When a machine (the generator and its prime mover) is running in Droop Speed Control mode while synchronized to a electrical distribution and distribution system with other generators and their prime movers the speed AND frequency of all the machines synchronized together is controlled by the frequency of the transmission and distribution system. For the frequency of the system to be at or very near rated frequency the amount of generation connected to the system must be equal to or very nearly equal to the amount of load the system is supplying at any instant in time. Any excess power being produced by the generation assets connected to the system at any instant in time will result in the frequency of the system being greater than rated; any deficiency of generation versus the load being supplied by the system will result in the frequency of the system being less than rated. (Remember—without some sort of energy storage capability no more power than required to supply the load connected to the system. So any excess generation causes the frequency of the system to increase. And, unless there is any energy storage to supplement the amount of generation on a system if the amount of generation is less than the load connected to the system the system frequency will decrease.)
It is the electrical transmission and distribution system operators that have to be very diligent and understand the peaks and valleys of energy consumption of the system that have to balance the generation to match the load in order to keep the system frequency from drifting too far above or below the amount of load connected to the system. It’s a big job.
So, that’s what causes the system frequency to deviate from normal (rated). AND, because machine speed is directly related to frequency any change in system frequency will cause the speeds of all the generators and their prime movers to vary as frequency varies. Some parts of the world have very good system operators and controls; others, not so good).
And when machine speed varies because the frequency of the system the machines are synchronized to is varying that causes the loads of the machines (prime movers and the generators they are driving) to change as system frequency changes.
MW control is a method of using a load setpoint to control the load of a particular machine (generator and its prime mover). As long as the frequency of the system the machine is synchronized to is at or very near rated MW control works pretty well for its purpose. But, when the system frequency starts changing quickly with large swings in load then MW control usually doesn’t work very well. Most power plant operators and their supervisors and managers think when a system is experiencing large frequency changes and load swings THEIR machine should be producing power at a constant load and stable frequency while the rest of the machines synchronized to the same system are experiencing frequency variations and load swings. But one , or seven or 17 or 47 machines can’t run at frequencies different than any other machine synchronized to the grid—that’s just not how AC power systems work. No matter what anyone thinks or believes they should work.
Droop Speed Control doesn’t try to return a machine’s frequency or the frequency of the system a machine is synchronized to back to rated when the system is experiencing frequency and load swings. THAT’S the job of the system operators to work with individual machines and power plants to return the system frequency and have a stable system load. In fact, Droop Speed Control RELIES on an error between a machine’s speed reference and its actual speed to work properly—and one of the critical secondary aspects of Droop Speed Control is to change a machine’s load to try to support system frequency and load during periods of frequency excursions and load swings.
The topic of Droop Speed Control has been covered MANY times before on Control.com. It is a very simple and ingenious method of allowing multiple machines to all work together to share in producing a large amount of electrical power as if one machine was producing all the electrical power of a system to supply all the loads connected to the system. And Droop Speed Control does all of this without special dedicated wires connecting all the machines and power plants synchronized to a system. Because it’s all related to the frequency of the system—which is stable and at rated when the amount of generation matches the amount of load at any instant in time. Which, again, is the system operator’s responsibility.
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