isolating mains

[Dan21]

hi

 

New here to the forum..

 

I am looking for a way to automatically isolate the mains if I have a power cut so that the hybrid inverter doesn't leak back into the grid.

 

Is it a simple as a 100amp relay on the incoming mains?

[ProDave]

You are over thinking it.  A grid tied inverter is designed to shut down when it loses it's grid connection.

[Dan21]

thanks for the reply.

 

Yes a grid tied but a hybrid with battery backup will continue to produce power and if the incoming mains is not disconnected power could go back through the grid.

 

[Rob99]

No. Hybrid inverter will disconnect itself from the grid automatically when it loses power and will only reconnect when the grid connection is restored. They are designed this way and only approved for connection to the grid on this basis. 

[SuperJohnG]

I know what you mean, Givenergy instructions give you the pros and cons. E.g. 

 

Doc also attached. 

 

 

EPS Configuration v2.pdf

[Dillsue]

11 hours ago, Dan21 said:

Yes a grid tied but a hybrid with battery backup will continue to produce power and if the incoming mains is not disconnected power could go back through the grid.

It will(should) only produce power on the back up/UPS/EPS AC output. The AC connection to the grid will shutdown if the grid supply is lost.

[Dan21]

OK, I get that, but that would mean you only have limited power, say to a power socket, if you want the whole house to work you have to disconnect the incoming main with an ATS or a manual one.

[ProDave]

33 minutes ago, Dan21 said:

OK, I get that, but that would mean you only have limited power, say to a power socket, if you want the whole house to work you have to disconnect the incoming main with an ATS or a manual one.

And it still would not produce any power as it has no grid to synchronise to.

 

You are just limited to the low power UPS output.

[Dan21]

Is there no way around this?

 

[Dan21]

I mean, am I really over thinking this, I cant actually remember the last time we had a power cut !!

 

I just  like to look at every option before design.

 

If I were to put a ADS on the incoming mains is there any way of getting the inverter to believe there is power there?

 

Is it possible to use the APS which the inverter will switch to in a power cut and connecting it back to the inverter??

 

I am looking at the Growatt SPH5000, thoughts on that too.

 

Many thanks

[SuperJohnG]

10 hours ago, ProDave said:

And it still would not produce any power as it has no grid to synchronise to.

 

You are just limited to the low power UPS output.

It will via the battery. 

 

It's also limited by the battery output and storage, its not necessary a low power only output.

[Dillsue]

20 hours ago, Dan21 said:

Is there no way around this?

 

I believe there's plenty of systems that will island the house and give several kw of power to the house. I've no personal experience but beleive tesla powerwall does it using their gateway, solaredge now support whole house backup and I think there's lots of others. Whatever you do make sure is certified for UK grid connection and from a reputable manufacturerer- you don't want to be powering the grid when a linesman is trying to fix a fault!

[SteamyTea]

1 minute ago, Dillsue said:

you don't want to be powering the grid when a linesman is trying to fix a fault

Or worse, powering your neighbours house as they run the tumble dryer on a sunny day.

[RichardL]

What do you actually 'need' in a power-cut,  
Understood - depends on your local situation -  how often you get them and how long for etc..

Freezers will last a fair while if sensible about opening them,
Cooking can be put off usually until power comes back/live on cold snacks
....

Think it through and work out if a full blown house disconnect and backup is worth it, vs (in my case) an off the shelf UPS which keeps the internet router up for work for a couple of hours and laptop/phone batteries maintaining a similar uptime.

 

Medical equipment accepted - I'd assume that sort of kit has its own local backup option rather than relying on a whole house/site solution?

[TonyT]

You could have a an essential consumer unit fed from the UPS circuit?

 

 

[Alan Ambrose]

Anyone else think that this 'using your battery as UPS' subject needs properly designing and sorting out so it becomes the standard UK set-up?

 

Something like a separate CU or 'high integrity CU' with a separate section for freezers, lighting, fire alarms, IT equipment etc that will be powered additionally by the battery and inverter when the main supply goes out.

 

Also, the standard 'islanding' thing with its 'NE bonding relay' is a kludge. If we can't depend on the imported main earth, then let's just always use a local one just like a lot of countries do - we often need one for car chargers anyway.

 

And it would also get rid of all the 'PEN fault detection' nonsense. When PME was designed to save the costs of a conductor, probably nobody guessed what the modern day PV, battery and car charger situation would look like.

 

Yes, I appreciate that this needs a proper local low impedance local earth connection, not any old earth rod banged into the ground.

Edited March 23 by Alan Ambrose

[Mattg4321]

PME has the downsides you’ve listed, but it’s also a very cost effective way of achieving very low Ze. 
 

If we went down the route of TT earthing everywhere, it would then be routine to rely on the operation of RCD’s for fault protection. I come across countless RCD’s not operating properly/at all every year. I’ve only very rarely come across a lost PEN conductor.

 

Both situations dangerous. TN-S earthing is probably the best, but not done anymore. This can fail too though. A supplementary earth rod to TN-C-S (PME) systems looks likely to be brought in and would be a good idea imo. 
 

FYI neutral/earth bond relays are needed to give neutral a reference to the earth. Without it an RCD will not operate correctly. It’s a slightly separate issue to loss of PEN conductor, which won’t usually matter when in islanding mode as grid is switched out.