Generator load sharing methods


Thread Starter


I've heard of the following methods, but need a "Dummies Guide" explanation of what these are and how they work - Layman's Terms Please.

Methods heard of:

- Synchronous or Asychronous or Sonchrounous!?
Was said so quickly I didn't catch this term properly, I guess it's one or the other...

- Astatic Loop

- Droop (Voltage droop is it?)

Any help anyone can provide me with, even if it's only a pointer to a good website for explanations of these.


Will the two units be operated independently of any electrical grid (sometimes referred to as "island mode")?

Will there be any transformer(s) between the outputs of the diesel's generator and the gas turbine's generator?

In general, generators which are connected to each other and also in parallel with other generators on an electrical grid are all operated with their prime movers' governors (the diesel or the gas turbine control system) in what's known as Droop Speed Control mode. Droop Speed Control allows generators to "share" the load of an alternating current (AC) electrical grid without having to worry about the frequency of the grid. (The concept is that there is some other "larger" generator and prime mover out there on the grid that is acting to modulate its power output in order to maintain a constant frequency. The reality, especially for large, or "infinite," grids is usually different.)

When an prime mover and its generator are being used to control grid frequency, the prime mover's control system is usually operated in what is known as Isochronous Speed Control mode--and the governor very tightly controls the power output of the unit to maintain a stable grid frequency as loads (lights, motors, heaters, air conditioners, etc.) are turned on and turned off.

If your installation is going to be operated while not connected to an electrical grid with other generators, then one of the units will need to be operated in Isochronous Speed Control mode and the other will need to be operated in Droop Speed Control mode. This is sometimes referred to as "island mode"--where the load being supplied by the generators is like an island in an ocean, not connected to any other islands.

If you use the search feature of you can find LOTS of discussions of Droop Speed Control mode as it is implemented on GE-design heavy-duty gas turbine-generators. The concept is basically the same for other machines, they just phrase their feedback and control loops differently. It's VERY difficult to connect AC generators (more correctly called alternators) in parallel with each unless the prime movers are operated in some kind of Droop Speed Control mode. If two or more generators with their prime movers in Isochronous Speed Control mode are connected in parallel with each other, they will usually "fight" each other for control of the load and the frequency--and the result is usually wild load- and frequency swings, sometimes even black-out!

There is also an excellent website operated by a Canadian entity:

This link is to some information about synchronous generators (also known as alternators or AC generators). There is also some other really good information on that site about Droop- and Isochronous Speed Control modes.

The manufacturers of the control systems for the prime movers at your site should have some information about the various speed control modes available on the systems.

If there are not transformers between the outputs of the generators, then the generator exciter regulators will need some tuning, also (sometimes called "voltage droop"). You can find some references to that in the exciter regulator manuals, possibly even on the Canteach website.

You were probably listening to a Texan tell you about paralleling generators--that's why it was difficult to understand....and most probably, incorrect, too! It sure would be nice to know who told Texans they were the only people who knew how to build and operate power plants ('cause it just ain't true!). They might figger out this or that, but it's the extremely rare Texan who can 'splain it to anyone else so they can understand what is happening or how to do it. They're also famous for saying, "That's not how we did it on the last job!" (Of course, they never seem to remember exactly how it was done on the last job--only that this is different from the way it was done on the last job!)

And this author has always wondered why most Texans are always just starting a new job or looking for a new one.?.?.? Guess that's so they can always say, "That not how we did it on the last job!"


(Seriously, though, we shouldn't pick on Texans. They make it too easy!)

pls tell me when we feed a load independent of the grid, what will be no-load frequency and nominal load frequency? if it is like 62 hz and 60 hz (nominal load) so why is this so? I mean do we have half load and 61 hz so why cant we have 60 hz at half load?
Under stable load condition in the range from 0-100% the out put frequency shall be 60 HZ the variation in frequency to higher side is when loads are normally thrown off and lower side when load are added the variation in frequency and stabilization time depends on the governor normally classified as A1,....etc.

Vinay Jadhav

Reply: yes is it Voltage droop method & one more method is Isochronous for Load sharing.

>Methods heard of:
>- Synchronous or Asychronous or Sonchrounous!?
>Was said so quickly I didn't catch this term properly, I
>guess it's one or the other...
>Reply: yes is it Voltage droop method & one more method is
>Isochronous for Load sharing.
>>Methods heard of:
>>- Synchronous or Asychronous or Sonchrounous!?
>>Was said so quickly I didn't catch this term properly, I
>>guess it's one or the other...

Generator exciter control systems (AVRs; exciter regulators) can use voltage droop for controlling generator terminal voltage under some applications.

One has to remember: The generator is really a pretty dumb device. It converts torque into amperes (in the same way that motors convert amperes into torque--only in reverse; in fact motors and generators can be used as generators and motors if conditions are right, physically there's little difference mechanically between motors and generators (besides the differences between induction and synchronous machines)). It's really the prime mover governor that controls how much torque is applied to the generator to be converted into amperes.

Prime movers (driving generators) are usually operated in one of two basic modes: Droop Speed Control or Isochronous Speed Control. BOTH have been covered in great depth here on (speed) (sometimes it feels that way!). Droop speed control is NOT to be confused with voltage droop control--which is one method of controlling generator terminal voltage, and generator terminal voltage is not usually controlled by the prime mover governor.

Generator terminal voltage is usually controlled by the AVR, or exciter, or exciter regulator.

The amount of power (Watts) produced by a generator is controlled by the prime mover governor.

The amount of reactive power (VArs) produced by a generator is controlled by the generator AVR/exciter/exciter regulator.

Lastly, Droop Speed Control is the prime mover mode that allows multiple generators and their prime movers to be synchronized together on a grid supplying electrical loads much larger than any single generator could supply by itself. Synchronization means all the various generators connected together ("in parallel", or synchronized together) are operating as one very large generator, supplying one very large load (which is the sum of all the lights and motors and televisions and tea kettles and computers and computer monitors receiving energy from the grid). And, it's Droop Speed Control that allows all the generators and their prime movers to behave smoothly and stably while synchronized together.

If all prime movers and generators were synchronized together and operating in Isochronous Speed Control the grid would be VERY unstable, and the prime movers and generators would fight each other with very damaging results.

So, it's Droop Speed Control that allows the prime movers to smoothly and stably control the energy flow-rate into the prime mover, which controls the amount of torque being applied to the generator, which determines how many amperes are produced by the generator.

And since Power is equal to Volts multiplied by Amperes, and since generator terminal voltage is usually fairly constant, if one wants to produce more power one has to produce more amperes--which means one needs to apply more torque to the generator from the prime mover. Less power from the generator? Reduce the amount of torque being applied by the prime mover by reducing the amount of energy flowing into the prime mover.

It's all as simple as that. Really; it isn't any more complicated than that. There's generators--which can be driven by (have torque applied by) reciprocating engines, or steam turbines, or combustion (gas) turbines, or hydro turbines, or wind turbines. The prime mover is just a means of producing torque, which when applied to a generator which is synchronized to a grid converts the torque into amperes. Wires transmit the amperes to MANY different locations which can be quite far from the prime mover and generator, and those devices at the ends of the wires then convert the amperes back into some form of energy (light, heat, torque, "virtual torque" (computers and computer monitors and televisions).

And, the prime mover governor controls the amount of energy flowing into the prime mover which the prime mover converts to torque which the generator converts to amperes. And the governor can be controlled in one of two basic modes: Droop Speed Control, and Isochronous Speed Control.

And, that's that.