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Grid tie PV in a blackout

 
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Tarrel



Joined: 29 Nov 2011
Posts: 2447
Location: Ross-shire, Scotland

PostPosted: Wed Jan 08, 2014 6:42 pm    Post subject: Reply with quote

A technical question that hopefully someone can answer:

If I have a grid-tied solar PV system and we experience a power cut, the grid-tie inverter's "islanding" system will kick in and shut down the inverter to protect the line workers trying to repair the grid. All those lovely solar panels doing f**k all when I need them most. Sad

Now, if I had a deep cycle battery, kept charged through a mains charger, feeding into my household circuits through an inverter, would this be enough to trick the grid-tie inverter into thinking grid power was still present and avoid it shutting down?

So, procedure in event of power cut would be:
- Switch off mains switch to isolate house from grid
- Feed power into household circuits from battery/inverter (using a "male to male" mains lead going from inverter to any 13A socket)
- Enjoy power to household circuits provided by the solar panels via the grid tie inverter, which would be working because it senses power in the circuits.

Any thoughts?
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RenewableCandy



Joined: 12 Sep 2007
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PostPosted: Wed Jan 08, 2014 9:01 pm    Post subject: Reply with quote

It sounds like a theoretical possibility.

However there are also inverters for boats, that have a "ship" (power island) and a "shore" (getting welly from Mains) setting: I wonder if buying one of these and plugging it in during power cuts would also work...would rely on connector types being compatible.

I worry about male to male connectors. It means you've got mains voltage on the metal prongs. Very good for "accidentally" getting rid of unwanted housemates but otherwise...erm...
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Mean Mr Mustard



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PostPosted: Wed Jan 08, 2014 9:09 pm    Post subject: Reply with quote

Seem to recall Adam described them as ' suicide leads' when describing cretinous activity during an extended outage at Dartford... Rolling Eyes
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adam2
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PostPosted: Thu Jan 09, 2014 9:44 am    Post subject: Reply with quote

Cant recomend it, and it probably wont work anyway.
A grid tie inverter senses the presence of the mains and wont run without this, it is unlikely that a basic power inverter will produce a good enough qaulity output to "fool" the grid tie inverter.

Also the design of the grid tie inverter assumes that the grid can take all the power produced (up to the maximum output of the grid tie inverter) A stand alone battery inverter can only supply power, not absorb it, unlike the grid.
What would happen if the grid tie inverter was producing 2,000 watts, but the load suddenly dropped to say 1,000 watts ? At the very least the grid tie inverter would trip, and I suspect that something expensive would break.

And yes I advise strongly against the use of cable with a male plug on each end, they are called suicide leads for a reason.
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Tarrel



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PostPosted: Thu Jan 09, 2014 6:29 pm    Post subject: Reply with quote

I'm take it as a "no" then! Very Happy
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adam2
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PostPosted: Fri Jan 10, 2014 12:21 pm    Post subject: Reply with quote

Yes.
In a short term or rota power cut then the options regarding electricity are
1, manage without
2, use a generator
3, use a battery bank, with or without a power inverter.

In case of a long term or TEOTWAWKI situation then it could be worth re-purposing the modules of the grid tied array for battery charging. This is somewhat innvolved and would breach any waranty, therefore not worth it for short term breakdowns or rota cuts.
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Pepperman



Joined: 10 Oct 2010
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PostPosted: Fri Jan 10, 2014 3:06 pm    Post subject: Reply with quote

RenewableCandy wrote:
It sounds like a theoretical possibility.

However there are also inverters for boats, that have a "ship" (power island) and a "shore" (getting welly from Mains) setting: I wonder if buying one of these and plugging it in during power cuts would also work...would rely on connector types being compatible.

I worry about male to male connectors. It means you've got mains voltage on the metal prongs. Very good for "accidentally" getting rid of unwanted housemates but otherwise...erm...


