3 Phase - various questions

[Mart]

Hi all.

I persuaded my sister to get 3phase installed a few years ago, as it was just across the lane, and they have a large property (combined with business), and a roof that was ideal for a huge 10kWp PV system.

So, was recently chatting about battery storage, which they'd like, but not yet, maybe next year.

Q1 - I understand that Tesla's solution is to stick a Powerwall on each phase. But I can see issues with that, as excess on one phase reaches the battery max, or demand is on another phase, where the battery is empty.

So, are there battery/inverters on the way, where all of the energy across the 3 phases is taken, and stored in one big pot, and can then be distributed to meet demand across all phases?


Q2 - An installer recently told me that if you have 3phase and a smart meter, then most(?) should net off leccy import/export. So for instance, if they are generating 6kW, with 2kW on each phase, and they have 6kW of demand on one phase, and none on the other two, then the export of 4kW (not being used), would balance out the import of 4kW on the phase in use. Does that sound correct, reasonable, rational?


Will probbaly think of other Q's but that'll do for now. Thanks.

[dan_b]

Q1 - there are hybrid 3-phase solar inverters out there already - so you connect the DC solar and the DC battery into the inverter for generation, which then does its best to balance the production/export across all 3 phases in real time. SolarEdge and GivEnergy both make them, I'm sure others do.

Q2 - that sounds odd to me. We have a 3-phase SolarEdge system at work and an Octopus Smart Meter. Not sure how I would notice if it was exporting on one phase an importing on another - although the monitoring portal shows the output on each phase.

[jonc_uk]

Q1 - There are a number of 3-phase inverter/chargers around now - Sunsynk springs to mind. https://www.bimblesolar.com/sunsynk-three-phase

It can also be done with 3x single phase units connected to one battery stack (but you will need 3x battery communication outputs - easy when you have loads of batteries, not when you only have a few) - I would not recommend this method these days. I previously ran 4x Sofar ME3000SP over 3 phases but have now switched to 3x SMA Sunny Islands.

Q2 - I _think_ net-metering is standard. Certainly now I have the Sunny Islands, it seems they only work this way. Whereas my old setup would typically show zeros on each of the 3 phases, the new one looks like this every day:



but showing the net only:



.. and of course, Octopus confirms the readings are net metered:

[Paul_F]

Q2 - An installer recently told me that if you have 3phase and a smart meter, then most(?) should net off leccy import/export. So for instance, if they are generating 6kW, with 2kW on each phase, and they have 6kW of demand on one phase, and none on the other two, then the export of 4kW (not being used), would balance out the import of 4kW on the phase in use. Does that sound correct, reasonable, rational?

I'd be shocked if it didn't - anybody on 3-phase will also have significant loads which only use 1 phase. Normal practice is to share loads across the 3 phases, but you're never going to be perfect. If it only measured 1 of the 3 phases, you could work out which one that was and put all your loads on the other 2 phases!

[Mart]

Q2. Thanks guys, good to know it should be net metered.

May sound silly, but as I encouraged them to get 3phase (it cost about £2k), I was worried about potential negatives. One thought was if granny charging a BEV (in the future), and generating 3kW, then if each phase was getting 1kW, it could impact leccy bills. I shall share the good news with them. The same could apply to a 7kW BEV charger, though it would probably be better to get a 3phase charger anyway.

Don't know if anyone else recalls this, but I seem to remember a member on an older forum once saying that they had 3phase, but ran the house mainly on one phase, but there PV install was on a different phase. So PV exporting, whilst home/bill importing. This was about 10yrs(?) ago, and I'm certain was a mechanical meter. Funnt how things stick in the mind sometimes ...... though there is equally as much chance I'm totally mis-remembering.

Just had another thought - they still hope to get a 3phase ASHP to replace their oil boiler, but some concerns about the ability to get hot water to the far opposite end of the property (16m away). A suggestion was for an electric power shower, so net metering would help with that too (if PV generating).


Q1. Hadn't thought of DC side batts to be honest. That would mean replacing the inverter, but that just balances out against needing one for the AC side batt system. They also have an enormous loft, with many external walls, that are now internal (due to extensions) for wall mounted, or installing a strong point for floor mounted batts shouldn't be hard either, though getting them up there will be fun.



