Questions about Isochronous and Droop

S

Thread Starter

SAK

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??
 
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] <b>synchronized</b> 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 <b>synchronized</b> to a well-regulated grid <b>it's speed DOES NOT CHANGE WITH LOAD.</b> 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 <b>when synchronized to a grid</b> 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?
 
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, <i>compensation</i>) 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!
 
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 <i><b>synchronized</i></b> 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. <i>After</i> 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?
 
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 <i>reduced</i> 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!
 
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.
 
Hi CSA,

>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.

Can you please share the thread of that equation you are mentioning here?
 
esdauto,

In maths, formula and equation mean (describe) the same thing. In my post to this very thread above, of 17 October, 2017, 6:30 pm, the equation (formula) is stated very clearly:

>The general Droop speed control formula is:
>
>Energy flow-rate into prime mover = ((prime mover speed
>reference - prime mover actual speed) * Gain) + Offset

The equation (formula) is of the form y=mx+b, or f(x)=mx+b, where x=(prime movers speed reference - prime movers actual speed), m=Gain, and b=Offset. This is the equation (formula) for a straight line in maths, and the only difference between the equation (formula) for a straight line and the general Droop speed control formula (equation) is the 'x' term (or variable) is actually two terms: prime mover speed reference and prime mover actual speed.

The 'x' term (variable) is the <b>speed error</b> of the Droop speed control equation (formula). Error is another way of saying 'difference.' (Aren't mathematicians wonderful people, using different words to mean or describe the same thing? The English language is so flexible, isn't it?!?!?)

On an AC grid, the presumption is the prime mover actual speed will be stable and constant--because the prime mover speed is directly related to the grid frequency (by another formula (equation)). And the operator, or the turbine control system, or the grid operator (via some remote control means) varies the prime mover speed reference to change load of the turbine-generator. So, when the grid frequency is stable and constant (as it should be and normally is), turbine-generator load is changed by changing the prime mover speed reference, which changes the speed error, because the prime mover actual speed is stable and constant.

BUT, if the prime mover speed reference is stable (which means the turbine-generator load is stable!) and the grid frequency changes then the speed error between the prime mover speed reference and the prime mover actual speed changes, which means the load of the turbine-generator will change.

Turbine control system (governor) manufacturers can use their own signal names to substitute for the variables and Gain and Offset, and even for the result of the equation (formula)--and do. But the basic formula never changes--it is so necessary for multiple prime movers and generators to be able to be synchronized together on a grid and produce power and to properly respond to changes in grid frequency that it can't change very much. And, it's the method by which all the prime movers and generators synchronized together on a grid "communicate" with each other--because they are all running at speeds that are directly related to the frequency of the grid they are synchronized to. That's the beauty of the AC power generation, transmission and distribution system! It's very simple; it doesn't require all the prime mover governor's (control systems) to be interconnected for the multiple prime movers and generators to respond properly to grid frequency disturbances.

So, we've had a maths lesson and an English language lesson today. And a generator prime mover governor (speed control system) lesson (be it a flyball governor or a digital electro-hydraulic governor). I'd say it's a pretty good day. So far.
 
Hi CSA ,

Going through the explanation you have given for droop mode, it&#8217;™s very clear now the droop speed control.

Isochronous mode is selected on which stage or conditions? My understanding is that once it is synchronized, droop mode is normally selected.

Please explain.
 
esdauto18,

Isochronous speed control mode is the prime mover governor mode that tells the governor to do whatever is necessary to the energy flow-rate into the prime mover to make the actual speed equal to the speed reference--and the speed reference is usually the speed corresponding to rated frequency (50.0 Hz, or 60.0 Hz). So, a two-pole synchronous generator operating on a 50.0 Hz grid would be spinning at 3000 rpm at 50.0 Hz. If the governor of the prime mover driving that generator were in Isochronous speed control mode any change to the frequency (speed--because the two are directly related) will cause the prime mover governor to immediately change the energy flow-rate into the prime mover to return the actual speed (frequency) to the reference (the speed that corresponds to 50.0 Hz).

