STANDARD DROOP/SPEED CONTROL compared to other LOAD/SPEED CONTROL

PungentReindeerKing is correct. The WORST mistake most people make--especially with GE-design heavy duty gas turbines--is to operate in a mode that's always trying to maintain a load setpoint, and adjusting the turbine speed reference in doing so. In GE-speak it's called Pre-Selected Load Control, and in it's most basic form what happens when the grid frequency deviates from normal which causes the actual speed to deviate from normal is that Droop Speed Control changes the fuel flow-rate because the speed error changes, which changes the actual load--making it deviate from the setpoint--and then Pre-Selected Load Control changes the turbine speed reference to try to return the load to the setpoint, and from there it's a horse race! In other words, the load fluctuates, sometimes by a lot. And this in turn contributes to the grid instability--making it worse!

GE will let Customers pay for a fix for their (GE's) problem--it's called PFR, Primary Frequency Response. And, it will sense a deviation in actual speed--which is an indicator of a grid frequency problem--and will temporarily disable Pre-Selected Load Control so that Droop Speed Control can do what it's supposed to do. It works; sort of. It's just a band-aid, actually.

In reality, no one really needs to operate with Pre-Selected Load Control enabled. Why? Because once the desired load is achieved (by the operator raising or lowering the load to the desired value) and stops using the RAISE- and/or LOWER SPEED/LOAD buttons the turbine speed reference stops changing. So, the speed error stops changing. And when the grid is stable, the speed error doesn't change. So, the fuel flow-rate doesn't change. So, the load stays the same. In most cases, the load is actually more stable than if Pre-Selected Load Control is not active!

But, no one can convince operators, or their supervision, that the unit will remain stable if Pre-Selected Load Control isn't active. Not no how. Not no way. They won't try it for one minute (60 seconds). Or even for 30 seconds. Because the just KNOW that if Pre-selected Load Control isn't active the unit load is going to drift, or worse--it's going to jump. And even worse, it's going to be completely unstable. And someone will lose their job. So, good luck trying to convince anyone to try operating the unit without Pre-Selected Load Control inactive. It just won't happen.

So, what happens when the grid frequency does deviate from rated? Well, if the unit has been outfitted with PFR and Pre-Selected Load Control is active, the fun begins. Not only is the unit load changing because of the grid frequency instability, but it's even more unstable because of the Pre-Selected Load Control fighting with Droop Speed Control. Which only makes the grid frequency more unstable.

And, those same operators and their supervision who believe in the heart and soul that the unit will be unstable if not operated with Pre-Selected Load Control active also believe in the heart and soul that their unit should NOT change it's load under any circumstances--even if the grid frequency is unstable! So, the Mark* is deemed to be the cause of the problem--when it's only making the problem worse because of the way it's being operated.

But, good luck convincing anyone of that. "If GE had wanted Pre-Selected Load Control to self-cancel when the desired load setpoint was reached, they would have programmed it to work that way!" That's what Pre-Selected Load Control was designed to do--make it easy for the operator to change load without a lot of button-pushing, or handle twisting, or clicking. It was intended that once the setpoint was reached, the function would be cancelled--because, again, when the speed reference and the actual speed are both not changing, then the speed error is not changing, and the fuel flow-rate will not change, and the load will not change. BUT, they just forgot to do that one little thing--automatically cancel Pre-Selected Load Control when the desired setpoint was reached. They left that up to the operator, but, operators simply can't be bothered to take that extra step--especially when they firmly believe if they do the unit will go unstable, and someone will lose their job.

Anyway, as for the rest of what PungentReindeerKing wrote, it's all true. A lot of these functions are "layered" on top of each other (in the way Pre-Selected Load Control is layered on top of Droop Speed Control) and they need to be tuned or adjusted to all work together properly. In some cases, they can't be made to work together--but that doesn't stop people from continuing to try to force them to work together. And, in the end--it's always the Mark*'s fault that they can't work together.

Such is life.
 
Hi CSA ,

Thanks a ton for your out of the box explanation and deep learning. My OEM doesn't say it as speed control rather its defined as load control. looking at the algorithms and your explanation I could understand that GAIN is what they derived from the equation.
Droop setting : 4 %
Base Load : 200 MW
Thus the frequency range is from 50 to 52 % ( At full load speed reference @52 Hz ?)
GAIN = 200/2 = 100 (I inferred this as gain of P controller as per your explanation )
Error = Speed Ref - Actual Spd
FR MW = Error*100 (Loaded or Deloaded depending up on the error whether +/-)

Does this make sense as simple P controller? Does this work in real life for paralleling turbo-generators ?

