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For obvious safety reasons my residential PV system disconnects from the grid if it notes the grid is down. The thing is it also shuts itself off so that during a grid blackout rather than providing me power but detaching from the grid the inverter disconnects itself from both the grid and the panels leaving me without power. I want to safely defeat this safety feature that is to say I want to be able to use solar power when the grid is down without frying the guy in the cherry-picker working on the lines to fix them.

Edit to add: I was hoping I could do something more clever along the lines of : 1) manually throw knife switch to disconnect grid from my home. 2) turn on little liar circuit that puts out a drip of 240vac (powered by say a motor cycle batter) to make my inverter think the grid is live and then uses the inverter power feed to keep the 240vac liar circuit powered (as well as the battery charger for the motorcycle battery). Seems safe for the line workers and simple enough to build assuming the inverter will be fooled.

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I would get a licensed electrician to do this. –  Brian Carlton Nov 14 '12 at 23:02
    
Reasonable advice though I want to understand the solution before I allow someone to come in and implement it. –  Ram Nov 14 '12 at 23:10
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really a -1 on the questionand no reason in the comment - bad form no? –  Ram Nov 14 '12 at 23:22
    
Anti-islanding is mandated by regulations and switches which allow islanding need regulatory approval. Islanding detection can be complex and hard to fool with a "non stiff" reference source. –  Russell McMahon Jul 3 at 16:24
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4 Answers 4

If your inverter shuts down completely when there is no line power, it is probably not capable of putting out AC without AC present. Some inverters assume the power line is essentially a 0 Ω source. They look at the voltage and decide what current to dump onto it. These types of inverters don't actually synthesize the 60 Hz themselves. They count on that being a input.

If you want to be self-powered during a blackout, you have to get the right inverter for that, which probably costs more. Then the inverter also has to deal with not being able to hold up the line to the proper voltage, and what to do about it. It gets complicated, which is why many inveters require the voltage waveform to already be there.

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Thanks for that insight Olin. What am I looking for in the inverter specifications to see if that is the case? [It's an Aurora PVI-6000 with all the docs online power-one.com/renewable-energy/products/solar/string-inverters/…. –  Ram Nov 15 '12 at 0:57
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If your inverter is currently grid-tied, I don't think the anti-islanding feature is something you'll be able to defeat, since it's integral to the safety approval of the inverter (and is a gross violation of most electrical codes)

What it sounds like you need is a separate, off-grid inverter with an automatic transfer switch, that will keep the inverter isolated from the mains when not in use but power your loads after the mains drop out (with some small delay, of course). These schemes generally also involve battery banks to stabilize / provide at least some power when clouds or other solar obstructions occur.

I cannot say for sure if the particular model you've targeted can operate in off-grid mode. (People I know who have this model use it grid-tied.)

Considering the costs involved with a second inverter, you may be better off just going with a gasoline-fired generator for your backup power.

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The problem with gas fired is I still have to dump substantial $. I was hoping I could do something more clever along the lines of : 1) manually throw knife switch to disconnect grid from my home. 2) turn on little liar circuit that puts out a drip of 240vac (powered by say a motor cycle batter) to make my inverter think the grid is live and then uses the inverter power feed to keep the 240vac liar circuit powered (as well as the battery charger for the motorcycle battery). Seems safe for the line workers and simple enough to build assuming the inverter will be fooled. –  Ram Nov 15 '12 at 18:36
    
Hard to say if the inverter would be fooled by that. The anti-islanding feature often minutely adjusts the inverter frequency (to move the phase minutely) to detect how the grid 'pulls' and 'pushes' on the inverter ("infinte sink" or "infinite source" depending on the phase relationship). If the pushing and pulling isn't what the normal grid would do, I doubt the inverter will enable. I don't think there's anything safe about a knife switch grid disconnect either, but I digress... –  Madmanguruman Nov 15 '12 at 19:04
    
The knife switch grid disconnect is mandatory for all PV and generator systems where I live (I would have thought it be be NEC) this is in addition to the main service breaker in my main panel. I guess I have some reading to do to figure out how this inverter behaves and how easy it is to fool –  Ram Nov 15 '12 at 19:48
    
I don't think a knife switch would fly in Ontario. –  Madmanguruman Nov 16 '12 at 2:22
    
I probably used that term in a less jargony way. I was thinking about my friend's model train set but I suspect the guts of this thing are the same... –  Ram Nov 16 '12 at 21:36
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You would need a separate UPS that only provides standby power to a load it can handle and deal with diverting the DC power from the PV to charge the UPS instead of the grid supply. So all the Statement of work is done at the DC side.

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I have an aurora 6000 and just spoke with their techs. What you want can Be done according to them. You must have a manual transfer for input And use code compliant disconnect for isolation from grid. Quality of Signal in voltage and frequency is critical as the aurora will Shut itself down even if Input signal is out of parameters. Definitely Get a qualified individual to help you design and inspect prior to commission

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This answer would be much better with proper spelling ("you" not "u") and more information besides a product recommendation. As it stands, it doesn't say much about electrical engineering. –  Phil Frost Jul 1 '13 at 18:27
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