Solar power during a power cut - could this work?

[EightyTwo]

I have been musing, especially as the Scottish Islands have 17,000 people without power in the middle of the winter...

I have a dozen solar panels on the roof of my house and they are capable of generating a couple of kilowatts. As I understand it, the inverter will not supply any 240Vac until it has got itself synchronised with the mains (good idea!!). When there's a powercut, the inverter will not provide any 240Vac at all. Would it be possible to isolate the grid at the main switch in the CU and, using a length of flex with a three pin plugtop on each end (I know, I know...) plug a cheap 12V inverter into a socket. The cheap inverter could run from a 12V battery (I have alarm batteries) and this would provide a crappy 240Vac signal for the solar inverter to synchronise. I'd then have upto a couple of kilowatts (on a good day) from an alarm battery!! Enough to run the gas boiler and the fridge-freezer...

My main concern is that the solar inverter will produce a voltage higher than the pseudo-mains coming from the cheap inverter and this might kill the latter. There's also the question of whether the solar inverter will sync to the cheapy at all.

Any thoughts?

 

[ban-all-sheds]

I have been musing, especially as the Scottish Islands have 17,000 people without power in the middle of the winter...

Buy a generator with a diesel engine which will also run on kerosene, heating oil...

(BTW - Winter doesn't start for another 11 days.... )

using a length of flex with a three pin plugtop on each end (I know, I know...)

Well if you know, don't do it!

plug a cheap 12V inverter into a socket. The cheap inverter could run from a 12V battery (I have alarm batteries) and this would provide a crappy 240Vac signal for the solar inverter to synchronise.

Speak to Eric about the perils of quite good inverters, let alone cheap crappy ones.

Your solar inverter will be designed to look for a genuine mains supply - you might find that the expensive blue smoke escapes if you give it a poorly regulated non-sine wave or modified sine-wave supply.

I'd then have upto a couple of kilowatts (on a good day) from an alarm battery!!

It would be an extraordinarily good day indeed if you could get 167A out of a 12V alarm battery.

Or are you hoping that the solar panels will power the boiler and F/F reliably at the times of year when power outages are common?

Any thoughts?

Sounds like a pretty flaky way to arrange an off-grid standby supply.

 

[Robin0577]

What he said....

Plus...

If you're going to use a generator, use it as an independent supply rather than trying to get the solar inverter to sync to it for the extra couple of kilowatts. I suspect they won't like each other. I suspect they will fight. I don't know which will win but I suspect it will be messy and blue smoke laden, especially if the generator has electronic voltage regulation.

 

[bernardgreen]

The solar powered invertor doesn't have to synchronise to any mains network source unless it is connected to and can supply power into that network ( feed in ).

Provided the output from the invertor is TOTALLY isolated from the network supply ( and any other invertor) then it can run "free" and supply the house with power.

So if solar is going to be used to supply a house during a power cut fit an isolation switch ( and find out how to make the invertor run "free"

 

[JohnW2]

The solar powered invertor doesn't have to synchronise to any mains network source unless it is connected to and can supply power into that network ( feed in ). ... Provided the output from the invertor is TOTALLY isolated from the network supply ( and any other invertor) then it can run "free" and supply the house with power. ... So if solar is going to be used to supply a house during a power cut fit an isolation switch ( and find out how to make the invertor run "free"

Whilst it would obviously be technically possible to get the inverter to 'run free' (even if it involved a can opener!) is it actually allowed that these things can have a means whereby a normal user can get them into a 'running free' mode? It sounds all a bit dicey to me!

Kind Regards, John

 

[Robin0577]

Whilst it would obviously be technically possible to get the inverter to 'run free' (even if it involved a can opener!) is it actually allowed that these things can have a means whereby a normal user can get them into a 'running free' mode? It sounds all a bit dicey to me!

Kind Regards, John

I think not, I think grid tie inverters are deliberately designed in such a way as to make it very difficult to get them to run free, requiring significant re-engineering and/or re-writing of software. I also believe that such modifications would void any approvals for connecting them to the grid.

 

[bernardgreen]

Some of the early Feed In invertors were free running invertors modified to ensure they could not run without there being a network supply.

