Droop control

Z

Thread Starter

ZestE0Man

We have a client who has 4 generators in a power farm primarily providing power for their own needs but also exporting power to the grid.

We supplied one turbine-generator; the others were existing. The problem is that in island situations our turbine often trips where the others don't. The client keep pointing at us that our governor is not responsive enough.

The complete picture is as follows:

1. Back-pressure Steam turbine-generator, 32MW, mechanical governor with direct plant load of 20MW with rest being "exported" to other parts of plant with 11kV distribution.

2. Reactive Steam turbine-generator, 34MW, mechanical governor with direct plant load of 28MW with rest being "exported to other parts of plant with 11kV distribution.

3. Our Back-pressure Steam turbine, 48MW, electronic governor with direct plant load of 5MW with rest being exported to grid.

4. Gas turbine-generator, 45MW, electronic governor with all it's load distributed in plant.

On top of this our turbine has steam extractions providing to the steam consumers on the plant as well, all individually controlled.

We have pointed out that because we have to shed 90% of our load during an island, whilst maintaining stable output to the steam consumers, the turbine cannot respond any faster and that they need to load our turbine more directly with the plant.

They keep saying it is our droop settings causing the instability (turbine trip on overspeed).

Your comments please!
 
<b>Based on the information provided,</b> a unit with one or more controlled extractions can't be expected to shed 90% of load quickly while maintaining pressure/flow to the extraction lines. It wouldn't seem that a single controlled extraction should be so difficult to tune, but multiple extractions might be more problematic.

You DIDN'T say if the GT or any other unit was switched to Isochronous control when the plant is islanded. If not, there must be some kind of 'power management system' or load shedding system to try to control frequency. It would seem to be more ideal to use the GT for frequency control since it's much faster (or should be!) to frequency fluctuations--especially if switched to Isochronous when islanded.

But, again, based on the information provided about the new unit it would seem expectations are too high--unless some salesperson made some promises that were a stretch of the possibilities with confirming with engineering. (I know that <i>NEVER</i> happens, but it could some day, somewhere, I suppose.)
 
CSA--

Sorry to hi-jack this thread, but I want to ask you a favor. I am new to the power generation business and I am trying to educate myself. Do you have any suggestions for textbooks that cover the usual subjects? (VArs, speed droop, AVR, etc.) Specifically, I am working in hydropower and am not afraid of higher math. Thanks for any help!!
 
Dignan,

There are many power generation references, such as 'Power Generation Handbook' and others. A good way to search for reference books or textbooks by name is to use Amazon.com or Alibris.com and search by topic.

The physical principles of AC power systems are all the same--regardless of prime mover. So, whether the prime mover is a hydro turbine or steam turbine it's all the same.

As for a good written description of Droop speed control, I have been is search of such a document for more than thirty years and have yet to find a decent, comprehensive and understandable explanation. Many are written by ivory-tower egg heads who try to say that generator and prime mover speed changes with load--and that's just plain theoretical hooey.

There has been a <b>LOT</b> written on control.com about Droop and Isochronous speed control--and while most of the detailed descriptions are for GE-design heavy duty gas turbine control systems the basic physical principles are the same. If you're not afraid of high maths (I prefer not to confuse people--myself included--with it) then Droop speed control is proportional control and Isochronous speed control is proportional plus integral control.

If you have specific (non-high maths) questions we can try to answer them here.
 
Thanks for the tips, CSA. I'll get started with those resources before I bother everyone with yet more questions on droop speed control. (I've already read several of your posts on the topic.) I have taken a course in process control (ChE) but it was nearly two decades ago. I can't really talk at work in terms of proportional control or PI. If people haven't studied process control they look at me like I've lost my mind. Process Control was the most difficult/fascinating course I ever took, and the math does get crazy.

Thanks again.
 
Are the PLANT connections of the 4 generators in parallel, or separate loads?

Is the 11kV distribution parallel, or separate loads?

Is the GRID tie opened under load, or is there a gradual reduction of export power before disconnection?

> We have a client who has 4 generators in a power farm primarily providing power for
> their own needs but also exporting power to the grid.
 
Dignan,

Look at www.basler.com, you will need to get a logon for downloads.

They have a lot of application information on voltage regulations, VAR sharing, etc.

Remember that all grid tie applications must be isochronous control. In islanded applications, droop can be used with no connection, or communication, between generators.
 
T
> The client keep pointing at us that our governor is not responsive enough.

If your turbine is tripping on over speed which is directly related to frequency, you might be able to raise the over speed setting on your machine so that it trips after the other machines. That would leave your machine running after the others are down.

> We have pointed out that because we have to shed 90% of our load during an island, whilst
> maintaining stable output to the steam consumers, the turbine cannot respond any faster
> and that they need to load our turbine more directly with the plant.

It may not be possible to maintain stable steam output when you shed that much load. Steam going through the machine makes MW. When the MW is limited by loss of the tie to the utility, the frequency will rise, with the speed until other machines are forced to back down by the droop setting or the plant grid frequency will increase until something trips.

SO, when the utility tie is lost, either the steam flow must be reduced to control MW generation or other machines must reduce generation to prevent over speed tripping.

> They keep saying it is our droop settings causing the instability (turbine trip on overspeed).

The droop setting does not matter if you are still trying to maintain steam flow/pressure at the rates present before the separation from the utility. If the MW load is lost, steam flow must be reduced to control speed. This may also cause the steam pressure control to go out of control.

> Your comments please!

Are the other machines in a parallel load sharing scheme when the plant goes island? If they are, and you are not tied into that load sharing system, your machine may indeed be the problem. It will not share load in island mode and indeed will try to maintain pressures on the controlled steam headers in spite of an over or under frequency condition.
 
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