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Fuel Stroke Reference Doubt

what is meaning of "%" in FSR?

By knm on 14 June, 2018 - 1:29 am

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If FSRN is 32% at any instant of frame 6B machine...what does it tell?

what do we come to know from 32%?

FSRN/any other FSR, is '%' of what?

what decided the Full speed no load FSR(FSRKn_1 which is 15.9%) and Droop FSR(FSRKn_2 which is 11.2%)?

By CSA on 14 June, 2018 - 2:28 pm

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khoriwal000,

Doubt is not the right word; you want clarification and explanation of FSR. To doubt is to question the truth or fact of something. There can be no question about the truth or fact of FSR. There might be some explanation required in order to clarify one's understanding of the topic/term, but there can be no doubt.

Fuel Stroke Reference is a value, in percent, of the amount of fuel that should be flowing to the unit through opening(s) in fuel control valves (NOT the Stop/Ratio Valve, but Gas Control Valve(s) and Liquid Fuel Control Valves).

FSRN, Speed Control FSR (most usually Droop Speed Control FSR), is really only active above approximately 95% speed, which is essentially "full speed," and when loaded (when the generator breaker is closed.

Droop Speed Control--as has been covered ad nauseum on control.com--is the governor control mode for prime movers and their generators that allows them to share in supplying to a grid with many other prime movers and generators in a stable fashion. It does so because of a very fundamental principle of AC (Alternating Current) power generation that states that the speed of a prime mover and generator is directly proportional to the frequency of the AC power being produced. Or, said another way, the frequency of the AC power being produced is a function of the speed of the prime mover and generator.

When generators and prime movers are ** synchronized** together with other prime movers and generators on an AC grid

And, because the speed of the generators and the prime movers driving the generators is directly proportional (a function of) the frequency, the speeds of all the generators and their prime movers is controlled by the grid frequency. If the nominal grid frequency is 50.00 Hz, but the actual grid frequency is 49.71 Hz, then the speeds of all the generators and prime movers synchronized to the grid will be LESS THAN rated (that is, below synchronous speed). And, if the actual grid frequency is 50.65 Hz, the speeds of all the generators and prime mover synchronized to the grid will be MORE than rated (that is, above synchronous speed).

So, when a grid is being properly operated and grid frequency is being properly regulated (by balancing generation and load), the grid frequency will be at or very near rated and the speeds of all the generators and their prime movers will also be at or very near rated--and stable. That's important, because when the grid frequency is stable, the speeds of all the generators and their prime movers synchronized to the grid will also be stable.

And, Droop Speed Control operates on the premise that the grid is at or very near rated usually, so the speeds of the machines are at or very near rated--and are not changing (they are stable, at approximately 100%).

So, when the operator wants to increase the power production of a generator, unbeknownst to him/her they change the prime mover speed reference--trying to increase the speed of their prime mover and generator. BUT, the grid WILL NOT allow that to happen (not by any appreciable amount!).

By increasing the prime mover speed reference when the actual speed cannot change (because it is being held constant by grid frequency, which is stable and not changing) the error between the speed reference and the actual speed of the generator and prime mover increases.

This increasing error causes the amount of energy being admitted to the prime mover driving the generator to increase, which would TEND to cause the generator (and prime mover) speed to increase--but, again, the grid isn't going to let that happen, and the generator, doing what generators are known for doing, converts the additional torque being produced by the prime mover because of the increased energy flow-rate into the prime mover because of the increased error between the speed reference and the actual speed of the machine causes more amperes to flow in the generator stator winding--which increases the electrical power output of the generator.

So, the operator is watching the MW meter as he/she is clicking on RAISE SPEED/LOAD and and stops clicking or RAISE SPEED/LOAD when the MW meter reaches the desired load. But, what's really happening in the background is that RAISE SPEED/LOAD is increasing the error between the speed reference and the actual speed (which isn't changing!) which is increasing the energy flow-rate into the prime mover which the generator is converting to amperes to make the MW meter increase it's value.

That's basically what Droop Speed Control does--it looks at the error between prime mover speed reference and prime mover actual speed and changes the energy flow-rate into the prime mover, which changes the electrical power output of the generator.

