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Questions about Isochronous and Droop
I understand droop and isochronous modes, but have a couple of questions.

CSA,

I have been reading your post since 2009 and i must say they have been quite helpful in understanding the concept of Droop/ISO responses. However, i personally work in a power plant and i am stuck at one thing. i hope u will have answer for it.

Lets take an example and u correct me if i am wrong:

Lets just say, there is a machine on droop whose rated RPMs are 5000, and its Droop is set at 4% at a 60 Hz frequency grid. It will be on full load at droop compensation of 5200. Right?

Lets say machine was running at 50% load, so its droop compensation would be 5100 at that time. Correct? Lets say a machine on the same grid trips, and the frequency drops to 59.4 hz (i.e. 1% drop in frequency). the machine will take 25% load instantly as per its Droop setting of 4%. Now i have 2 questions:

1. Will the Droop compensation that was previously 5100 also change automatically or not? which i think it won't?

2. Then my second question is, basically the Droop compensation is defining the load of the machine, right? As in the above example if the droop compensation is 5050 RPM, it means machine is at 25% of rated load, if it is 5150 RPM then machine is at 75% rated load. So if the droop doesn't change automatically, then will the try to come back to its original load of 50%, because in the above example we assumed that the droop compensation 5100, so after taking 25% load and stabilizing the frequency of the grid will the machine try to get back to its 50% load??

1 out of 1 members thought this post was helpful...

SAK,

I do not remember ever referring to "droop compensation." When a new and clean machine operating at rated ambient conditions with 4% droop [regulation] synchronized to a well-regulated grid it will be at full load when when the speed reference is at 104%.

If that same unit is synchronized to a grid that is at rated frequency, and is operating at a speed reference of 102%, then it will be producing 50% of its rated load. Every 1% difference in speed error for a machine with 4% droop will cause the load to change by approximately 25% of rated load.

If the grid frequency drops by 1% while the unit is operating at a speed reference of 102% the speed error will increase to 3% (102-99) and the load will increase by 25% of rated, to 75% of rated load.

Droop [regulation] is about the speed error--anything that causes the speed error to change will cause the rated load to change. The equation has been covered many times.

When a unit is synchronized to a well-regulated grid it's speed DOES NOT CHANGE WITH LOAD. The speed reference is used to change load, but the actual speed does not change. That's the problem with most written descriptions of droop speed control[regulation]--they say that speed changes with load, but it doesn't when synchronized to a well-regulated grid. Load changes when the error between the speed reference and the actual speed changes.

When a unit with 4% droop, synchronized to a well-regulated grid operating at a speed reference of 101% experiences a grid frequency increase if 1% the speed error changes to 0%--and the load decreases to 0% of rated. If the frequency continued to increase the speed error would become negative, and the load would go negative; as the frequency continued to increase (and the continued to increase in the negative direction) the unit would eventually trip on reverse power to protect the prime mover.

Droop speed control does not cause a unit's speed to change as it is loaded and unloaded. Load changes as the speed error between the speed reference and the actual speed changes. The actual speed of a unit synchronized to a grid is controlled by the grid frequency. If the grid frequency is at rated and stable then the actual speed is 100%. The operator changes load by increasing or decreasing the speed reference to change the speed error--which changes the load. If the speed reference and the actual speed are stable, and the actual speed changes--because of a change in grid frequency--that also causes the speed error to change.

The only time I could envision the actual speed being at 104% of rated speed would be if the speed reference were at 104% and the grid frequency was at 104%--which would make the speed error 0% (for a unit with 4% droop), which would make the load equal to 0% of rated load.

It's the speed error that changes the energy flow-rate into the prime mover that changes the load. And--again--the speed doen't change with load on a well-regulated grid operating at rated frequency. The load changes the EITHER the actual speed OR the speed reference changes--which causes the speed error to change which cause the energy flow-rate into the prime mover to change. Load doesn't change the actual speed--when the actual speed changes the load changes. And on a grid the frequency should be at 100% which would make the actual speed equal to 100%.

