GEK Wiki / How to Grid Tie
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How to Grid Tie

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Saved by jim mason
on March 29, 2010 at 11:51:56 pm

return to Practical Engineering



This article understates the issues and safety equipment needed when doing grid tie via an inducation generator/motor.  While it can be this simple in theory, in practice there are a variety of error states still possible that can create islanding and thus back energizing the grid when it is otherwise down.  This is a very serious issue in rural and dev world power, where a small generator might infact run a long branch distribution circuit, endangering linemen possibly working on it.  For a complete tour what is needed for the induction asynchronous generator scenario, see the article here: Induction Generators: What can go Wrong?



Why Grid Tie?

Most of us would like to eliminate, or at least drastically reduce, our electric bills.  If you're like me, you've had visions of setting up a large generator to power your household needs, preferably on some almost free fuel such as wood.  This probably sounds great unless you've tried it before, which I have.  Are you really willing to listen to a generator run constantly?  Do you understand how many household loads you can start and run simultaneously?  What if your wife decides to start baking, drying clothes etc without telling you,or your electric water heater kicks on while running your pool pump?  There's a reason most of us have large services to our houses - typically 100A at 240V.  That's well over 20KW available, or roughly 30hp not counting generator and other losses.  Most of the time your house is only drawing an average of 10-15% of that capability, but when you need to start a large motor such as an air conditioner that extra capacity is a necessity.  Motors will draw several times their rated power when starting, so your generator must be very big, respond quickly, and not drop voltage or you will trip breakers or burn out the motor.  From what I have learned from the GEK people, gasifiers aren't well suited for highly variable loads.  Finally, do you really enjoy reprogramming your TV and setting clocks every time you shut down the generator?


Utilizing grid tie will allow you to run your generator when you please, start and run anything you normally would at any time and other people in your house won't give you dirty looks because the digital clocks are flashing.  You can send a smaller amount of power back through the line for a longer period of time, to have the same net effect on your power bill.  You can also set your gasifier flow rate to produce a steady, high quality output.


Is Grid Tie Affordable?

In a word, yes.  A DIY installed mechanically driven induction generator can give you much more power than electronic means, and do it for only a couple hundred dollars.  More specifics later.


So How Do I Hook Up My Generator?

You don't!  When most people mention generators, they are talking about something called a synchronous generator.  If the generator you own is designed to give you power off the grid, i.e. back-up power, you have a synchronous generator and it would be very expensive and complicated indeed to hook it up to the grid.  RPM must be strictly monitored, and phase matched exactly to the grid precisely using expensive equipment before bringing it online.  It's not cheap, and it's not easy to do even after you have it set up.  The most difficult step I had to take was to remove the synchronous generator head from my genset and replace it with something called an induction generator, which can actually just be an old motor of the correct type.


The Induction Generator

An induction generator has several advantages over synchronous generators for tying to the grid.  They take their cue for frequency from the grid, so as long as the generator is turning above the minimum RPM threshold, changes in RPM are not a problem.  This could be good for some phases of GEK syngas production where gas quality or quantity are variable.  However, for practical purposes there is an upper RPM limit you will want to observe to prevent burning the windings or tripping the breaker.  The upper limit can be somewhat self regulating though if the engine driving the generator is not too big.  If you can't put enough horsepower into your generator to burn the windings, then there's no problem.  Try to match the rating of the induction generator in horsepower to the motor you'll be driving it with.


Bringing the induction generator online is also very easy.  As a general rule of thumb, a running induction generator doesn't actually generate any electricity until it's hooked up to the grid (exception is if capacitors of any kind are installed - more about that later).  Just bring the generator up to speed and flip the breaker - no pops, sparks etc.  You will just hear your motor working harder.


Another advantage (from a safety viewpoint) is that the induction generator stops generating in the event of a power outage (again, when no capacitors are present).  This helps keep the people that are working to restore power safe.  However, I think it is wise to install a motor starter relay to ensure that the generator won't come back online automatically because it can be problematic during those periods when the power rapidly starts and stops.  I knew a guy once that blew up his electric meter like that - fortunately the utility company didn't ask too many questions.


Yet another advantage is that if you need more power, you can add more induction generators, even to the same motor if the generators share similar characteristics.


Where do I buy an Induction Generator?

Don't be silly - those cost too much.  Induction Generators are really just squirrel cage motors, and you will have good luck finding used ones at your local motor shop - probably for less than $200.  Don't worry - most large motors are squirrel cage motors.  Compressor motors are plentiful and fairly large - up to about 5HP.  To keep things simple, I would recommend a single phase, 220V motor.  Try to find one with a high power factor rating or you will not convert your input energy as efficiently.  I would shoot for 80% or higher.  Also, look for the RPM in a suitable range for your combustion engine,  Pulleys can be used to overcome RPM differences using the appropriate ratios.  Most motors are either approximately 1800 or 3600 RPM, thought the nameplate ratings typically show loaded ratings of 1750 and 3450 RPM.


Hooking Up the Induction Generator

Some motors only rotate in one direction, so pay attention to that.  Also, remove any capacitors that may be installed on the motor.  These are usually found in rounded humps of metal attached to the top of the motor.  Some motors have only start capacitors, and some have both start and run capacitors.  Neither are safe, and you'll be using a combustion engine to start the generator anyway.


Wire the motor so you can hook it up to 220V, that way you can push more power through smaller wire.  Of course, use recommended breakers, motor starting relays, whatever is recommended by the national electric code, your utility, friends and neighbors.  I'm not responsible for anything.  I did hear that one utility near me, PGE, is starting to prosecute people for Grid Tying illegally.  Might be a lot of red tape to cut through for an individual system, but eventually somebody will get a cookie cutter system approved that they will like.


Bringing the Induction Generator Online

  1. Start your engine
  2. Try to set RPM slightly above nameplate rating (e.g. around 1800 RPM for 1750 RPM rated device, but not more than 1850 or so)
  3. Use a clamp on meter to measure current in the line
  4. Flip breaker or whatever to connect to the grid
  5. Note amperage
  6. Increase RPM slightly
  7. If amperage goes up with RPM increase, you are already generating
  8. If amperage goes down with RPM increase, it was running as a motor and is not generating yet.  Increase RPM some more.
  9. Increase RPM until amperage is great enough for your needs, but never exceed amperage on nameplate.

Note that generator is more efficient when amperage is not at maximum, because heat is generated in the windings.  This increases resistance, which in turn causes more heat to be created to a certain extent.  Another consideration is the location of the generator - if it's in a hot generator shed 5 inches away from your combustion engine, it may get too hot even at lower amperages.


The Fun Part

Watching your meter "turn backwards", even if it is a digital meter, is fun.  However, due to the somewhat complex topic of power factor, the meter will not register as much as you seem to be generating based on straight voltage*amperage calculations.


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