Now, as we all know, it takes a certain amount of energy to get the prime mover and generator up to rated speed (100% speed) and to keep it at 100% speed--and that's FSNL FSR. Let's just say it's 20%. And, let's say that when the prime mover and generator--when operating at rated speed (100% speed and 100% frequency--50 Hz in this example (it could be 60 Hz in some parts of the world)--requires 70% FSR to produce rated power (when the prime mover is in a new and clean condition and the inlet air filters are clean, and the ambient temperature and pressure are at nameplate rating, and the fuel being burned matches the expected fuel characteristics supplied to the prime mover manufacturer). That's a a difference of 50% (70%-20%). And for a machine which has a Droop Setpoint (characteristic) of 4%, that means that for each 1% change is speed reference the FSR will change by 12.5% of rated. And, the energy flow-rate into the prime mover will change by 12.5% (again when all conditions are at or very near rated). So, if the machine is rated at 40 MW, for example, when it's producing 0 MW the FSR will be 20% (the amount required just to maintain 100% rated speed). When the unit is producing 10 MW (1/4 of rated load, or 25% of rated load), FSR will be 32.5% (20% + 12.5%). And when the unit is producing 50% of rated load, FSR will be 45 % (20% + 25%). And when the unit is producing 75% of rated load, FSR will be 57.5% (20% + 37.5%). And, when the unit is producing rated power output, 40 MW, FSR will be (20% + 50%).

Who (not What) decides FSKRN1 and FSKRN2? The factory decides that, because they were given the expected fuel characteristics that would be available at the site when they were designing the turbine. And, they used that information to calculate the fuel control valve internals (plugs and seats) and fuel nozzle orifices. They know how many BTUs are required to produce 100% speed and 100% load (rated load) and they choose valve internals and fuel nozzle orifices so that the expected fuel flow-rates will produce desired speeds and loads in a certain range of values (and approximately 20% FSR is about right for FSNL; and approximately 70% FSR is about right for Base Load (rated output) FSR).

So, it should be getting clearer that FSR is related to the amount of energy flowing into the prime mover, which is also related to the speed of the prime mover and generator (when they're NOT producing electrical power) and to the load of the prime mover and generator (when they ARE producing power). And, for machines burning gas fuel, the gas control valve internals and fuel nozzle orifices are chosen such that the energy flow-rate through the valve(s) is proportional to the opening of the valve(s)--the "stroke" of the valve(s). So, when the Gas Control Valve is at 32% of stroke, that's relative to some portion of the expected energy flow-rate through the valve at 32% of position (opening).

Using your values of 15.9% for FSKRN1 and 11.2% FSKRN2 and 32% FSRN, and presuming a Droop setpoint of 4% (typical for heavy duty gas turbines), it would seem that the unit should be producing 100% of rated power when FSR was at 60.7% (15.9% + (4*11.2)). And when FSR is at 32%, the unit would be producing approximately 36% of rated power (32% - 15.9%)/(4*11.2))*100=~35.9%, or ~36.0% of rated. If the unit was rated at 40 MW, the power output would be approximately 14.4 MW (40 MW * ~36%).

FSR is widely considered to be equal to the Gas Control Valve stroke (opening; position) when burning gas fuel)--for units with a Gas Control Valve. (Some units have multiple Gas Control Valves to control the fuel flow-rates to various nozzles in the combustors, and the sum of the flow-rates through these valves add up to the FSR--the total energy flow-rate through however many valves are in service.)

But, FSR is really about how much (in percent) fuel is flowing into the turbine--because the gas turbine designers chose fuel valve internals and fuel nozzle orifices such that expected fuel valve positions (strokes; openings) would correspond VERY CLOSELY to fuel flow-rates (expected fuel flow-rates) when the unit was at rated speed and producing power. Fuel valve stroke (opening; position) is proportional to fuel flow-rate. And fuel valve stroke is expressed in percent of valve travel, 0-100% One wouldn't want the fuel valve to be 100% open when the the unit was producing rated power output, because the valve isn't really "controlling" fuel flow-rate at that point. So, instead of rated load occurring at 100% of valve opening, the designers chose the valve internals so that the valve will be at somewhere around 60-80% of rated travel when the unit is producing rated load--so that the valve can control fuel flow-rate should anything unexpected happen. (Safety margin, in other words.)

Does this help? Because it's really all fact, not subject to doubt. It is what it is. It's not anything else. When FSR is 32%, the Mark* is calling for a percentage of fuel flow-rate, and the percentage it just so happens is proportional to valve position (stroke; opening) as a function of the rated opening of the valve. (That's made possible by the SRV--which controls the pressure upstream of the GCV, thereby making flow through the GCV proportional to the stroke (opening; position) of the GCV.

It's very similar for liquid fuel, too.

Pretty cool, huh? (I thought you'd like it!)

Before DLN combustors, there was just one GCV (Gas Control Valve). And FSR was almost always very nearly equal to GCV position (stroke; opening). And that's expressed in percent of maximum opening.

Have a look at Section 5 of your Control Specification. It will be very enlightening.

And, if you have questions--we're good at questions. Doubts, ..., well, ..., we don't deal in doubts. This is a technical forum, based on physics and scientific principles--facts and truths and formulae. Nothing is doubtful--it is what it is. It may not be intuitively obvious, but it is not subject to doubt.

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I am highly thankful to you and your team.

While calculating FSR at any point of time, you have always considered (TNR-TNH)% to be droop value i.e.4%.

