DIY grid connect Inverter

[RP]

I believe you could put this together yourself.  You'll need:

1.  An inverter large enough to supply all your electrical needs.
   
2.  A smallish battery capable of carrying the invertor for at least a few minutes at full load.
   
3.  A grid tied battery charger
   
4.  Your RE power source to also charge the battery.
   
   

Under normal circusmstances the battery is charged by your generator.  When the battery voltage falls too low (due to insufficient generating capacity) the grid tied battery charger kicks in and supplements the generator as needed.

I think what you're describing is kind of a UPS but backwards.  Normally drawing from the battery unless it gets weak and switching to the grid.  I wonder if a standard UPS could be modified to do this?

[craig110]

Congrats on the nice outputs from your turbines!  Trying to figure out what to do with excess power is a nice problem to have.

One possible option shy of going full grid-tie is to set up a hybrid on-grid / off-grid configuration.  The goal of this configuration is to be able to manually select which circuits in your house are powered by the grid electricity or by your off-grid power.  This would entail getting a decent sized inverter (where "decent sized" is up to you to decide) and some batteries.  Feed the output of the inverter into an auxiliary distribution panel and put appropriate sized breakers into this panel for the circuits that you want to be able to power off-grid.  To keep the flexibility of easily adjusting your off-grid load to your power production and battery state, put a switchbox between the circuit and the two panels so that you can easily switch whether that circuit is fed by the grid or by the off-grid power.  As long as you make sure that no failure can allow the two distribution panels to feed each other, and mechanical switches are rather error resistant, you should be able to use your extra power at will without having to deal with the UL testing and grid-tie permission problems.

To avoid any synchronization problems that can be caused by quickly switching a device from one power source to another unsynchronized source, I'd suggest using "A-off-B" switches and pausing at the "off" state for a few seconds when switching something.  (Using batteries to smooth out the supply and usage, my hunch is that pretty quickly you'll have a stable set of circuits feed by your off-grid power and will rarely have to switch sources for any of the circuits.)

Craig

[scottsAI]

Hello RP,

UPS could be a good choice. Check the efficiency first.

A modified UPS could do the job, not a simple task.

Conversion would be as simple as a relay from the mains to the ups. When you want to come off the battery turn off the mains.

Automated would require a controller with battery state of charge.

When the MAINS are on line, UPS will start charging the battery, could be good or bad? Might require another change in the UPS, depends on where you want to go with it.

Have fun,

Scott.

[powerbuoy]

Working with UL is not that hard ... even though they tend to milk your finances. I can not see how this is a non profit organisation, ... maybe it is not. Anyway, there are also ETL, TUV Rheinland, CSA and other test labs that can certify to UL standards. Any NRTL can test to UL standards. First thing is to buy the required UL standard, read it carefully. Then contact a local UL office, get the name of the manager responsible for testing to this very standard and start talking. He/She will let you know excactly what they will test, how many units they need, what the time frame and expenses will be and what you need to do to make your inverter rock solid.

[Nando]

Now that the FLAK has subsided I would like to give some comments.

ISLAND = GRID TIED external power source feeding POWER to the GRID.

The regulations for ISLAND protection is due to several reasons in the use of multiple ISLANDS in a single large branch of a GRID section.

GRID section, like a 1000 KVA, may have several sub-branches, in a short condition, the circuitry may order a power OFF for a down line of the GRID section,or under a manual disconnection the circuitry includes, as well, opening the lines in the sub-branches for easy re-connection that is done in segments to avoid initial ON very high peak currents.

If a section or a sub-branch may have several connected ISLANDs that may have enough power to feed power to that sub-branch, the power stays up sometimes with good wave form because some of the ISLANDS may have sine wave form capabilities, specially if they have higher power capabilities and the effective Q, in the whole section or sub-branch, is greater than the damping effects of the loads .

If the loads are greater than the capabilities of one or more ISLANDs the power dies down with certain limitations, like around 100 cycles or so.

The regulations have some age, when the electronics available were in their infancy, but with the present available technology a GRID connected Inverter does not need to supply a wave form, but to supply a AC current that is determined by the incoming sine wave of the GRID.