I live on a canal boat and we have:

- a charger/inverter which can take AC power either from a shore line (if we are on a proper mooring) or the generator (if we're cruising). This has a DC output which is connected to our leisure batteries and an AC output which powers the 230V circuits
- our main engine also has an alternator which charges the batteries while we're moving
- our PV array is connected via a separate MPPT controller to our leisure batteries and does not go via the charger/inverter
- I haven't come across a charger/inverter that can take an AC connection from the grid and a DC connection from solar panels but there might be something out there.
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Pepperman



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PostPosted: Fri Jan 10, 2014 3:08 pm    Post subject: Reply with quote

Actually, here's one that can do everything
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Tarrel



Joined: 29 Nov 2011
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Location: Ross-shire, Scotland

PostPosted: Fri Jan 10, 2014 3:26 pm    Post subject: Reply with quote

adam2 wrote:
Yes.
In a short term or rota power cut then the options regarding electricity are
1, manage without
2, use a generator
3, use a battery bank, with or without a power inverter.

In case of a long term or TEOTWAWKI situation then it could be worth re-purposing the modules of the grid tied array for battery charging. This is somewhat innvolved and would breach any waranty, therefore not worth it for short term breakdowns or rota cuts.


I must say, our main strategy is, and has been, No. 1 (manage without). However, it would be useful in a long-term blackout situation to be able to charge the chainsaw and keep a freezer going. Short term or rota blackouts are most likely to occur in winter, as a result of poor weather or excess demand. Where we live, outside temperatures are at a point where food that would normally be kept in a fridge can be kept outside.

My original plan was to produce a modest DIY grid-tie system and forego the feed-in tariff. We'd still get a reasonable return on the investment, as it is likely we would use most of the power generated. I would build this around 12V panels, connected in strings, so they could be readily re-purposed to charge 12V batteries in a long-term power down. I think I'll stick with that plan, and have a simple "Batteries+charger+inverter" set up to provide critical power for the short-term.

(Pepperman, that equipment you linked to looks awesome, but I shudder to think of the cost!)
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PS_RalphW



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PostPosted: Fri Jan 10, 2014 3:28 pm    Post subject: Reply with quote

That pdf is unreadable on my browser (firefox). One of the fonts is missing.
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Pepperman



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PostPosted: Fri Jan 10, 2014 3:32 pm    Post subject: Reply with quote

http://www.jaycar.com.au/pt_richelec_offgrid.asp
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mikepepler
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Joined: 24 Nov 2005
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Location: Rye, UK

PostPosted: Fri Jan 10, 2014 5:46 pm    Post subject: Reply with quote

Tarrel wrote:
adam2 wrote:
Yes.
In a short term or rota power cut then the options regarding electricity are
1, manage without
2, use a generator
3, use a battery bank, with or without a power inverter.

In case of a long term or TEOTWAWKI situation then it could be worth re-purposing the modules of the grid tied array for battery charging. This is somewhat innvolved and would breach any waranty, therefore not worth it for short term breakdowns or rota cuts.


I must say, our main strategy is, and has been, No. 1 (manage without). However, it would be useful in a long-term blackout situation to be able to charge the chainsaw and keep a freezer going. Short term or rota blackouts are most likely to occur in winter, as a result of poor weather or excess demand. Where we live, outside temperatures are at a point where food that would normally be kept in a fridge can be kept outside.

My original plan was to produce a modest DIY grid-tie system and forego the feed-in tariff. We'd still get a reasonable return on the investment, as it is likely we would use most of the power generated. I would build this around 12V panels, connected in strings, so they could be readily re-purposed to charge 12V batteries in a long-term power down. I think I'll stick with that plan, and have a simple "Batteries+charger+inverter" set up to provide critical power for the short-term.

(Pepperman, that equipment you linked to looks awesome, but I shudder to think of the cost!)

If you use a charge controller that does MPPT, then you don't have to stick with 12V panels. I'm using panels with a MPP of about 30V to run my 12V batteries through a Morningstar TriStar MPPT 60A. Higher voltage is more efficient for a given cable size, and more importantly gives you a much wider (cheaper) range of panels.

I'm struggling with the opposite problem to you, as my system is off grid already, and I don't have a use for the excess power in the summer. (can't easily feed it into the grid)

If I was starting from scratch, and had plenty to spend, I'd either do:
- grid tied system with inverter/charger and battery - SMA and Studer make systems that can do this, but it adds £2-3k to the grid tied system.
- normal grid tied system with batteries on float charge, with a off-grid charge controller that could handle the panels as well (the one I have can take 150V input). Then I'd do a manual switchover in a power cut.

I guess the ideal is the first of the above options, but with some clever stuff to use the batteries a bit at night to maximise your own use of the PV output. But that would probably cost even more!
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