Faking It for the Grid

[LowGear]

OK, I bought a Sunny Boy grid tie inverter and as you all know it won't invert unless it senses the proper value on the line.  No grid power = No SB inverter power. 

But let's say someone goes over to the main and turns the grid off.  And then has a a second inverter that is a stand alone and it starts acting like it's the grid.  Will it fool the Sunny Boy into AC generation from it's DC sources?  How big or powerful does the pretend grid have to be?

Casey

[rcavictim]

OK, I bought a Sunny Boy grid tie inverter and as you all know it won't invert unless it senses the proper value on the line.  No grid power = No SB inverter power. 

But let's say someone goes over to the main and turns the grid off.  And then has a a second inverter that is a stand alone and it starts acting like it's the grid.  Will it fool the Sunny Boy into AC generation from it's DC sources?  How big or powerful does the pretend grid have to be?

Casey

Might be able to fool it with a cheapo cigarette lighter plug car mod-sine inverter.  That will save blowing up an expensive sine wave inverter which certainly won't much care for what will be effectively mains power parked on it's output.  Now the current ought not be much to trigger the Sunny Boy so maybe placing a couple of resistors between the trigger inverter output and the SB might ease the burden the starter inverter might see.  Use a small wattage 120 volt incandescent light bulbs if you don't have resistors.  I'd say 10 watts for a try. In theory the SB is going to sync itself to the trigger source and this is essential if the starter inverter is gonna live.  As we all know theory has been known to make smoke.

This whole business of spending a small fortune on solar panels and an inverter and then having no power at home when the utility goes down doesn't make a whole lot of practical sense to this rocket scientist.

[Carlb]

why not just add a sunny island inverter to your setup, that is what it is made for.

[mike90045]

I believe the grid-tie inverters, as part of their "5 minute stable grid test" (before beginning to invert) also "pulse" the grid to measure it's impedance. A small "micro grid" will fail this test.   As it ramps up it's output, the "micro grid" will quickly overvoltage (that's how GT inverters work, they produce a higher voltage to feed the lower voltage grid) reset, and then restart the 5 minute stability test.

[LowGear]

CarlB,

I'm pretty sure the Sunny Island is not grid tie.

Casey

[Carlb]

CarlB,

I'm pretty sure the Sunny Island is not grid tie.

Casey

This inverter can also be utilized as a back-up system in grid-tied applications. Its seamless transfer allows existing Sunny Boy inverters to be utilized during daytime grid outages. The Sunny Island 5048U is an extremely robust and sophisticated unit providing both off-grid and back-up users with the highest quality available in the industry today.

Here is a link the the specs on alt e web site


http://www.altestore.com/store/Inverters/Grid-Tie-Off-Grid-Capable-Inverters/SMA-Sunny-Island-5048U-OnOff-Grid-Inverter/p6071/

carl

[LowGear]

Hi Carl,

SMA's next generation of Sunny Island inverters combines the most advanced technology used in their Sunny Boys with an outstanding feature set for off-grid PV applications.

and

his inverter can also be utilized as a back-up system in grid-tied applications. Its seamless transfer allows existing Sunny Boy inverters to be utilized during daytime grid outages.

Maybe I'm misreading your link but it sounds like the Island unit becomes a fake grid?

It's a shame it's panels can't be harnessed to feed the grid once the batteries are topped up.  It'll take AC from a generator too - about 5 KW I think.

Casey

[mike90045]

The problem happens when the solar/wind begins to generate more power then is being consumed.  At that point, the inverter becomes an unregulated battery charger, and if your batteries are full, they first get a stout equalization charge, which does not stop till the "local grid"/island voltage climbs over the inverters overvoltage cutoff (~255VAC)
(Grid tie inverter H bridge outputs are bi-directional)

[LowGear]

Wow Mike,

You're so far over my head it's scary. 

I wish the ads were more informative - valid and less "damn the torpedoes and sell the box" for these products.  When I read about the Xantrex XW it would seem it could do all these things we've covered in this thread.  The Xantrex support here on the Big Island isn't good enough to be rated so they suffer bad press when compared to SMA.

Casey

[mike90045]

Here's a link to the "official way" http://www.xantrex.com/web/id/25/learn.asp  and the outback version http://www.outbackpower.com/GT_Inverters.htm

This thread here :  http://www.wind-sun.com/ForumVB/showthread.php?t=2616  covers some of what happens if you have GT inverters and a battery inverter sharing the domestic 240V system. 

As the battery inverter powers up, the GT inverters track it for 5 minutes, and then start feeding their power into your "mini grid".  If you loads slack off and become less than the solar fed GT inverter is supplying, the excess power will reverse flow into the battery inverter, and start charging the battery.   not bad for a short while, but eventually, the battery fills up, the battery inverter does not know how to properly charge a battery, and continues to feed and cook the battery. Eventually, your mini grid goes over voltage, and trips the GT inverters off-line. Then they re-sync, and it all starts over again..

Still looking for the right doc to steer you to.

other threads:
http://www.wind-sun.com/ForumVB/showthread.php?t=8089
Attached  Xantrex AC coupling tech paper .pdf