Hope this helps! It's very important to understand that the speed of a generator and its prime mover are proportional to the frequency of the generator, and that all generator and their prime movers when synchronized to a grid are operating at the same frequency--and proportional speed. No generator and its prime mover can operate at any different frequency--and proportional speed--than any other generator. That's the definition of synchronism.

Thanks CSA,

Its been very helpful and about the "Droop Compensation" term, its just the terminology we used for reference speed, speed reference or difference between actual and reference speed.

I completely understand your view point, i mean to say that the reference speed is in our hands. Right?

Its doesn't change automatically.. Am i right?

Like in the above example, a machine with actual speed of 5000 RPM at 60 hz frequency grid with 4% droop will be at 100% load when the reference speed is set at 5200.

My question is, if the conditions remain same and we set the reference speed at 5100, machine will be at 50% load. At that point if there is some disturbance in the grid due to tripping of some machine and frequency drop to 59.4 Hz (i.e. 1%), this particular machine will take 25% load (i.e. it will go up to 75% load) right?

If yes, then my point is that the reference speed is set at 5100 and now the grid is back to its normal frequency of 60 Hz, and this particular machine was at 75% due that transient period of load disturbance. Will this machine try to comeback to its 50% load or not?

Also I have seen in Droop machines take load in milliseconds. I mean if a certain change in frequency suggest that the machine should increase load from 25% to 75%, how exactly the machine pick up load in milliseconds. Is there some kind pre-positioned command that causes the governor to open up immediately or what?

1 out of 2 members thought this post was helpful...

SAK,

Well, ..., er, ..., uh, ..., I wouldn't use the same term to describe different things. The difference between the speed reference (if you want to call it 'droop compensation', more power to you) and the actual speed is the speed error. It shouldn't be called anything else (in my personal opinion).

Many control system manufacturers prefer to use speed reference and actual speed in percent of rated; that makes it easy to have different rated speeds for different machines and still use the same logic/application code. (A machine rated at 5100 RPM (that's the prime mover speed--NOT the generator speed) would be at 100% speed when it was operating at 5100 RPM. A machine rated at 3600 RPM would be at 100% speed when it's speed was at 3600 RPM. All one would need to do would be to change the rated speed--and everything else would be the same.)

You are correct about most everything. The speed error is used to determine how much fuel is going to go into the machine--and fuel flow-rate is directly proportional to the electrical load being produced by the generator. Increase the fuel flow-rate, and the load on the generator increases. The speed DOES NOT increase when synchronized to a well-regulated grid--the amount of torque being produced does. And, since the torque CAN'T increase the speed (because the generator is synchronized to the grid) the generator converts the additional torque into amperes--which is what makes the load increase.

Droop speed control is how the amount of fuel flowing into the machine is controlled. And, any change in the speed error (whether it be by the operator changing the speed reference, or the grid frequency changing changing the actual speed of the prime mover) will change the fuel flow-rate. Immediately. It's direct--no ramp rate.

And, when the frequency disturbance is over and the grid frequency returns to normal, if the speed reference was 5100 RPM (102%) for a machine rated at 5000 RPM with 4% droop (regulation--or, compensation) which was running at 75% load when the frequency decreased by 1% (and the speed decreased to 4950 RPM), the load will automatically decrease by 25% when the speed increases by 1% back to rated (5000 RPM). Because the speed error will return to 102% (from 103% when the frequency was 99% and the speed reference was 102%--a difference of 3%). So when the actual speed returns to 100% and if the speed reference remained at 102% during the frequency disturbance (the operator didn't try to change load, or Pre-Selected Load Control wasn't active, or Remote Load Control wasn't active) then the speed error will return to 2%.

A speed reference of 5100 RPM for a machine rated at 5000 RPM with 4% droop (regulation or compensation) would translate to a speed error of 100 RPM, which is 100 RPM out of 200 RPM (full load)--which means the load should be approximately 50% of rated. Or, a machine with 4% droop and operating with a speed error of 1% would translate to 25% of rated load (this, of course, presumes the actual speed it 100%--which it should be for a well-regulated grid).

