3 phase planning

[Jeremy Harris]

5 minutes ago, Ed Davies said:

If you have a three phase meter do exports on one phase (e.g., from PV) cancel imports on another? If not, as I suspect, then it's probably best that any loads likely to be running when any PV is generating be on the same phase as the PV.

 

 

No, they don't.  All three phases are separately metered, so you can be exporting on one phase and still importing on another. 

[ProDave]

6 minutes ago, JSHarris said:

 

 

No, they don't.  All three phases are separately metered, so you can be exporting on one phase and still importing on another. 

So if fitting single phase in the house and fitting solar pv, make sure the solar pv is connected to the same phase as the house loads.

[le-cerveau]

18 hours ago, DaveH said:

One thing I wasn't sure though - from a safety point of view are there any issues having multiple phases in one area? I have been advised so far to keep each floor on a separate phase.

 

Only if you cross connect between equipment.

[joth]

On 03/01/2019 at 16:15, ProDave said:

So if fitting single phase in the house and fitting solar pv, make sure the solar pv is connected to the same phase as the house loads.

 

So if fitting 3-phase inverter (and possibly batteries in future), presumably then the goal is to always split demand as evenly as possible  across phases? For summer cooling, this could almost become a reason to use a 3ph ASHP on its own.

 

I was hoping the balance of phases would be summed prior to metering, as stated here https://www.solarquotes.com.au/blog/3-phase-solar-what-you-need-to-know-about-connecting-solar-to-your-3-phase-supply/ (that's in Australia,  but it was the only reference I could find)

Hmmm.

 

[Jeremy Harris]

22 minutes ago, joth said:

 

So if fitting 3-phase inverter (and possibly batteries in future), presumably then the goal is to always split demand as evenly as possible  across phases? For summer cooling, this could almost become a reason to use a 3ph ASHP on its own.

 

I was hoping the balance of phases would be summed prior to metering, as stated here https://www.solarquotes.com.au/blog/3-phase-solar-what-you-need-to-know-about-connecting-solar-to-your-3-phase-supply/ (that's in Australia,  but it was the only reference I could find)

Hmmm.

 

 

 

The snag is that most of your domestic loads will probably be on a single phase, as it would be unusual for a house to need more than the ~23 kVA that a single phase supply can deliver.  Ideally you want your PV to be connected to the same phase as the household loads, so that any PV generation offsets normal use.  If PV is on 3 phase, then only around 1/3rd of the generated PV energy will be offset against the house domestic loads.

[Onoff]

17 minutes ago, JSHarris said:

 

 

The snag is that most of your domestic loads will probably be on a single phase, as it would be unusual for a house to need more than the ~23 kVA that a single phase supply can deliver.  Ideally you want your PV to be connected to the same phase as the household loads, so that any PV generation offsets normal use.  If PV is on 3 phase, then only around 1/3rd of the generated PV energy will be offset against the house domestic loads.

 

If you do have 3 phase coming in can't you fit a higher capacity PV system so as not to be limited to the <4kW? 

 

There was a while back a call by some of the DNOs to try and make 3 phase to all new homes mandatory. 

[Jeremy Harris]

7 minutes ago, Onoff said:

 

If you do have 3 phase coming in can't you fit a higher capacity PV system so as not to be limited to the <4kW? 

 

There was a while back a call by some of the DNOs to try and make 3 phase to all new homes mandatory. 

 

 

Sometimes, it depends on the local network.  I asked for the maximum the DNO would allow under G59 (as it was then) and they told me that I could go up to 10 kWp on our single phase supply.  We've got 6.25 kWp, as that was as much as I could fit on to the roof.  It's useful, though, as the spare capacity we have in our authorisation means that fitting a 3.6 kWp battery system stays within the existing approval (it's bizarre, but for some reason DNOs seem to assume that battery systems will export to the grid, even if they physically cannot).

[Ed Davies]

1 hour ago, Onoff said:

If you do have 3 phase coming in can't you fit a higher capacity PV system so as not to be limited to the <4kW?

 

Yes, the actual limit is 16 amps/phase so, so long as they're properly balanced across phases, you can go to ~11 kW without prior DNO approval.

 

1 hour ago, JSHarris said:

it's bizarre, but for some reason DNOs seem to assume that battery systems will export to the grid, even if they physically cannot

 

What stops your battery system from exporting to the grid?

[Jeremy Harris]

21 minutes ago, Ed Davies said:

 

What stops your battery system from exporting to the grid?

 

It measures import/export power at the incomer, and ramps the inverter power down so that it never exports. 

[ProDave]

2 hours ago, Ed Davies said:

 

Yes, the actual limit is 16 amps/phase so, so long as they're properly balanced across phases, you can go to ~11 kW without prior DNO approval.

