from the MARK V department...
 Posted by Mark V on 29 September, 2012 - 10:07 am
Dear Experts

I work in a CCPP. We have two GT and a ST which each of GTs F9E and 123 MW. I need to loading curve mw/sn when they operation on the Base,Presel Load. The machine How is response to frequence fluctuations. Which BBL is control? For your reply thanks.

 Posted by CSA on 29 September, 2012 - 3:21 pm
Mark V,

If your turbine-generators are synchronized to a large or infinite grid with other prime movers and their generators, then the frequency of the generator output at your site is completely a function of the grid frequency.

All the turbine generators on a large or infinite grid are synchronized to each other, and are, for all intents and purposes, operating at the same frequency--including those at your site.

And for those generators directly coupled to their prime movers the speed of the prime mover is directly proportional to the frequency of the generator, which is the same as the grid frequency (under normal circumstances).

Typically, when turbine-generators are being operated in parallel with other prime movers and their generators on a large or infinite grid (synchronized), the turbine control systems are being operated in Droop Speed Control, or Droop Governor Mode. That means that some other "entity" is in charge of controlling the frequency of the grid, and the speed of the generator rotors and their prime movers.

Droop Speed Control is PROPORTIONAL control--ONLY. There is no Integral or Reset, or Derivative, applied to Droop Speed Control. It's just proportional control. Period. Full Stop. If the grid frequency varies, then the generator output frequency will vary, and the turbine speed will vary. Period. Full stop.

If the turbine-generators are being operated independently of a large or infinite grid, in other words in "island mode", then the turbine control system of at least one of them should be in Isochronous Speed Control, or Isochronous Governor Mode. Isochronous Speed Control means that the turbine control system is going to adjust it's fuel flow-rate as necessary (but within limits!) to keep the turbine speed--and hence the generator frequency!--equal to the reference, 50.0 Hz in your case. In actuality, there will be some slight variation, for Speedtronic turbine control systems being operated properly in Isochronous mode the variation should be no more than approximately plus-or-minus 0.13% of rated, or from approximately 49.915 Hz to 50.085 Hz. But, this is ONLY for units being (properly) operated in Isochronous Speed Control mode independently of a large or infinite grid.

And ALL of the above is ONLY true for units which are ***NOT*** being operated:

1) at Base Load,

or,

2) not being operated in Pre-Select Load Control Mode.

When a GE-design heavy duty gas turbine with a Speedtronic turbine control system is being operated at Base Load, there is no speed or frequency control, whatsoever. The turbine is being commanded to produce as much power as possible without overfiring (without putting excessive fuel into the machine, which would decrease parts life and/or damage the machine). Speed, and frequency, are "irrelevant" when Base Load is selected and active. Power is king when Base Load is selected and active, and frequency (and speed) take a back seat to power when Base Load is selected and active.

When a GE-design heavy duty gas turbine is being operated with Pre-Selected Load Control enabled and active below Base Load (in other words, at "Part Load"), if the grid frequency changes then the generator frequency--and turbine speed--will change. Droop Speed Control will then try to change load to try to correct the frequency--but Pre-Selected Load Control will say, "NO! The operator has selected Pre-Selected Load Control, and therefore the load will not be allowed to change!"

Which is the exact opposite of what the grid regulator wants to have happen when the grid frequency drifts from nominal.... One of the main functions of Droop Speed Control is to help support load, and therefore frequency, by changing load as necessary. And when Pre-Selected Load Control over-rides the Droop function then, well, the grid is not being "supported" as it should be.

Now, the algorithm or BBL that controls Droop- and Isochronous Speed Control is the FSRN block. There is a "switch" input to the block that selects either Droop- or Isochronous Speed Control.

And, again, when a unit is operating at Base Load, there is no speed control being done by the Speedtronic. None. Zilch. Zero. Nada. Whether or not the unit is in Droop Speed Control or Isochronous Speed Control.

I think the problem here is that someone doesn't understand what the turbine and generator is supposed to do AND what the turbine and generator are capable of doing, and, as is quite frequently the case (almost always, sadly!), the first thing that gets blamed is the Speedtronic turbine control system.

If the grid in your part of the world is experiencing frequency disturbances, there's little your turbines can do (unless the grid they are being operated in parallel with is small in relation to the total output of your plant). Grid frequency control is function of how much generation is on line and producing power. The total amount of generation must EXACTLY match the total load for the frequency to be stable. If the amount of generation is less than the load (the amount of lights and motors and computers) then the frequency will decrease. And it's the grid regulator's responsibility to increase generation, through secondary frequency control ("AGC") or some other method, to bring the grid frequency back to nominal. The opposite is true when generation exceeds load.

If the gas turbines at your site are being operated at Base Load, then when a grid frequency disturbance occurs they will do nothing to attempt to support grid frequency. Period. Full stop. They can't. They are already putting out as much power as they can. Period. Full Stop. (Unless there is some special sequencing or application code running in the Speedtronic panel, that is. Usually called "grid code" for short.)

Also, if the gas turbines at your site are being operated at Part Load with Pre-Selected Load Control enabled and active and a grid frequency disturbance occurs, unless there is some special sequencing or application code running in the Speedtronic turbine control panels, they will NOT respond appropriately to the frequency disturbance. Period. Full stop.

So, if your site is being "accused" of not appropriately responding to grid frequency disturbances then you need first to examine how the gas turbines are being operated. If they are operating at Base Load or at Part Load with Pre-Selected Load Control enabled and active they will NOT respond to grid frequency disturbances. Period. Full stop. End of discussion.

Steam turbines at CCPPs are usually operating in what's commonly called "following mode" meaning the control valve(s) is(are) fully open all the time and the amount of power being produced is a function of how much steam is being produced by the heat from the gas turbine exhaust. There is no speed control active in this mode; there can't be, because the control valve(s) is(are) wide open and being held wide open and power production is dependent on steam flow which is a function of gas turbine exhaust heat and flow.

Do not fall into the trap of automatically assuming that something is wrong with the Speedtronic just because someone says the turbine(s) is(are) not responding to grid frequency disturbances they way someone perceives, or says, they should. First, understand what they are capable of doing, then look into how they are being operated and it's pretty certain you will find the disconnect between what is happening and what someone thinks should be happening.

Otherwise you will spend a great deal of extremely frustrating time trying to make something happen which can't happen, or trying to understand why something isn't happening that can't happen.

The topic of Droop Speed Control has been covered ad nauseum on control.com. Use the 'Search' function of control.com and you will find many threads and explanations of Droop Speed Control.

Your use of this site is subject to the terms and conditions set forth under Legal Notices and the Privacy Policy. Please read those terms and conditions carefully. Subject to the rights expressly reserved to others under Legal Notices, the content of this site and the compilation thereof is © 1999-2013 Nerds in Control, LLC. All rights reserved.

Users of this site are benefiting from open source technologies, including PHP, MySQL and Apache. Be happy.

Fortune
Pure drivel tends to drive ordinary drivel off the TV screen.
Help keep our servers running...