3 phase planning

[joth]

57 minutes ago, JSHarris said:

Bear in mind that smart three phase meters will almost certainly record kVA, rather than kWh.  I strongly suspect that what you hope for, the offsetting of current export in lightly loaded phases against current import on a loaded phase, will be (quite reasonably) penalised by the supplier (as the local grid has to be reinforced to allow for imbalance).  Single phase meters (at the moment) only measure (for billing purposes) kWh, not kVA (although some can record kVA).  Three phase supplies for non-domestic use have pretty much always been billed in kVA, hence the reason that most industrial three phase users incorporate some form of PF management.

 

Politically it'd be very tough to impose  kVA metering on domestic 3ph users as part of the already widely disliked smart meter upgrade. And by the same token why not do single phase users too?

 

Also I think there's a large risk of unintended consequences here: the world you describe of penalties for anyone using 3ph in a non-perfectly balanced way will force everyone to stay on (or request a downgrade to) single phase. If the goal is balancing demand as the energy market electrifies, they'd be better off doing everything to encourage 3ph installs in homes even if only the occasional appliance (e.g. the heat pump) would use it at first.

 

Anyway, back to my previous question - do you have any source for information on how 3 phase smart meter will work, or is it educated guesses? 

Thanks.

 

Edited August 12, 2019 by joth

[joth]

This page also confirms via experimentation that existing (non-smart) 3ph meters only charge the net import, in fact explaining it in terms of the 1Wh bucket the same as a single phase meter uses to smooth over import/export fluctuations

https://openenergymonitor.org/forum-archive/node/1613.html

 

I.e. each phase contributes into a single shared by bucket

 

I'm going to assume the statement upthread that phases are metered independently only applies to commercial users (which will potentially also be liable to kVA and phase balancing metering too)

 

 

Edited August 13, 2019 by joth

[Jeremy Harris]

I think it's worth examining this from the suppliers perspective, bearing in mind that all "smart" meters have the ability to bill in KVARh (it's something that's on the wish list of suppliers to implement already, has been for some time).  Suppliers buy in wholesale energy on the half hourly, 24 hour buy ahead, auction.  They pay around 5p/kWh or so on average for it (varies a great deal, January this year averaged about 6.255p/kWh, by April this had dropped to about 3.55p/kWh). 

 

A domestic three phase supply is exactly the same to a supplier as three separate domestic single phase supplies.  Why should a supplier be happy with a domestic installation that consumes energy on one phase whilst exporting energy on the other two phases, offsetting export against total consumption and which really means the supplier giving the consumer energy on the loaded phase for free?  It's a bit like me asking if my neighbour's bill can be reduced by the amount we export, on the basis that the cable that feeds both of us (on different phases) is supplying less net power across all phases.

 

It may well be that domestic three phase kWh net metering may possibly exist for a time, but frankly I can't see the suppliers putting up with it for long.  Sooner or later they will want to address the imbalance, if only to keep the network operators happy.

[ProDave]

This is an interesting discussion.

 

Your typical 3 phase meter has just 1 dial used to take readings and produce your bill (talking ordinary meter here not smart meter)

 

So I assume there are 3 current transformers and the electronics of 3 separate meters and they are summed to give the total reading.

 

One would have thought in a modern meter, each sensor only meters and counts import.  So if 1 phase was exporting, that would not reduce the import reading, it would just be ignored, just as it is on a single phase meter.  The export might be stored in an internal register but not normally displayed on the meter.

 

[AliG]

I just started looking at this thread as I have three phases and have been checking out the documentation on my PV install.

 

I hadn't looked at it before and I wasn't at home when it was installed and signed off.

 

It looks like they have used three single phase inverters.

 

Looking at this test certificate, would it be safe to assume that they are attached to a different phase each? 

 

 

On the other hand, this suggests that they are all on the same phase, but I think he maybe just wrote in the wrong boxes. You shouldn't install 5kW on one phase.

 

 

Anyway, the question I have is regarding a PV diverter. I had asked for one to be installed, but it wasn't.

 

Looking now, though, it seems that none of these are 3 phase. Theoretically I would need to install 3. Or can I try and figure out which phase has the lowest load on it and just install one on that phase?

 

 

Edited August 13, 2019 by AliG

[ProDave]

The first certificate suggests it is one inverter per phase, with 10A MCB's.

 

The second which I assume is the DNO notification seems to be a work of fiction. They won't all be on the same phase, and if I am reading that correctly two are 2KW inverters and one is a 1.5Kw inverter, so that one will have a different power surely?

 

It will be interesting to see if the DNO raises any questions about the notification.  They did with ours as they saw the inverter had "4000" in it's model number and assumed it was 4Kw output and thus over 3.68Kw. Only solved when I forwarded the manufacturers declaration that it was limited to 3.68Kw.

