Re: Renewables in power cuts

In message <pan.2005.10.01.16.00.21.496215@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>, Andy Baxter <news4@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> writes
John Beardmore said: 

A mains connected inverter must match the frequency and phase of the
mains supply to deliver energy and avoid damage, i.e. it must be a
synchronous inverter.  Further, to meet UK specifications, it must
shutdown if the grid fails, if only to avoid scenarios like putting
power out onto a line when it has been shut down for maintenance.  The
specifications G57, G77 or G83 as above etc, are pretty much cast in
stone.  The inverter manufacture has little scope for whimsy or
innovation.


Yes, but it would be technically possible to have the inverter disconnect
from the grid, but continue to supply current to the house from a battery
bank (maybe with a very short break when the grid came up again while it
resynchronised with the grid phase).

Yes - it's possible to have it manage the switch over, but I don't think there's any kit that can do that legally in the UK.

I think Trace (now Xantrex ?) did something like this for the US market place, but it's more complex than it sounds as the inverter may not have the capacity / storage to run the whole house anyway.


Effectively what these
manufacturers have chosen to do is use the grid as the battery bank,

You can see it that way from an 'island' perspective, but from a grid perspective, it just makes the site look like a smaller load or a small generator.


and
count on the fact that in most places it's pretty rare to have a 100%
power cut. I.e. they've gone for a simpler spec to make the whole thing
cheaper, which is what I meant - the reasons are more economic and
regulatory than technical.

I disagree, though in a sense it depends on the objectives of the end user.

Batteries have an enormous environmental foot print. If you want to save the world they are a very poor tool compared with connection to a local power grid. There are huge advantages to eliminating the use of batteries.

Automatic switch over to isolated async operation from any batteries you happen to have isn't necessarily simple because you need to manage the load to make sure that its one the inverter can handle, and that the highest priority devices continue to get power.

Probably simplest and cheapest to use the G83 (synchronous whatever) inverter to dump any excess renewable energy into the grid, and have a UPS with big enough batteries to run critical loads to keep the things you really need running during outages. Charge the UPS batteries directly from your renewable sources if possible to keep things efficient ?

I looked into this myself a few years ago after a "wrong kind of snow" event precipitated a 10 day power cut, but although I build an enormous UPS with many kWh capacity, the batteries have all long since sulphated.

Is this sort of thing worth it at a domestic level ? I'll tell you next time we have a 10 day power cut...


Cheers, J/.
--
John Beardmore
.

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