Synchronous Generators are operated in a standalone pattern where the machine is simply connected to a load. It is said to be operating in Solo Mode. In Solo mode, the active and reactive loads are determined by the distribution.
Island Parallel Mode
When the generator is running in parallel with more than one generator in a synchronized condition, it said to be operating in island parallel mode. It is called "island mode" as the machine is not connected to the grid utility. In Island Parallel mode, the total plant load is determined by the distribution. However, the individual machine loads can be adjusted.
Grid Parallel Mode
When the generator is running in parallel to the utility, it is called Grid Parallel Mode. Once synchronized with the grid, the plant operator does not have any control over the frequency and the voltage. However, the active and reactive power on the machines can be controlled from the plant.
The three different modes have different mechanisms for control of the active and the reactive power.