Hi,

Can someone provide me a good resource on PLC based Gas Turbine Control System? I am looking for internal workflow of PLC from IO devices.

Regards,
Rajesh

 

Hi Rajesh,

It would be too much to describe here. Of course there is lot of information on web sites if you search. But if you need anything specific, you can write to my mail id mahajanravi @ yahoo. com.

ravi

 

PLC-based gas turbine control. Sounds sweet; seems like a good idea.

Are you considering replacing some original turbine manufacturer's turbine control system with a PLC?

One of the many problems with doing this is that many gas turbine manufacturers use devices and instrumentation which are not very commonly used in applications where one would normally find PLCs. This means one either has to replace the devices or instrumentation with ones compatible with the PLC, or some kind of "interface" device must be used.

For example, LVDT excitation and feedback. These devices are primarily used on fuel control valves and variable axial compressor inlet guide vane actuators. Many PLCs don't have input/output cards that LVDTs can be directly connected to, so people resort to using a "converter", some kind of device that requires a power source (sometimes 110/220 VAC) to provide the excitation source for the LVDT, and then converts the LVDT feedback into a 4-20 mA signal which is then connected to the PLC.

So, the "converter" represents yet another possible point of failure. *And* on most applications it greatly complicates calibration of LVDT feedback. And, in my experience, those who provide PLC-based gas turbine control panels do not provide sufficient instructions for how to perform calibrations with these "converters". Also, sometimes the time delay in conversion is unacceptable. Finally, this represents yet another spare part which must be stocked. A 4-20 mA signal for a device which might have an effective travel of three inches or more and which can have a huge impact on stability and even performance just doesn't have very good resolution

Another really important example of protection which most PLCs can't perform like systems designed for turbine control and protection is speed sensing. Many PLCs can't deal with 5000 Hz speed signals which can change extremely quickly, so they also are run through some kind of "converter" which "smooths" the speed sensing into something the PLC can accept. I've even seen speed signals from 5000+ ROM machines rung through converters that change the signal into a 4-20 mA signal. Not recommended; one wonders if the owner's insurance company knew about that what they would have to say about it.

And this represents another point of failure and another spare part which must be stocked. I have seen several of these speed sensor interface cards which were proprietary and only available from the PLC packager.

Add many types of flame detectors to the list; many require very high levels of excitation voltage (335 VDC), and output a frequency at that voltage. Most PLCs can't handle that, and so another "converter" is required. Another point of failure, and another spare part to purchase and stock.

The electro-hydraulic servo-valves used by some original turbine manufacturers require a bipolar current output, which many PLCs aren't capable of, requiring still another "converter". Which means still another point of failure, as well as more spare parts to purchase and stock.

Will you be programming a PLC to control and protect a turbine, auxiliaries and generator?

As for the "internal workflow", you are likely referring to the PLC program which would be used to control and protect the gas turbine. That would have to be developed from the existing turbine control system. I have seen some very excellent PLC programming in PLCs used for turbine control, but, by and large, most of the examples I have seen have been really bad. Most have been written and or modified by more than one person (and it's obvious), and the transition from modes like Droop Speed Control to Exhaust Temperature Control were kludgy or simply just couldn't be made to work at all.

I know of at least three sites where the turbines NEVER run on CPD-biased exhaust temperature control except in the coldest winter months, because the Droop Control "limiter" (which itself couldn't be understood or explained) clamps the gas turbine output. So, the owners aren't running their units at optimal power output most of the year.

In some of the PLC-base turbine controls I've seen there were several examples of really brilliant programming by some people who were obviously very knowledgeable about programming that particular PLC. But, they obviously weren't knowledgeable about turbine control and protection, and couldn't create logic and sequencing which duplicated what was in the turbine control system which was being replaced in order to be able to properly control and operate and protect the turbine.

I've been to sites where the owners and operators wouldn't give up their PLC-based turbine control for any amount of money! But those sites are few and far between in my experience. Most people would swap their PLC for an original turbine manufacturer's turbine control system, if they hadn't spent so much for the PLC-based system, the spares required, and the man-months of service they've purchased just to try to make them work. Many owners have withheld money from the supplier because the systems couldn't be made to work properly. Some suppliers have gone out of business, or don't have anyone to send to site to support the outages or troubleshoot the systems. Some owners will never do business with the supplier of the PLC-based system again (and some will never do business with the original turbine manufacturer again, either, which puts them in a very difficult position!).