[joth]

21 hours ago, AliG said:

 

Looking now, though, it seems that none of these are 3 phase. Theoretically I would need to install 3. Or can I try and figure out which phase has the lowest load on it and just install one on that phase?

 

Great! Do you already have the immersion heater? (Or, a standard kettle) and is the system already commissioned?

If so you can answer this question for both of us by waiting until a sunny day (>3kW generation), and turning on the 3kW single-phase source, and seeing if your import meter increases or decreases. You'd need to run it for >20mins to see 1kWh of usage. 

 

I presume you also have an export meter? In which case it'd be equally useful to know how much that increases by. 

[AliG]

@joth I am afraid that I do not have an export meter, just a generation meter.

 

However, my electricity meter in an Elster A1140 which is a "smart meter" and can measure export. I am not sure it is set up to do this. It was installed before the solar panels.

 

When I get home at the weekend I will see if I can try this. The meter is quite user unfriendly, it is a hassle just to get a reading from it.

 

We have enormous electricity usage in the house, I am fairly certain the meter never goes backwards even if I am exporting power. 

 

It seems that under the system that is supposed to replace FiT payments we will be paid for actual export, but the metering system is not yet in place.

 

If I was paid for export then there would be no point in having a PV diverter, it would be better to get the export payments and use gas to heat hot water.

[joth]

@AliG ah without an export meter it'd be very tricky to do any useful measurement unless you can remove all the other load (e.g. trip all breakers briefly).

 

Btw 3x inverters could be ab expensive way to do this, one 3-phase inverter is less than the price of 2 single phase inverters, in the quotes we got

 

The A1140 is the one I pasted a link to install manual for upthread. Useful to know that they're used by some suppliers, can I ask which co. installed it for you? 

although there's about 8 different ways they can be configured so it depends on the specifics of the install.

Indeed, I spoke to new connections team at two suppliers yesterday and they were both hesitant to make any statements about how a 3phase meter gets installed other than say it's a specialist job, takes longer, costs more, and they often have to go back and fix it when they discover it's been configured wrong or running backwards or whatever. The smaller supplier actually recommended going with a Big 6 supplier to have it installed, then switch to then after it's in.

None has any timeline for availability of SMETS2 polyphase meters, but at least when they do eventually become available the metering is well defined in Part C of the spec: it will indeed sum usage across phases prior to metering so no penalty for exporting on 2 phases while importing on a third. 

 

Also speaking to DNO they are totally fine with installing 3ph to the cutout and actually said they don't care one jot how we load the phases - we could entirely not use 1 or 2 or them, that's regularly what they see with London houses split into flats. 

 

 

[AliG]

I think EON was my original supplier who installed the meter. There don't appear to have been any issues with it.

 

The PV quote was based on three phase and I expected one inverter. Looking on line, one 5kW three phase inverter is about £120 cheaper than the three inverters, so I suspect they just had them available.

 

The 1.5kW inverter is odd, although the spec says it will accept 1.8kW of DC input, but with a max output of 1.5kW. It theoretically reduces my possible output, but I would need to know how often I will get the maximum output from the panels. It this is say 10% of the time over a year then I have lost less than 0.5% of potential output so I probably won't argue about it.

[Dan F]

@joth  Did you every to the bottom of the 3ph net metering thing?

 

We're at the stage where we are trying to work out if we should go with single phase or three phase, but there are no clear answers about how UK 3 phase smart metering works, or will work.  The Telsa Powerwall Gateway also uses the approach described in the Sunny video you shared, using current meters on all three phases, but unsure if this is supported/enabled in the U.K.

 

Reading the manual you shared:

 

Quote

Registration of Quantities Total import and total export quantities are registered separately. The measurement discrimination is such that, as the power factor of any load from 0.05Ib to Imax is varied over 360°, the import and export registers will never advance together. The resolution of registration is 1mWh

kVAh:  Note: Real and reactive energy for each phase is respectively summated prior to kVAh calculation.

 

This seems to say that there is no net metering between import/export, but the import/export across phases is netted out. (unless I'm reading it wrong).

 

Edited May 6, 2020 by Dan Feist

[joth]

11 minutes ago, Dan Feist said:

 

This seems to say that there is no net metering between import/export, but the import/export across phases is netted out. (unless I'm reading it wrong).

I wouldn't read too much into that manual, it's for one particular (albeit popular) manufacturers meter that is used in many countries and for commercial as well as domestic installs, hence supports many different metering modes.

 

I found the SMETSv2 polyphase spec easier and more trustworthy, as described here

(Tip: search for polyphase for more discussion on it)

 

Snag is no-one makes / supplies those meters yet, and looks like they may never do so, but AIUI existing non-smart domestic installs are expected to work the same way. 