The theory of AC power generation, transmission and distribution is that on any grid there is only one prime mover and its generator operating in Isochronous speed control mode. And, all the other prime movers and their generators are operating in Droop speed control mode. When the load on the grid changes--when a motor is started, for example--the immediate effect on the grid would be for the grid frequency to decrease by an amount proportional to the load being added to the grid. BUT, the governor of the prime mover operating in Isochronous speed control mode immediately senses the change in frequency (speed) and increases the energy flow-rate into the prime mover of the governor it is driving to return the frequency (speed) to rated (50.0 Hz). All of the other generators and their prime movers are operating in Droop speed control mode, happily producing the power they are being asked to produce while the Isochronous machine does whatever it can to control the frequency of the entire grid as the load on the grid changes. The Isochronous governor is the governor that immediately responds to changes in load (which cause changes in frequency/speed) and does whatever is necessary--within it's power range--to keep the grid frequency stable and constant at the nominal frequency, which is 50.0 Hz in our example.

Many small power distribution and transmission systems (sometimes called "power islands") have only one or two or three or four generators providing the power of the transmission and distribution system, such as at a refinery that is not receiving power from a national grid, or on a container ship or cruise ship on the ocean, or an oil rig in a gulf somewhere. Usually, these systems have one machine's governor operating in Isochronous speed control mode and one or two other generators, synchronized all together on the grid, and the other two generator prime mover governors are operating in Droop speed control mode.

Droop speed control mode doesn't really care about what the frequency is. As we have seen, when the frequency changes the Droop speed control prime mover governor does NOT try to restore the frequency. The Droop speed control mode sees the difference in frequency (speed) and tells the governor to increase or decrease the energy flow-rate into the prime mover to change the LOAD of the generator, but NOT the speed or frequency. In fact, if there is no error between the speed reference and the actual speed the fuel flowing into the prime mover of a Droop speed control unit will not change--it's only when the difference (error) between the the reference and the actual changes will the energy flow-rate into the prime mover change. And it does nothing to return the actual speed or the error to it's previous value--the speed remains the same, be it higher or lower than it should be. Only the load of the unit changes--the actual speed does not return to its nominal value.

When an Isochronous prime mover governor senses a change in actual speed it changes the energy flow-rate into the prime mover to cause the actual speed to return to the nominal value--50.0 Hz in our example. And because all of the generators and their prime movers <i>synchronized</i> to the grid together <i>all operate at speed that are directly related to the frequency of the grid they are synchronized to</i> the speed of ALL the generators will rise and fall together. And, it's the Isochronous machine's prime mover governor that is controlling the speed of all the generators and prime mover synchronized to the grid.

There's that word: synchronized. It's a very powerful word. It's a very powerful concept. No single generator synchronized to a grid of any size, anywhere in the world, can operate at any speed other than the speed which is directly related (proportional) to the frequency of the grid it is synchronized to. That speed is called "synchronous speed"--and synchronous is derived from synchronized. On a 50 Hz grid, one machine can't be operating at 48.76 Hz, another operating at 50.3 Hz, another operating at 49.99 Hz, another operating at 52.1 Hz. They ALL have to be operating at the SAME frequency, be it 50.09 Hz, or 49.95 Hz, or 50.0 Hz--in order for the power at the outlet in the wall where the TV or the tea kettle or the fan is plugged into to provide a nominal 50 Hz. That's also why the process of synchronizing is so important when connecting a generator to a running grid. The frequency of the incoming generator and it's prime mover has to be in phase and very nearly equal to the frequency of the running grid during synchronization when the breaker is closed to connect--synchronize--the incoming generator to the running bus. And, a LOT of people then think that when synchronization is complete as they load the newly synchronized generator to the grid it's speed will change as the unit is loaded, and when it's unloaded. BUT, IT DOESN'T CHANGE once it is synchronized to the grid. The speed of the generator and it's prime mover is now controlled by the frequency of the grid--really by the Isochronous machine on the grid!

