As a rule of thumb, for every 1% change in turbine speed reference, the power output of the turbine should change by 25% for a unit with 4% droop setting.
But in our plant, I have noticed that the speed references in the units are always around 103% at base load when actual speed is 100%. And I have noticed that the speed reference at around 50% of base load is as expected.
The gas turbines work perfectly fine and all the other parameters are changing as expected. I request someone to give an explanation for this.
I request you to supply:
1) Nameplate rating of GT (ambient temperature, ambient pressure, output--everything copied exactly from the turbine nameplate)
2) Ambient conditions at time when TNR was 103% and TNH was 100%--including axial compressor inlet temp (CTIM), CPD, barometric pressure
3) Exact mode of operation selected at the time--Base Load, Pre-Selected Load Control (including Pre-Selected Load Control Reference), or Part Load
4) Load at the time; exhaust temperature at the time
5) IGV angle at the time
6) Age of machine; time since last scheduled maintenance outage; time since last off-line compressor water wash
7) Does the GT exhaust directly to atmosphere, or into a heat recovery steam generator (HRSG)?
That will do for a start to attempt to answer your request.
The ratings of the GT are as follows:
1) Speed GT/ Load: 5089/3000
2) Site conditions: 95 deg F ( 35 deg C) 14.7 PSIA (1.013 bars)
3) Nameplate rating : Base: 19520 KW
Peak: 21230 KW
4) Fuel system : Gas
At the time of concern;
1) Mode of operation : Base Load
2) Ambient conditions : 31 deg C, 1008 mb
3) CTIM : 32 deg C
4) CPD : 8.43 kg/cm2
5) Load : 21.2 MW
6) Exhaust Temperature : 507 deg C
7) IGV angle : 83 deg
The turbine is 27 years old. Last Major Inspection was conducted in 2010 and Combustion Inspection in 2014. The GT exhaust into an HRSG and the last offline waterwash done around 3 months back.
It's noted that the nameplate data you provided doesn't exactly match the operating conditions you listed. If the Peak Load rating is 21.230 MW and the unit was operating at Base Load and producing 21.2 MW, then it's pretty likely that some turbine performance upgrades have been done over the 27 years since the unit was first installed--especially since the original nameplate rating for Base Load was 19.520 MW--and the turbine nameplate wasn't changed to reflect the upgrade(s).
Gas turbine output while operating on CPD-biased exhaust temperature control (Base Load or Peak Load) is a function of MANY things, including inlet filter cleanliness, axial compressor cleanliness, IGV angle, axial compressor mechanical condition (clearances; blade condition; etc.), fuel make-up (BTU content; UHV; LHV; etc.), exhaust back pressure, hot gas path part condition (nozzles; buckets; buckets/shroud clearances; etc.), ambient temperature, ambient pressure, and ambient humidity. ALL of these things can each have an impact on turbine output individually--and when combined they can have an even larger impact.
You were exactly 100% correct in the way you described Droop Speed Control--it is, first and foremost, the way to describe how load changes with changes in speed error (the difference between speed reference and actual speed). A machine with 4% Droop would change load by 25% for every 1% change in speed error.
When a gas turbine is in new and clean condition, with clean inlet air filters, a clean axial compressor with tolerances within specified limits, with turbine hot gas path parts in new and clean condition with bucket clearances within specified limits, with an exhaust duct back pressure within specified limits, with a fuel make-up that matches that used for configuring and designing the fuel nozzles and fuel control valves, with properly calibrated fuel control valve LVDTs, with properly calibrated IGV LVDTs, and with ambient conditions at rated--the power output will be at nameplate rated when the turbine speed reference is 104%. 27 year-old machines almost never have all those conditions all at the same time when operating at Base Load.
(It should be noted that for machines with Peak Load Rating on the nameplate with 4% Droop and with control systems configured to provide Peak Load, that when in a new and clean condition and at rated ambient conditions the turbine will produce Peak Load rated power output when the turbine speed reference is 104%. That means upgraded units with nameplates changed to reflect the upgraded output, too--which this unit doesn't seem to have (based on the information provided).)
Rated power output also only occurs when the unit is operating at rated speed (for those who operate turbines in regions where the grid frequency is unstable).
So, if turbine speed reference is 102% (approximately) when the unit is at 50% power output, and if turbine speed reference is at 101% (approximately) when the unit is at 25% power, just because it's NOT at 104% (approximately) when it's operating at Base Load (or Peak Load as noted above) <B>DOES NOT MEAN the Droop Speed Control is not working. In fact, it IS working--load is changing by approximately 25% for every 1% change in speed error. It's just that current operating conditions (machine conditions and ambient conditions and fuel make-up) are not all at rated and within specified limits when the unit is operating on CPD-biased exhaust temperature control on any given day.
That's why Performance Correction Curves for gas turbines are provided by the manufacturers--so that off-rated operating conditions can be corrected back to rated conditions to assess machine performance.
I would venture to say based on the information provided that the IGVs are not properly calibrated or something is amiss with the servo-valve and/or the Null Bias Current setting or regulator gain--based on the 83 Degree information you provided (because the typical IGV reference angle for Base Load for most GE-design Frame 5 heavy duty gas turbines is 84.0 Degrees). IGV angle has a LOT to do with Base Load (and Peak Load) performance. Compressor cleanliness, inlet filter cleanliness and exhaust duct back-pressure also have a lot to do with performance. And, lastly, fuel make-up and gas control valve LVDT calibrations also have a lot to do with performance--and with how the turbine control system deals with the speed error when at Base (or Peak) Load.
Again, if a 1% change in turbine speed reference below Base (or Peak) Load results in a change of approximately 25% of rated power output on a machine with 4% Droop, then the unit is operating properly. Just because it can't get to 104% turbine speed reference when being operated at Base (or Peak) Load does not mean Droop Speed Control is not working properly. A lot of factors affect Base (and Peak) Load performance, and it's rare that at any time in a gas turbine's life that all the factors are as they should be--even during initial commissioning, it's difficult to get Mother Nature to keep ambient temperature and pressure and humidity to nameplate rating (which is why there are Correction Curves).
Droop Speed Control is about how much the load changes for a given change in the speed error for the machine. Not about whether or not rated power output occurs when Droop is at the setpoint.
Hope this helps!