Should it not be real difference (in %) of TNR and TNH. I mean if TNR=100.5% and TNH is 100%, then (TNR-TNH) comes out to be 0.5%.

so using formula, FSR=FSRKn_1+(TNR-TNH)*FSKRn_2 = 15.9 + 0.5*11.6=21.7%.

Please correct me if i am wrong.

One more clarification i need is, during changeover from one type of FSR to another type of FSR-(consider start up FSR i.e. FSRSU to speed control FSR i.e. FSRN)- will the last value of FSRSU be the first value of FSRN?

By CSA on 17 June, 2018 - 9:41 am

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Khoriwal000,

Can you not use the 'Search' feature of control.com? This topic has been covered tens of times before, with the answer to your first question.

4% droop means that when the speed reference is at 104%--meaning the speed error is 4% (when the grid frequency is at 100%)--the United will be at rated (Base) Load (when it's in a new and clean condition and the ambient conditions match the turbine nameplate conditions).

For your last question, the answer is yes under normal operating conditions with normal parameters and expected fuel characteristics.

The 'Search' feature of control.com is very powerful and one can learn a lot by using it. The GE-design heavy duty gas turbine control community has been very active here on control.com for more than one-and_a-Half decades (yes, more than fifteen years!). Most questions have been asked and answered at least once--but the subject of droop speed control has been asked and answered so many times several people have wondered if the name of this forum had been quietly changed to droopspeedcontrol.com.

By CSA on 19 June, 2018 - 11:29 am

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Khoriwal000,

>While calculating FSR at any point of time, you have always

>considered (TNR-TNH)% to be droop value i.e.4%.

The droop characteristic, or droop regulation, or droop setpoint, is a number that is chosen based on many criteria, usually how quickly the prime mover can respond to changes in the error between the prime mover speed reference and the prime mover actual speed. The droop characteristic for heavy duty gas turbines is typically 4%; for steam turbines it's typically 5%.

What this means is that when the speed error equals the chosen/specified droop setpoint the unit will be at rated power output (the rating of the prime mover--NOT the generator), when the ambient conditions are at or very near the nameplate rating of the prime mover and when the prime mover is in good mechanical condition ("new and clean condition").

I believe GE tries to disguise the droop setpoint of their heavy duty gas turbines to some extent. Why? Because there are WAY too many people who think that by changing the droop setpoint they can "improve" the performance and/or responsiveness of their prime mover and generator. They would be changing the droop setpoint all the time if it were just a simple number (Control Constant). Why? Because they can. And because they haven't taken the time to understand what droop speed control is and how it works, and how GE uses it to control their heavy duty gas turbines. It's part of a very large control scheme, that includes loading and unloading rates--which many people think that changing the droop setpoint is the proper way to change loading or unloading rates. And it's not.

Droop speed control is about the error between speed reference and the actual speed--and how it relates to the specified/chosen droop characteristic (setpoint; regulation) for the specific prime mover.

If you will refer to Sect. 05 of the Control Specification provided with the Mark* turbine control system at your site, you will find a sub-section titled 'Expected Fuel Characteristics.' It contains a LOT of very useful information about the **expected** operation of the unit based on the expected fuel characteristics at the time the control system was being configured (fuels do change over time and also because people calibrate LVDTs differently changes can also occur; and as mentioned above, some people like to "tinker" with Control Constants without understanding the knock-on effects). You will see expected FSRs and fuel flow-rates for FSNL, 1/4 rated load, 2/4 rated load, 3/4 rated load and 4/4 rated load. If the unit was configured for Peak or Peak Reserve Load, you will also see data for those conditions, too.

If your machine isn't operating exactly per the data in the table, that **does NOT** mean that something is wrong. It can mean MANY things, especially if the fuel characteristics have changed since commissioning. But, it should be a very good guide to the ideal performance of the machine and how it was intended to operate when it was new and the fuel matched the expected characteristics.

The droop characteristic (regulation; setpoint) is related to the speed error between the speed reference and the actual speed. It defines how much the fuel flow-rate will change for a given change in the speed error. When the speed error is stable and not changing, the fuel flow-rate will be stable and not changing--and the load will be steady and not changing very much.

Droop speed control is not something which is intuitively obvious to the vast majority of people. It has taken me decades since I was first introduced to the topic in university to understand it. However, it is fairly simple and it's universally used for most prime mover governors. Once a person begins to really understand it, and how most prime movers use it--one can see how really and truly powerful it is. It's really an ingenious concept in several ways. Frequency is critical to an AC power system, and frequency and speed are related. If speed is fixed by the frequency of the AC power system (a well-regulated AC power system), then one of the variables in the droop speed control equation will be fixed, and changing the other one will change the fuel flow-rate. Which will change the amount of torque being produced by the prime mover. Which will tend to try to change the speed of the prime mover--but because the grid frequency is holding the holding the speed of the prime mover fixed that extra torque is converted by the generator to amperes--alternating current amperes. And that is what changes the electrical power output from the generator.