Looking good, and thanks again for the thoughts and comments.

[John_S]

Whilst it would be nice to think that there should be net metering across the phases of a 3 phase meter, I don't think that is how it works in practice, nor should it. We reluctantly accept that there is a cost of providing a grid supply and we cannot expect to pay nowt if import = export, although with Octopus Intelligent Go and export we could actually make money at the moment. The same applies to 3 phase netting.

I recall a few years ago reading of this issue in Australia with consumers expecting net metering across the phases and the DNOs/suppliers saying that is not how it works.

In the UK street supply is 3 phase with every third property generally being on a different single phase. This should ensure that the draw on each phase is well balanced given enough properties downstream of the sub-station.

However on a 3 phase supply, a lazy installer might connect all or most of the load onto one phase with perhaps a big intermittent load on another phase. Only if the loads was randomly connected to different phases, ie not most of the load to brown and the intermittent load to black and nothing to grey in every situation, would there be a chance of loads evening out. Adding export would exasperate the situation.

Thus, I sympathise with DNOs wanting no net metering and shifting the issue of balancing the phases back to the installer/consumer.

It should also be remembered that 3 phase can operate in either delta (without a neutral) or star/Y (with a neutral) configuration. Whilst the final leg from the substation might have a neutral, the high voltage legs back to the power station won't and thus the DNOs need to ensure that the phases are properly balanced at all stages. As I understand it, this is the biggest driver of DNOs limiting export as the distribution system cannot cope with unbalanced loads.

In conclusion, don't assume that net metering across phases exists, or if it presently does (like the old meters that could go backwards), the situation will continue forever. Ensure that the phases are balanced as much as possible and that the batteries and their inverter/charger can balance this out optimally.

[AlBargey]

An expensive but completely adjustable and programmable option is 3 x Victron inverters set up as 3 phase.

It can be setup for phase compensation (Net Metering) or individual phase regulation:

https://www.victronenergy.com/media/pg/ ... ation.html

[Paul_F]

In the UK street supply is 3 phase with every third property generally being on a different single phase. This should ensure that the draw on each phase is well balanced given enough properties downstream of the sub-station.

However on a 3 phase supply, a lazy installer might connect all or most of the load onto one phase with perhaps a big intermittent load on another phase. Only if the loads was randomly connected to different phases, ie not most of the load to brown and the intermittent load to black and nothing to grey in every situation, would there be a chance of loads evening out. Adding export would exasperate the situation.

Thus, I sympathise with DNOs wanting no net metering and shifting the issue of balancing the phases back to the installer/consumer.

Reality is that this isn't an issue with a large enough population: not only does the loading average out across the phases, but there is a strong time averaging as well (After Diversity Maximum Demand) which means you can plug a much larger load into the grid than it ever needs to supply.

It should also be remembered that 3 phase can operate in either delta (without a neutral) or star/Y (with a neutral) configuration. Whilst the final leg from the substation might have a neutral, the high voltage legs back to the power station won't and thus the DNOs need to ensure that the phases are properly balanced at all stages. As I understand it, this is the biggest driver of DNOs limiting export as the distribution system cannot cope with unbalanced loads.

Doesn't work like that: the Neutral is there to provide a voltage reference so the individual phases all have the same voltage when there are unbalanced phase currents.
Issue with unbalanced loads is that it reduces the efficiency of the generator and distribution system (losses go with the square of current) and means you need to over-size it to cope with the largest single phase current. Since there is essentially zero cost to the DNO from requiring loads to be more or less balanced by their customers, that's the way they'll go.

In conclusion, don't assume that net metering across phases exists, or if it presently does (like the old meters that could go backwards), the situation will continue forever. Ensure that the phases are balanced as much as possible and that the batteries and their inverter/charger can balance this out optimally.

I'd have to see your Australian example, but my suspicion is that this is at the root of it - someone wanted to dump a load of power onto a single phase and get credit for it on the other two, and was told that wasn't allowed. They may then have blamed the metering situation rather than the grid rules for it.