Turbine Generator Simulation

D

Thread Starter

Dave

Hello,

I'm working of a simulation of a turbine generator that will be used for operator training. So far its working pretty well, but I have a question about the load limiter screen. the turbine itself is a GE steam turbine. using a Mark VI control system. On the example of their load limiter screen that I have from the Mill, there are 3 digital representations of analogue gauges on the bottom. They are labeled VI COIL CURRENT(R) V1 COIL CURRENT(S) AND VI COIL CURRENT(T) respectively. As an example, the bottom left "gauge" is reading 8.44, with a range of -30 to 30. Below it are 2 readings labeled Reg reference, and Reg Feedback, and they read 8.4% and 75.7% respectively. What are these numbers actually measuring/controlling? At this state, the Unit is producing 22.9 MW and 7.6 MVAR and is at 110% Load limit. The unit is running in exhaust pressure control. My guess is that this has to do with the 3 phases on the stator, but what specifically it measures or controls, I'm not sure. Any info would be appreciated.

Thanks
 
The "V1 COIL CURRENT(R), V1 COIL CURRENTS(S), and V1 COIL CURRENT(T)" are the currents through the servo controlling the main steam inlet valve (V1), from the R, S and T controllers in the Mark VI. Ideally, they should all be the same, although in the real world there are usually small differences among them.

I don't remember exactly what the Reg Reference and Reg Feedback are.
 
Thanks,

That makes sense on a load limiter screen. Now that I look at it, the Reg feedback numbers are about what the control valve opening is. So still not sure what the Reg reference is, but in the screenshot I have, they are all the same (at 8.4%).
 
Reg reference = regulator reference

Reg feedback = regulator feedback

Regulator is another (GE) term for servo-valve output.

The reg reference and -feedback numbers don't make sense to me, but then we don't know what the null bias currents are set to which might be the reason for the odd numbers. And we don't know what signal pointnames are being used for the values for these descriptions.

Since you're probably only using printouts of CIMPLICITY displays, you can't look at the signal pointnames used for the values; you can only see the description the display creator/editor used for the displayed values. And, there is no standard in GE that displayed values and their descriptions should match, so people are free to use whatever descriptions they want--which often leads to confusion, especially when they use abbreviations.

Aren't GE HMIs wonderful and fun?
 
Thanks,

Yes, we are looking at printouts of the what we would call "Mark VI screens". We also have copies of some of the other DCS screens pertinent to the simulation, like Steam header, and power distribution displays.

Just to be straight on nomenclature, I've been treating anything labled "reference" as a setpoint, and Feedback as a field input. The difference between them would be the error for a given controller. If that's correct, that would mean the error at the time these screens were taken was about 67% for all three coils. We will of course straighten it all out with the mill as time goes on, but we do need to design something that behaves like their unit.

I also have a question about Load control. On the screens I have, they have the load reference set to 110%, and the load reference command is at 110%. What is the value that this is 110% of? Is it whatever MW their unit is rated for? For example, if the unit was rated for 25MW at .9PF, and they were running 27.5MW, would that be how they would measure the load%? Can the unit be operated in more than 1 control mode at a time? They typically operate in exhaust pressure control. Is it also possible to have a load control setpoint in place, so that if that load setpoint is reached, the unit will back down, regardless of the exhaust pressure at the time?

Thanks
 
Dave,

Wow! So many questions....

You are generally correct, but when looking at GE CIMPLICITY displays for GE-design turbines one can NEVER ass-u-me anything. That's because the standard at GE is: There is no standard. Everyone who can edit a CIMPLICITY display is free to use whatever terminology they want--and the steam turbine people at GE use different terminology for the same values then the gas turbine people. And the never the twain shall meet. At least not because of any standard at GE, that's for sure.

Steam turbine load control. Many sites use load control improperly. By inputting a setpoint (reference) higher than rated they believe the turbine will continually produce rated output (maximum) when possible. So, some sites input a value of 110%; others 125%. This is not really a good idea, especially for gas turbines--but we're talking about a steam turbine here and let's not stray.

Some steam turbines have controlled extractions; some have a single controlled extraction; some have two controlled extractions; some even have three controlled extractions (though GE doesn't like to do tripe controlled extraction steam turbines). In this case, there is a kind of "cascading" control selection, where one value (say, pressure) is the primary control loop, and if certain conditions exist then the secondary control loop, say load, might become the primary loop.

It can be very confusing to understand--and even more difficult to explain. Without knowing more about the turbine at the site and it's various control modes and possibilities, it's really impossible to say.

However, remember this when it comes to steam turbines at paper mills: their primary purpose is to act as a pressure-reducing station. Steam can produced more efficiently at a higher pressure than most mills can use the steam. So, rather than just have a big pressure-reducing/regulating valve to drop the pressure they use a steam turbine to drop the pressure while at the same time producing some electricity--which makes the whole process that much more efficient! (Genius, really!)

So, the steam turbines are usually operated in some kind of pressure control mode--either inlet pressure control, or extraction pressure control, or sometimes even back-pressure control (as some turbines do not exhaust into a condenser). And, whatever electricity they produce is just "gravy", "icing on the cake," a by-product, really--though a very valuable by-product.

But the real purpose of the steam turbine is just to reduce the steam pressure for some use(s) in the mill, and so they primarily operate the turbine in some kind of pressure control mode, with a load reference as a "back-up". So, if for example, a paper machine tripped off line or is shut down and the steam flow to that machine is no longer required then the steam turbine could produce more power--but only up to the rating, regardless of the setpoint (which is, again, just a wrong-headed method of telling the steam turbine to produce as much power as possible when it can because it shouldn't produce more than rated, or if so only for a brief period).

That's about all the help I can give. And, I'm not really all that good with steam turbine control philosophy, because, when it comes to steam turbines they literally break the mold every time they build one. They are very unique machines, and every one is just a little bit different--again, because they broke the mold every time they make one (literally)--and because every mill is at least just a little bit different, sometimes a lot bit different. As steam turbine design and materials improved, the turbines evolved, and so did the control schemes (to a certain extent). So, understanding how a particular steam turbine at a particular mill operates is very much dependent on how the mill operates, and how the operators are taught to operate the steam turbine.

And, SAX (Single-Automated Extraction) control and DAX (Double-Automated Extraction) control are not easy to understand or explain. In fact the Mark VI algorithm descriptions for these two types of controls (which I wouldn't think would be present on the same machine) just shows some inputs to a rectangle (a "block") and some outputs, and what happens inside the block is magic because it's not described anywhere or if it is, not very well.

It's really kind of something operators just learn to accept and live with--whatever they are told is how it works (which is how most operators operate turbines--just by rote memorization and based on what they were told, right or wrong).

Best of luck!
 
Thanks,

Yes, this unit is basically used to control the pressure on their 150# header, and produce some electricity. It is normally operated in backpressure control, and this particular unit has no extraction (it used to, but it was removed when the unit was rebuilt). The information flow from the Mill hasn't been that free-flowing, so I just wanted to get a basic understanding of how different control schemes might interact. I will eventually travel to the site, and I'm sure I'll get detailed information then. I just want to have a good understanding of their unit and screens so when I do go down and spend a few days with the operators / engineers, I'll be able to ask intelligent questions and understand the answers. I will say that I've learned a lot about the subject from just searching this forum.

Thanks again
 
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