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Clarification about droop and frequency control
Engineering and workplace issues. topic
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Posted by Mik on 16 September, 2009 - 6:34 am
Hello,

First of all, I need a clarification about droop characteristic. To my knowledge, droop can be seen as a speed error or mismatch of turbine speed in case of load change. In other words, it is said that droop of governing system is let's say 5% if change of 100% of load is causing 5% of change of turbine speed. Droop can be understood by characteristic frequency-output power and is approximately linear.

Now, I have found one example. It says basically this: "When frequency changes in the grid, every turbo-generator unit reacts and adjust its generated power. For instance, when frequency falls by about 0.1%, generation of a unit should be increased by 20% with droop of 5%."

Now I cannot understand this example. How 20% of more generation with 5 % droop can correct 0.1% in frequency drop? I guess this example assumes that there is only one unit in the grid which is allowed to correct frequency error.

What if there are more units on the grid? Due to primary regulation every turbine controller system (governing system) will try to correct this.

Thanks


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Posted by CSA on 17 September, 2009 - 5:58 pm
There has been a LOT written on droop speed control on control.com. Use the fine 'Search' feature and you will find more than you probably ever wanted to know.

Synchronous generators. Their speed is a function of their frequency (or, it can also be said that their frequency is a function of their speed). But because they are synchronous generators no generator can go faster or slower than the speed that is dictated by the frequency. And, because they are all connected together and their rotors are locked into synchronism with each other (magnetically), the prime movers which are mechanically coupled to the generators can't change their speeds either.

When a prime mover driving a synchronous generator is connected to a grid with other generators and their prime movers, particularly a large or "infinte" grid, the frequency of the generator is controlled by the frequency of the grid. And, since the speed of the prime mover is a function of the frequency of the generator (I'm talking about prime movers that are mechanically coupled to synchronous generator rotors), then the speed of the prime mover is fixed.

No synchronous generator (nor its prime mover) can run faster or slower than the other synchronous generators (and their prime movers) on the grid. It's just not physically possible for one generator to be running at 50.134 Hz and another to be running at 52.27 Hz, and still another to be running at 49.65 Hz if they are all connected to the same grid which is operating at 50.001 Hz. It just can't happen.

So, while a lot of texts try to describe droop in the way you did, in the real world, it just doesn't work that way, except when there's just one machine driving a load independent of any other machine. And in that case, the prime mover control system is usually operating in Isochronous Speed Control, not in Droop Speed Control.

To make a prime mover (which is providing the torque input that the generator is converting to amps that is being converted to torque by motors which are also connected to the grid) stably control its power output while connected in parallel with other generators and prime movers on a grid, the control systems employ straight proportional control. And that proportional control is called droop control, droop speed control.

By the way, stably controlling power output when connected to a grid in parallel with other prime movers and their generators is my definition of "sharing load."

Droop speed control looks at a prime mover's speed reference and it's actual speed, which is a function of the grid frequency.

To increase the power output, the speed reference is increased. But, since the speed can't actually change the increased error between reference and actual speed is converted to increased fuel flow. That increased fuel flow, which would tend to increase the speed, which can't increase appreciably, is still extra torque. And the generator converts that extra torque into more amps. All of this is done very smoothly and all the prime movers and their generators behave nicely and work together to provide the load. If the "load" on the generator is to be increased, then the turbine speed reference is increase again, the error between the actual speed and the speed reference increases again, which increases the fuel flow which increases the torque which increases the amps.

When turbine operators are watching the watt meter and twisting the governor handle in the Raise direction to get the watt meter reading to increase, they aren't changing the watt reference they are changing the turbine speed reference.

Droop speed control is straight proportional control in the strictest, purest sense of the word. There is no reset or integral action to increase the fuel to make the actual speed be equal to the reference. It can't be equal to the reference (if the reference is more than the actual speed); it's not physically possible. And droop speed control makes use of that impossibility to stably control the fuel in proportion to the error.

The error can change for either of two reasons: a change in the speed reference, or a change in the actual speed. It's not common (in most parts of the world!) for the actual speed (grid frequency) to change. But when the grid frequency (and hence the actual speed) does change, the control system "automatically" reacts to the change because the error changes and adjusts the fuel to try to compensate for the change in actual speed relative to the speed reference.

So now you can imagine what happens when the turbine speed references of all the prime movers connected to a large, "infinite" grid are all fairly constant, which means the errors between their speed references and their actual speeds are all fairly constant, and the grid frequency changes. Since frequency and speed are directly proportional, the error between actual speed and speed reference changes. If the grid frequency decreases, then the error increases which increases the fuel--of all the machines because they are connected together to the grid.

Each prime mover's governor (control system) will respond to a change of frequency as a function of the amount of droop that the control system is programmed to have. A 1% change in frequency on a machine with 5% droop will result a 20% change in load, nominally, supposing the machine was running at 80% of load or less to begin with. A unit with 4% droop will respond with a 25% change in load, nominally, again presuming the machine was running at 75% or less than rated load to begin with.