Personally for a new install I'd plan to install 3ph to the cut out but plan to put a single phase meter on it, unless something truly demanded 3ph from day 1.

 

Edited May 6, 2020 by joth

[Dan F]

1 minute ago, joth said:

Personally for a new install I'd plan to install 3ph to the cut out but plan to put a single phase meter on it,

 

That's exactly what we already have (just need to get the SMART meter installed)!  The old bungalow we demolished already had it..

 

We have no immediate need for 3 phase (apart from faster charging of a future EV), so really just trying to work this out from a PV planning perspective.  A three phase inverter probably makes the most sense given the size of the array (even though the DNO will apparently let us export 10kW), but we wouldn't want to use a three phase supply/inverter and for this to significantly impact self-consumpton.

 

None of the PV suppliers I've talked to really know much about any of this at all ?

[joth]

9 hours ago, Dan Feist said:

 

None of the PV suppliers I've talked to really know much about any of this at all ?

Our PV installer told us to ask an electrician. I asked a few, and the DNO and three different suppliers (ovo, BG, Bulb) and none of them knew. Ovo was perhaps most enlightening as the customer services guy said he had no idea how 3ph export metering was supposed to work, but that they must be complicated as he fields a lot of calls getting an engineer out to put it right.

The DNO confirmed that for residential installs they have no expectation or requirement of the usage being balanced a across phases, and billing (inc penalties for unbalanced use) is not in their control.

 

If you go 3ph, one mitigation would be to sign up to the Octopus Tesla energy plan, as that charges the same for import and export so the metering is then moot. 

 

9 hours ago, Dan Feist said:

A three phase inverter probably makes the most sense given the size of the array

 

Definitely would make sense if the suppliers vaguely supported it!

 

We're having 8kW PV and originally thought a 3ph inverter would be marginally cheaper vs 2 single ph (but not by enough to offset having the new supply installed), but we're having solaredge optimisers anyway and their new optimizer-optimized inverters will go up to 10kW on a single phase in a single box and actually works out cheaper than the 3ph one.

 

On the plus side: inverters only last a decade or so, so you should have plenty of opportunity to change your mind over the lifetime of the house.

Edited May 7, 2020 by joth

[Dan F]

Another interesting element to the net metering across phases is that, in theory (based on Sonnen/Tesla documentation) you can connect a battery to a single phase and it can compensate for usage on other phases by exporting.  What this means is you can effectively, in an indirect way, use a single phase battery for consumption across three phases without import costs!  There are a number of caveats to this though:

 

- This won't work when there is a power cut (so everything you want backup for should be on phase that the battery is on)

- Noone has any concrete proof that:
   i) UK 3-phase SMART meters will work in this way (although everything seems to point to the fact they will given the documentation that @joth referenced).

   ii) It's unclear if this battery "phase compensation" approach, supported by Sonnen/Tesla is enabled/supported/allowed in the U.K.

 

If this does work and is supported it would be the preffered approach for us I think, as there would then be no downside and we'd have 3-phase for a car-charger, could potentially use a 3-phase ASHP and I think I read somewhere that having a 3-phase 13A system, is probably preferred to a 40amp single-phase system (impact on voltage?).

 

This is some (U.K) Sonnon documentation which mentions the battery compensation: https://midsummerwholesale.co.uk/pdfs/sonnen-net-metering-technical-note.pdf

 

Edited May 7, 2020 by Dan Feist

[joth]

5 minutes ago, Dan Feist said:

This is some (U.K) Sonnon documentation which mentions the battery compensation: https://midsummerwholesale.co.uk/pdfs/sonnen-net-metering-technical-note.pdf

 

 Very useful link. Do you have the Tesla equivelent? 

"This will only work when the customer has a three-phase meter capable of Net metering. This can be checked with the customer electricity supply company."  if only it were possible in practice! But it does seem to suport the general assumption.

For more motivation: Germany has a lot of domestic 3ph, and all the German PV and battery manufacturers seem to assume domestic installs have net metering and the SMETS2 spec is based on some EU conventions which Germany surely had a hand in writing, hence why I feel this is the longer term trend even if it will take a decade or 3 for UK to converge towards it.

 

 

[Dan F]

56 minutes ago, joth said:

Very useful link. Do you have the Tesla equivelent? 

 

This isn't offical, and the guys not even an electrician, but it does explain how this would work in theory and matches the Sonnon documentation.

https://www.youtube.com/watch?v=YDFfqqlAJEw

 

Also, this is someone in South Africa that see this behaviour: https://teslamotorsclub.com/tmc/threads/battery-self-consumption-across-all-three-phases-with-1p-pv.185180/

 

[Benjseb]

Just to add...