 

 

What stops your battery system from exporting to the grid?

It depends on what you install the batteries for and how it is designed surely?

 

Most will install modest battery capacity in order to maximise PV self usage so store PV generation that you cannot use in real time, to be used later.

 

But larger, particularly storage using EV vehicle batteries is all being designed to export to the grid at peak demand times.

 

2 completely different systems for different reasons.

[Ed Davies]

3 hours ago, JSHarris said:

It measures import/export power at the incomer, and ramps the inverter power down so that it never exports. 

 

Right, so if there's any problem with that measurement it could export. DNOs only approved certain (one?) of the PV-to-hot-water diverter systems to limit larger than 3.68 kW PV systems to 16 amps/phase presumably because they had doubts about the robustness or fail-safe abilities of other systems.

 

1 hour ago, ProDave said:

It depends on what you install the batteries for and how it is designed surely?

 

Of course, I was asking @JSHarris about the specific system he had in mind that physically couldn't.

[Jeremy Harris]

Just now, Ed Davies said:

 

Right, so if there's any problem with that measurement it could export. DNOs only approved certain (one?) of the PV-to-hot-water diverter systems to limit larger than 3.68 kW PV systems to 16 amps/phase presumably because they had doubts about the robustness or fail-safe abilities of other systems.

 

 

The inverter carries the appropriate approval to ensure it cannot export, though.  Presumably if there is a problem with measuring the power at the incomer it does the same as the PV inverter does when that senses something like the loop impedance being above limits and just shuts down.

[Ed Davies]

5 minutes ago, JSHarris said:

The inverter carries the appropriate approval to ensure it cannot export, though.

 

Just to be clear, the inverter can produce more power than the approved amount of export power as long as the balance is taken as self-consumption (e.g., to the kettle, charging the car or house battery)? Interesting, I didn't realise any were like that.

 

Does it use a current-sense clamp or something more solidly wired in?

[ProDave]

1 minute ago, Ed Davies said:

 

Just to be clear, the inverter can produce more power than the approved amount of export power as long as the balance is taken as self-consumption (e.g., to the kettle, charging the car or house battery)? Interesting, I didn't realise any were like that.

 

Does it use a current-sense clamp or something more solidly wired in?

I thought you could only have an inverter capable of exceeding your export limit if you had an "approved" device to ensure the export limit was not exceeded.

 

My home made PV diverter would not count. If that failed, and nothing else at all in the house was consuming power, then I would export what the inverter produced.  Irelevant to me as my inverter limits the power to 16A.

 

Of course if I ever add batteries they would be "off grid" (cough)

[Jeremy Harris]

5 minutes ago, Ed Davies said:

 

Just to be clear, the inverter can produce more power than the approved amount of export power as long as the balance is taken as self-consumption (e.g., to the kettle, charging the car or house battery)? Interesting, I didn't realise any were like that.

 

Does it use a current-sense clamp or something more solidly wired in?

 

It uses 2 current transformers, one on the incoming supply and one on the PV inverter output.  The battery inverter is approved to not allow export to the grid, according to the paperwork, as it has an integral G100 export limiting relay.

[joth]

On 09/08/2019 at 12:32, JSHarris said:

 

 

Sometimes, it depends on the local network.  I asked for the maximum the DNO would allow under G59 (as it was then) and they told me that I could go up to 10 kWp on our single phase supply.  We've got 6.25 kWp, as that was as much as I could fit on to the roof.  It's useful, though, as the spare capacity we have in our authorisation means that fitting a 3.6 kWp battery system stays within the existing approval (it's bizarre, but for some reason DNOs seem to assume that battery systems will export to the grid, even if they physically cannot).

 

I didn't know / think of requesting "as much export as allowed". Our G99 was completed by the solar supplier and they've just approved the exact system designed (8kW) and not 1W more. The real pain is the approval is only valid for 90days, the lead time on manufacturing and supplying the exact dimension panels is longer than that! And we're months away from being able to measure up to place the order.

Hence my now looking into 3phase as a less bureaucratic plan B.

 

@JSHarris do you now have a battery storage or still in the planning stages? Is it AC or DC coupled? (To the existing invertor?)

It's interesting it needs to be figured into the total export allowance. That's another good future proofing reason to go with 3ph.

I'm actually wondering if a future 3ph battery storage could be used to optimize phases. Charge from or feed into each  phase independently to minimize thr export on each one, via a shared DC battery store, dynamically rebalancing them.

 

In lieu of that, ASHP, plant room (immersion element(s), washing machine), and perhaps the ovens and induction hob, would be candidates to shift to different phases. Ideally the ASHP could be 3ph but we only need 7kW so totally overkill. 