The AC Current level is defined by the available power source and if the source is variable, the AC Current will be as well variable -- the Incoming AC Sine Wave defines the AC Current along the Sine Wave slope which in a sense represent the reverse behavior of a resistor as a load.

There are several forms to produce a GRID tied power source -- power source to indicate that it may not be a totally electronic piece of equipment, it could be one like such (most commonly available for higher efficiency) - BUT there are other methods, like :

1. Wind Mill feeding a DC/AC GRID tied Inverter.
   
2. Wind mill feeding a battery bank and this feeding a DC/AC GRID tied inverter
   
3. Wind mill feeding a DC motor feeding an AC Induction Motor as generator ( Grid biasing)
   
4. Wind mill with an Induction Motor as generator with electronics to connect/disconnect the GRID as the motor is within the GRID frequency, sometimes with a dynamic load for longer connection time.
   
5. Wind mill with a 3 phase induction motor, as an Induction Generator with additional capabilities to make use of the 3 phase to supply energy to the GRID in single phase configuration.
   
1. & 2) types require a DC/AC electronic Inverter -- highest efficiency around 90+ %
   
3. the easiest for the "experimenter " to play with, because the controlling requirements may be low to implement, simple electronic controller is needed . Efficiency around 45 to 55 %
   
4. A bit more complicated because a micro controller is needed with additional loads and electric - fluid brakes ( like in a car) with limited time -- over all efficiency around 40 %
   
5. A MICROPROCESSOR is required and as well a not-too-complicated electronic to convert the 2 additional phases to the primary phase EFFICIENCY around 80 to 85 %
   
   

Other ideas are available, but is sufficient for now.

Nando

[powerbuoy]

Nando:

Idea # 3 appeals to me because it's such a nice understandable (straight forward mechanial) approach. I just wonder how to obtain the controller/regulator for the DC motor.  Probably easier to do when there is a battery bank inbetween, so that the DC motor can be fed more steadily. Still, the inverter is the way to go, having no moving parts seems to be the future.

Powerbuoy

[Nando]

The difference between the two of them is price, in one, is the money to buy the DC/AC GRID rated inverter.

The other is the AC induction motor that can be bought for a low price if used.

then a DC motor capable of running the power -- or an AC induction motor with a RPM controller to maintain the AC RPM above the Slip value.

With a DC motor a Voltage PWM controller to maintain the RPM constant (with feed back).

A 5 Kw built for around $600 or 800 dollars (Wind mill not included )

Of course life limitations of the mechanical parts.

Though I know of some that have lasted for more than 10 years with maintenance .

Nando

[The Crazy Noob]

T've done the third thing as a test; here are the results:

I've tested the thing with a 12V motor from a fan to cool a car-radiator connected to a 12V/15A power supply and a 100W induction motor (1 phase, i don't see a capacitor for a phase shift...)

When I just turned on the AC motor, it drew about 0.5A (230V~ input). I put a kWh meter in between and it turned at a certain speed. (the DC motor was generating about 8V)

When I applied current to the DC motor (it drew around 8A (wich is close to the 100W from the AC motor)) I could see the kWh meter turn slower (less power used, still not turning backworths) but the amps that were flowing between the grid and the induction motor rose to around 0.7A or so.

Does annyone have an explanation why the amp-meter showed higher power being used and the kWh meter showed that less power was being used? Does it have something to do with the voltage being out of phase so that the meters can't detect it the same? (something with vectors and those things?)

I need to get my hands on a better and more powerfull DC motor, I also have another 100W indcution motor lying around that has got a capacitor for a phase shift but the axis is of a differint size and I don't have the righ parts to hook it to the DC motor.

[pemtek]

First of all Merry Christmas,

I have been doing some work on this idea throughout the year and have managed to test a few circuits on the bench but as yet I have not made a proper prototype.

In my bumbling around I have been following a few threads over here in the UK about Grid connect inverters and use on networks.

It seems the standard HERE for backfeed connection G83 is an advisory standard and is not law (atleast not in the UK). Some Electricity providers will ask for a G83 cert but some will not. All they ask is that the equipment be compatible with G83 / BS7671.

There are a number of companies that sell none certified grid connect inverters and people just fit them without even speaking to their supplier. This apparently is because they dont expect to export anything because their continous load is above the output of the inverter.

Phil