Dear Gentlemen:

In our power plant we have 5STG’s in island mode operation (not connected to the grid) they are in AFC mode (Automatic Frequency Control):

- Steam turbine generator 120MW / 60Hz

I would like to know how many STG we can put in Load limiter mode (not AFC mode: AFC OUT), and if there is a calculation to be done please give me the formula.
Thank you in advance

 

It would be necessary to have quite a bit more information than you have provided, and this kind of project should really be done by a professional who has access to some detailed information about your plant and how the AFC works and/or can be made to work.

The short answer is: It's probably possible, but the magnitude of expected load swings and the sizes of the steam turbines and their ability to respond to expected load changes all would need to be known and considered, along with more detailed information about the AFC scheme/control system.

It's suggested you hire the services of a knowledgeable professional to avoid frequency swings and potential blackouts by trying to implement what would only be estimations by anyone trying to help you via this forum, even with more detailed information which would absolutely be necessary to avoid misunderstandings and problems.

 

Thanks CSA:

It is clear that you are professional in the field. Kindly help me to solve this issue and I will send you any documentation you request.
Regarding the load swing is about 11MW not always but we don’t consider the tripping.

I want to share your experience and tell me which parameters can we check to know the capability of the STG in AFC and also how many STGs can we put in load limiter mode:

- If we put 2STGs Load limiter and 3STG’s AFC control: What are the advantages and the disadvantages.

- If we put 1 STG in load limiter and 4STG’s in AFC control what are the advantages and disadvantages.
This is my Email [email protected]

Thank you

 

Henry,

Flattery will get you almost anywhere with some people. This is a subject that is near and dear to my heart, but I won't take it on over the Internet in a forum like this. I think many people could benefit from the exercise, but there are just too many intangibles and too many possibilities for misunderstanding and miscommunication.

Thanks for the kind words, but, please find a knowledgeable person who can come to site, observe the present operation, learn how your site operates and is operated, and can effectively address your needs and provide recommendations and assistance with keeping your plant reliable while improving the efficiency and operation.

Many times when a plant is built there is a guiding document that is used to describe how it is to be operated and why, and that document is usually provided with the manuals at turn-over to the owner/operator after commissioning. The commissioning people use that document to make the plant run as it was designed to run.

It's possible that the designers envisioned it would be best to use AFC on all the units to respond to load changes for reliability based on their understanding of how the plant was going to be operated. But it turns out the plant is not now operated in the same way, so in these cases it's necessary to understand how the plant is operated, how it can be operated (how the steam is produced; how many STGs; what their size is; what the largest possible load change could be (including tripping one or more of the STGs off line, and when they are taken down for maintenance, etc.)); and how best to program or configure AFC to achieve the most economical and reliable operation--and if it's even possible with the current equipment.

That would best be accomplished by writing another document to describe how the plant is to be operated based on current operating philosophy and conditions to achieve economy and reliability, then program/configure the units and control systems to achieve those goals, and then re-train the power plant operators so they can respond to normal and abnormal conditions and keep the lights in the plant on.

That's just too much to do in a forum like this. And, in my current position I can't accept any outside work (I already give away a great deal of my time to control.com).

I will say this, though, about what you are proposing. <b>If I understand Load Limiter operation from my limited steam turbine experience,</b> what you are trying to do is to run two or more STGs at a constant load and use one or two STGs to control frequency. If you try to do that by setting a continuous load setpoint and there are any situations that the units in AFC can't respond to, then the frequency excursions will be worse than they otherwise might have been.

Units operating in pure Droop Speed Control mode at Part Load without any load reference or setpoint will respond to frequency disturbances by increasing their output if the total load increases and the AFC unit(s) can't produce any more power and the system frequency decreases. This is one of the things that Droop Speed Control does--helps to stabilize load and frequency.

However, if you have units operating at Part Load with some kind of control that tries to maintain a constant steady load regardless of frequency changes, then if there is a load increase that the AFC unit(s) can't respond to then the Droop units will NOT increase their power output to help stabilize load and frequency, and that's NOT what you want to happen; not at all.

If by Load Limiter you mean that you put a reference of, say 50 MW, into the STG control system and the control system adjusts steam flow to hold 50 MW at all times, even if frequency changes, then you are likely asking for problems if there can be any load swings which the AFC unit(s) can't handle--such as one of the units (Droop or AFC) tripping off line.

There are a lot of variables. Load swings can also include loss of load, like some part of your operation or process shutting down or the breaker that feeds it opening for some reason. If the total system load suddenly decreases and the AFC unit(s) can't unload to maintain frequency (they hit their zero load setpoint, and perhaps even trip on reverse power--which is entirely possible under transient conditions) and the Droop units are not decreasing their load because they are trying to maintain the Load Limiter setpoint, then that's also asking for blackout conditions, depending on the severity of the situation.

If you're going to try running any of the units off AFC, I suggest you just take it off AFC and then adjust the load on that unit to where you want it to be while it's just on normal, pure Droop Speed Control (which is something we don't know if the control systems have!) and see what happens. Remember--the load of this unit WILL change if there are frequency disturbances, that's what's supposed to happen during frequency disturbances! But, if the AFC can keep the frequency stable then it's power output will remain as you "set" it using the Droop Speed Control speed reference setpoint.

Best of luck on your endeavour!