It's all about the speed error--whether you want to think of it in terms of RPM or percent (it's really easier in percent, but it took me years to understand that myself...). And speed error directly affects the amount of fuel flowing into the machine, which directly affects the electrical load being produced by the generator.

i will be so grateful If you allow me to contribute in this discussion and try to help.

about your question, YES it will comeback automatically, and i will try to explain why. so once the machine is loaded 50% so the speed reference is 102% (50% load), and the actual speed is 100% because the grid frequency is stable 60Hz. so the difference between the speed reference and the actual speed will affect directly the fuel flow in the prime mover(in our case 102%-100%) = 2%.

so now once we have a grid frequency disturbance like you said, and as Mr CSA said yesterday that the actual speed is relevant to the synchronism speed of the grid. so the actual speed would fall by 1%.

following your example:

so now what we will have? the speed reference 102% (we haven't change it) and the actual speed 99%. consequently the difference between them will rise to 3%. like i said this difference affects the fuel flow in the prime mover, and so the fuel flow would rise and this is why this machine will take 25% load to be 75%. once the grid frequency stablise to 60Hz >, actual speed will go back to 100%, and the difference between it and the speed reference will be 2% again > the fuel flow will lower. so this is why i said it will go back automatically.

Hope i answered the question!

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The general Droop speed control formula is:

Energy flow-rate into prime mover = ((prime mover speed reference - prime mover actual speed) * Gain) + Offset

The Offset is the amount of energy required just to achieve and maintain rated speed when the unit is un-synchronized. If it takes four times as much energy to achieve rated load as it does to achieve and maintain rated speed (when the unit is un-synchronized), the Gain would be 4. In either case, BOTH the Offset and the Gain are constants--they are not variable.

Howsoever, BOTH the prime mover speed reference AND the prime mover actual speed are variables. It just so happens, that on a well-regulated grid the prime mover actual speed is stable and at rated as long as grid frequency is at rated and stable (which is the desired operating condition for most every grid I've ever worked on). So, the normal way to change load is to change the prime mover speed reference--which increases the speed error, which increases the energy flow-rate into the prime mover. And, to operate at Part Load, one just changes the speed reference until the desired load is achieved, and the speed error will remain unchanged (as long as the frequency remains unchanged).

[By the way, Droop speed control is usually NOT used during acceleration to rated speed, nor deceleration from rated speed. Just in case anyone was wondering. It's only used during loaded operation on many machines (after the generator is synchronized), or just slightly before or when rated speed is achieved ("synchronous speed").]

NOW, since frequency and prime mover speed are directly related, if the grid frequency changes, the prime mover actual speed will actually change, which will change the speed error which will change the energy flow-rate into the prime mover. Before there were transducers which could measure frequency and provide a signal to the control system, all there was was speed--actual speed which was equivalent to frequency. So, it was logical to create a system which used a speed reference to control load AND which would adjust itself to control frequency when needed. That's the beauty of Droop speed control--it does TWO things with one equation.

When either the electrical power being produced is not equal to the power being consumed (such as when a prime mover and generator trips off the grid) OR the electrical power being consumed becomes greater than the electrical power being produced (because the grid operators didn't increase generation to match load), then the grid frequency will drop. The load IS what the load IS--it's not going to change even if the frequency changes. So, Droop speed control allows units which are operating at Part Load (so at speed errors less than 4% for units with 4% droop) to increase their load in proportion to the frequency decrease (which is proportional to the load increase) to help support the electrical load and grid stability. If no units did picked up load during a frequency decrease, then a death spiral of ever-decreasing frequency would occur until there was a brown-out followed by a black-out (cause by the underfrequency relays tripping the generators).

When either the electrical power being consumed decreases below the electrical power being produced (because the grid operators weren't decreasing generation as load decreased), OR some large block of load is tripped off the line (such as a substation breaker opening and blacking-out a neighborhood or factory), then the frequency will increase. And Droop speed control allows those units not running at full load to reduce their generation to maintain load and grid stability.

If the grid frequency were to drop and remain at some value less than rated--say because some very large generator had tripped off-line and wasn't expected to be able to re-synchronize any time soon, the operators of those units which had picked up load (those units operating at Part Load) would only need to manually start increasing their prime mover speed references to begin restoring frequency.

Usually, grid operators use something called AGC (Automatic Generation Control) to control the prime mover speed references of some units synchronized to the grid--and they send RAISE- or LOWER SPEED/LOAD signals to those units to adjust the loads of those units to maintain grid frequency.

