GEK Wiki / How to Grid Tie
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How to Grid Tie

This version was saved 13 years, 2 months ago View current version     Page history
Saved by Jason Poteet
on January 17, 2011 at 10:48:06 am
 

return to Practical Engineering

 

Warning from All Power Labs:

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?

 

Warning Discussion from Article Author

The warning above posted by All Power Labs is understandable, especially as they are based in California where lawsuits abound.  It is also understandable as killing people, no matter their size, occupation, health status or productive worth to society is unquestionably evil.  However, the main point that I wish to make in my article is that induction generators are inherently good at preventing islanding and back energizing the grid when the grid power is down.  One must work hard to enable an induction generator to produce any voltage when not tied to the grid, which is why I specifically warn users not to connect capacitors to the induction generator.  Even if one were to ignore my warning and add capacitors, most small induction generators would not be able power all of the loads suddenly placed on them when grid power is lost and would not be able to self excite in that situation anyway.

 

Regarding the article "Induction Generators: What can go Wrong?" it should be noted that most of what they are talking about deals with complexities introduced when using wind turbines, multi-phase (e.g. 3-phase) or commercial billing.  Other problems arise from improper sizing of input HP availability to induction generator.  I'm adding a section at the bottom of the page to address issues specifically.

 

Based on the fact that All Power Labs hasn't removed this article, and that some of them find it interesting, I decided to come back and add clarification.  Since I don't work for them they can still be held blameless for liability purposes.  Hopefully this information will somehow work to further the cause of affordable alternative energy distributed electrical generation with the specific goal of enabling small single phase induction generators to come online. 

 

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 custom essay, 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.

 

Regarding Article "Induction Generators: What Can Go Wrong?"

At the top of the page there is a warning with a link to an article about all the things that can go wrong when using induction generators.  After reading it casually one is tempted to mortgage the house and spend $100k paying electrical contractors, permits and utility company inspection fees.  Luckily restricting the operating conditions and applying a bit of common sense can make internal combustion engine grid tie electrical generation safe and cost effective.  Here are the restrictions one must apply to remain safe and cost effective:

 

  1. Single Phase Only - do not connect to 3 phase grid (3 phase induction motors can make efficient single phase induction generators though due to their inherently high power factor)
  2. No capacitors - remove motor starting capacitor if it has one, and do not add any additional capacitors
  3. Only install a small system recommend 5 HP or less - capable of only a few kilowatts (oversized generator good for efficiency, just don't use a large engine to drive it)
  4. Match, or undersize the internal combustion engine to the generator
  5. Do not use a windmill - we're talking about internal combustion engines here, probably powered by wood gasification

 

 Below are some specific points from the article that I will address in more detail:

 

Phase Imbalance 

This is a valid point for multi-phased systems.  The easiest and most cost effective way to address this issue is just to not grid tie to a multi-phase system.  Granted our power grid in the US is all multi phased distribution at some point, but I'm referring to the number of phases available at your meter.  If you have a single phase meter, then power will either be on or off.

 

Lightning

This is a valid concern for windmills because they are typically installed in open locations and elevated by towers.  Though it may be tempting to rent a crane and install your gasifier, motor and induction generator on top of a tall tower in an open field, try to restrain yourself.

 

Overload from Prime Mover

While this is a concern for almost any system, it is harder to solve for when working with windmills.  It's always desirable to have an oversized windmill to handle lower wind conditions but wind is variable and there can always be temporary conditions of high winds.  The main point here is that it is possible to put too much energy into an induction generator, as is the case with a regular generator.  This is easily solved when using an internal combustion engine instead of a windmill - simply use an undersized motor in relation to the induction generator.  In fact, it is desirable for efficiency to do this.  A 5 HP engine (reduced to 4 HP when running on wood gas) driving a 10 HP induction generator would be a great setup.  The oversized generator will run cooler (avoiding electrical loss due to resistive heating) and also produce a better power factor (another measure of efficiency - bottom line it will create more useful energy for local use or to run your meter backwards).

 

Wandering

Again referring to windmills, the induction generator may wander between generating mode and motor mode if not controlled properly due to varying speed of the wind.  For an internal combustion engine, this almost never happens unless the engine isn't functioning properly or runs out of fuel.  In normal operation with a properly sized engine, a suitable amount of ungoverned throttle can be applied and the induction generator will allow the rpm to increase and generate up to the available amount of power available from the shaft.  If the engine can't provide more power than the induction generator an take, there's no problem.  If the engine does run erratically or die for some reason, then the induction generator turns into a motor and you will be paying the utility company for the power it is drawing.  For this reason, unattended operation for the span of day or more is not recommended.  However, there's not really any harm in using a motor to turn over your induction generator.

 

Fluctuating Line Voltage

Induction motors are actually designed to tolerate a fairly wide range of line voltage, though below a certain point they will draw huge amounts of power from the virtually unlimited source of the grid.  However, the source of power when used as an induction generator is the drive shaft.  Low grid voltage is only a problem if the input shaft power is too great for the induction generator.  With the ungoverned input of a small internal combustion engine that I mentioned earlier, the rpm will drop to the point where the voltage slightly exceeds the grid voltage.  Remember with an induction generator that RPM doesn't determine frequency - the grid determines the frequency.  The point here is that if you supply your induction generator with a constant power input, e.g. 3HP, then it will automatically output 3HP worth of electricity at the current line voltage and frequency.  However, you should still put an appropriately sized breaker between the induction generator and the grid.  Also note that you should use an induction generator wired for the intended voltage - e.g. wire induction generator for 240 volts when hooking it up to 240v lines.

 

 

 

 

 

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