 

We have 3ph power with 3 ph solar

 

We are billed for import currently even when we’re exporting on 2 phases

 

i emailed our supplier Bulb who finally, after 2 months, confirmed our meter was t set up correctly to bill for net import

 

This is the sticky point, they know it’s not right but think the only way to fix it is with a smart meter. And there’s not many 3ph smart meters being installed. 
 

Ofgem have advised that you can get a normal 3 phase meter that’s suitable we may need to move to a big 6 supplier to get it!

 

so looks like we’ll have to go back to the dark side to get a meter fitted then switch back 

[Dan F]

18 hours ago, Benjseb said:

We are billed for import currently even when we’re exporting on 2 phases

 

i emailed our supplier Bulb who finally, after 2 months, confirmed our meter was t set up correctly to bill for net import

 

This is the sticky point, they know it’s not right but think the only way to fix it is with a smart meter. And there’s not many 3ph smart meters being installed.

 

So the issue you have is that you are exporting, but given you don't have a smart meter this is being ignored.  Have you been told that exporting will be counted when you have a smart meter, or specifically that import/export across phases will be netted out once you have a smart meter?

 

Out of interest, how much PV do you have across the phase?

 

 

[Dan F]

So, a PV supplier/installer in our area just confirmed that they have personal expeirence of:

- Net metetering across three-phases.

- Powerwall exporting to compensate for consumption on other phaaes and this working in the U.K.

 

This doesn't mean 3 phase wiring or 3 phase inverter necesarily makes sense for us (given we've been told we can export 10kW), but is good to know.  In fact we could upgrade to three phase supply in the future, but still use single phase inverter (if our array isn't  larger than 15kW) without an issue and a 3-phase car charger on a bight summers day would still utilize PV fully via the 3-phase net metering.

Edited May 12, 2020 by Dan Feist

[Benjseb]

Yes in Theory it works but good luck finding an elec company that will supply the correct meter. They seem pretty clueless! 

[Dan F]

47 minutes ago, Benjseb said:

Yes in Theory it works but good luck finding an elec company that will supply the correct meter. They seem pretty clueless! 

 

Agree, but this time next year hopefully these meters are avaialble.  So it's still worth understanding from a planning perspective.

 

Did you go for 3-phase inverter because you weren't allowed to install/export much on a single phase, or for some other reason?

 

 

[joth]

12 hours ago, Dan Feist said:

Agree, but this time next year hopefully these meters are avaialble. 

Is that based on any inside info, or just expression of hope?

 

Not sure when it was first published but the SMETS2 spec was had its last round of revisions in 2014, yet in 2020 still no-one is making/installing polyphase meters to that spec, and with the backlash against smart meters in general I don't hold much hope for them focusing on 3ph anytime soon...

[Dan F]

1 hour ago, joth said:

Is that based on any inside info, or just expression of hope?

 

Expression of hope, more than anything else.  But I think there is some kind of legislation about smart meters being used everywhere by end of 2020.

 

I think we have a plan though for our build:

- Move our existing 3-phase connection to garage and get our curent single-phase meter upgraded to a smart meter.

- Wire the house on a single phase (unless loading calculations show this is an issue, but it shouldn't be).

- Use a single phase 10kW inverter, which will allow for oversizing the array to up 15.5kW of installed capacity across east/south/west roofs (there isn't room for much more anyway unless we go for more expensive 360W+ panels, and 15kw is a lot).

- If required we can upgrade to 3-phase further down the line if/when fast car charging is important. At this point we wouldn't actually need to do anything to PV as this would export generation on phase 1 and could be consumed by a car-chager on phases 1/2/3.

- The only potential issue with staying with single-phase is if we decide a second battery makes sense, as it's unlikely the DNO would allow 10kw + 2 batteries on a single phase.

 

Also there is one downside to a three-phase inverter; unless you have an ac-coupled battery on each phase the inverter will switch off when there is a power-cut or you go "off-grid".  With a single-phase inverter on a three phase supply you don't have this issue.

Edited May 13, 2020 by Dan Feist

[Dan F]

So Octopus have now confirmed that:

 

1) They are installing 3-phase smart meters, but this is paused due to COVID-19.

2) Metering is "vector sum" across all three phases. (This confirms what @joth concluded from his investigation, and means imports/exports on different phases at the same time are netted out.

 

Based on this, siwtched back to 3-phase now rather than upgrade later, as it seems there is nothing to loose and we already have a 3-phase supply on-site.  Need to do load-calcs for the house, but this will probably still use the a single primary phase for house consumption.

 

 

Edited May 19, 2020 by Dan Feist