 

[Jeremy Harris]

Still in the pre-delivery stage for the battery system, but it will be AC coupled and limited by the equivalent of a G100 relay to prevent export, so it doesn't (as I understand it) need any specific additional DNO approval.

 

Three phase would be useful for a high power EV charge point, if you really believe that you will have a genuine need to regularly charge at home at around 88mph or so.  I can charge at 7 kW, but never do so, as I've not yet had a need to charge that fast, and charging at a lower power (typically around 2.5 kW to 4 kW) during the day means that charging is mostly free.  In the 8 months of so that I've owned an EV I've never had a need to fast charge either at home or at a destination, as the car invariably ends up sat at a destination charge point for several hours, often overnight, so 7 kW is more than enough.  An EV capable of a 250 mile journey would need a charge time of around 9 hours or so at 7 kW, which is generally fine for an overnight charge.  Also worth bearing in mind that not many EVs are capable of accepting a 22 kW AC charge.  Mine can only charge at 11 kW from a three phase 22 kW charge point, although it will charge at around 50 kW or so from a DC charge point.  The emphasis on car fast charging has shifted to DC, as the weight and volume of an onboard 22 kW AC three phase charger is a penalty that many manufacturers think is unacceptable, especially given that there are few 22 kW AC charge points around anyway.

 

Be extremely wary of running domestic loads on different phases, as there is a high peak voltage between phases (415 VAC).  For this reason, you would need to split out the house supply by physical area, rather than load, when arranging the single phase supplies from a three phase incomer.  Typically this would be done by floor, so the ground floor would be on one phase, the first floor on another, etc.  The snag is that this most probably won't come close to balancing the loads, as most of the high load stuff will probably be on the ground floor.

 

I'm personally far from convinced that three phase makes much sense, unless you have a very specific high power requirement that needs it.  The loss of offset from PV generation is a big negative - having a three phase connected PV system means you need to synchronise loads across all three phases in order to take advantage of the power being generated.  This is easy enough to do on a single phase supply, but not at all easy on three phase.  I would guess that you would end up "wasting" at least half, maybe more, of your generated power, and importing from the grid a great deal more to make up for this.

 

[scottishjohn]

will it really not depend on how big a PV array you have and if you intend to sell lots back to grid in the summer ?

that is only reason i can see for a 3 phase supply 

and at present rates that is a long term payback for that extra generation capacity 

 when batteries come down in price and you can be classed as a grid smoothing terminal --eg they buy it back from your batteries when needed to keep grid happy --

then maybe 

lots of  variables to work out to make that choice .

I have been getting quotes from PPA solar farm installers and the difference in how much per acre the different companies are quoting is wide for 25 acres --anything from 5mw to 7.5mw 

 and thats with them basically renting ground and just paying rental for it

 

 

[Ed Davies]

1 hour ago, JSHarris said:

Be extremely wary of running domestic loads on different phases, as there is a high peak voltage between phases (415 VAC).

 

Totally irrelevant but something I find curious: apparently in Norway 3-phase into the home is normal but rather than the star connection used in most countries it's Δ so 230 volts between phases. So you run your downstairs sockets between, say, L1 and L2, upstairs sockets between L2 and L3 and lighting between L3 and L1 or whatever. Neutral/earth floats around somewhere in the middle of the triangle depending on your distance from the substation and neighbour's leakages on each phase to ground. Seems a bit bizarre but it does give the copper-saving benefits of three phase without having such quite high potential differences in the house. Sort taking the 110-0-110 systems common in the US to their logical conclusion.

[ProDave]

1 hour ago, Ed Davies said:

 

Totally irrelevant but something I find curious: apparently in Norway 3-phase into the home is normal but rather than the star connection used in most countries it's Δ so 230 volts between phases. So you run your downstairs sockets between, say, L1 and L2, upstairs sockets between L2 and L3 and lighting between L3 and L1 or whatever. Neutral/earth floats around somewhere in the middle of the triangle depending on your distance from the substation and neighbour's leakages on each phase to ground. Seems a bit bizarre but it does give the copper-saving benefits of three phase without having such quite high potential differences in the house. Sort taking the 110-0-110 systems common in the US to their logical conclusion.

That will be DP circuit breakers and DP switching on everything including lights then? (it would if done like that in the UK)

[Ed Davies]

3 hours ago, ProDave said:

That will be DP circuit breakers and DP switching on everything including lights then? (it would if done like that in the UK)

 

I'd imagine so for circuit breakers and light switches. Don't know about socket switches; continental sockets tend not to be polarised so they're a lot more relaxed about the line/neutral distinction anyway and often don't switch the socket anyway.