The somewhat greedy people who take advantage of the high price paid for the Feed In tariff have to accept the down side. No mains .. no power even if the sun is shining brightly

While the less greedy who don't take the Feed In tariff can run their appliances during a day light power cut.

 

[JohnD]

The solar powered invertor doesn't have to synchronise to any mains network source unless it is connected to and can supply power into that network ( feed in ).

You would however have to buy an "off-grid" inverter.

The inverters widely sold in the UK for ordinary domestic PV installations synchronise to the grid, and turn themselves off if there is no incoming AC supply. So they will not work in a power cut, or if there is no mains supply. You are not allowed to connect an ordinary generator, or an independent inverter, so that it can backfeed the network otherwise.

Generator installations that are not intended for feed-in have a changeover switch so that the network and the generator can never be connected. This is a requirement of the Electricity Act.

 

[leeco]

I set up a system on my boat but I cost me about £2k.

A sterling combi pro s 3000w inverter 3 stage charger which you can plug mains 240v to charge a bank of batteries up that cost £800.

750w of panels and a outback fm80 MPPT controller which cost me over £1000 but the controller is top notch.

A bank of 6v trogon batteries giving me a 600ah bank at 12v.

The solar at this time of year isn't giving me a great deal so I either plug into my moorings 240v of if out on the cut I use my genny with the sterling combi.

 

[EightyTwo]

Thanks Guys - that's where my thinking had got to, too. Letting the magic smoke out of my solar inverter would be a bad idea - it's so hard to get it back in!

I'm way too mean to buy a generator or an off-grid inverter and changeover switch. It just seems a shame that I have all this generating capacity and can't use it when I need it most. Still, the 50p a unit I get when it is running makes the pill sweeter to swallow.

 

[JohnW2]

Whilst it would obviously be technically possible to get the inverter to 'run free' (even if it involved a can opener!) is it actually allowed that these things can have a means whereby a normal user can get them into a 'running free' mode? It sounds all a bit dicey to me!

I think not, I think grid tie inverters are deliberately designed in such a way as to make it very difficult to get them to run free, requiring significant re-engineering and/or re-writing of software. I also believe that such modifications would void any approvals for connecting them to the grid.

Indeed - that's what I had always assumed. Anything else would be plain dangerous!

Kind Regards, John

 

[ban-all-sheds]

It just seems a shame that I have all this generating capacity and can't use it when I need it most. Still, the 50p a unit I get when it is running makes the pill sweeter to swallow.

The somewhat greedy people who take advantage of the high price paid for the Feed In tariff have to accept the down side. No mains .. no power even if the sun is shining brightly.

Ever thought where the money comes from which allows an electricity company to buy your electricity at 4x what they can sell it for?

 

[winston1]

using a length of flex with a three pin plugtop on each end (I know, I know...)

I know what you know, but what you don't seem to know is that they are plugs, not plug tops.

 

[JohnD]

sadly, instead of just killing you, they might kill someone more valuable.

 

[Lectrician]

Domestic self contained inverters would not be able to have the shutdown protection enabled without hacking, and this would void all approvals.

Larger sites with larger outputs do have "free run" outputs, which do syncronise to the mains, but do not shut down when the mains vanishes. In order to have these connected to the mains supply, all suppliers will force the installation of a G59 protection relay, which will monitor the grid and shutdown the generation should it vanish.

A site we maintain has a consumption of around 300-400kVA, and a turbine with an output of 800kVA (which is ramped back a little to satisfy the requirements for the FITs payments - as you go larger, your payments get smaller). The turbine for the most part will supply the site and feed the grid, the consumption from the grid over a month being very low. When there is a power cut however, the turbines G59 relay opens. (a 11kV RMU is tripped open). The turbine then sees no load and shuts down after a time. The site therefore still has to maintain a backup generator. Trying to explain this to the MD was quite a task. When power is restored, the 11kV RMU is electrically switched, the turbine sees a supply/load and begins to generate again.

There is one huge windfarm near me of such a size that it is controlled by the national grid remotely, and is treated as "a power station". It doesn't have G59 protection - Its output is controlled.