Speed Drop in Steam Turbine with Woodward 505e

Hello,

We've been facing a problem with our 15 MW steam turbine recently. It has Woodward 505e governor controller installed and configured in 4% droop mode, although normally it is running alone. At other times (during startup), it is synchronized to other DGs.

The problem we faced recently was that when our turbine went up to about 11 MW (with all other DGs shut down), it kept normal for about 10-20 minutes. Then all of a sudden, without an increase in the load, its speed suddenly dropped from 3000 to 2950 rpm. Operators tried to increase Woodward speed setpoint manually, but it did not respond. Operators then switched off some 1.5 MW load manually and the turbine speed normalized to 3000 rpm. All this happened within 30-40 seconds. During this time there was no change in load, no change in steam pressure or flow (extraction was off at the time), no physical change in HP or LP valve.

The two questions which are looking to be answered are:

1. Why would the turbine suddenly slow down?

2. Even if somehow the turbine slowed down, why did 505e not respond by increasing its output to maintain the rpm?

Then 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?

Any suggestions would be most welcome!
 
electrocuted,

The fact that the operators were unable to change load suggests that manual load change signals were blocked--most likely by some automatic load, or frequency, control scheme which was active at the time. Your recent post about frequency control on a small power islandwhere all the units are operating in Droop Speed Control suggests that your site may have such a system, and if so it may not be working properly. Most such systems are not very well designed, implemented or understood by the operators, technicians and supervisors who have had little or no formal training with the system which was custom designed and provided with little or no documentation, and was usually extensively modified during commissioning with little or no documentation about what was changed.

Please write back to let us know what you find!
 
CSA,

Thanks for your response. We do not have any PMS installed on the site; yet all generators are running in droop mode. This scheme has been going on for ever since I can remember, and we haven't experienced this kind of problem before. The only change is that the total load requirement of our plant has increased by about 2.5 MW (25% increase).

What would be the possible problems faced if all generators are in droop without any PMS?
 
electrocuted,

How is the frequency of the islanded system (isolated from a larger grid) at your facility maintained? Do the operators make manual adjustments to the loads of the generator-sets when the frequency of the system changes as the facility load increases or decreases?

No one has said an islanded system would be problematic.

I generally find that sites that operate independently of a large grid that don't use a single machine in Isoch mode to control frequency do so because of past problems with being able to balance loads or the unreliability of one or more units. Some sites are designed to have all units running in Droop mode--but with a PMS or similar control system monitoring frequency and sending signals to one or more units to automatically control frequency. In my experience, having operators perform frequency control, especially if the facility load is not relatively stable or load changes cannot be anticipated, results in constantly changing and drifting system frequency--and management doesn't usually like that and are looking for a way to have the system frequency be more stable.

You haven't told us how stable the plant load is, how stable the plant frequency is or how the plant frequency is controlled(by the operators, or???) So there's a lot we don't know. Some facilities and management have more tolerance for frequency swings than others, and some don't want to spend the money required to make frequency control automatic. And that's a decision that has to be respected.
 
I think if you review this post which references information from way back from a Sept '90 issue of Motion Control, it might help you with #2. https://www.axcontrol.com/blog/2019/09/03/pid-and-servo-tutorial/

>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?

>Any suggestions would be most welcome!
 
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