Now, in reality, on most very large grid, there is NOT a machine with it's prime mover governor operating in Isochronous speed control mode. BUT, there are SO MANY machines operating in Droop speed control mode that it takes a large load change to cause a change in frequency--such as everyone waking up at about the same time in the morning and turning on lights and tea kettles and computers and computer monitors and televisions and cooktops. And, grid operators can actually see the effects of people waking up in the morning by watching the grid frequency--and they direct some Droop speed control machines to increase their power output to keep the grid frequency close to rated. If there is an Isochronous machine on the grid, the grid operators have to watch the output of that machine to see that it doesn't max out at rated--because once it does that it can't help control the frequency if more load is added to the grid. So, they tell one or more Droop speed control machines to increase their load, which causes the load on the Isochronous speed control machine to decrease, which gives the Isochronous speed control machine some more room to produce power should the load on the grid increase--the Isochronous machine could increase it's power output to respond.

And the Isochronous machine responds automatically to changes in load (which cause changes in frequency (speed)). If there is no Isochronous speed control machine on the grid, or the load on the Isochronous speed control machine is near zero or near maximum, the frequency of the grid is going to change as load decreases further or increases further--because the Isochronous machine can't respond. In this case the grid operators have to tell Droop speed control machines to change their load to help control grid frequency--and they do this all day and night. They usually have contracts with several power producers that allow them to have more "direct" control of the load(s) of their generators and prime movers--and they can change the loads from their central control room to help control grid frequency all over the grid. (This control scheme is sometimes called "AGC", or Automatic Governor Control, among other similar names.)

But, having one Isochronous machine--usually a machine with a very large and powerful prime mover--can help control grid frequency without the grid operators having to make a lot of changes/calls to power producers as grid frequency changes. The Isochronous machine, as long as it is operating within it's power range, and not too close to the minimum or maximum limits, will respond to changes automatically and maintain the frequency (speed) of the whole grid. Only when the load on the Isochronous machine gets close to zero or rated do the grid operators have to make changes to Droop speed control machines to return the Isochronous machine to near mid operating range.

Now, some small grids use machines which are in Isochronous Load Sharing Mode--which is just a de-tuned Isochronous speed control mode. Or, they have a machine in Isochronous Standby mode (maybe two machines). There are also schemes where an external control system, sometimes called a Power Management System (PMS) can send signals to multiple machines, operating in Droop speed control mode, or Isochronous load sharing mode, to control frequency as load on the grid changes. Some of these work very well; most don't.

I like to say that Droop speed control machines don't care about the frequency (speed) of the grid, and that confuses some people. Yes, the load of a Droop speed control machine will change when the grid frequency (speed) changes--but the governor of a Droop speed control machine will not automatically return the frequency (speed) of the grid to nominal, rated frequency. That's NOT the job of a Droop speed control machine--to control frequency.

An Isochronous speed control machine, on the other hand, DOES care about the change in frequency (speed) of the grid--and will respond VERY quickly to restore the grid frequency (when it can) to nominal, rated frequency. The Isochronous speed control machine's load will change when it's doing this frequency (speed) control--because as 100 kW of load is added to or removed from the grid the Isochronous speed control governor will change the energy flow-rate into the prime mover it is controlling to increase the load by 100 Kw or decrease the energy flow-rate by 100 Kw, respectively. (And, it's not even really looking at the amount of load that is being added to or removed from the grid--it's looking at the change in frequency (speed) the addition or subtraction of load causes.)

The last thing about Isochronous speed control governors--there should generally be only one operating on a grid at any one time. Because, if there are more than one Isochronous speed control governor on the same grid the Isochronous speed control governors will fight each other for control of frequency--and that cause one or both of the governors to trip the unit off the grid, which can lead to further bad problems for the grid's Customers. (Again, there are some "de-tuned" Isochronous speed control modes which are used which allow multiple Isochronous speed control governors to be synchronized together, but they are not common, and some of them don't work very well at all.)

