Dear all,

I am hearing about Model Based Control too often recently, and how it out perfoms the traditional control especially in Gas Turbines industry. I was wondering if someone could please give us an overview of MBC in comparison to Traditional Controls.

Thank you,

 

Model Based Control (MBC), sometimes also called Model Based Predictive Control (MBPC), is a technique where the PID functionality is replaced by a dynamic process model.

Thus the controller computes its actions based on quantitative process knowledge (whereas the PID actions could be seen as “smart trial and error”), it “knows” e.g. what the final value of the MV/controller output must be in order to reach a new setpoint.

This allows more precise actions but the main advantage is that it can handle process deadtime much better than a PID. Some more advantages exist for cascades and when a feedforward is used (avoidance of the “double kick”). Furthermore, multivariable control is practically impossible without MBC (except for small 2x2 systems).

Especially wherever a loop exhibits long deadtime (esp. ratio deadtime/time constant > 2) one should not hesitate to apply MBC instead of endless (and never fully satisfying) attempts to tune a PID, which inherently cannot deliver good performance in this case. We have once developed an MBC controller for a batch reactor in just 10 hours whereas PID tuning would have taken us several days – without reaching the same performance.

The models can be quite simple and with proper model error handling (in contrary to popular belief) quite large model errors can be accommodated without excessive performance deterioration of even instability. Of course, more detailed models will improve the performance.

We do have a small, simple, easy to use MBC controller that is very robust against model errors and can be easily implemented in every DCS. We also offer specific training courses on the subject. More details on www.act-control.com under Technology, Training and Upcoming Events.

 

Excellent basic explanation of MBC by Hans E. Eder.

studmad, I presume you are or represent an owner or operators of gas turbines. The implementation of MBC for gas turbines is one OEM's way to differentiate its offering from other OEMs. First there was TMR, now there is MBC. And we all know how well TMR is documented and understood--even by the OEM's personnel.

In gas turbine control, MBC is being used to optimize performance and efficiency by programming the turbine control system with an ideal "model" of a running turbine and then monitoring operating machine parameters and adjusting various outputs to achieve and maintain optimum performance and efficiency.

To do this well it's <b>critical</b> that all instrumentation be maintained and kept in proper calibration, and I just don't see that commitment in many power plants around the works today.

It's also critical that the model be capable of recognizing and reacting to errors in instrumentation and make the operators aware of problems so that optimum performance and efficiency can be maintained. And this means <b><i>MORE</b></i> alarms--and more training for operators and maintenance technicians. And in my experience, most power plant management/ownership is not willing to invest in training, willing to rely instead on the control system to protect the turbine against all possible operating problems. They are becoming all-too-reliant on automation.

And turbine control systems just aren't capable of doing that--even with MBC. One of the problems with MBC is that the OEM is going to keep all of the model "internals" closed by claiming they are proprietary--making troubleshooting and understanding turbine operation even more difficult for <b>BOTH</b> the OEM's technicians <b>AND</b> site technicians.

This makes documentation by the OEM <b>CRITICAL</b> to power plant operators and technicians, and some OEMs have proven they are loathe to produce and provide useful documentation.

MBC is a great concept, with great potential. But, the Devil is in the details and for gas turbine control there is a very steep learning curve--even for the OEMs. There are going to be some hard lessons to be learned as this technology is rolled out and tested and adjusted and improved. Without good documentation for both the OEM technicians and operators and site technicians in this early implementation I think there are going to be some difficult and costly lessons to be learned because it's going to take trained and knowledgeable personnel to recognize <b>both</b> proper and improper operation in order to be able to tell when the turbine is operating properly or is in danger of catastrophic failure.

Error detection is one of the most critical aspects of MBC for gas turbine control. Make sure operators understand when they are seeing an error (as well as when they are not!)--but more importantly that they know and understand what action to take to protect the turbine. And training and documentation is key to developing that operational expertise.

As with all promising new technologies, there is a learning curve. Ask the OEM about their training and documentation--it's <b>every bit as important as MBC itself</b> and it will be crucial to reliability and availability.

But it's not just the OEM documentation and training, it's also the plant's commitment to personnel training and development of SOPs (Standard Operating Procedures) for the new technology.

Again, don't forget the closed, proprietary nature of the technology; you're going to own it long after the warranty expires. And you're going to have to operate and maintain it during the warranty period, also--and without documentation and training from the OEM that's going to be challenging.

We'd really appreciate hearing your thoughts and impressions, studmad, of what you're hearing about MBC. And how it's impacting your decision-making.

Lastly, there are very few slow, long loops in gas turbine control that would benefit from some of the parts of MBC as described by Mr. Eder.

 

Dear studmad,

My name is Carlos Scott, Product Line Leader for Power Generation at GE Control Solutions. Our team is responsible for control system retrofits for the GE HDGT fleet.

GE first applied MBC to a heavy duty gas turbine in 2007 and now has more the 1,000,000 hours of MBC experience, on more than 100 units. In gas turbine control, MBC replaces static control schedules used in legacy control system software with real-time turbine simulators to enable fast, accurate, adaptive control. This enables continuous automated combustor tuning, enables performance gains, handles degradation, and enables handling of very fast transients, such as grid frequency excursions, to prevent turbine trips.

GE recognizes the need for robust sensor fault handling with MBC and so designed our MBC software with extensive FDIA capability to both monitor sensor health and handle faults and avoid trips – leveraging Aviation practices. We’ve also developed formal onsite training courses through our Energy Learning Center for customers to learn MBC operation and troubleshooting.

I have more detailed information on control systems as they evolved through time, including our Model Based Control suite and can share that with you directly if you contact me at: [email protected] or my colleague, Robert Bellis, OpFlex Product Manager, at [email protected].

Alternatively, if you are a turbine operator, using Controls Connect gives you access to our knowledge database on MBC and other topics like GEH-6810 on ETS or 6811 on AutoTune. There you can also find knowledge articles, manuals, application guides, TILs, white papers, etc.

CONTROLS CONNECT NOTE: If you have a GE SSO#/Login and are a GE turbine operator, business associate or EPC firm, please send me an email with the subject "Controls Connect Access" and include your SSO#, Name and Company. Note that we do not accept generic email addresses (e.g. @yahoo, @hotmail, @gmail, etc). Company based email addresses are required for security reasons. Your registration will take approximately 3 days. If you do not have a GE SSO# you can register at http://site.ge-energy.com/online_tools/controls_connect.htm or just perform a Google search on "Controls Connect". Just click on the register button to start the process. Registration forms should require less than 5 minutes to complete. If you have any questions feel free to contact me at [email protected]

 

Dear all,

Thank you very much for your explanations.

I am a new controls engineer and I enjoy searching and reading posts in this forum. I want to learn more about MBC but did not find simple explanation to start with in older posts.

Thank you,
Regards,
Studmad