[joth]

On 03/01/2019 at 16:08, JSHarris said:

 

 

No, they don't.  All three phases are separately metered, so you can be exporting on one phase and still importing on another. 

 

I've been researching more into this, and now wonder how you know this - I guess it's based on observing existing (non-smart) 3 phase meters?

For metering export, the long term answer will be up to whatever Smart meters will measure. But for all I can find no one yet will supply a 3-phase smart meter, so it's hard to have any empirical evidence how they will meter simultaneous import+export.

 

I'm starting to convince myself that smart meters might sum the phases before figuring a net import/export measure of usage, based on sources of varying provenance e.g.:
1/ "ted" at the end of this thread
2/ This australian(?) presenter from a German inverter manufacturer (SMA)

3/ This australian supplier

4/ The best guess of the tech support guy at my solar supplier (GB sol, via email)

 

The nearest I've found to a technical manual on the subject is for the Honeywell Elster A1140. But page 13 got serious about metering of inductive vs capacitive loads, power factors and mismatched phases, at which point I figure that's for commercial rather than domestic usage anyway; and certainly not UK "smart" meter.

 

The only supplier offering smart export tariff today is Octopus, and they don't yet support it for 3 phase.

 

Putting it all another way: the Smart export guarantee hasn't yet started, 3 phase smart meters don't yet exist, so between the two it looks like second-order case of "anyone's guess".  Fair?

 

 

[Jeremy Harris]

34 minutes ago, joth said:

 

I've been researching more into this, and now wonder how you know this - I guess it's based on observing existing (non-smart) 3 phase meters?

For metering export, the long term answer will be up to whatever Smart meters will measure. But for all I can find no one yet will supply a 3-phase smart meter, so it's hard to have any empirical evidence how they will meter simultaneous import+export.

 

I'm starting to convince myself that smart meters might sum the phases before figuring a net import/export measure of usage, based on sources of varying provenance e.g.:
1/ "ted" at the end of this thread
2/ This australian(?) presenter from a German inverter manufacturer (SMA)

3/ This australian supplier

4/ The best guess of the tech support guy at my solar supplier (GB sol, via email)

 

The nearest I've found to a technical manual on the subject is for the Honeywell Elster A1140. But page 13 got serious about metering of inductive vs capacitive loads, power factors and mismatched phases, at which point I figure that's for commercial rather than domestic usage anyway; and certainly not UK "smart" meter.

 

The only supplier offering smart export tariff today is Octopus, and they don't yet support it for 3 phase.

 

Putting it all another way: the Smart export guarantee hasn't yet started, 3 phase smart meters don't yet exist, so between the two it looks like second-order case of "anyone's guess".  Fair?

 

 

 

 

A three phase inverter will always export evenly on all three phases.  If, say, the PV system is generating 9 kW, then the inverter will be exporting 3 kW on each phase (doesn't matter what sort of meter is fitted).  Therefore, if there is a load on, say, the brown phase, then that can only be offset by the generation on the brown phase.   The export on the black and grey phases will just go through the meter to the grid without being offset by self-consumption on the brown phase.

 

Because the house will have to be wired as single phase on each floor, it's unlikely that it will be practical to arrange to load all three phases equally, in order to be able to make best use of the generated power at any instant.  The only time that all the export will be able to be offset would be for a 3 phase load, as that will be drawing power equally from each phase.

 

 

[joth]

17 minutes ago, JSHarris said:

 

 

A three phase inverter will always export evenly on all three phases.  If, say, the PV system is generating 9 kW, then the inverter will be exporting 3 kW on each phase (doesn't matter what sort of meter is fitted).  Therefore, if there is a load on, say, the brown phase, then that can only be offset by the generation on the brown phase.   The export on the black and grey phases will just go through the meter to the grid without being offset by self-consumption on the brown phase.

 

Because the house will have to be wired as single phase on each floor, it's unlikely that it will be practical to arrange to load all three phases equally, in order to be able to make best use of the generated power at any instant.  The only time that all the export will be able to be offset would be for a 3 phase load, as that will be drawing power equally from each phase.

 

 

 

Indeed - I'm not questioning how a 3 phase inverter works, the question is how a 3 phase smart meter works. If the meters will actually sum the phase power usage prior  to metering, then it doesn't make any difference if you feed generation into 1, 2 or all 3 phases, or in what balance: in all cases generation will be automatically optimized to reduce to domestic consumption prior to contributing to export.

This is the point the SMA video most clearly makes. It'd be useful to know country that's aimed at, but regardless -- if that's the metering mode  that inverter manufacturer’s are all designing for then there will be a good deal of pressure on SEG 3ph metering to accommodate it.

 

 

 

Edited August 12, 2019 by joth

[Jeremy Harris]

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.