On those site where the owners and technicians were most happy with their PLC-based control systems, a lot of the devices and instrumentation on the turbine and auxiliaries had been replaced with devices which were compatible with standard PLC I/O cards. Some admitted it was a learning experience, and they just happened to have a technician or two who grasped the concept of turbine control and protection and took it on themselves to learn the PLC and all the interface "converters" and wrote procedures and participated in finding the right bits and pieces and making the necessary modifications to get the units to run as they had before, and better in some cases.

The attraction of using a PLC to control a turbine is much, much sweeter than the reality. The cost when all is said and done is usually comparable to or even higher (when conversions take longer than expected and combine with rough start-ups to impact production) than purchasing a system from the original turbine manufacturer. There's the issue of interfacing all the devices and instrumentation to the PLC, plus all the "converters" and the potential points of failure they represent. Many of these "converters" are used in many industries, but try to get some support from one of them when you say you're using it on a 120 MW gas turbine instead of a 20 HP hydraulic pump. The ones I've called have said there device wasn't intended for the application, right before they hung up the phone.

I've yet to see even a good PLC-based system that comes with sufficient documentation to understand how to calibrate and troubleshoot the system. Granted, some of the documentation provided by original turbine manufacturers isn't all that great, but at least there resources around now like control.com.

It's been said before on control.com: Before choosing any PLC-based control system from any supplier, make sure you know the supplier. If you don't already have a relationship with them, ask for references. Ask to speak to their programmers to see how much turbine control experience they have. For gas turbine control and protection, ask how they will handle speed sensing, LVDTs, flame detectors, and servo-valves. And, ask to see their documentation, the procedures to calibrate and troubleshoot the system.

Hindsight is always better than foresight.

 

Yes I can. I have been associated with a control conmpany that has done numerous control systme retrofits on GE Westinghouse, Ruston and others over the past 15 years. Of those that they have installed I have been project manager and startup engineer on approx. 40 of those units. They have sucessfully retrofited both gas and steam turbines. If you could repsond and advise me of the type of turbine and what you specific needs are. Perhaps I can assist you.

Sincerely,
Kevin Guthorn

 

Hi Kevin,

I'm looking for information about how to retrofit a Ruston TA1500 Gas turbine Driven Pump. Would you please assist me?

Sincerely,
Reza

 

Markvguy's post eloquently addresses the compatibility AND the safety issues involved in using a PLC for gas turbine control in the thead at
http://www.control.com/thread/1026223561#1026223660

 

Early this year I was at a site in California that had a General Electric Mark IV controller for a gas turbine. I have previous Mark V experience and was employed by an Operations & Maintenance that was to take over operation of the unit after an upgrade from the Mark IV to a very-common PLC.

During this 'upgrade' I was merely an observer. That did not stop me from reviewing the electrical drawings and immediately asking why 4-20 mA analog signals were being used to drive the bi-polar servo coils for the gas control valves (I already hear gasps from CSA and markvguy). I was told by the controls engineer doing the upgrade that it was not a problem. Then I saw that the controls engineer was having difficulties with the servo controls for the gas valves, offered my assistance and was turned down as this 'engineer' could do it himself.

Long story short this 'PLC' company managed to blow up the unit as the aft end of the turbine and the exhaust was filled with natural gas when the unit got a fire signal during a start test. It was not a pretty sight nor sound. I don't know if the unit is operational yet or not. I doubt it as I was not recalled to work. It did chase off the persistent pigeons for about two hours.

This was the second 'upgrade' that this 'upgrade' company had ever done. Their replacement panel had to be rewired because they did not realize that Mark IV panel layouts for terminals and so on are standardized; that told me that they did not have GE experience nor technical documentation. The field wiring came up short in some areas. They wanted to immediately fire the unit after the wiring was done; I convinced the client to follow some sensible procedures such as checking that the e-stops worked, crank commands worked, and a false fire would trip the unit. This lack of experience... well, you get the picture.

The moral of the story is this: If you are insisting on a PLC upgrade, find a company with a proven record otherwise you will see first hand the results of poor operation of critical equipment such as gas control valves. Fortunately no one was injured. Under the circumstances I would not want to be responsible for operations and maintenance of that unit without a careful review.