Thank you
 
mariselvan,

Let's consider a machine with 4% droop regulation (as it's typically called by the ivory tower references and OEM documentation). Further, let's say the machine is a prime mover (steam turbine, or gas turbine, or reciprocating engine) driving a 50 Hz generator. AND, let's further say the machine is capable of producing 200 MW and is operating on Droop Speed Control and IS NOT synchronized to any other prime mover/generator or grid. This is important--this is the part that most texts and references and OEM documentation DON'T SAY!!! If the load on the unit, with the generator breaker closed and powering the transmission and distribution system is 0 MW, the output frequency of the unit will be 50.0 Hz. As soon as motors and lights and televisions and computers and computer monitors and tea kettles are turned on the load will increase (based on the number of devices turned on and the power being drawn by the devices which were turned on). Let's say the load increased to 50 Mw, which is one fourth of the rated output of the prime mover and generator. That would mean that the frequency of the grid had decreased to 49.5 Hz, which is a 1% decrease in the nominal frequency, which is equal to 25% of the machine's capacity with 4% droop regulation. Let's now say that some more loads were switched on, and the load increased to 150 MW, which is 75% of the machine's rating, which is 3% of the 4% droop regulation. Now the frequency of the machine (and the transmission and distribution system--and all the loads connected to the system!) is at 48.5 Hz. If the load increases to 200 MW, the frequency will decrease to 48.0 Hz.

That is proportional control.

Now, let's say the same machine--NOT synchronized to any grid or other prime mover/generator--is operating at 50.0 Hz with 200 MW of load. If 50 MW of load is turned of, the frequency of the output will increase to 50.5 Hz; if another 50 MW of load is de-energized, the frequency will increase to 51.0 Hz, and if another 50 MW of load is turned off the frequency will increase to 51.5 Hz. And, if all the loads are shut off, the machine frequency will increase to 52.0 Hz.

That is also proportional control.

Again--the part that most references and texts and OEM documents FAIL to include is the bit about a single unit, operating under Droop Speed Control, unsynchronized with any grid or any other prime mover/generator when they are talking about speed changes. It's unfortunate, but that's the problem with most written descriptions of Droop Speed Control--they almost ALWAYS say the speed of the system changes when load changes-- and that's simply NOT true for one or even multiple machines synchronized together on a well-regulated grid with other prime movers and generators. The concept of AC (Alternating Current) power generation is that is done at a fairly constant frequency. And, the concept of multiple generators synchronized together means they operate at the SAME frequency--all of them!!! No single generator synchronized to a grid with other prime movers and generators can operate at any other frequency (speed) than entire grid. That's what make synchronized and synchronism such an important and critical concept. And, yet people with little or no experience--and poor critical thinking skills--still keep espousing this idea of individual machines changing speed when synchronized to a grid with other machines. It's just doesn't happen in the real world. (Actually, it doesn't even happen in the theoretical world--it's a fallacy.)

Now, let's talk about what does happen to a grid with multiple machines synchronized together when the load exceeds the generation capacity (such as when a machine trips off the grid and all the other machines are running at or very near their rated capacity). In this case, the grid frequency starts to decrease, and that means the difference between the actual frequency (speed) and the frequency (speed) reference starts to increase. This causes the amount of energy flowing into ALL of the machines NOT operating at their rated output to increase. This in turns help to support grid stability--but it DOES NOT return the grid to rated frequency. Operators (usually grid operators) have to do this, either manually or automatically. The number of loads and the current drawn by the loads is not changing--but the amount of energy being produced by the machines synchronized to the grid is not what is required to supply the load(s) AND maintain rated frequency (speed).

I know people don't like my bicycle analogies, or the train analogies--but they are perfectly analogous to this situation. It's all about carrying a load (or loads) at a constant speed (frequency) and dealing with changes in load or changes in the torque being supplied to maintain the constant speed (frequency). An AC grid is "carrying" load(s) at a constant frequency (speed) and to do so it has to supply the loads AT the rated speed, otherwise the frequency will deviate from rated.

Proportional, Droop Speed Control says the amount of energy flowing into the prime mover of a synchronous generators is a function of (proportional to) the difference between the actual speed (frequency) of the machine and the machine's speed (frequency) reference. Increase the error between the actual speed and the speed reference--by changing EITHER the actual speed OR the speed reference--and the amount of energy being supplied to the synchronous generator's prime mover will also increase. Decrease the error between the actual speed and the speed reference--by changing EITHER the actual speed OR the speed reference and the amount of energy flowing into the prime mover will decrease. And, the amount of change is a function of (proportional to) the Droop regulation percentage.

But, when a prime mover/synchronous generator is synchronized to a well-regulated grid with other prime movers and synchronous generators two things should be very apparent: First, the speeds--and frequencies--of all the machines synchronized together on the grid constant, and the frequencies of all the machines are the same, and all the machines are running at their particular synchronous speeds (which are constant). Second, the machines can ONLY share in carrying (supplying) the loads (the motors and televisions and computers and computer monitors and tea kettles) if they are operating in Droop Speed Control--or, proportional control. Base it on load or speed--it has to be something. And beginning over a hundred years ago--it was speed, because speed and frequency are directly related. Mechanical governors for prime movers back then had no way of understanding amperes--only speed. And since all the machines synchronized to the grid had to run at their synchronous speeds to produce the same frequency Droop Speed Control--using speed--was "chosen" (really; there wasn't any other way to do it!).
 
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