Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building
"The most popular book on green building in the UK today."
New fourth edition in two volumes!

Order both books now for the combined price of just £9.95
and free delivery!

(free delivery applies to UK addresses only).

Or get both books for just £7.90 if purchased at the same time as a subscription to Green Building magazine





Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




  1.  
    I am planning on installing 10 kWp of solar PV at my home which currently has a single phase supply. My DNO may not allow me to do this because of over-voltage in my rural location. My question is about how a Smart Meter nets off production and consumption across a 3 phase supply.

    I am considering two solutions if they refuse permission. The first is to install 10 kWp but to restrict the output of the inverters, by having a lower capacity inverter e.g. 6 kWp , or by using a fancy Solar Edge inverter system which monitors export to the mains and then throttles back to a configured limit (although I am not sure this is G59/3 compliant at the moment). The second solution would be to pay to have a 3 phase supply installed - a distance of 6m down my drive from an overhead telegraph pole.

    If I had a 3 phase supply I would have about 3.3 kWp of solar PV on each phase. It is likely however that the consumers in the house would remain on the existing single phase. I am also planning on installing a small ASHP to provide heating baseload in the winter again most likely on the main single phase supply to the house. I am also aware that DECC are looking to install Smart Meters in everyone's homes and potentially use them to remove the deemed export mechanism on renewable installations.

    The question then is how would the meter calculate import and export across the 3 phases. To provide an example, lets say I was producing 1 kW on each of the 3 phases i.e. 3 kW in total ,and consuming 3 kW in the house. Would the meter deem I was not consuming at all (i.e. it would net of the aggregate of the import and export) or would it deem I was importing a net of 2 kW on one phase (3 kW - 1 kW) while exporting 2 x 1 kW on the other 2 phases? The alternatives have cost implications the first would be cost neutral, the second would cost me 20p/hr (i.e. 2 kW at 15p/kWh net import on a single phase minus 2 x 1 kW export on the other 2 phases at 5p/kWh).

    I know Smart Meter's can display and log both import and export, but does anyone know whether for 3 phases meters whether they aggregate the 3 phases to determine import or export of treat the 3 phases individually? I tried approaching a Smart Meter company but they haven't bothered getting back to me.
    • CommentAuthorEd Davies
    • CommentTimeNov 15th 2015 edited
     
    Sorry, no idea about smart meters.

    Posted By: ActivePassiveby having a lower capacity inverter e.g. 6 kWp
    If they're going to refuse you permission then you'd likely be stuck with the legal maximum of 16 amps (typically accepted as 3680 W). 6 kW would be too much.

    or by using a fancy Solar Edge inverter system which monitors export to the mains and then throttles back to a configured limit (although I am not sure this is G59/3 compliant at the moment).
    Even if it's G59/3 compliant it still might not be accepted as limiting the export suitably. AFAIK it's up to particular DNOs what they'll accept but it's usually only particular hardware that's allowed. Maybe your DNO will accept a Solar Edge system but you'd have to check.

    I am also planning on installing a small ASHP to provide heating baseload in the winter again most likely on the main single phase supply to the house.
    If you had three phase, wouldn't it be better to put the ASHP on its own phase or is that not practical?
    • CommentAuthorowlman
    • CommentTimeNov 15th 2015
     
    AFAIK all the switching devices like Immersun etc, which divert excess solar to other sources only supply to resistive loads, immersion heaters, electric UFH, and the like which in practice means they can cope with the varying output of Solar, think clouds.
    ASHPs are inductive loads e.g. electric motors etc. so I guess you'd have to interface the solar supply to the ASHP via e.g. battery storage before you could drive an ASHP which run 24/7. Unless you're prepared to import then you're going to need batteries anyway to cater for night time.

