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Droop

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Droop

  1. 1. here has been a LOT written on droop speed control on control.com. Use thefine Search feature and you will find more than you probably everwanted to know.Synchronous generators. Their speed is a function of their frequency (or, itcan also be said that their frequency is a function of their speed). Butbecause they are synchronous generators no generator can go faster orslower than the speed that is dictated by the frequency. And, because theyare all connected together and their rotors are locked into synchronismwith each other (magnetically), the prime movers which are mechanicallycoupled to the generators cant change their speeds either.When a prime mover driving a synchronous generator is connected to a gridwith other generators and their prime movers, particularly a large or"infinte" grid, the frequency of the generator is controlled by thefrequency of the grid. And, since the speed of the prime mover is afunction of the frequency of the generator (Im talking about primemovers 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 thanthe other synchronous generators (and their prime movers) on the grid.Its just not physically possible for one generator to be running at 50.134Hz and another to be running at 52.27 Hz, and still another to be runningat 49.65 Hz if they are all connected to the same grid which is operatingat 50.001 Hz. It just cant happen.So, while a lot of texts try to describe droop in the way you did, in the realworld, it just doesnt work that way, except when theres just onemachine driving a load independent of any other machine. And in thatcase, the prime mover control system is usually operating in IsochronousSpeed Control, not in Droop Speed Control.To make a prime mover (which is providing the torque input that thegenerator is converting to amps that is being converted to torque bymotors which are also connected to the grid) stably control its poweroutput while connected in parallel with other generators and primemovers on a grid, the control systems employ straight proportionalcontrol. And that proportional control is called droop control, droopspeed control.
  2. 2. By the way, stably controlling power output when connected to a grid inparallel with other prime movers and their generators is my definition of"sharing load."Droop speed control looks at a prime movers speed reference and its actualspeed, which is a function of the grid frequency.To increase the power output, the speed reference is increased. But, since thespeed cant actually change the increased error between reference andactual speed is converted to increased fuel flow. That increased fuel flow,which would tend to increase the speed, which cant increase appreciably,is still extra torque. And the generator converts that extra torque intomore amps. All of this is done very smoothly and all the prime moversand their generators behave nicely and work together to provide the load.If the "load" on the generator is to be increased, then the turbine speedreference is increase again, the error between the actual speed and thespeed reference increases again, which increases the fuel flow whichincreases the torque which increases the amps.When turbine operators are watching the watt meter and twisting thegovernor handle in the Raise direction to get the watt meter reading toincrease, they arent changing the watt reference they are changing theturbine speed reference.Droop speed control is straight proportional control in the strictest, purestsense of the word. There is no reset or integral action to increase the fuelto make the actual speed be equal to the reference. It cant be equal to thereference (if the reference is more than the actual speed); its notphysically possible. And droop speed control makes use of thatimpossibility to stably control the fuel in proportion to the error.The error can change for either of two reasons: a change in the speedreference, or a change in the actual speed. Its not common (in most partsof the world!) for the actual speed (grid frequency) to change. But whenthe grid frequency (and hence the actual speed) does change, the controlsystem "automatically" reacts to the change because the error changesand adjusts the fuel to try to compensate for the change in actual speedrelative to the speed reference.
  3. 3. So now you can imagine what happens when the turbine speed references ofall the prime movers connected to a large, "infinite" grid are all fairlyconstant, which means the errors between their speed references andtheir actual speeds are all fairly constant, and the grid frequency changes.Since frequency and speed are directly proportional, the error betweenactual speed and speed reference changes. If the grid frequencydecreases, then the error increases which increases the fuel--of all themachines because they are connected together to the grid.Each prime movers governor (control system) will respond to a change offrequency as a function of the amount of droop that the control system isprogrammed to have. A 1% change in frequency on a machine with 5%droop will result a 20% change in load, nominally, supposing themachine was running at 80% of load or less to begin with. A unit with4% droop will respond with a 25% change in load, nominally, againpresuming the machine was running at 75% or less than rated load tobegin with.Some manufacturers use the above scenario, what happens to the poweroutput of a machine whose primer mover is operating in droop speedcontrol when the grid frequency changes as their definition of "loadsharing".Its important to note that if a machine is operating at rated power output ondroop speed control and the grid frequency decreases, the prime movercannot increase its power output any further. And a LOT of primemovers connected to a large "infinite" grid at any one time are operatingat rated power output. So even though they are in droop speed controlthey cant pick up any additional load by increasing their power outputwhen the grid frequency decreases. Theyre just along for the ride at thatpoint. And if the turbine is a combustion turbine, a single shaftcombustion turbine, well thats not good for the grid. But, thats foranother thread.Now, a machine which is said to have 5% droop will nominally reach ratedoutput when the speed reference reaches approximately 105% of ratedspeed. Nominally is the operative word here. A machine with 4% droopwill reach its rated power when the speed reference is 104%.
  4. 4. So, mull this over. Search the archives of control.com with the Searchfeature and ask more questions. This is not a complicated subject, but alot of what has been written in many texts is very theoretical and veryconfusing.I like to describe droop one way like this: We are telling the turbine to runfaster than it can when we increase the turbine speed reference. In otherwords, 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 theactual speed is relatively constant and use the error between the twosignals to control the amount of fuel or steam or water or whateverenergy source is being used in the prime mover.As a side benefit of this, if the actual speed does change for some reason thecontrol will automatically respond in the appropriate way to try to helpmaintain the grid frequency.Its a wonderful thing, droop speed control. Its so simple, and so powerful. Allat the same time.

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