Re: [LUG] Mains power control and measurement.

On Mon, 4 Mar 2019 at 11:47, Gordon Henderson <gordon+lug@xxxxxxxxxx> wrote:

On Sat, 2 Mar 2019, Tom via list wrote:

> It seems that FIT is no longer paid for new PV so electricity companies will
> be getting free power on sunny days. As such I'm wondering how to stop this
> happening and this of course comes with a more generalised power control (you
> want to turn on your immersion and/or storage heaters when you are not using
> all you generate. I've had a look around and there are devices that come
> close but they seem ludicrously expensive. Anyone got any experience of these
> at all - I was thinking of making something with a PiZero to opensource to
> try and stop this madness!

This is a solved solution - there are power switches used in e.g. data
centers that will take 2 feeds in and produce one out. (Search for
Automatic Transfer Switch) If one input fails then they automatically
switch over to the other, so make the PV power source the primary and the
incoming mains as the secondary.

Same idea as automatic generator backup too.

These switches are not cheap - especially as the amps increase, however it
might give you something to go on.

That doesn't solve your wastage issue (where waste here is defined as
excess PV that you can't use or sell).

Also feed PV into a battery then into the switch. The downside here is
that the switch is working at 230v, so there will be a small inefficiency
in the conversion from PV DC to 230v AC, but you need to do that anyway.
That also gives you a whole house UPS, assuming you have enough storage -
lead acid is still cost effective if you have the space for them, same for
NiFe cells.

The switch also provides the isolation needed to stop you back-feeding
your PV into the grid. This is essential when the mains incoming fails -
e.g. in a deliberate situation when people are working on it....

And you don't even need a Pi to do this, although a good MPPT solar
controller to do the battery charge and feed to the inverter would be
good.

If you want to DIY the transfer switch then (much better than a Pi for
many reasons) you could use an ATmega (Arduino) and a great big 2-pole, 2
throw relay/contactor to switch from one to the other. The down-side there
is that you'll lose a few cycles during the changeover period - that may
be acceptable though. You power the ATmega from the inside power via it's
own small supercapacitor or LiPo cell with the normally closed position of
the contactor being the incoming mains.

Or, a fix for this might be to put a double conversion mode UPS unit after
the switch to soak up the transfer time. A down-side then is that the feed
from the mains or solar would need to be double if trying to charge flat
batterys after an extended outage (or you accept a much longer charge
time). Double conversion units ar not cheap, nor are they that efficient -
I used a 2KVa one at home for a while some years back and it idled at 60w
with no load )-:

I'd stick to little UPSs on sensitive kit on the inside.

Gordon--
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