There have been a plethora of posts on droop speed control and Isochronous speed control modes of governors. At the right-hand edge of the menu bar at the top of every control.com page you will find a 'Search' term field and a '?' Help icon. I suggest you use the search help and then search for speed control and similar terms on control.com and you will find a lot of useful, and some not so useful, information.
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A speed governor is pretty much what it says: It's a governor (mechanism, control system) for a prime mover (a reciprocating engine, a turbine, etc.) for controlling the speed of the prime mover, usually controlling the amount of fuel or steam or water that is admitted to the prime mover. (A prime mover is the device used to supply torque to some other piece of equipment that produces some usable quantity but requires torque to do so. Examples are electrical generators and compressors and pumps; they all require torque to produce electrical power, move a compressible fluid, or move water or other fluids, slurries, etc.)
Speed is a pretty important control parameter for any prime mover. If a prime mover's speed is uncontrolled then usually the torque output of the prime mover will be unstable, and therefore unusable. If the speed is excessive, the prime mover can be catastrophically damaged, even resulting in personnel injury and death; so preventing overspeed conditions is very important. Controlling speed is pretty much at the heart of every prime mover control system in order to be able to provide usable and stable torque to some other piece of equipment.
In electrical power generation, speed control is very important because the frequency of an AC (alternating current) electrical system is very important; frequency stability is very highly desired (at least in most parts of the world).
Speed governors, sometimes just called governors, are pretty simple devices, and very ingenious devices, as well. They have been around for centuries, and have run the gamut from mechanical fly-ball governors to digital electronic governors. (You can learn about governors from studying the very simple, and old, fly-ball governors; big hint here!)
Many control systems used for prime movers do much more than simply control the energy being admitted to the prime mover; they control auxiliaries and other process-related equipments, to be an "all-in-one" control system for the entire power-producing complement of equipment.
Power doesn't just refer to electricity; it can be torque to drive a compressor or a pump, as well.
Use your preferred Internet search engine to start your search by looking for 'governors' and 'governor action' and similar terms. As you find relevant results and learn more about them, you can ask more specific questions here.
If you are in university or have access to a library with technical references, even simple encyclopedias, you can also find a lot of texts and reference materials on governors.
If you are a fresher at an industrial- or power plant, you might seek out one of the more mature engineers or supervisors and ask if they have any reference books or manuals or literature they would share with you and if you can ask questions if you have any after studying the materials.
So, remember, a speed governor is really a very simple, and limited, device. Most control systems used on prime movers these days have a speed control "governor" (algorithm; regulator; controller) as part of the system, but the control system does much more than just control the speed to control the power being produced, produce power stably, and protect the prime mover against catastrophic overspeed.
Hope this helps!