Steam Turbine Valve Time Constant 1

[sparkyken]

Hi,

I am new to this forum. I am working on this project where we are trying to model our power generation system. This system consists of 4 steam turbine alternators and 2 gas turbine alternators. Each alternator is controlled by Woodward 505 governor(turbine) controller. The generators are small sized, with a rating of 5MW.

At the moment i am trying to model the dynamics of the control system for the steam turbines. The controller send signal to actuator, which in turn dictates the hydraulic amplifier to control the valves.

I need to find the time constant of the valves that control the steam. So this is the time taken for the valves to go from fully closed position to 63% open position. I have tested this using the woodward 505 controller to tell the valves to open completely in one step. The only problem is that the 505 controller has rate limiters and the result was that I get a ramp response instead of a step response.

I would like to know if there is another method to stimulate this kind of response. I have thought about using a signal generator to send a "fake" signal to the actuator making it open the valves instantaneously. The only drawback is, by using this method i am considering an open loop control so this may not provide correct information in modelling a closed loop control system.

Any thoughts or ideas will be greatly appreciated. Please let me know if i have put this in the wrong forum.

Thanks for your help

 

[waross]

Please don't double post. You may however post a request in another forum for the participants to look at this posting.
If you add this thread number, thread408-231894, it will automatically link to this thread.
Try the "Electric power engineering" forum.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Likely forum1083 is an even better fit for this question: Peter Nachtwey does this sort of stuff for a living and he'll probably give you a very good answer.
I have a few ideas from some performance characterisation tests we did on the hydraulic servo valves on our steam turbines a few years ago, but you'll likely get a better analysis from Peter.


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If we learn from our mistakes I'm getting a great education!

 

[BigInch]

If the valves are opening in one step, your steps are probably way too big.

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[PNachtwey]

I am assuming a hydraulic actuator is moving the spool that controls the steam flow. I am assuming that at governor is getting speed feedback from the turbine and it is trying to control the speed at 3000 or 3600 rpm. The governor in turn is sending a 4-20ma position reference to the hydraulic amplifier card and that is doing a simple closed loop control to move the steam spool to the reference position.

A simplified and linearized model for the steam valve spool position is K*omega^2/(s*(s^2+2*zeta*omega*s+omega^2))
The limit you see is probably caused by the gain K which has units of steam spool velocity/control output. Therefore if the control output is 100% you are still limited to some velocity. You will not have a time constant in the sense that the spool position will approach the set point in a exponential manner. It is a little trickier than that.

Omega is the natural frequency of the spool and hydraulic actuator, zeta is the damping factor. The damping factor is hard to calculate as it changes with respect. The controller is probably tuned very conservatively.

To do the system identification you need to move the 4-20ma spool position reference to the controller or go into a manual or open loop mode where you can send a open loop control signal from the controller to the hydraulic servo valve that is controlling the spool. From this data one can determine the gain K, natural frequency and damping factor. Our controllers have an auto tuning feature that will compute this data for you. The controller collects control signal and hydraulic actuator position and uses the Levenberg_Marquardt algorithm to that finds the Gain, damping factor and natural frequency that minimizes the sum of error between the estimated position and the actual position. This isn't trivial. Get a math package like Scilab or Matlab to do this.

NOTE!!! You don't want to do be exciting the hydraulic actuator while in operation.

 

[davefitz]

I'm not sure if this is useful, but I recall that when simulating an overspeed trip, one may assume the stop valve will close within 0.2 seconds. This value is also used for steam hammer analyses, and for modeling of the response of the steam turbine bypass system and the dynamic response of teh steam line/ bypass system, and steam generator/ boiler.See tech papers by Sulzer and Siemens circa 1980.

The steam turbine mfr likely has a datasheet that may provide add'l dynamic response data for your particular steam turbine and the valves supplied .