Solar PV on a three phase system - how should it be working?

Ashworks

Hi All,

We recently had 7Kw solar panels, a Lux Squirrel Pod and two batteries (total of 6.4 Kw) fitted to our new house, which is on a three phase system due to 1960s underfloor electric heating (ouch). The Pod and batteries are connected to Phase 1, which supplies the main house circuits while phases 2 and 3 supply the heating. We are planning on decommissioning the heating this year and replacing with electric radiators, connected to the same two phases.

When we commissioned the system, we were under the impression that it would route the generated power to the phases according to demand. However it has become apparent that it is actually splitting the power equally across the phases, which means that when the heating is not on we are discharging two thirds of the power straight back to the grid.

The solar company have agreed it has been installed incorrectly, and are suggesting they set it up so that when the heating is off, the surplus power feeds the batteries, so we would only discharge to the mains once they are full. This sounds reasonable but it is not how we understood that solar PV with a 3 phase system would work. 

Are we wrong to think it should it be possible to have a system which routes the power according to need?

I hope this makes sense - all advice gratefully received!

Ectophile

Your first paragraph seems to contradicting the second one.  Is the inverter connected to phase 1 only, or to all three phases?  Most domestic-sized inverters are single phase only.

tacpot12

Is it possible that you have three Squirrel Pod Inverters? Often multiple single phases inverters can be linked to produce three phase power, and it sounds like this what you have and your installer is suggesting that the link between them is broken, so that each inverter syncs to the single-phase power it sees, and supplies the load it sees (or charges the battery if there is no load, until the batteries are full, at which point the eneration is exported). 

I think if the heating is off, this would mean that two of the inverters would charge the battery, while the third would pull power from the Battery DC bus for the single phase that the rest of the home uses (as well as using its own PV string). So the third inverter would have access to power from the PV panels connected to the other two inverters. 

You are lucky having a three-phase power supply at home, as you are well set up for EV charging and a heat pump.

Ashworks

Ectophile said:

Your first paragraph seems to contradicting the second one.  Is the inverter connected to phase 1 only, or to all three phases?  Most domestic-sized inverters are single phase only.

We have a three phase inverter connected to all three phases, which splits the power equally across them. We then have a squirrel pod and batteries connected to the first phase only.

This is because the heating would flatten the batteries straight away and cause them to wear out quickly. It is VERY power hungry!

The supplier is suggesting that surplus power from phases 2 and 3 could also go into the battery if they connected it differently. We could then use this power on Phase 1.

The question is whether (instead) it is possible for the three phase inverter to route the power flexibly across the phases according to need, rather than splitting it equally three ways.

I hope this is clearer - it's not a straightforward system!

CSI_Yorkshire

That entirely depends on the inverter.  Ask the manufacturer.

Standard practice is to balance equally across all three phases, which it seems is how yours is at the moment.

Ashworks

CSI_Yorkshire said:

That entirely depends on the inverter.  Ask the manufacturer.

Standard practice is to balance equally across all three phases, which it seems is how yours is at the moment.

Thank you, that's helpful to know. 

ComicGeek

The 3 phase inverter will spread it equally across the 3 phases to balance the load, due to it being 400V rather than 230V. That's why we look to spread building loads across the 3 phases, rather than creating unbalanced situations like yours.

You would need to check that your meter reads net metering, ie that it works out what is being consumed and exported on each phase and calculates the net. You may need to change to a smart meter if you don't already have one. 

The setup sounds strange to me though - the time when the PVs are producing their highest (both instantaneous and daily) generation is when the electric heating is not going to be operating. Where on earth are they thinking this circa 50% of generation is going to be used in this current setup?

The surplus power from the 2 heating phases should be connected back into the battery. The batteries should be set to not discharge when you're using heating - you would have to do this via the schedule on the battery inverter, and could be annoying having to change this daily/weekly to suit heating usage. The battery inverter output is going to have a limit though, depending on inverter size, so isn't going to instantly drain the batteries. 

You really should be considering heat pump technology rather than electric radiators - you may not find much running cost difference with electric radiators, don't believe the nonsense spouted by companies like Fischer etc.

And what's the plan for the excess PV generation in summer that you can't use or store? Do you have a good export tariff? I have 4kWp PV and 20 kWh batteries, 2 EVs and air con units, but still have periods on a sunny day when I can't use it all! You have double the PV output, 1/3rd of the storage, your battery is going to be full by mid morning.

Ashworks

Thanks ComicGeek, thats really helpful. 

markin

Electric radiators won't be much different in cost than the underfloor heating, You would need Modern night storage heaters on a TOU or E7 tariff to really save money, And many are replacing or supplementing them with 2,3 Mini split A/C units, Also known as Heat pumps!

You also want to move to Octopus energy for the best Export rates and Tariffs.

And as Comic points out, There's not a lot of solar production in Nov,Dec,Jan when you need heating.

chris_n

You would be better off with 3 single phase inverters connected to one set of batteries.