Windmill speed and generation control

Discussion in 'Power Electronics' started by Dawsonh4, Aug 12, 2018 at 4:47 PM.

  1. Dawsonh4

    Thread Starter New Member

    Apr 11, 2017
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    i understand that power generating windmills adjust their blade pitch to control speed. They also have a gearbox that can adjust the amount of power being generated.

    My question is - is it possible for windmills to control speed and power output by adjusting the load/resistance of the generator.

    For example in extreme winds the generator would be harder to spin and create more power per rotation than it would in low winds.
     
  2. dendad

    Well-Known Member

    Feb 20, 2016
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    You could do it that way, but in strong winds, the current may be high enough to damage the generator.
    Another way is to swing the windmill out of the direct alignment to the wind. Some do it that way with a mechanical arrangement the swings the tail.
    If you are to switch a resistive load onto the windmill when your batteries are full, use a water cooled one, a heating element in your hot water cylinder, so the power is not wasted.
     
  3. oz93666

    Active Member

    Sep 7, 2010
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    Not just damage the generator but the whole structure .Extracting more power exerts greater resistance to the wind , more force on the blades and the tower , so the whole thing has to be redesigned to handle the higher stress and load.
    Wind energy is a technology of the past , it was only justifiable when solar panels were expensive ... after a decade of falling prices ,PV is the only game in town.
     
  4. Dawsonh4

    Thread Starter New Member

    Apr 11, 2017
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    Thanks for the reply. I appreciate the fact that wind technology does not currently make sense, I am just using that concept for my question.

    How would someone go about designing a generator system with variable load that fed into a power system (electricity grid)? I believe generators are generally set at a fixed voltage and based on the resistance the amps can change. Does that mean in order to do something like this you would have to put variable resistance on the generator?
     
  5. oz93666

    Active Member

    Sep 7, 2010
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    I believe most generators have a permanent magnet rotor , with coils on the stator ...three phase arrangement is most efficient ... basically just a brushless motor ... for small turbines a motor used for RC aircraft would be good ...

    So three leads come off the motor/generator giving a three phase supply the voltage will be proportional to the speed , you can rectify so you have DC ... and then you need electronics to extract the optimum power for a given voltage ...

    It's a bit similar to solar panels I don't know if the same regulator can be used , output usually charges a battery.

    On massive turbines I think they use gears and variable pitch to keep the generator turning at a speed to give 50Hz so it can go directly into the grid ...not sure...
     
  6. dendad

    Well-Known Member

    Feb 20, 2016
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  7. Reloadron

    Distinguished Member

    Jan 15, 2015
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    Before I retired I noticed some strange goings on at the Lincoln Electric (the guys who make welding machines) plant adjacent to my plant property. They began construction of a 2.5 MW turbine so my entertainment was watching this monster be built. The reason for the variable pitch prop was similar to an automotive transmission changing gears to optimize the torque. You don't start off in a car in high gear, rather you start in low gear and shift up to optimize the power train. That's how their engineers explained it. This thing is huge standing about 443 FT (135 meters) base to blade tip. Each blade is about 164 feet long from center of hub to tip of blade (50 meters). Interesting because the absolute maximum speed is about 15 RPM and at 14.1 RPM the blade tip speed is about 165 mph (265.542 Kilometers per Hour) at a hub rotation of 14.1 rpm.When winds become too high the prop is feathered and remains stationary.

    The actual generator on the top is a large DC generator producing 690 VDC at 3,600 amps which is sent down the tower to a power sub station where the DC is inverted to AC 4160 Volts 60 Hz 3 Phase. The 4160 is a common US industrial voltage for sub stations in large manufacturing facilities. So on a good day that 2.5 MW represents about 10% of Lincoln Electric usage. The cost of this turbine was right about $6,000,000 US Dollars and 1.5 million (25%) was subsidized by the US government. It was impressive watching it get built and I have no idea how well it has performed over the past 5 or 6 years.The turbine was designed and manufactured by KENERSYS of Germany. It features KENERSYS' innovative SYNDERDRIVE TECHNOLOGY – a proven, load-optimized mechanical drive train system. You may want to look up a few of the large scale companies which make these things as to the technology used and why.

    Ron
     
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  8. crutschow

    Expert

    Mar 14, 2008
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    Especially at night and when it's cloudy. :rolleyes:
     
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  9. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Well, that gearbox, the drive train, is a mechanical multiplier so from some maybe 10 to 50 rpm you go to what I believe is 1.000 rpm (or maybe more?). Google for actual values.

    The pitch control is aimed to keep the blades rpm within the acceptable range to the point that if wind goes higher than an predefined limit, blades are "taking" the minimal wind possible (or should I say, no wind at all?) Not sure when the brakes come into play here.

    Power control, whatever that could be, seems to be happening after the drive train.

    Just in case I am thinking of the big generators in the windmill parks.

    20180803_110027.jpg

    20180804_100642.jpg
     
  10. Dawsonh4

    Thread Starter New Member

    Apr 11, 2017
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    I am not exactly sure what the generator charge controller does, but it seems to be on the correct path for the idea I am getting after.

    Bare with me here, I am very new to this - If I am trying to take a 3 phase AC generator/motor and delivery power into the US grid (240 V I believe) I would have to do it at a fixed voltage (240) but could vary the current which would in turn change the amount of watts. The faster the generator spins the more current it would produce. At some point the generator begins to spin to fast and decreases in efficiency.

