How to operate a grid-tie inverter w/o a grid?

[dan_aka_jack]

Is it possible to persuade a grid-tie inverter to run if there's no grid? 

As I understand it, all grid-tie inverters are programmed to shut down if there's a powercut (to protect the engineers as they try to get the mains working).  Is there a way to get a grid-tie inverter to run even when there's no grid?  For example, could I some how trick it into believing that the grid is still live by using a small battery-powered inverter?  Would the grid-tie inverter damage the battery-powered inverter?

Many thanks,
Jack

[RichardKB]

I would think that as this is a legal requirement it can not be easily overcome, trying to trick it with a small inverter would most likely end in tears.

The small inverter would have to be a true sine wave inverter and if the grid tie is delivering more power than the power required by your system i.e. you have no load or only a couple of cfl's running the small inverter would be asked to absorb the load from the grid tie.

Because I believe a grid tie inverter supplies all the power that is going into it from its source so if your source is providing 300W then the grid tie inverter will supply 300W minus losses to its output.

So I think the answer is probably no.

Rich

[dan_aka_jack]

Thanks for the reply.

[Ted]

There is the SMA Islanding setup that delivers what you suggest (as a hybrid system with batteries and grid-tie) but it is definitely not something that you could cobble together on a shoestring budget.

[Alan]

You can synchronise an off grid system if say the solar inverter say ( 500 watts ) is very small in comparison to the largest generator on your private grid ( say 7.5 K.W. diesel )  Small to large means no significant circulating current in the off grid system, which in turn means no significant hunting ( Under speed / over speed governor protection required at the diesel generator )

If the items to be synchronised were roughly the same size say ( 7.5 K.W. ) solar cell and  say ( 7.5 K.W. ) diesel the problems of compensating for say clouds blocking the sun light to the solar cell system would cause very large current but low voltage differences on your grid system. ( Circulating currents, very undesirable. ) because the mechanical changes required to correct the engine speed are very slow in comparison to the electrical change. ( Electric moves at 186000 miles per second compared to the milliseconds required for a mechanical lever to change the engine speed )

In an off grid environment it is much more straight forward to use a common D.C. system for joining various things together, control wise very straight forward compared to an A.C. system.

As per your original post there would be two lots of conversion efficiency loss. One amount in your grid inverter and one amount in your sine wave inverter.  These could be overcome by keeping the distribution grid system at a D.C.

Much better if the grid is available to connect all your various generating things to it and use what energy you require in the normal way. ( what ever you generate in comparison to the grid capacity is very insignificant there fore circulating currents are also very insignificant. )

Regards

Alan

[Paulh_Boats]

Dan,

If the grid goes down the law says you cannot put power into it .... so you MUST switch the fridge/freezer etc over to a second supply. You can do that with a big switch manually or automatically.

The second supply could be a sine wave inverter (cheapish), a big capacitor/battery and DC charger fed from PV or whatever. You don't need a big expensive battery pack if you want to use the power immediatly..and you know the grid will be back up usually within 8 hours. Also I cannot remember when we last had a power cut that caused us any problems...so for me its not worth worrying about.

So there really is no point in making the grid-tie invertor work off grid.

Maybe for your video editing a UPS or a battery permanently on mains trickle charge (1 or 2 watts) to run a laptop for 8 hours is all you will ever need? Or simply pick up the phone and tout for new business for 8 hours? 

[dan_aka_jack]

Hi folks,

Thanks loads for the replies... very interesting stuff.

I guess my best option is to start my solar PV install with:

~ 250W of PV + a grid-tie inverter

Then, a little later, I can think about buying an array of batteries + charger + sinewave inverter if it looks like the grid will start failing.  Sure, the grid has served us well up until now but I'm getting increasingly worried about the UK's energy security.

[Paulh_Boats]

Dan,

One very simple contingency is a long 12V extension lead to plug into your car cigar lighter and a 12V adaptor for your laptop. For the investment of a few quid you can run your laptop for another 5 hours, depending on the state of the car battery.

-Paul

[billi]

or like others said get a small honda inverter generator.....