What are generators in parallel?
Generators running in parallel means that two or more units (or a single unit and the mains supply), of the same or different sizes are running with their output cables connected onto a single electrical bus. The process of getting generators to run in parallel is called synchronising. Synchronising generators involves matching the frequency and voltage waveforms of each unit prior to closing an output circuit breaker to combine the electrical supplies together.
Attempting to parallel generators when they are not in sync is not usually done. When it is done, all the supplies that are paralleled together (be they mains, or a generator) must be in sync instantaneously and this puts severe strain on the engines mechanically and the alternators electrically. Paralleling generators together without putting them in sync first is called "Dead Bus Synchronising" and is used in mission-critical applications where the speed of response requirement is so great, catastrophic failure is an acceptable risk, while generally unlikely.
Why use generators in parallel?
Generators are used in parallel to increase the total system size and increase redundancy. It may also be more cost effective to combine several smaller units in preference to one larger unit.
Increase Power by Paralleling Generators
Imagine you own a factory. You you have a single generator, 500kVA which you have had for 2 years. Business is really good and you want to expand, but you don't have enough power. In this instance you might consider adding a second generator to your system, lets say another 500kVA. This will give you 1000kVA of power.
Increase Redundancy by Paralleling Generators
By adding additional generators to your system you increase the redundancy and make the system more reliable. For example, you have a total power requirement of 2000kVA, but you need to buy a bigger generator to cover load-acceptance issues, say 3000kVA. Instead you could buy 6 x 500kVA units in parallel. In the event of a single failure, in the first solution, the 3000kVA unit would not be able to run, you would have no power. In the second situation, while one of the 6 x 500kVa units could not run, you would still have 2500kVA available, giving you some redundancy.
Reduce Capital Cost by Paralleling Generators
Adding additional generators may seem like it is an expensive exercise, but for many use cases the total system can actually be cheaper compared to other solutions. Diesel engines up to about 660kVA are mass produced, as they are used for many applications. Above this size, the number of applications and quantity produced falls significantly and because of the reduction in volume the price increases. Usually this means the cost per kVA is higher, when you get to 3000kVA in our previous example system above, the price can be several times higher. The costs of transport to some destinations is also a factor, with larger units needing special transport on RORO vessels, as opposed to standard container shipping vessels for smaller units.