Some manufacturers use the above scenario, what happens to the power output of a machine whose primer mover is operating in droop speed control when the grid frequency changes as their definition of "load sharing".

It's important to note that if a machine is operating at rated power output on droop speed control and the grid frequency decreases, the prime mover cannot increase its power output any further. And a LOT of prime movers connected to a large "infinite" grid at any one time are operating at rated power output. So even though they are in droop speed control they can't pick up any additional load by increasing their power output when the grid frequency decreases. They're just along for the ride at that point. And if the turbine is a combustion turbine, a single shaft combustion turbine, well that's not good for the grid. But, that's for another thread.

Now, a machine which is said to have 5% droop will nominally reach rated output when the speed reference reaches approximately 105% of rated speed. Nominally is the operative word here. A machine with 4% droop will reach its rated power when the speed reference is 104%.

So, mull this over. Search the archives of control.com with the 'Search' feature and ask more questions. This is not a complicated subject, but a lot of what has been written in many texts is very theoretical and very confusing.

I like to describe droop one way like this: We are telling the turbine to run faster than it can when we increase the turbine speed reference. In other words, we are "allowing" the turbine speed to droop below the set point, AND we are making use of the the fact that under normal conditions the actual speed is relatively constant and use the error between the two signals to control the amount of fuel or steam or water or whatever energy source is being used in the prime mover.

As a side benefit of this, if the actual speed does change for some reason the control will automatically respond in the appropriate way to try to help maintain the grid frequency.

It's a wonderful thing, droop speed control. It's so simple, and so powerful. All at the same time.


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Posted by Mik on 18 September, 2009 - 1:17 am
Dear CSA,

thank you for your effort to expain this matter.
Actually, my questions came after reading this overview:

http://www.scribd.com/doc/6152992/Steam-Turbine-Governing-Systems-O verview

As I assumed in the article there is a mistake about 0.1% of frequency drop instead of 1% that will cause 20% of power output increase with 5 % droop.

However, your explanation rise another set of questions. Normally, as I understand the article and what I know from practice, prime mover control system usually has three modes of operation:
1. Speed control
2. Load control
3. Pressure control

In the start up phase, when generator circuit breaker is open, prime mover is controlled in speed control mode. Adjusting control valves changes total amount of steam flow entering steam turbine (assuming steam turbine as prime mover) which is directly proportional to speed of rotation.

Once generator is synchronized to the grid, control system is in either load or pressure mode (depends what is preferred by operators). For now let's assume it's load control mode. In this mode, control error that represents input to the controller is formed as a difference between set point and actual load.

What you have written in you response about changing speed reference is really strange to me.

I didn't find anywhere this information "To increase the power output, the speed reference is increased". I used to work in a steam power plant not that long ago and never seen that speed reference is changing in order to control output power Maybe it was going on "under the hood" but I don't think so. Control error was formed by comparing load set point with actual output power.

Although I can agree that both control modes uses the same controller, just in principle, there is a switch between speed difference (control deviation) and load difference (load control deviation). Either of these signals (which depends on control mode) is lead to a P-only controller. Speed set point and Load set point are entered separately (speed set point is meaningful only prior to synchronization).

If there is a mismatch then for example turbine's CVs (control valves) would open more to allow more steam flow. Since more steam flow cannot cause turbine to rotate faster, it will increase load (torque) angle and raise power.

There are many discussion about whether this action increase turbine speed a little bit or not at all. I wasn't able to observe this, since speed measurement is masked with process and measurement noise (+ or -2 rpm to 3000 rpm) so couldn't tell for sure.

Again thanks for the explanation, I must admit I don't have a clear understanding of droop control concept yet. Need to dig more....


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Posted by CSA on 18 September, 2009 - 6:57 pm
I was explaining classic droop speed control which as been around since the beginning of governors; flyball governors were droop speed control proportional controllers. And for decades afterwards, governors and control systems were configured and programmed (when they became digital, programmed systems) to mimic the actions of flyball governors.

I'm not familiar with the kind of droop control you describe, and it raises some questions for me. If the control is strictly based on load, how does the control system respond to changes in frequency? Without somehow sensing a change in speed and responding to that change to try help support the grid when the frequency changes, the "droop" doesn't seem to be "droop" at all.

I have seen load-biased droop speed control, but that doesn't sound like what you're describing.

If you put in a load set-point, and the control makes the load equal to the set-point, then where's the "droop"? That's not strict proportional control. How does the system respond to changes in frequency, which will change the speed of the prime mover? That's one of the critical aspects to droop (speed) control for most grid operators and regulators.

You haven't said what kind of control system, and it wasn't clear from your original post that you were referring to steam turbines. My experience is primarily with gas turbines manufactured by GE.

A change of 0.1 Hz on a 50 Hz system represents a change of 6 RPM; a change of 0.2 Hz represents a change of 12 RPM.

I would be very surprised to learn there is no speed component at all in the droop control scheme on your unit.