    Is it feasible to electronically put additional resistance on the generator that would slow it down? Is it also possible that the extra resistance has a way of increasing the current produced?

    Basically trying to substitute out the gearbox for an electronic equivalent
     
  11. oz93666

    Active Member

    Sep 7, 2010
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    Well of course both wind and solar require a grid as backup for when there's no wind or it's cloudy ...or battery storage ..

    I was referring to price .... all wind projects I know of are subsidised , see post 7 , this project had a 25% taxpayer subsidy and I bet if you checked on this project now you woulds find it underperformed and lost more money...
    This is not now true for solar , developing countries are undergoing a massive transformation because solar can now produce at a lower price than existing methods ...
    Political games (and lower sun levels) are preventing this change over in Europe and the US.
     
  12. oz93666

    Active Member

    Sep 7, 2010
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    That's not right ... the generator (if AC ) has to spin at the same constant speed no matter what amount of power you are giving the grid , that speed determines the frequency 60Hz...and that always must be perfectly in step with the grid , the connection to the grid forces it to be instep and if the wind stopped the generator would act like a motor /fan and draw power from the grid ( if there were no electronic circuitry to stop this)...

    What happens when the wind blows harder is that the generator produces a wave that is more out of phase with the grid (but at the same frequency) this pushes more current and more power into the grid ....

    What scale are you thinking of doing this on ??? You are guaranteed to lose a lot of money !!
     
    Last edited: Aug 14, 2018 at 5:58 AM
  13. dendad

    Well-Known Member

    Feb 20, 2016
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    Usually, the wind generator will be rectified, then a DC to AC inverter produces the power.
    BUT! You cannot home make something like this to feed the mains. It is illegal to do so here in Australia, and I would think most everywhere else too. You would run a very large risk of electrocuting someone. So if you want to feed into the grid, please be responsible and use a correctly designed and certified inverter.
    Whet you are wanting to do is not simple! There are many gotchas involved.
     
  14. oz93666

    Active Member

    Sep 7, 2010
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    Certainly the bureaucrats and inspectors wouldn't allow this ... but this is all red herring to protect established interest ..... there is no danger (except to the installer ) ...

    No one inspects the wiring in a normal house before a grid connection is allowed ... a domestic consumer cannot cause damage to the grid ...fuses would blow .... and the same with people who want to feed their solar or wind power into the grid ... they cannot cause problems to the grid .... only electrocute themselves , just as an ordinary consumer can do if he does something stupid ....

    This excessive snooping , bureaucracy , "certification" and inspection stops innovation and DIY activity
     
  15. dendad

    Well-Known Member

    Feb 20, 2016
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    That is a flawed argument.
    Admittedly, if the home grown unit is correctly designed and built, there is no more danger than a commercial one.
    But the questions of the TS indicate quite a lack of expertise.
    One of the biggest dangers is back feeding power to the grid when it has been turned off. Just think of the poor electrician who turns the mains off then gets killed as the home brew inverter still feeds power into the "dead" circuit.
    To encourage a probably unskilled person to build and connect one is irresponsible at least.
    I have been designing and building industrial control electronics for over 30 years, before that, years working on 100KW radio transmitters, and would not attempt such a project.
    Over the years, I have developed a healthy respect for dangers of electricity and have noticed on this forum a few times some others do not share that attitude. It does puzzle me. Remember, there are novice electronic builders reading this forum and we do owe a duty of care to give good safe advice.
    Also, it is my understanding that this sort of build would go against the conditions of this forum.
     
    Last edited: Aug 14, 2018 at 9:07 AM
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  16. crutschow

    Expert

    Mar 14, 2008
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    I don't know if it's commercially used but an induction motor can serve as a generator.
    If an induction motor is driven faster than synchronous speed it returns power to the grid at the synchronous frequency.
    The speed does thus not have to regulated, as the faster the blade tries to drive the motor the more power it returns to the grid, thus keeping the speed just a little above synchronous.
    So it's a very simple way to make a wind generator.

    One obvious restriction is that the motor must be disconnected from the line if the speed drops below synchronous, otherwise it reverts to being a motor and starts driving the blade.
     
  17. Dawsonh4

    Thread Starter New Member

    Apr 11, 2017
    24
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    I don't plan on doing anything myself. Just looking for some education. Specifically curious for controlling generators connected to deep ground air storage for electricity generation.

    So in the windmill case - what happens when it speeds up and the frequency gets too fast for the grid?
     
  18. digsys

    New Member

    Jun 27, 2018
    10
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    100.1% agree with dendad !! NO one should ever suggest connecting anything un-approved to the grid or at least inspected.
    There are other dangers as well.
    If your supply is less than the grid supply, VI will immediately go out of phase and there'll be a BIIIG bang !! .. that's your grid-tie unit :)
    If somehow you can supply more power than your grid, then there'll be MANY BIIIG bangs !! ... that's the grid and everyone on it :)
     
  19. Dawsonh4

    Thread Starter New Member

    Apr 11, 2017
    24
    1
    Understood, but windmills obviously spin at various speeds. How are they controlling the frequency?
     
  20. digsys

    New Member

    Jun 27, 2018
    10
    4
    Sorry, mis-understood you. DFIG (doubly fed induction generator) .. papers like this - https://ieeexplore.ieee.org/document/5414859/
    explain it a heck of a lot better than I could :) It is a well established principle used in many applications.
    " ... operate like a synchronous machine where the field ( the rotor windings) are given independent excitation ... "
     
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