I have been studying the GE MK III Governor circuit and descriptions. I am puzzled as to how the Governor achieves Droop control. It was my understanding that droop involved the load change being sensed, being used to modulate the speed setpoint. With what I understand about the MK III it modulates the output as a proportion of speed change.

Can any one shine some light on my understanding of this?

regards
Andy

 

Andy,

I've seen a lot of GE Speedtronic heavy duty gas turbine controls, but have never seen a Mark III Speedtronic heavy duty gas turbine control. The Speedtronic heavy duty gas turbine line "skipped" from Mark II to Mark IV, though there are rumors that a Mark III was produced (actually more than one), but I've never seen one or heard of one in operation.

I wonder if you have a Mark III EHC (Electro-Hydraulic Control) Governor steam turbine control system that was applied to a non-GE gas turbine, but that's just a guess. Maybe the Mark III Speedtronic panels that were produced were only applied to non-GE gas turbines. The Mark series EHC controls were produced primarily for steam turbines, and although they were also called "Marks" they were very different from the Mark Speedtronic heavy duty gas turbine controls.

It wasn't until GE started using the Mark V Speedtronic for steam turbine control that the gas- and steam turbine controls became more common (from a hardware perspective).

In general GE does droop speed control by comparing a turbine speed reference (setpoint) to actual turbine speed. In some cases, it is biased by load feedback, but not always. Since speed is proportional to frequency, a frequency change is proportional to a speed change, and droop speed control is about trying to "support" grid load during frequency excursions. If the turbine speed reference is constant but the actual turbine speed changes, then the error between the reference and actual changes, and the energy input to the turbine changes. If the error increases, the energy input will increase; if the error decreases, the energy input will decrease.

Not being familiar with the Mark III at your site, it would be really difficult to say how it's being done.

In your previous post you seemed to be saying that you were upgrading the controls, or maybe that you were looking at integrating some kind of external load control into the existing control system. If it's the former, then one would think the integrator or supplier of the control system would be capable of providing a system with droop speed control which would be acceptable to the grid regulator(s) in your area, be it done with speed control or load control, or some combination of the two.

If it's the latter, you should be able to see how the operators increase or decrease load (with push-buttons or bat-handle switches or some kind of potentiometer), and then determine how to integrate the external load control into the system.

If you're trying to determine what the droop setpoint is, for most gas turbines it's usually about 4%, sometimes 5%. But, you should have some kind of line-up instructions or control system adjustment instructions or control system settings drawing(s) to explain what the droop setpoint is and how to adjust it or calibrate it. (That may not be on the main drawings, but on some other related document supplied with the control sytem.)

But, it would seem that others here on control.com are not familiar with GE Mark III controls (EHC or Speedtronic) either, or they would have written in earlier.

 

Hi Andy and CSA,

Nice explanation from CSA.

I'm particularly interested in Steam turbine droop (deviating from the topic a bit I reckon). In my steam turbine, when in Speed control (and connected to the grid), there is a (considerable) difference between the Speed Reference and Speed Actual. About as much rpm as the machine load in MW. Thus, in essence, the Speed reference never equals the speed actual and there is always an error. Is that normal how a stock Speed Controller works for the Steam Turbine.

If you want the details, I can surely provide.

Ni10.

 

This is precisely how most governors work in droop speed control. Droop speed control is straight proportional control. As the error increases, the amount of steam (or fuel) is increased, and the amount of torque produced increases. If the error decreases, the amount of steam (or fuel) is decreased, and the amount of torque produced decreases.

It's important to note that for a synchronous generator connected in parallel with other synchronous generators to a large load, the speed of all the synchronous generators (and their prime movers) is the same, they are running in synchronism. No one generator can go faster or slower than the others. They can try, but they can't.

So that's why proportional (droop speed) control is used for controlling the power output of a prime mover driving a generator. The actual speed doesn't change when connected to a grid with other synchronous generators, and that fact is relied upon. By changing the speed reference, the error between actual and reference changes and that error is used to control the amount of steam (or fuel) that is admitted to the prime mover, which controls the amount of torque being produced by the prime mover, which the synchronous generator converts into amps, which is the variable component of the power equation for a synchronous generator (since the generators run at basically a constant terminal voltage, plus-or-minus a very slight amount).

So, to change the power output of a generator, one needs to change the amount of torque being supplied to the generator from the prime mover, and this is all being done at a constant speed (in most parts of the world; others, well, they have issues).

 

Hi CSA,

Please accept my thanks and appretiation for your time and response.
I gained a lot from both your replies. I have reverse enginered (conceptually) the governor from circuit drawings and set up procedures, it ties in with your explanation about proportional response to a speed error.

For your information governor is a Type GSEU 102 MK3. Its output amplifier origionally went to a hydraulic servo drive which also used P2 feedback pressure for something. Correct me if I'm wrong but that something is to provide feedback to the servo valve either as a positioner or as a limiter against too much fuel (Natural Gas).
The servo valve has since been replaced by a S.T.A.R. electronic control unit produced by Alsthom.

This application is for a Rolls Royce Avon in a GEC package.

Again many thanks!

 

Thanks for the feedback! It's what makes control.com threads so valuable, being able to search them and see what has worked or been helpful, and what has not worked or not been helpful.

I'm not familiar with the control system or the application you cite. On a GE-design heavy duty gas turbine, P2 pressure refers to the pressure between the Stop/Ratio Valve (SRV) and the Gas Control Valve (GCV). This pressure, during starting and acceleration, is a function of turbine speed, and once the unit is synchronized the pressure reference is constant.

I suspect the reference to P2 pressure for your application is different than what I'm accustomed to, and probably for a different reason than I'm accustomed to.