On GE heavy duty gas turbines, when they are running in load control, the Speedtronic system changes the turbine speed reference to make the actual load equal to the load reference.

How does the control system at your site have 4% 0r 5% droop? What is the reference and what is the feedback for the droop control? Because if it's just load, and the load is always equal to the load reference, how do you know if the unit has 4% droop or 5% droop?

How will your unit respond to a 1% change in grid frequency when it's being operated in droop control?

Because that's part and parcel of droop control in my understanding: How a control system can be predicted to respond to a change in frequency. And most grid operators and regulators are keen to know this as well.


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Posted by Mik on 19 September, 2009 - 6:22 am
Dear CSA,

like I said, I worked for the power plant a long time ago. And even then, my main occupation wasn't turbine governing system. It was a steam power plant (coal fired). However, there was a misunderstanding. I wasn't talked about "my" power plant and droop about 4%. I was referring to that article in the link above. I didn't understand the example. Now I see there is a mistake. It was written 0.1% instead 1% of frequency change.

Now back to the load/speed controller. If I recall correctly, before synchronization control error is formed only from speed (set point - actual speed). After synchronization, turbine controller system is in load mode which means that control error is formed mainly from load deviation. Here I wasn't clear enough. I admit it. I was trying to say that concept of load control was not to artificially change speed set point. I'm pretty sure about that. Control system was digital but I don't recall which one (Alstom or Siemens I think, but never mind). There were separate speed set point and load set point on the operator screen - HMI. In load control mode primary controlled variable was power output, but there was a so called correction factor from speed. If I'm not mistaken, speed error (set point - actual speed) was added with some gain factor to control error which was formed from load.

I have already written that both control modes (speed and load) use same controller (as opposed to pressure control mode).
Anyway in steady state when there is no frequency (speed) error, and actual load is equal to set point, control deviation is zero and everything is OK. Now if load starts to change, there would be load error and controller would move control valves in order to correct it.

In the same way, if load is equal to set point, but there is change in a speed, so called speed correct factor would become different than zero. Now, even so load deviation is zero, there will be an overall control deviation (caused by speed correction factor), so again controller would change the position of control valves (HP and IP) to eliminate the error.

This way no set point is ever changed (they are adjusted by the operator) but speed/load control is done in a different way. Overall control deviation is formed this way:

CTRL_ERROR = (LOAD_SP - LOAD) + Kf*(SPEED_SP - SPEED). Some folks call this a cascade control, but it is not since for cascade control more than one controller is needed. Maybe feed-forward!?!, but in any case no set point is changed.


Posted by Mik on 1 November, 2009 - 2:09 pm
CSA, you didn't comment previous post. I wonder if you have seen that control concept before. There is one more thing if anyoone can clarify it.

Droop control is a pure proportional-only control. Recently I have found out that steam pressure control by turbine controller is PI (proportional plus integral) control. Can anyone explain, why PI control is necessary in this case?

I can understand that integral component of a controller is needed every time when steady state response experiences offset. If that is not desired, then integral component must be added to integrate error and thus eliminate steady state error.
However, I don't fully uderstand why this is important when controlling steam pressure.


Posted by angel on 7 July, 2011 - 6:15 am
Hey guys. In the plant that I'm working on now (CHP plant) there are two modes of operation. So called "frequency response" mode and "load control" mode. In the first case droop speed control takes place and the plant responds to the frequency variations automatically. In the second case load setpoint is coming from the central dispatch center which is monitoring the grid frequency and distributing setpoints proportionally. I hope that solves the problem.


Posted by bruce on 11 April, 2014 - 11:23 am
Dear mik,

Droop is a function in the turbine controller to help controlling frequency on the grid. If too high output is generated to the grid the result is a high frequency. At high frequency all producing turbine has to decrease their output in correspondence to the frequency on the grid and the opposite for too low frequency.

Droop is expressed as a percentage of speed change which will cause a 100% change in output. The frequency control error added together with the output control error will be the input to the Frequency/Output controller. The droop will be a pre-adjusted figure normally set between 3 and 10%. Normal default setting ,5%.

See also Dead band below.

Droop control is used when connected to utility or when frequency is controlled in island operation together with a limited number of other units.


Posted by Arjun Sharma on 14 April, 2014 - 3:44 am
Dear All,

I am working on Hitachi Gas Turbines and Shin Nippon Steam turbines. There are three modes of control provided for control of power.

1. Isochronous or Auto Frequency Regulator
2. Droop Mode
3. Load Limiter Mode or Auto Power Control Mode

Droop mode has been rightly explained by CSA. And of course the Isochronous mode is well known.

In Load Limiter Mode or Auto Power Control mode, the operator shall specify a certain value of load to be generated by the generator. When we keep the system in this mode, the generator load is fixed to the specified value. And this mode is enabled only if the Grid tie breakers are closed or when running in an islanded mode, the second generators tie breaker is in closed position and running in Isochronous.

This will have no control, and I seriously don't know how the machine may behave if a sudden load throw off is conducted.

This is just for your kind information.

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