If you can monitor grid frequency in the morning when people wake up and go to work, you will usually see the frequency is a little low. Sometimes, often, actually, the grid operators have to ask some power plants to start up and go on line.

And, then late in the evening when people are going to bed , turning off their computers and televisions, and it cools off and the fans and air conditioners are shut down, you will usually see the grid frequency be a little high. Sometimes, often, actually, the grid operators have to ask some power plants to shut down, or to go to minimum load.

This is how grid operators try to match load and generation--which must be equal for frequency to remain stable (even if the frequency is low or high). Load always has to match generation. Units which are operating at Part Load are sometimes referred to as "spinning reserve"--because they can be loaded "quickly" if necessary, again by a changing speed error (either because the speed reference increase OR the actual speed decreased).

And, if there's no "support" for the grid as it experiences frequency disturbances (caused by load and generation mismatches), then the grid will become unstable. And that's when the lights have the biggest chance of going out.

Droop speed control is an amazing thing, when you think about. It doesn't need fancy transducers or control systems (which power transmission and distribution systems didn't have in the earliest days of AC power systems. They just used what they had: speed. And, if you search for "flyball governor" and you will see what the earliest prime mover governors looked like. They were completely mechanical--no electrical or electronic parts at all. Just speed. And spring pressure (simulating speed as the speed reference).

It's really not that difficult a topic--as long as you think about the two variables (speed reference and actual speed), either of which can change at any time. And the difference between the two--the speed error--is what's used to control the energy flow-rate into the prime mover.

Sure, control systems are MUCH more sophisticated these days and don't have to use speed--they can use actual load. BUT, what about all the older units (and there are still some VERY OLD governors still in use around the world today). How would they work? As controls systems (governors) became more sophisticated, they still had to work with the old governors. And, what is the ONE thing that all units synchronized to a grid have in common? Grid frequency--which is proportional to prime mover speed. All power plants synchronized to a grid all "communicate" with each other--by means of grid frequency. Nicola Tesla was a VERY smart man, indeed, recognizing the potential of AC power--in more ways than one!

It's all about synchronism and speed error. And it's really all very simple. Under normal circumstances, when the grid frequency is stable at 100%, one of the variables is stable at 100%. Change the other, and the error between the two changes, and the power being produced by the prime move and the generator changes. And, we can both control and predict how much the power will change by using Droop speed control.

It's all really amazing--once one understands synchronism and speed error. (And understanding the earliest flyball governors can be very helpful--as long as one understands synchronism.) It all builds from synchronism. N=120F/P for all synchronous generators and their prime movers when synchronized to the grid. No synchronous generator can run at a frequency other than the one it is synchronized to. Period. Synchronism.

Most power plant operators concentrate on the prime mover, not realizing how the generator works, and how the two work when the generator is synchronized with other generators. They all see the speed increasing as the energy flow-rate increases when the unit is accelerating to rated speed ("synchronous speed"), and then they think that after the unit is synchronized (a very important process of speed- and voltage matching, by the way!) that as the energy flow-rate is increased the speed also increases, and vice versa. They are shocked and amazed to learn the speed shouldn't change--and usually doesn't. And, really, that's all about magnetism.... But, we digress (to the beginning).

More than you wanted to know I'm, but I have some time this afternoon, waiting for mechanics and pipe fitters--and managers. After power plant operators, mechanics, pipe fitters and managers are the bane of my existence.

Sir,

I want to get something right here: In this your 1st paragraph below do you mean FSNL when you use Full Load?

"I do not remember ever referring to "droop compensation." When a new and clean machine operating at rated ambient conditions with 4% droop [regulation] synchronized to a well-regulated grid it will be at fULL LOAD when when the speed reference is at 104%"

CSA,

I have seen the responses of Droop machines. With the disturbance in grid, the load on the machines increases/decreases in milliseconds. I mean they take/give up 10-15 MW in miliseconds (in our machines, obviously for bigger machines and grids this would be even greater). Now we are out of stoneage and using modern electronic governors and other electronic transmitters for frequency, speed etc. How the does these governor respond so quickly in contrast to the old fly ball governors.

