For a ship genset, the set frequency is 60HZ so the prime mover has to be at 1800 Rpm and generates about 1800kva. Now when load increases or decreases (due to electrical demand), let's say the ship needs only 1000kva, how to provide the output power without messing with the speed of the prime mover (maintain the 60HZ)?
Shipboard genset, eh?
This response is to your question--a single shipboard genset (you didn't mention anything about multiple gensets operating in parallel!).
The governor of the genset prime mover should be operating in Isochronous Speed Control. Isochronous speed control will automatically adjust the fuel in order to supply the load AND maintain the proper speed--which is directly proportional (related) to frequency. If the load is 1800 kVA, the prime mover governor will supply the required energy to the prime mover to produce 1800 kVA while maintaining 60 Hz.
That's what a governor operating in Isochronous Speed Control mode does--specifically maintains rated speed (frequency) while adjusting the energy flow-rate into the prime mover to also maintain rated speed (frequency). The operator does NOTHING. The governor looks at speed (frequency), and adjusts the fuel as necessary to maintain rated speed (frequency) as load varies--as long as the load doesn't exceed the rating of the prime mover, nor go below zero kVA. The governor, when operating in Isochronous Speed Control mode maintains speed (frequency) AUTOMATICALLY and WITHOUT any operator intervention/adjustment. Period. Full stop.
The operator just chooses Isochronous Speed Control mode, and the governor does the rest. If the load is approaching the full load rating of the prime mover AND the load can be expected to exceed the full load rating of the governor, then the operator will need to synchronize a second generator and its prime mover to the first generator, with the second generator's governor set to operate in Droop Speed Control mode.
Some small "grids" (like a ship--which many ships aren't that small anymore!) may have a "Power Management System" which is an external load/frequency control system that attempts to maintain frequency as load varies automatically, without any human intervention. Most PMSs are NOT very good at this, though some are better than others.
But, the answer to your original question is: The operator does nothing more than choose Isochronous Speed Control mode for the genset's prime mover governor, and it will automatically adjust the energy flow-rate into the prime mover to maintain rated speed regardless of load (as long as the load doesn't exceed the full load rating of the prime mover). That's what Isochronous Speed Control mode does--and it usually does it very well, and without any human intervention at all.
It's only when a second genset is synchronized to the first that operator intervention is usually required, and then proper training is key to keeping the lights on and at rated frequency. But, that wasn't your question, and hopefully the above has answered your question. At least with the information provided (a single shipboard genset supplying the ship load while maintaining frequency (speed)).
In an AC power system, speed is directly related to frequency. And as load is added to or removed from an AC power system the initial reaction of the power system is to slow down (when load is added) or speed up (when load is removed). The Isochronous Speed Control mode of the prime mover governor immediately senses the change in speed--brought about by the change in load--and adjusts the energy flow-rate into the genset prime mover to return the frequency (generator speed) to rated (in this thread--60 Hz). It's as simple as that. The operator doesn't need lift a finger once the prime mover governor is set to Isochronous Speed Control mode--the governor does everything.