Each generator connects through its own breaker to a common paralleling bus in the switchgear. Feeder breakers on that bus supply the transfer switches, which serve groups of loads ranked by priority. Paralleling controls start the sets, synchronize them, close their breakers, share the load and add or remove loads as capacity allows.

A typical paralleling arrangement. Each generator closes onto the common bus through its own breaker once it is synchronized. Feeder breakers on the bus supply transfer switches grouped by priority, so life safety loads are connected first and optional loads last.

Synchronizing: matching before closing

Two AC sources can only be connected when their voltages match in size, frequency and phase angle at the instant the breaker closes. Closing out of sync forces the machines into step violently, with large currents and mechanical shock that can damage the engine, alternator, couplings and switchgear. The paralleling controls adjust each incoming generator's speed and voltage until it matches the bus, and a synchronizing check prevents the breaker closing until conditions are within limits.

Matching before closing. On the left the incoming generator's waveform is shifted from the bus, and closing now would jolt both machines. The controls adjust speed and voltage until, as on the right, the two waveforms line up, and only then allow the breaker to close.

Sharing the load

Once on the bus, the generators must share the load in proportion to their ratings, both real power (kW, set by the engines' governors) and reactive power (kVAR, set by the alternators' voltage regulators). If sharing is poor, one set can be overloaded while another idles, or reactive current can circulate between them and trip breakers. Modern paralleling controls handle this with communication between the generator controllers, and commissioning checks it at several load levels.

First-up, load add and load shed

In an outage, all sets start at once. The first set to reach rated voltage and frequency closes onto the dead bus, and the highest-priority loads, typically life safety, are connected to it straight away. Codes set how quickly emergency loads must be powered, so the design cannot wait for every set to synchronize. As the other sets synchronize and close, lower-priority loads are added in steps.

Loads follow capacity. The first generator on the bus carries the most important loads, and lower priorities are added as more sets close. If a set trips later, the controls shed the lowest-priority loads so the remaining sets are not overloaded and priority 1 stays powered.

Load shedding works the other way. If a set trips or the bus frequency sags because the load is too heavy, the controls disconnect the lowest-priority loads until the remaining capacity can carry what is left. The priority list is a design decision that has to be agreed with the owner and, for life safety loads, must meet code.

A typical case

A hospital expansion needs more standby capacity than its single existing generator provides. The owner wants life safety and critical care loads protected even if one generator fails, and wants to service any generator without an outage window.

The design adds two new sets of the same model and paralleling switchgear, with the three sets sized so that any two can carry all life safety and critical loads. Optional loads such as some kitchen equipment and non-essential mechanical plant are ranked lowest and are the first shed. The existing generator is evaluated; because its controls cannot parallel with the new sets, it is reassigned to serve a separate building on the campus rather than forced into the new system.

Commissioning includes pulling the utility supply with the building occupied in a planned test, failing a set to start, and tripping a set at load to confirm that load shedding leaves the essential loads untouched. The sequence of operation and the priority list are then written into the facility's emergency procedures, so staff know which loads to expect back first.

Paralleling with the utility

Some systems also parallel the generators with the utility, briefly for a closed-transition return to utility power without a blink, or for longer periods for peak shaving or testing under real load. Any paralleling with the utility needs the utility's approval and interconnection requirements, and protective relaying to separate the generators from the grid if the utility fails. Rules differ by utility and province. Our guide to transfer sequences explains open and closed transition.

Design points that are easy to miss

Single points of failure. The paralleling bus, the master controls and the DC control power can each become one. Look at what happens if any one of them fails.
Grounding. Multiple generators on one bus need a consistent grounding arrangement to avoid circulating currents and to make ground fault protection work.
Protection coordination. Breakers and relays must be set so a fault on one feeder trips only that feeder, not the generator breakers.
Matching sets. Generators of the same make and model share load most easily. Mixing types is possible but needs more careful control design.
Testing. Commissioning should test real outages, a set failing to start, a set tripping under load and load shedding, preferably with a load bank. See generator load bank testing.

Switching and testing are specialist work. Paralleling switchgear operates at high fault levels, and manual paralleling or changes to settings can damage equipment or injure people. Leave operation outside the automatic sequence, and all changes to settings, to trained and qualified technicians following written procedures.

How ehZee can help

We design paralleled generator systems, including switchgear arrangements, priority and load-shed schemes, protection and sequences of operation, as part of our emergency and standby power design service. Drawings are sealed by a licensed professional engineer where required and installation is by licensed electrical contractors.

Common questions

Can we add a second generator to our existing one?

Sometimes. It depends on whether the existing generator's controls can parallel, on space for paralleling switchgear and on how the distribution is arranged. In some cases a second generator serving a separate set of loads is simpler than paralleling.

Is paralleling more reliable than one generator?

With N+1 redundancy and a good design, it can be, because a single set failing does not drop the load. Without redundancy, more sets and more controls can mean more ways to fail. The design has to be built around redundancy to gain the benefit.

How long does synchronizing take?

It varies with the equipment and settings. Because of that, the design makes sure the most important loads do not depend on every set synchronizing; the first set on the bus picks them up.