As i mentioned in my previous post, that suppose a change in frequency suggest that the machine should increase its load up to 25%(previously running at 50% load, now had to increase load to 75%). How does this signal goes to governor and how the governor respond.

I mean if its a simple PID controller it can never increase its load that quickly. What i think is there must be some pre-defined opening of governor inside the program against each load. So if the machine is to increase its load to 75% that predefined opening will suggest the machine to open its governor upto 75% of the fuel demand?? Am i right?

SAK,

>I have seen the responses of Droop machines. With the
>disturbance in grid, the load on the machines
>increases/decreases in milliseconds. I mean they take/give
>up 10-15 MW in miliseconds (in our machines, obviously for
>bigger machines and grids this would be even greater).

When there is a grid frequency disturbance, the governors need to respond quickly. That's actually a desired feature of a prime--the ability to respond quickly to frequency disturbances. Grid regulators like gas turbines, because the can respond very quickly (when NOT operating in Pre-Selected Load Control and at Part Load) to frequency disturbances. Many reciprocating engines, hydro turbines, and large steam turbines, can't respond as quickly and for as long as heavy duty gas turbines can (some aero-derivative gas turbines can respond even faster!).

>How the does these governor respond so quickly in contrast
>to the old fly ball governors.

Hydraulics. And, some electric valve actuators can now respond just as fast as hydraulic actuators. Hydraulic actuators can respond very quickly--especially to shut off fuel flow during a trip.

>As i mentioned in my previous post, that suppose a change in
>frequency suggest that the machine should increase its load
>up to 25%(previously running at 50% load, now had to
>increase load to 75%). How does this signal goes to governor
>and how the governor respond.

For GE-design heavy duty gas turbines, the result of the basic Droop speed control equation drives FSRN--FSR, Speed Control, which is what controls fuel flow during Part Load operation (so anywhere from 0 MW to Base Load). Any change in the speed error causes an immediate change in the Fuel Stroke Reference (FSR).

>I mean if its a simple PID controller it can never increase
>its load that quickly. What i think is there must be some
>pre-defined opening of governor inside the program against
>each load. So if the machine is to increase its load to 75%
>that predefined opening will suggest the machine to open its
>governor upto 75% of the fuel demand?? Am i right?

There is no predefined limit or value--it's all based on the speed error. If the turbine speed reference (TNR for GE-design heavy duty gas turbine control systems) is constant and the actual speed (which is controlled by grid frequency) changes very quickly, then the speed error is going to change just as quickly, and there are no predefined values or limits (for typical sequencing). And, GE-design units don't use a traditional PID loop for fuel control.

The issue which many people (wrongly) complain about is that they believe their unit's load should remain constant during a grid frequency disturbance and they think the control system is not properly configured. And, that's just not true--the part about their unit's load remaining stable during a grid frequency disturbance. As I wrote, if generation on the grid doesn't match the load BAD things can happen (usually black-outs). Units need to--and grid regulators need units to--respond appropriately to frequency disturbances.

GE-design heavy duty gas turbines which are operating at Part Load on Pre-Selected Load Control DO NOT respond appropriately to grid frequency disturbances, and in fact they actually make the problem worse. They try to keep the unit load constant during frequency disturbances, but they actually pulse load up and down while doing so (by pulsing the FSRN value up and down). So, many grid regulators are banning the use of Pre-Selected Load control, requiring power plant operators to use "Free Governor Mode"--which is another term for Droop Speed Control.

Another problem is that in some parts of the world, grid frequency disturbances can be very erratic, almost "violent." And, they don't just drop below rated and stay there, or go above rated and stay there--they oscillate below and above rated. (And units that employ Pre-Selected Load Control actually contribute to that oscillation--at the very least they don't help stabilize it).

Another problem is that in many parts of the world the droop percentages of MANY governors is not what it should be. And, that causes problems for grid operators, too. In fact, grid regulators in North America are now, or soon will be, requiring most power plant operators/owners to periodically test their Droop regulation and provide the test results to the grid regulators. Units which were supposed to have 4% or 5% droop have been found to have 7% or 8% or more--which is not good for grid stability.

