3 Phase Inverter or 2 x Single Phase...

[deanwilts]

Hi all,

I'm still in the process of working out the best design for my use case. I have 3 phase power to the house with 2 of those phases in use - 1 is for the electric underfloor heating and the other for the rest of the household. We're all electric (no oil or gas - rural location etc).

I've applied recently for planning for up to 30kW of ground mount PV array to go into a field. I haven't yet spoken with the DNO - my strategy is get planning first (or get refused/reduced) then hone the system into whatever fits into planning office and DNO requirements.

But I've started to think about inverters. Assuming I can get 30kW, I think I have two choices - buy a 30kW 3 phase inverter or get 2 x 15kW single phase inverters with one on each phase.

One of the decision factors will be if I can run all power into one phase - for example, in the winter I would want any solar generated going straight into the floor, so would want 100% going into that phase. Is this type of unbalanced loading possible with a 3ph inverter? Obviously during winter the output will be low, but I'd at least want to put all that I do have into the floor.

Does anyone have any experience of 3 phase inverters or doing a pair (or three) of single phase ones instead?

Ultimately I'd like to add a battery at some point and I assume with all options I would basically need a battery per phase? Which again would probably be focussed on heating first - in fact I probably wouldn't bother with more than the heating phase having battery supply.

I have no intention of selling power back to the grid and don't need that capability. I'll either use it or lose it.

Very grateful for any thoughts/experiences...

 

[binky]

do you have a 3phase meter if you are only using 2? Do you have multiple distribution boards?

 

[deanwilts]

Hi Binky,

I believe so yes. These are what the two outside boxes look like. On the meter I see references to L1, L2 and L3 and 3 sets of cables in/out.

Do you have multiple distribution boards?

Yes - two separate consumer units.

 

[binky]

proper 3 ohase setup then only 2 going into the house, bit of an add one. I take it you have 2 single phase boards inside?

 

[deanwilts]

Sorry to answer with photos again but I'm not an electrician so I don't know if I'd be answering correctly...

Looking at the main breakers at the bottom, I'd say quite possible that I do, although the warning label implies it's just connected to 1 ?

 

[deanwilts]

Actually, I just noticed the L1, L2, L3 horizontal labels - never noticed those before

So it looks like a 3 phase board maybe, but I was told by the electricians that did the build some years ago that the whole of this consumer unit is just connected to one phase.

This is the main household one, I have an identical board for the underfloor heating, which I was always told was on phase 2.

So I'm a little confused now. It's possible that there was a change of plan and actually I'm using all 3 phases here? EG based on the labelling, it looks like garage and gate (not used) are on phase 1, middle sockets and hob on phase 3?

 

[Andy™]

looks like a 3 phase board used as single phase, theres no L1 from the meter so cannot be 3 phase, and the MCB's are not split only to L2 or L3

 

[deanwilts]

looks like a 3 phase board used as single phase

That is exactly what I was told by the electrician that did the install.

bit of an add one

The property was fully refurbished in 2014 and when the underfloor heating was put in SSE said that the supply needed to be upgraded to be able to handle the new load. I'm not sure whether the use of 2 of the 3 phases was suggested by SSE or the electricians. It works fine and hasn't given us any trouble - it's just a little confusing now when it comes to researching solar PV and what I need to work with this.

If I don't have 3 phases coming in, does that mean I'm looking at 2 x single phase inverters? Do I have the option to just make the whole thing 3phase by bringing in the 3rd phase and re-configuring the two consumer units to be proper 3 phase units? Any advantages in doing that as part of the solar plan?

Thanks for the input - really helps me to get my head around it.

 

[kerching]

Well the yellow label says single phase so my years of experience coupled with the extrapolation of available data, Forurier analysis and Heavysides Unit step function ( if you know, you know ) draws me to,the ( possibly flawed) conclusion it's single phase
Just saying

 

[Andy™]

Well the yellow label says single phase so my years of experience coupled with the extrapolation of available data, Forurier analysis and Heavysides Unit step function ( if you know, you know ) draws me to,the ( possibly flawed) conclusion it's single phase
Just saying

3 phase meter and L1L2L3 is marked as being live on it so does look like 3 phase is there

 

[binky]

Looks like 3 phase head with two boards used on single phase, bit weird, but it happens. I don't suppose the underfloor heating board is located close to the house board?

Ideally you would get everything connected to a single 3 phase board, and the main board converted back to 3 phase, with the third phase connected to that board from the isolator. That will simplify the whole installation and allow use of a 3 phase inverter which in turn will give a balanced output across the phases. This will also help avoid melting the single neutral cable.

 

[deanwilts]

I don't suppose the underfloor heating board is located close to the house board?

Yes - it's right next to it.

Ideally you would get everything connected to a single 3 phase board, and the main board converted back to 3 phase, with the third phase connected to that board from the isolator. That will simplify the whole installation and allow use of a 3 phase inverter which in turn will give a balanced output across the phases. This will also help avoid melting the single neutral cable.

Yes - this makes perfect sense to me, and I'm a fan of making things simple.

If I had that done, is there a recommended inverter or brand of them that I could start looking at? I'd ideally like a battery at some point also, although this would likely be only on one of the phases (in the new single board I would probably put certain circuits on the phase with the battery, if that makes sense - like hot water, certain rooms for heating and maybe a couple of more utilised sockets like the kitchen).

 

[binky]

In that case I would get the board connected up as 3 phase as it looks like you have that upto the isolator ( confirm this with the DNO) incorporate all the cicuits into a single board and install a 3 phase inverter which will feed energy to all the cicuits without the need to piddle about trying to feed one phase or the other. Likewise get a 3 phase battery.

 

[deanwilts]

Thanks Binky.

Likewise get a 3 phase battery.

So something like this?



Do I need to consider/match the 30kW rating of the solar inverter with the battery inverter in any way?

I'm struggling to find a 3 phase battery inverter that's more than 10kW.

I'm eyeing up a Solis 30kW 3 phase solar inverter at the moment - looks like it has all I need, just not sure how I might approach the battery one.

I need to also be sure I can limit (to zero) what I send to the grid.

without the need to piddle about

I'm so on that page . I love keeping things simple - I don't mind spending a bit more money to achieve that.

 

[binky]

I quite like Solis, I've been fitting these for a few years now, albeit never used one of their larger units.

Battery inverter, 10 kw is going to run a lot, but bear in mind it could flatten a 10kw battery in an hour. Battery inverters come with an amp clamp or small meter type gadget to monitor energy flow. If it detects energy going to the grid, it wil redirect to the battery. It can be used to stop or limit export to the grid. Chances are you will need to set some form of export limitation for the panel inverter to meet DNO requirements. Have a read of the Solis manual, it has various settings to achieve different things know as work modes. Trying to get battery charged and ufh soaking up energy is not so easy, I think an Eddi may help with energy distribution. I suspect, mid winter you may have warmish floors and flat battery or vice versa. I tend to fit Solic immersion controllers to redirect energy to water, but haven't done much with batteries in this respect, so don't know what works best.

 

[deanwilts]

I suspect, mid winter you may have warmish floors and flat battery or vice versa.

Mid winter the focus would be on the floors and I'd be less concerned about having a charged battery.

Long term I'm hopeful that an energy supplier with a night rate tarrif will support 3 phase so I could charge the batteries at cheap rate overnight.

I'll have a read through the Solis manuals to confirm they can have zero export to grid. I think you're confirming that the battery inverter can be smaller than the PV inverter?