Isochronous speed control is very tight speed (frequency) control. Droop speed control is very "loose" speed (frequency) control. Droop speed control machines rely on some other "entity" to keep the grid frequency (speed) constant--they don't perform the grid frequency control. They can be used to control grid frequency--either by humans or by automatic control schemes--but left in typical operating mode Droop speed control does NOT control frequency. Isochronous speed control mode is typically the mode that is used for a machine (prime mover and generator) to control frequency while also supplying the load connected to the grid. If there is just one generator supplying a load, even a small gas engine-powered generator powering a small home, it's governor will be operating in Isochronous speed control mode trying to keep the frequency of the generator output constant.

Hope this helps! It's a VERY HIGH-LEVEL view of a topic that is very mis-understood. Suffice it to say, if you are synchronizing a generator to a grid with other generators you will want the incoming generator to be in Droop speed control mode, and to remain in Droop speed control mode while synchronized to the grid.

If you are using a synchronous generator to power a grid from a zero power state (called "black" or "dead bus"), you will want the generators' prime mover governor to be in Isochronous speed control mode--so the frequency of the generator output will remain at nominal, rated as loads are added to the grid, and removed from the grid. As other generators are synchronized to this grid, they will be synchronized in Droop speed control mode, leaving the frequency control of the grid to the Isochronous speed control machine. The Isochronous machine designation can be changed, and it is usually done at a time when the load is fairly stable and not changing, and the Isochronous machine is then switched to Droop speed control, and very shortly after that one of the Droop speed control machines is then switched to Isochronous speed control.

It is the OPERATOR'S responsibility when operating a grid (of any size) with a machine in Isochronous speed control mode to balance the loads of the generators synchronized with the Isochronous machine to keep the Isochronous machine's load above zero and below maximum. And, that is done by decreasing the load on one or more Droop speed control machines (to increase the load on the Isochronous machine), or to increase the load on one or more Droop speed control machines (to decrease the load on the Isochronous machine). It is a BIG falsehood that one can just put a machine in Isochronous speed control mode and it will automatically, and by itself, adjust it's load at all times and under all conditions to keep from tripping on reverse power or reaching maximum power output. People, or well-programmed external control systems, have to monitor the load on the Isochronous speed control machine to keep the load in a range the Isochronous machine can respond to changes without going to negative power output (reverse power) or to maximum power output (when it can't increase its power output any further).

It can be a little overwhelming in the beginning, trying to put all of this together--but after a while, it should gradually become clearer and clearer.

Droop speed control mode is the mode that allows multiple synchronous generators and their prime movers to be synchronized together and act as one generator--running at a single frequency!--to power a load or loads that no single generator and prime mover could supply. It allows the prime movers to "share" in supplying a portion of the load of the grid the generators they are synchronized to are powering. If one tried to synchronize multiple Isochronous speed control machines together on a grid to power a load--it would be VERY unstable (the frequency and the power level) because Isochronous governors do not like to share load with other Isochronous governors. It's like trying to have two or more Kings all trying to do the same things--it won't work very well.

Droop speed control mode is the mode that allows for multiple generators to stably contribute to the power being produced by multiple generators all acting as one single generator--operating at a single frequency!--without instability. Droop speed control mode does NOT care about speed (frequency), some other entity (person or control system) has to do the tight frequency control.

And, when enough prime movers driving generators and operating in Droop speed control mode get synchronized together they all have an inertia that contributes to grid stability (frequency and voltage)--but it can be disturbed pretty easily with big changes. Most large (sometimes called "infinite") grids are made up only of generators operating in Droop speed control mode. And, the grid operators have to balance the loads of many of the machines in order to maintain a stable grid frequency.

Enough for today, eh?
 
CSA,

I have found your post very helpful and informative, although i still have some questions.

I am trying to synchronize two generators in an isolated bus. The prime movers are steam turbines, a 3MW and a 6MW generators.

