Droop Control on Steam Turbine

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Thread Starter

stor138

We have a 30 MW generator connected to a steam turbine. The turbine is a steam extraction unit, with HP inlet steam (~600 psi), MP steam extraction port (~120 psi), and LP steam exhaust (~60 psi). There is a inlet valve and an extraction port valve. There is not a valve on the outlet port.

The existing PLC-based governor has the following controls (described only when grid-connected):

1 - load control (MW); controls inlet valve
2 - speed-droop control (turbine rpm); controls inlet valve
3 - backpressure control (psi); controls inlet valve
4 - extraction port pressure control (psi); controls extraction valve

Mode 1 is a cascade control loop which provides a speed set-point to the speed-droop loop. Mode 3 is the normal operating mode and controls only on pressure - it does not consider the unit speed. Mode 4 operates independently of the other loops.

We are having issues with compliance of the governor with the local regulations - the utility that the generator is installed in requires the governor to operate in droop at all times for system stability. Since this unit is normally in backpressure control, it will not normally respond to system frequency changes.

My questions are:

1 - is droop functionality while in exhaust pressure control an operating mode that should be expected for governors on extraction/exhaust turbines? From what I can tell, I believe that this should be standard, but can stand corrected.

2 - if droop functionality is expected to be in operation while the turbine controls exhaust pressure, how does the unit react on a sustained change in utility frequency, if the back-pressure control loop is modified to be cascaded into the speed-droop loop? When the inlet valve opens in response to the system frequency change, will the corresponding pressure change seen on the header result in a counter-acting of the droop action? Are there any governor control schemes that are recommended so that the response to a change in system frequency is sustained, not momentary?

3 - how is this control mode handled in off the shelf governors? From what I can tell, a Woodward 505E handles exhaust pressure control as a cascade into a speed loop. Does a Woodward have logic to also maintain the proportional response to a frequency change once the steam outlet pressure is off setpoint, so it is not reversed by the pressure loop?

4 - any suggestions or experience on how we might achieve compliance with the utility?

I would like to better understand how systems are traditionally configured so I can better understand this specific situation.

I hope this email is clear - please let me know if there is any missing information that would help with a reply.

Thanks
 
stor138,

I believe that most industrial steam turbines with extraction control, including non-condensing (back-pressure) units, should have droop speed control active at all times--except, possibly, when the unit is running at "Base Load" (rated power output selected and the governor is putting as much steam into and through the unit as possible to make as much power as possible). (Even when Base Load operation is selected, droop speed control should still be "running in the background" as a back-up.)

You might have a copy of it, but Woodward's Application Note 83403 has some very basic information about controlled extraction and back-pressure units--and it says speed control (droop speed control) is always active.

In most control systems there is a "summer" or some kind of High Signal Select or Low Signal Select that looks at two or more inputs and chooses the one to be "in control". Speed, or, the error between the speed reference and the actual speed, should be one of those inputs. And when the error causes speed control to be the controlling reference then the governor should take appropriate action to the various outputs (inlet control valves, extraction control valves, etc.) to assist with maintaining grid stability by changing load as necessary.

Are you sure that there isn't some kind of signal summer or HSS or LSS function that looks at speed error when exhaust pressure control is active--or, really, when any control mode is active? Just asking, because it's a pretty standard thing, or at least it should be.

Sometimes it's a matter of tuning gains and such to make these various functions behave nicely with each other, also.

Hope this helps!

30 MW isn't very large, in relation to most grids in developed countries. Unless, you're at the end of some transmission line. And even then, one would think they're more worried about voltage support than frequency support (though they would claim both, I'm sure). I know that in some parts of North America, 30 MW is the cut-off for regulated versus non-regulated co-generation--which many paper mill applications were once, and many are still, considered.

Anyway, please write back to let us know how you fare with this!
 
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