Combine two or more of these problems, and it can get very messy during a grid disturbance. And, units which should respond very quickly and don't are making any grid frequency worse than it otherwise would be.

When a prime mover and generator are connected to a grid, they are subject to the grid conditions--and grid regulators need to rely on units to respond as they anticipate they will in order to help stabilize the grid.

To a certain extent, it becomes kind of a question of whether the dog is wagging it's tail or the tail is wagging the dog (meaning, which came first: the chicken or the egg?). Believe it or not, a large machine which can respond quickly is a good thing during most grid frequency disturbances.

I really don't understand what you're saying; you seem to be saying that the unit load should not change very quickly during frequency disturbances, and also saying that fast-acting control systems are the problem, yet you don't seem to want fly ball governors, either. Grid frequency disturbances are, or can be, very complex situations. And, unfortunately, poorly regulated grids need a lot of work to make them less susceptible to large oscillations. AND, in a perfect world, there would be "central" agency or control room which was connected (using fiber optic cables) to ALL generators and their prime movers synchronized to the grid and could change generation very quickly to respond to conditions which might otherwise cause severe grid disturbances. But, that isn't going to happen any time soon on this planet (though battery storage may help a lot).

I sincerely hope this helps. I don't seem to be answering your question(s) in the way you need them answered. I can only address basic droop speed control, and particularly as it's implemented on GE-design heavy duty gas turbine control systems, and try to explain why fast response is a good thing. Sure, it makes for a wild ride during the the worst grid frequency disturbances--but the opposite of that would be to just as quickly change fuel flow to keep load constant in spite of frequency disturbances. And that would require fast-acting control valves and control systems.

I suppose if speed were removed from the equation and only load were used, that would help with the (perceived) load stability during frequency disturbance issue. But, then again--as I wrote--in effect, all prime movers operating in Droop speed control DO communicate with each other--using frequency/speed--and on a well-regulated grid where they respond quickly and as expected the grid frequency disturbances are relatively short-lived and tend not to be so erratic. And, there are a LOT of governors which aren't as sophisticated and advanced as the GE Mark*/Speedtronic control systems. And, all generator prime mover governors need to be able to work together (at least that's always been the way it's been done in the past)--and Droop speed control has been effective method for more than a century.

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Rotimi88,

No; I meant full load (rated prime mover output--when the machine is in a new and clean condition, and ambient conditions are at machine rating, and the fuel is per specification) and the unit is synchronized to a stable grid operating.

When TNR (for GE-design heavy duty gas turbines) is at 104% (for a machine with 4% droop--which is common for many gas turbines) the load on the machine will be at Base Load (when the unit is on primary (CPD- or CPR-biased) exhaust temperature control). If everything is "normal" as the machine is loaded (by increasing the turbine speed reference) the unit will reach Base Load when the turbine speed reference is at approximately 104% (on a well-regulated grid).

And the basic Droop speed control formula is used by many control system designers to control fuel flow-rate at part load operation. It's actually pretty universal. (Some governor control system manufacturers do an "odd" thing--when the unit is at FSNL (unsynchronized) the speed reference is at, say 105% (for machines with 5% droop), and then as the unit is loaded, the speed reference is reduced to 100%. There is a second "inversion" in the calculation. It works, but it seems kludgey (at least to me). Woodward Governor Co. had been doing this for some time....)

In it's simplest form, Droop speed control is about how energy flow-rate into the generator prime mover is controlled when synchronized to a grid with other generators and their prime movers. For a machine with 4% droop, a 1% change in speed reference when synchronized to a well-regulated grid (so, a 1% change in the speed error) will result in a change of 25% of rated load. It's a way of being able to control loading/unloading ramp rates.

And for grid regulators, it's also a way of understanding how much "spinning reserve" is available on the grid to support grid disturbances. (Units running at rated output (Base Load) cannot generally increase their power above rated when the grid frequency decreases; only units which are not running at rated output (so, they are running at Part Load) can increase their output--but only to rated--when the grid frequency drops.) They can "calculate" (or, rather, their computers can calculate) how much each power plants' unit(s) can take during a frequency disturbance when they are running at part load.

Hope this helps!