Grid Tie Induction Generator article

[rl71459]

I stumbled across this.... I find it very interesting. I'm sure many will see it as a controversial article.

Being the "SmartAss" that I am, I had to post it.

http://gekgasifier.pbworks.com/How%20to%20Grid%20Tie

Rob

[Halfcrazy]

Ok after reading Bill Rodgers book I think that guy missed a few point's but it sure does seem about that easy from reading Bills work. The biggest thing is to wire the start windings backwards. Now to devise a fail safe to disconect it in the event of a power failure?

[Ronmar]

Yes it is pretty much that simple.  If you are not starting the motor with electric current, you do not need the start circuitry.  If the engine is spinning the motor below it's nameplate rating, closing the breaker will power the motor and draw current from the grid.  As soon as the engine rpm increases to drive the motor faster than it's nameplate rating, it will start to backfeed.  The more torque you apply, the more the rpm tries to increase and the more the induction motor tries to backfeed.

With a properly switched starter circuit, you could in theory use grid current to spin up and start the engine.  The engine once running would overspeed the motor to start generating back to the grid.
 
You need a circuit that positively will isolate the system if the grid looses power.  In theory the induction motor should stop generating if the grid that is providing the excitation goes down.  But because of some possible residual magnetism and some capacitive effects of the connected grid, the motor might keep right on generating.  I think an approvable anti islanding circuit is the hardest part of a legal grid tied induction generator.

[Ronmar]

Oh, the reason it dosn't smoke when you close the breaker is the design of the caged induction motor.  Because of the design the motor can easilly align/sync to the incomming frequency with only a minor change in phase/shaft angle, particularly if it is near it's rated RPM.  That is why they need start caps to spin up.  They need the altered phase that the capacitor provides to apply torque and accelerate the motor without damaging the windings.  A conventional AC wound motor has fewer poles, so the potential to be out of phase can be very great.  That is the problem with syncing conventional generators with other generators or the grid.  They must be properly aligned before the connection is made.

I mentioned that when you try to accelerate the induction motor you begin to apply the engines energy back to the grid.  I said try because that is what is happening, the motor RPM once synced to the grid just dosn't want to increase unless you have a grossly mismatched engine and motor.  if you have enough HP to drive the motor out of phase, massive ammounts of current will try to flow within the motor, and it will die a violent death.

The best analogy I can come up with to grid tied induction generation is a boat in the surf headed in the same direction as the waves.  The surf and repeating waves represents the grid.  The boat represents the induction motor.  With the engine driving the boat at rated RPM, When you close the breaker, the boat aligns with the waves and rides along in the trough under it's own power.  If you decrease the engine RPM slightly, the boat slows down a little and the wave behind it starts to overtake and lift the stern and the boat begins to surf along the face of the following wave, drawing energy(current) from the wave.  If you increase the throttle slightly, the boat will accelerate ahead and begin to climb the back of the wave ahead, applying it's energy to the wave.  The more power you apply, the higher up the wave you go and the more energy you apply back to the grid. If you either stop the boat completely, or accelerate it to a great speed, the wave energy will smash the boat to bits:)        

[Lloyd]

Ron,

Great analogy...I love a good visual representation(mind projection).

Flying Cloud loves a good following sea, if I could only figger a way to keep her synced up. When I get seas big enough to surf, it's short lived bc the waves are traveling faster than the boat. The hull friction slows her, so as the waves pass, she hits a huge drag, and it sucks(the next coming wave) her back causing the engines to temporally surge.

Give me a positive tidal current on the stern and I save lots of fuel.

Lloyd

[jimmason]

the wiki article that started this thread is a bit optimistic on the ease of the induction generator grid tie scenario.  yes, it is easy to do, but it is not without some potential failure modes that will create islanding.  these are more rare than with a regular synchronous alternator, but they still need to be attended too.

there is a very good article on the full monty of issues here:

Induction Generators: What can go wrong?
http://findarticles.com/p/articles/mi_qa3726/is_200010/ai_n8909370/

jim