And because all generators synchronized to an AC power system are running at the same frequency (different synchronous speed based on the number of poles of the generator--but the same frequency) all generators and their prime movers are sensitive to any change in frequency. And, if the grid frequency changes--as can happen (more in some parts of the world than others)--then all the generators, and their prime movers, will respond to the changes in frequency/speed in order to help support the grid stability *to the extent possible.* And, that is the other part of droop speed control. And all the generators and prime movers, because they all operate on droop speed control, will respond to changes in grid frequency (presuming they are not already at rated load, or at zero load)--simply because their speeds are changing with respect to their speed references. No special communications require--it's all done with frequency, and speed.

Look, there really has been a LOT written about droop speed control on control.com. And, I understand that everyone has a different perspective and there aren't videos and graphs and drawings on control.com to help everyone find their own way to understand the concept. Droop speed control, at it's very simplest, is how the energy flow-rate into a prime mover is controlled. The energy flow-rate has to be controlled based on something--and it's done based on speed. (Back when AC power systems were first being developed, there wasn't much else. It was virtually impossible to get amperes into the governor, which was a mechanical device, a fly-ball governor in most cases. There was no electronics back in those days--just spring tension and centrifugal force. And speed. And as governors and control systems evolved over the years they still had to connect to the same AC power systems as the older equipment was also connected to. And, droop speed control was continued for decades (more than a century) as the primary method of controlling the energy flow-rate into prime movers. It allows all the prime movers and generators synchronized to a grid to act as one large generator, sharing stably in the production of AC power at a stable frequency (when well-regulated).

So, it must not be a very complicated concept--in fact, as control schemes go it's one of the simplest. It's called proportional control, the 'P' in PID control (Proportional/Integral/Derivative). It's extremely simple in that it relies on an error--the speed error. There is nothing to "return" the error to zero--as in many other control schemes and control loops.

Grid regulators/operators need to know the droop characteristic or droop setpoint each prime mover and generator has--because when the grid frequency increases and decreases and the prime movers and generators change their load in response the change in the error between the speed reference and the actual speed--they need to know how much each generator will change their load by. And if the droop setpoints are different than expected, or if some scheme such as Pre-Selected Load Control is arbitrarily changing the load when it shouldn't be, then responding to grid frequency disturbances can be very difficult and lead to brown-outs and black-outs. This is yet another--the most important, actually--reason for never changing the droop setpoint without fully understanding all the knock-on effects. In some parts of the world power plant operators can be fined for changing the droop without notifying the grid regulators/operators. And, it's one reason why Pre-Selected Load Control is being "outlawed" in some parts of the world--because it can actually aggravate grid frequency issues.

Again, droop is about how the energy flow-rate into the prime mover is controlled. It's done using the error between the speed reference and the actual speed--the latter being a function of frequency, which is usually stable (presumed to be stable). So, changing the error changes the energy flow-rate. And, the droop setpoint, or droop characteristic, or droop regulation, defines how much the energy flow-rate will change for a given change in the speed error. Units with 4% droop will change their energy flow-rate by 25% of the no load-to-full load energy flow-rate for every 1% change in the speed error. Units with 5% droop will change their energy flow-rate by 20% of the no load-to-full load flow-rate for every 1% change in the speed error.

And, the error can change whenever the speed reference changes and the actual speed is constant, or when the actual speed changes and the speed reference is constant.

If you know the mathematical formula for a straight line (y-mx+b; or f(x)=mx+b are two of the most common formats), you can see how the droop speed control formula is a variation on that. The "x" term in the droop speed control formula is (speed reference - actual speed)--the speed error. The "m" term is a gain, and the "b" term is the offset. The "m" term describes how much the energy flow-rate will change as the error (the "x" term" changes. And the "b" term--that's how much energy is required just to keep the unit spinning at rated speed/rated frequency. The "m" term is the "gain" term, and the "b" term is the "offset" term.

Droop speed control can be described using a straight line. The difference is that the speed error really has two variables--one that's usually constant (the actual speed), and one that changes when the operator wants to change load. But, when the grid frequency changes the error also changes. Again, it's a pretty ingenious and powerful--yet simple--scheme. But, it usually takes a lot of thought and contemplation and consideration and study to fully understand it.

Hope this helps! I've dumped a lot on you here, but it's all related in the end. And, there are many ways to think of droop speed control--but it's all about how much the energy flow-rate into the prime mover will change for a given change in the speed error. And, that is a function of the droop characteristic, or the droop regulation, or the droop setpoint, of the generator's prime mover. (The generator is really very dumb--it just converts torque to amperes, just like a motor converts amperes to torque. Change the torque being supplied to a generator--and the amperes will change.)