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Droop speed control system
Adjustable droop in electronic control system.


I have read a lot about the droop speed control here on this forum. While the concept is clear to me, I still don't understand one important thing. How droop can be adjustable on droop control systems? Since most control systems these days are electronic systems, I wonder how droop can be adjusted. Is it adjusted by changing the gain of the proportional controller or in some other manner?


Since droop speed control is proportional control, the proportional gains are changed depending on the characteristics of the prime mover.

Is there a settable deadband for the droop speed control settings? If so, how?

My specific applications are on an MHC system, Mark I system, Mark II system, and Mark VI system.

Answers are much appreciated.

Apparently, there is no such software block as a deadband. It is all bout increasing or decreasing proportional gain. Since, in control theory, proportional control will always have some offset error between setpoint and actual value, if you increase proportional gain, you'll reduce this error and vie versa.

In my understanding, this is the way to set droop. I'd like to be corrected if I'm wrong.

I think, at least with the most recent versions of Speedtronic turbine controls, the droop setting is "hidden" more or less by design. The Control Constant is adjustable, but it's not really "visible" in that it doesn't just jump out at you with a name like "DROOP".

I'm a little unsure about the 'deadband' part of your question.

MHC is usually a control system associated with steam turbines, and there were Mark I and Mark II EHC (Electro-Hydraulic Control) systems also used for steam turbine applications, which were not the same as the Mark I and Mark II Speedtronic control panels used on GE-design gas turbines. Beginning with Mark V Speedtronic turbine control systems, Speedtronic controls were applied to both steam and gas turbines, so Mark VI Speedtronic is used for both.

What I'm getting to is that your questions seems to be more related towards steam turbine applications, and I'm not very experienced the Mark "n" EHC controls or the MHC controls ("links and levers" I think was how some people described them).

My experience with steam turbine controls beginning with Mark V and more recent generations of the Speedtronic line is that steam turbine control philosophy has morphed over the years, and they don't seem to apply droop in the same way as in the EHC and MHC days. I think there's something approaching droop speed control, but with most steam turbines operating in "load following" mode these days (with the control valves more or less full open and power output a function of steam pressure/flow) that droop isn't so much of a "concern" (and I'm not sure if that's the proper word, but I can't think of a better one right now). Most steam turbines these days are "smaller", combined cycle applications. And I think the concepts and philosophies used with these applications have spilled over to retrofits of large, utility steam turbine units as well. Precise control of frequency with large steam plants would seem to be difficult under the best of circumstances, but that's just a swag on my part.

If you're looking for specific Control Constant names for Speedtronic turbine control systems, I think those have changed over the years as droop has changed from straight droop speed control to constant settable droop speed control (for heavy duty gas turbines).

Your best bet would be to review the control schematics and elementaries, the line-up drawings/diagrams, and the sequencing and application code for a particular machine. It's there, and it's usually variable, but it's not always the same parameter name or value.

When I say it's "hidden by design" I think it's done because some parts of the world seem to have a propensity for changing the droop setting without understanding how it is used and interacts with all the other control schemes (loading and unloading rates, etc.). So, while it's still there, it's "incognito". Just to keep things on the up and up.

I wish it were easier, but it's not!

By Namatimangan08 on 22 May, 2011 - 1:10 am
1 out of 1 members thought this post was helpful...

Let us back to basic. I don't know much about new technology of the droop but I can assure you as far as power generation related applications, the basic principle for the speed droop hasn't change by that much.

The droop

Assuming you a have a prime mover (GT) that has this design parameter.

No load RPM =2850

Full load RPM =3150

Rated RPM =3000

What do these parameter mean? At 2850RPM, fuel throttling position of your prime mover (Says at 20% position) is set at no load condition. If you open the throttling position up to 100%, your GT will be supplied by 100% fuel flow and its RPM will increase to 3150. The design droop for your GT, using rated RPM as base RPM, becomes

Design Droop = (3150-2850)*100/(3000) =10%

We can define the droop in term of percentage speed as above or in term of frequency bias constant or proportional gain. Assuming your prime mover has rated capacity 100MW. The calculation is as follows.

Capacity =100MW

Total frequency deviation = 300RPPM =5 Hz.

Frequency bias setting = 100/5Hz= 20MW/Hz =2MW/0.1Hz

This bias setting will be used by the prime mover's governor to bias its output if RPM of its shaft deviates from the reference RPM used by the governor. For example, if the actual shaft RPM increases by 6RPM (0.1Hz) the governor will subtract 2MW output from the current load set point or from the current load. If shaft RPM is lower than the reference by 0.1Hz then the governor will add 2MW.

Now, due to some reasons you want to have your prime mover's response from +/-2MW/0.1 to 4MW/0.1Hz. What you have to do?

During the old days you can achieve this by changing the droop knob with resolution up to 0.1%. In this case you can turn the knob from 10% to 5%. How it effects the governor control? You are actual changing range of RPM from minimum fuel supply to maximum fuel supply. Your new range will be 150RPM (Previous was 300RPM). For example, at 2850RPM the fuel is throttled at no load condition and at 3000RPM your prime mover will be fed with the maximum fuel flow. The relationship is assumed linear.

Note that we have similar regulation in voltage regulation. Voltage regulation is defined as

% Voltage regulation = (No load voltage-Full load voltage)*100/(Full load voltage)

It is important to consider this issue if you want to change the droop set point from the "agreed" percentage. Otherwise you may have the scenario in which no load voltage is not coincide with no load RPM of the droop or full load voltage may not coincide with full load RPM of the droop.

It is important also to note that normally droop set point is sanctioned by grid operator.

Dead band.

There are possible dead bands that you are referring to. I call them them (1) design dead band or design resolution (2) operational dead band

(1) Design dead band is simply the smallest frequency deviation required to trigger the droop to activate.

(2) Operational dead band has little thing to do with the droop itself. It is more about to meet one specific requirement of a grid operator.

Normally the grid management defines frequency regulation areas into three zones: (i) normal operating regime (2) abnormal operating regime and (3) System major separation.

For a big grid system such as in the US and Canada the droop response is not required by that much. Therefore the grid operator may not want to have big intervention from all the droop commands to adjust the loading within the normal operating regime. Secondary response from the selected power plants that operate under AGC-constant frequency mode is good enough to match supply and demand. Some of the plants such as conventional thermals shall be allowed to operate under temperature limiter. For that reason there should be the way to set the droop so that it can be deactivated within the selected frequency regime such as from 49.925-50.025Hz. The droop shall be activated automatically if any of these upper and lower limits is violated.

I've had the pleasure of getting up close with GE EHC and MHC turbines.

On a MHC mechanical governed turbine, the drop is set by adjusting the pivot points in the linkages in the governor system. There should be a linkage diagram with all the original documentation. A Woodward UG-8 governor actually has a knob on the face of it to set the droop. Set the droop to zero and you have a isochronous unit. I guess we can say it's PI control, but would PI control give the same effect as an infinite gain in P? But I digress.

Two EHC units I deal with are Mk2 retrofitted to MkV. I've discussed some droop details on these boards previously, but the droop is well hidden in the logic for the MkV.

And for some reason, GE likes to hide droop and gain in the logic while other governor makers such as Woodward are explicit with "droop" and other gains/deadband....and they document it--just for CSA :)