single-phase battery back-up on 3-phase residential supplied DB

[mtkhan]

Hello All. My first post.

What are the considerations I should keep in mind if I'd like to create one phase of my 3-phase munic supplied DB as a "back-up" phase, let's say it's the blue phase. My idea is to get my DB re-configured such that all critical loads are placed on the blue phase, while all non-essential and large loads e.g. geyser, stove, etc stay on the white and red phases. I know the trick is to for the electrician to balance the loads as best as possible. 

If I then get a single-phase battery inverter/charger installed with the inverter's AC-out connected onto the blue/back-up phase, and the inverter's AC-in coming from the white phase, will I effectively be bridging the 2 phases together when the inverter is working in bypass mode during normal munic supply conditions? My concern is then how do you prevent these 2 phases not being bridged at the munic supply side? Do you need to disconnect the blue phase supply at the main breaker, and only allow all grid supply to the blue-phase during normal operation to come via the munic supplied white phase through the inverter, obviously keeping in mind not to overload the white phase then?

thanks in advance

[Ingo]

I did exactly the same a few years ago. I initially had 3P installed from the electricity supplier even though I didn't have any 3P devices, essentially I had three seperate 63A circuits into my house for this exact reason. To do what you are trying now I had to convert my 3P feed to a 1P feed @80A. On the DB side they just bridged all three phases with a link so I still had 3 feeds into the house with a combined Max of 80A. They also installed an 80A MCB but left the 3P 63A unit to bridge the circuits.

When I eventually added my back-up solution I moved all my heavy loads EG Geyser, Pool etc to my third 'phase' while leaving the other two bridged and feeding the rest of the house. The Inverter/Charger then supplied power to the original P1 & P2 with P3 connected to the AC Out 2. This one disconnects when running on battery while AC Out 1 provides power to critical loads.

Things to keep in mind when you do this:

1. Make sure the utility provider infrastructure is capable of handling 80A or unless you stick to 63A then you should be Ok.

2. They will charge you a fee for converting the 3P to 1P, be prepared for that. Mine wasn't much if I remember correctly but it was a good few years ago.

3. Have no 3P devices connected at all - ever.

4. EL must be single phase. In my case I had three separate units and not a combined 3P unit as I understand they look at phase load distribution as well (can't confirm).

5. Make double sure your feed to the electrical supplier does not exceed your contracted rate, match the MCB equal to the supplied current. With this I mean, in my case I could oversubscribe the supplier if I draw 30A on each phase totalling 90A. I have a 80A MCB before it enters my Main DB to prevent me from ever going over the limit.

6. If you have Home Automation devices (I'm talking Clipsal now), make sure to heed the specs to not have the power of a Dimmer unit on one 'phase' and the lights it controls on the other. Simply put, put Lights on P1 and have Dimmer power on P1 as well, same goes for P2 if you have a dimmer and lights on P2.

I am sure that today there are more/better ways to do what I did a while ago.

Ingo

 

[Coulomb]

7 hours ago, mtkhan said:

If I then get a single-phase battery inverter/charger installed with the inverter's AC-out connected onto the blue/back-up phase, and the inverter's AC-in coming from the white phase, will I effectively be bridging the 2 phases together when the inverter is working in bypass mode during normal munic supply conditions?

I don't think that this will ever happen, because as far as I know you never connect utility active to both an input and an output from either a hybrid inverter, and certainly not to an "off-grid" inverter. So in your scenario, you would not be connecting the blue phase to anything. (But I'm far less familiar with hybrid inverters than the so-called off-grid ones.)

If instead of the white phase, you fed the AC in to the inverter from the blue phase, and only had loads that formerly ran on the blue phase connected to the inverter's AC out, using either an off-grid or a hybrid inverter, that would be fine. You would then effectively have two separate systems; one (red and white phases) is utility only, and the other (on the blue phase) is a standard single phase system that has no connection to the other system other than via the neutrals. It would be like a standard single (blue) phase system with single phase neighbours on the red and white phases.

In any event, you certainly want to avoid connecting two phases to an inverter intended for single phase operation. That can only lead to trouble.

I note that your proposed system, if it has solar panels to minimise total power consumption, will lighten the load on the blue phase, which will unbalance your phases somewhat. I don't think this is a huge deal, but it will increase the current in your neutral conductor back to the utility. It may be that the neutral conductor has a smaller cross-section, and therefore will be able to carry less current than the phase actives. You'll likely have plenty of reserve, but depending on the situation, it might be something to consider.

[mtkhan]

Firstly, really impressed by the insight on my first post on this forum, and and time taken by you both to respond to my query. Thank you.

23 hours ago, Ingo said:

 To do what you are trying now I had to convert my 3P feed to a 1P feed @80A.

Ingo, regarding above, what does this entail both administratively and physically? Did you have to submit some applicaiton form to your munic and then they sent technicians out to your place and replace meter with a single-phase one? And then physically did the munic simply disconnect 2 phases and allow only one phase supply from meter to your main DB?

 

Thanks both for all the other tips, will surely factor into the planning.

 

 

[Ingo]

Quote

Ingo, regarding above, what does this entail both administratively and physically? Did you have to submit some applicaiton form to your munic and then they sent technicians out to your place and replace meter with a single-phase one? And then physically did the munic simply disconnect 2 phases and allow only one phase supply from meter to your main DB?

Yes, my provider (not municipality) did come out and they did all that you mentioned above. I am not sure how the process will work for your municipality but my provider buys from Eskom and then on-sells to us. My first step was a simple email to them with the 3P to 1P question and when they agreed the process started.