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Spoofing grid-tie inverter w/ signal generator - help please?

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klaksÉ™n

Mon Aug 03 2009, 04:24PM

Registered Member #2060 Joined: Fri Apr 03 2009, 01:49PM
Location:
Posts: 4

I decommissioned a grid tied solar install a while back.

After all was said and done, I have a Xantrex 4kw 240VAC grid tied inverter that accepts 48VDC as input (SW4048PV)

This inverter lacks any internal regulation - all it does is synch and match the voltage sensed across its output terminals once its checked for stability.

It will never sniff the grid again while in my possession & I hate to pawn off a 10-year old inverter onto someone on eBay.

What would be sweet is to be able to dedicate this inverter for off-grid 240VAC work, ie: power tools or well pump.

I think a start on specs required to spoof the utility grid includes:

1) no loading up even if output appears to be a short

2) be immune to back feeding

Since the Inverter was essentially free it sure would be nice to be able to get some use out of it...

Photo shows the inverter and its ground fault interrupter duaghter panel - combiner PCB.

Any notions or potions? thanks ! !

Steve Conner

Mon Aug 03 2009, 04:49PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Some grid-tied inverters (like the one I built) physically can't run without the grid. They operate as current sources by default and don't have the circuitry to run as a voltage source. But then again, it might be as easy as changing a jumper. I'd suggest calling (not e-mailing) Xantrex for info.

Selling it on Ebay might not be a bad idea, as those old Trace inverters have a good reputation, I imagine the resale value would be pretty decent. Maybe even enough to buy the kind of inverter you wanted in the first place.

Conundrum

Mon Aug 03 2009, 06:12PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

hmm... just a thought, but have you tried just using a spare mains transformer (suitably hacked) and then feed the input via a simple DIY pseudosine wave generator using a 4017 and resistors feeding the bases of a pair of 2N3055s?

this might work, watch the dissipation though. Just for safety I'd use at least a 3A winding on the primary and add overvoltage protection on the mains side.

-A

klaksÉ™n

Wed Aug 05 2009, 02:25PM

Registered Member #2060 Joined: Fri Apr 03 2009, 01:49PM
Location:
Posts: 4

Thanks for insight and suggestion:

Xantrex is a closed shop - even when their products are long obsolete no tech sheets make it out of authorized service centers.

Current source - Ive seen the panel dump 12A of 240VAC back into the grid, it just can't synthesize a voltage level and simply matches what it sniffs off the grid. Not quite understanding what you mean there...

I have no lab bench set up currently to fab component add-ons - and I hesitate to place a step transformer across the output but you are correct with fusing everything; when you say overvoltage protection on the mains how so? The surge on the SW line is easily 3x rating, be a bad dream to see my add-on go china syndrome for sure.

I was thinking a small 200W pure-sine inverter such as made for mobile electronics and increasing the cooling on for long term attachment to a step-up xfmr BUT it would be attempting to power the AC side load and give problems from that, not quite sure if it would be smart enough to load share w/o feedback.

I guess to get inside the inverter panel and see where circuitry was omitted from the normal SW4048 model - and trace the sensing circuit out, but I can't cut the link to grid feed as it needs to self-monitor...

argh.

Bjørn

Wed Aug 05 2009, 04:37PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Current source - Ive seen the panel dump 12A of 240VAC back into the grid, it just can't synthesize a voltage level and simply matches what it sniffs off the grid. Not quite understanding what you mean there...

You can't regulate both voltage and current without regulating the load. Since the grid is outside your control you can really only control the current because whatever you do the grid will deliver enough current that the voltage is completely decided by the grid.

Just imagine a 100 Ohm resistor, how can you adjust the voltage over it and current through it at the same time? You can't, you have to obey Ohms law.

...

Fri Aug 07 2009, 04:37AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I agree with the others in that this inverter is not designed for what you want to do, and making it work like you want will take more than feeding it with a low power ac signal.

From what I understand grid tied inverters, old or new, usually fair quite well on ebay or the likes--and finding a nice pure sine inverter for less than what you sell this one for should be pretty easy.

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