from the Automation List department...
Primary Frequency Regulation
Continuous process industries, DCS questions. topic
Posted by a_w_p_d on 8 December, 2007 - 1:03 pm
Hi, please bear with me on this one, even though it is connected to droop and freq I have a question that I can not seem to answer.
I have been through the past posts and the answer does not seem to jump out at me.

I do understand the relationship between droop machines and the grid/freq in that the machine speed is governed by the grid/freq.

• When at base load and the Freq drops, so the speed drops so the load drops (less speed/lower exhaust temp/less fuel...ect...ect...).
• When at part load and the Freq drops the diff in TNR – TNH will increase, so fuel flow will increase so load increases. (do I have that right? I think I have)?

Anyway the Question is….

If when in PLC Enabled (Plant Load Control) what is the function of the GT PFR ("Primary Frequency Regulation") in relation to droop control and frequency?

We had a situation were when at min load the and in PLC (PFR was not enabled), the freq dropped which pulled our speed down, which took our load down and took us out of pilot premix in to sub-pilot pre mix. This gave us a run back.

Since we have had the PFR enabled we have seen the freq/speed drop so low (2984rpm) that an excessive speed error alarm came in on the DCS put the load remained steady (=/- 2mw).

At present we enable the PFR when we are above min load and disable it again when at min load,


Should the PFR regulate with the freq and maintain load, or is it possible that we could go into another run back with the PFR enabled at min load?


Posted by CSA on 9 December, 2007 - 5:32 pm
You have the speed/frequency thing correct.

Without being able to look at the sequencing in your Speedtronic panel(s), it's not possible to say for sure what could or should be happening. There have been several iterations of Primary Frequency Response (also known as Primary Frequency Control), and it's not possible to tell which one is in use at your site.

The main objective of PFR (or PFC) is to allow a unit to be operated at part load with Preselected Load Control enabled *and* respond to frequency variations as if the unit were simply at part load in Droop Speed Control. When Preselected Load Control is enabled and the unit is at part load, the Preselected Load Control is kind of an "outer loop" to Droop Speed Control. If the frequency changes (which will cause a load change) Droop Speed Control will immediately respond by increasing or decreasing fuel in response to the increased error between the turbine speed reference and the actual turbine speed (which has changed), which will result in a change in load. However, because Preselected Load Control is enabled that function will sense the change in load and try to counter the fuel change to maintain the Preselected Load setpoint. So, what ends up happening is that Droop Speed Control and Preselected Load Control are each trying to adjust fuel--which doesn't help the grid frequency deviation at all.

PFR (PFC) is supposed to allow the unit to be operated at Part Load with Preselected Load Control enabled and when a frequency disturbance is detected the unit will respond by changing fuel in response to the change in the error between the turbine speed reference and the actual speed and allowing load to change to whatever it needs to be in order to try to support frequency until such time as the frequency disturbance is over and then the unit load will (should) return to the Preselected Load setpoint. This is what would happen if the unit were *not* being operated with Preselected Load control enabled: Droop Speed Control would increase or decrease fuel (and load) in response to the grid frequency disturbance. But, without PFR (PFC) the Speedtronic will try to maintain the Preselected Load setpoint regardless of the grid frequency disturbance, which is not what most regulators want to have happen during frequency disturbances.

One of the problems that many operators and supervisors have with frequency disturbances is that they believe the frequency of their unit should be returned to normal (nominal) by Droop Speed Control with or without PFR (PFC) enabled. But, if the unit is connected to a grid with other units the frequency of a single unit can't change.

Also, the magnitude of the change in load during a frequency disturbance is proportional to the magnitude of the increase in the error between the turbine speed reference and the actual speed. The load will only change when the frequency deviates from nominal, and will be returned to its previous value once the frequency returns to nominal.

Now, if you have Plant Load Control (sometimes, confusingly, referred to as PLC), then you have an "external" load controller for multiple units. I have never considered how that would, or should, operate during a frequency disturbance--but in considering it quickly just now, it probably would not operate optimally, especially since it's control setpoint and -feedback is load which needs to change during frequency disturbances. Hmmm..... That would take some serious consideration, more than a casual thought.

Now, if Plant Load Control is enabled, then if I recall correctly there is something akin to Preselected Load control active or enabled in the Speedtronic panel to respond to changes in load setpoint. It's been *many* years since I've worked on a site with Plant Load Control, and then it was only one, and there have been a couple of iterations of Plant Load Control, also. Again, in considering this quickly, it's possible you have a couple of non-integrated control functions, and they may also be of early generation(s).

Also, it's been said many times here on by at least two other contributors: DLN combustor-equipped units do not make good frequency control units (okay; specifically, they have been said to be poor Isochronous control machines, but the same goes for frequency response when being operated at Part Load). Especially if maintaining minimal emissions or emissions compliance is a primary consideration. The unit has to change load to respond to frequency changes, and if that means it has to change combustion modes and emissions will change, then it has to be.

From the sounds of it though, for this issue you might just need to change your minimum load setpoints. But that's just a suggestion made with the benefit of understanding how your plant is operated or what the operating requirements are. It's not a well-accepted fact, but it is a fact none-the-less: It's not possible to maintain emissions compliance over the entire load range of most heavy-duty gas turbines, so therefore, responding to frequency disturbances which will cause load to vary will also cause emissions to vary. Period. There's no such thing as a perfect anything (at least in this time-space continuum). The whole frequency disturbance thing has been turned upside down because the explosion in use of gas turbine-powered power plants. And, when emissions compliance is thrown in the mix, well, it gets downright ugly. Not so much from a technical perspective, but from a regulatory and political perspective.

I hope this has been of some help, but one would really need to be at your site to spend some time understanding how the units are operated, how they are to be operated, and how the Speedtronic panels are programmed/configured to be able to give you specific answers to your questions. There are just too many variations out there, and, your questions are complicated by the fact that you appear have multiple control functions enabled at one time.

Posted by a_w_p_d on 11 December, 2007 - 2:06 am
Just to say a quick thank you to you CSA.

I have just read through your reply and I must say how explanatory it is. I do however have to take some more time and go over it again (and again) in relation to my plant. Once I can relate your explanations to my own plant I shall reply again.

Thank you.

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