Woodward 505e Speed PID Output vs HP Output

[electrocuted1]

We have a Woodward 505e installed on our 15 MW extraction steam turbine. Yesterday we noticed that when we were running at 7 MW (without extraction), the HP output was 38% while the speed PID output had gone up to 83%. Then we opened up the extraction, and as soon as we did that, speed PID output came down to 37%.

My questions are:

1. What is the difference between speed PID output and HP output? I know there's a ratio/limiter block in between them, but how does it work? The ratio block has some equations written in the manual such as HP = K1S + K2P + K3. How do I calculate/find out the values for K1, K2, K3 etc?

2. I am afraid that if we had not opened up extraction and kept on increasing electric load, speed PID output would have been saturated. A similar incident happened a few days when were running at 11.2 MW with 42% HP output without extraction. All of a sudden the turbine rpm dropped. I am almost convinced that speed PID output would have reached 100%, though I do not have trends available for that.

3. I know the ratio block has something to do with the steam map/envelope fed into 505e, but am not sure how it works. Could somebody please explain that? Attached is the steam map that has been fed into our 505e.

 

[waross]

PID output by itself as a percentage is not a lot of help.
What is 100%?
The output is generally related to valve position, but the valve may be non-linear.
The output may be driving a valve positioner but again the PID output vs the factual valve position may be non linear.
HP output is directly related to HP in or steam flow.
Without extraction low pressure stage back pressure may be limiting the steam flow through the HP turbine.
When you reach 100% on a grid paralleled machine, any increase in load is picked up by the other machines.
When you reach 100% on an islanded machine, any increase in load will cause the set to slow down and the frequency to drop.

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

 

[electrocuted1]

Thanks for the response Waross. My concern is that in our case even though the HP output was only 38% (this also corresponds to almost the same opening in the field), PID output had gone upto 83%. We have calibrated the HP output to the physical valve over the whole range (0-100%). Is there a possibility that PID output can reach 100% and then restrict further opening of HP valve, although the valve may be open only 40-45%.