To start the small generator the bus is connected to the utility until enough steam is generated in the boiler to power the small turbine. once this is done and the turbine is up to rated speed, the utility tie breaker is opened and the small generator breaker is closed. The small turbines takes the load of the isolated bus and starts powering loads to produce more steam. Once enough steam is produced in the boiler to power the bigger turbine, the steam chest is opened and the big turbine is set to its rated speed. Once this is achieved, the smaller generator breaker is opened, and the 6MW gen breaker is closed. This procedure causes two temporary blackouts in the plant.

The 6MW turbine is governed with a woodward 505 and a DSLC-2 is available for synchronization. Our plan is to start the small generator the normal way (utility power then blackout) and then synchronize the 6MW gen with it using the DSLC-2.

Our questions come once the two units are synchronized. how can we unload the small generator and transfer all the load into the bigger one? The small generator is in droop mode governed with a UG40. The usual load in the bus is around 3 MW.

What mode should we set the bigger generator in in order to avoid motorizing or lose of power?

Thank you.
 
jaed,

>Our questions come once the two units are synchronized. how
>can we unload the small generator and transfer all the load
>into the bigger one? The small generator is in droop mode
>governed with a UG40. The usual load in the bus is around 3
>MW.

>What mode should we set the bigger generator in in order to
>avoid motorizing or lose of power?

My recommendation is as follows: Synchronize the smaller turbine-generator to the grid in Droop Speed Control mode (in other words, synchronize it with the utility tie breaker open). Then open the utility tie breaker, and very quickly switch to Isochronous Speed Control Mode--if the governor has that mode. That way, the operators don't have to do anything; the governor will automatically adjust load to maintain frequency as you bring the other turbine-generator on-line. If the governor doesn't have Isochronous Speed Control mode, the operators will have to manually monitor load and change load in order to maintain rated system frequency.

Bring the large steam turbine generator up to speed and synchronize it to the smaller unit; the large steam turbine generator should be in Droop Speed Control Mode when you synch it to the smaller unit if the smaller unit is in Isochronous Speed Control Mode. If the smaller unit doesn't have Isochronous Speed Control mode, you should still synchronize the larger unit to the smaller unit in Droop Speed Control Mode.

If the smaller unit is in Isochronous Speed Control mode switch it to Droop Speed Control mode.

Then switch the larger unit to Isochronous Speed Control Mode. Once the larger unit is in Isochronous Speed Control mode and the smaller unit is in Droop Speed Control mode, start reducing the load on the smaller unit--the larger unit will automatically (without ANY operator intervention) increase its load as the load is reduced on the smaller unit. Once the smaller unit reaches a few kW, open the generator breaker of the smaller unit.

What you want to avoid is putting or having both units in Isochronous Speed Control Mode at the same time when the plant tie breaker is closed and the two units are synchronized together.

When a governor is in Isochronous Speed Control mode, if the operator tries to change the load manually--all the operator is going to succeed in doing is changing the frequency of the unit/power island. When a governor is in Isochronous Speed Control Mode <i><b>it will automatically adjust its load to maintain rated speed/frequency</b></i>. The operator doesn't need to do anything. and, again--if the operator tries to change load of the Isochronous Speed Control governor the net effect will be to change the frequency (speed) of the unit and the power island.

When two units are synchronized together, and one of them is in Isochronous Speed Control mode and the other is in Droop Speed Control mode, if one wants to change the load of the Isochronous unit one changes the load of the Droop unit--<i>that's right, the Droop unit.</i> To put more load on the Isoch unit, one unloads the Droop unit. To reduce the load on the Isoch unit, one loads the Droop unit. But one cannot change the load of the Isoch unit with the governor of the Isoch unit. It will adjust its load automatically and as necessary to maintain frequency, and it will respond to changes in the load of the Droop unit automatically. Most operators wrongly try to change the Isoch unit with the Isoch governor controls--and that's incorrect.

If you want to keep the small unit synchronized to the unit with very little load, just reduce the load on the small unit with it's governor controls but do not go below the reverse power relay setting. Stay a few kW above the reverse power relay setting and you should be okay.

Hope this helps! Please write back to let us know how you fare with your units and preventing black-outs!
 
jaed,

>My recommendation is as follows: Synchronize the smaller
>turbine-generator to the grid in Droop Speed Control mode
>(in other words, synchronize it with the utility tie breaker
>open).

Sorry for any confusion! It should have read, (in other words, synchronize it with the utility tie breaker <b>CLOSED.</b>
 
Hello everybody,

In my plant we have three gas turbines running and one in standby. The gas turbines are solar mars 100, normally we use it in isochronous KW and KVAR load sharing with mode control run at rated enable.

When we need to change the load in one turbine, we disable the mode control run at rated and then in manual mode decrease or increase the speed setpoint. (increase or decrease the voltage if it influences on power factor). But if for example we start a pump and then the total load increase also, the load in this turbine increase.

Therefore I would ask you how can i fix the load value in this turbine? in such a way that if total load change this turbine keeps fixed its value.

Thanks
 
It seems someone, in their infinite wisdom, decided that an "external" power management system (load-sharing system) was required for the plant to operate. And it uses some kind of Isochronous Load Sharing scheme for the MW load control.

I presume the 'facility' is not tied to a grid??? In other words, it is it's own "power island."

If there was NO external Power Management System one unit could be run in Isochronous Speed Control Mode and the other in Droop Speed Control mode. The load of the Droop unit could be controlled by the operators, and the load of the Isoch unit would automatically vary to maintain frequency. If the load of the Droop unit was manually decreased the load of the Isoch unit would automatically increase. And if the load of the Droop unit was manually increased the load of the Isoch unit would automatically increase. The operators CANNOT manually control the load of the Isoch unit--it's job is to vary it's load to maintain frequency. The operators can manually vary the load of the Droop unit, which will have an effect on the load of the Isoch unit--but the Droop unit load will remain constant at the load set by the operator regardless of the load of the grid or the Isoch unit.

BUT, at your plant someone decided that an external load-sharing control scheme was required. You have to read and understand the instructions and the plant operating description to determine if what you want to do is feasible using some procedure that may--or may not--have been built in to the control scheme.

Since we don't have access to the documentation and drawings for your plant we can't make any recommendations--except to read and understand the instructions and plant operating description (usually provided by the plant designer for islanded systems like yours-- because they had to describe how the external Power Management System (load control) scheme had to be configured and operated). SOMEWHERE at your plant there are instructions and operating descriptions (sometimes called SOPs--Standard Operating Procedures); find them and study them and you will understand what your plant controls are capable of. That's the only--and the best--way to answer your question.

There are some equipment manufacturers that make load-sharing systems, such as Woodward Governor Company, but they only provide the "tools" for configuring and programming there equipment; the final decisions are made by the supplier of the load-sharing system.

Hope this helps. What you are asking would be possible--I think--if the load-sharing system were not enabl6, and one unit was operated in Isochronous Speed Control mode and the other unit(s) were operated in Droop Speed Control mode. And, there may be some method configured in the load-sharing system at your site to allow the same thing--but only the documentation and instructions and drawings at your site can provide that detailed information.

Plant designers often provide these kinds of load-sharing systems to try to automate plant operation, requiring operators with less skills and training (reducing costs!). Automation can be a good thing, but it can also be a stubborn thing if not properly configured and programmed. Actually, I would be surprised (but not shocked) to learn that your load-sharing system did not have a method to manually set a load reference for one of the machines; that's actually a common option. But, again, only the documentation and drawings and instructions at your site for your system have the details of the load-sharing system at your plant.

Please write back to let us know what you learn!
 
I tried SOOO HARD >>NOT<< to mess the initial description up!!! BUT, I did....

>And if the load of the Droop unit was manually increased
>the load of the Isoch unit would automatically <b>decrease.</b> <b>DECREASE.</b>

The Isoch unit load will (automatically) respond OPPOSITE to the manual load change of the Droop unit.

What can I say? It's a free reply.

My sincere apologies for any confusion!!!
 
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