    I guess because 3 phase to domestic properties is fairly unusual in the UK nobody has thought too much about it. Being domestic my guess is the FIT supplier would prefer deemed, and simply work off the generation meter ( x 3 maybe?? ) reading, they understand that.
  2.  
    Ed:
    1. I think there is a reasonable chance of refusal for anything above 4 kW, but given I have asked the DNO to do a network design I am hoping otherwise. A 8 kW system would be no different from 2 neighbouring houses installing a 4 kWp system so I am hoping they might be more flexible. If they refuse above 4 kWp I would either be looking at 3 phase or some form of inverter restriction down to 4 kWp
    2. I do have some concerns about the Solar Edge solution, and haven't managed to get hold of anyone who can provide an answer, their inverters are G59 compliant, but I suspect the ancillary equipment which monitors grid export and then throttles isn't. I could however put a G59/3 compliant switch/relay after the inverter and hope the inverter would reduce output before the G59 compliant switch turned the inverter supply off. I would presume the DNO would be happy with a standalone G59 relay as a solution?
    3. Its awkward but potentially practical to run the ASHP off another phase, but given the base load in the house averages 100W it is probably inconsequential. 3 phase ASHP's tend to be too large and not modulate down enough for my needs
    Owlman:
    1. yes, with clouds I will need to import some of the time, but am hoping to minimise this. One of the reasons I am going for a large solar array is battery storage, but not until prices come down. Chevrolet are now quoting $145/kWh for their Volt, which for a 10 kWh system implies battery costs of around £1,000 which would pay itself back on gains I would make on the 10p/kWh spread between the daytime export tariff (5p/kWh) and the overnight/cloudy import tariff of 15p/kWh. Unfortunately I will have to wait for a few years for the system prices of such battery systems to approach the $145/kWh Chevrolet are quoting for their cars.
    2. Yes. the installer is likely to go for deemed, but the recent DECC FIT consultation asked the question about switching all new installs, and perhaps retrospectively to a non-deemed smart meter system. I feel it inevitable over the life of the panels that we might move to a more dynamic pricing model for electricity to better balance supply and demand and I suspect deemed export will disappear once this happens?

    Thanks for your replies, I am still stuck with the question as to whether the export is netted across all 3 phases or not for Smart Meters going forward? This may determine what solar PV/inverter setup I go for in the end, depending on what my DNO says in response to my network application?
    • CommentAuthorEd Davies
    • CommentTimeNov 15th 2015
     
    Posted By: owlmanAFAIK all the switching devices like Immersun etc, which divert excess solar to other sources…
    I don't think ActivePassive is intending to use a diverter of that sort but rather just limiting the output of the inverters to at most the consumption of the house at the present time + the allowed export.
  3.  
    We have ten kW on a two domestic single phase properties and 6kw on loads of others.
    Bournemouth Council just did two adjacent houses with 8kW.

    No issues with the DNO.
    Common sense says a 100A incoming supply is capable of dealing with the load both ways.
    10kW is forty panels at around one amp each. In the very very very unlikely event that you are exporting 40A the incoming cable can easily cope.

    If you know the way to ask the question the answer is always yes.

    Never fitted Solaredge as don`t see the benefit as its a halfway solution. Enphase micros are far better with no single point of failure, no high voltage DC at the top end of solar products . Or SMA or Fronius string inverters for basic string systems.
  4.  
    The 8 kW was each house
    • CommentAuthorskyewright
    • CommentTimeNov 16th 2015 edited
     
    Posted By: moulesnfritesIf you know the way to ask the question the answer is always yes.

    Impressed by your confidence. Any advice appreciated.

    Here when SSE were asked, via the enquiry form available from their website, they wanted to add an extra pole to allow any increase at all on out existing 16A G83 setup.

    We are the second of 2 properties on our link to the local pole mounted transformer. The extra pole would reduce the overhead distance from approx 150m to 100m and mean that we weren't the second property along the line (currently the line goes past us, on the other side of the road, to the other property, then cuts back to us). In total 9 properties share the same pole mounted transformer. They estimated 'at least 6K' for tee off existing pole, erect pole & stay, and erect LV ABC conductor.

    And after all that we could have an extra....

    ...5kW maximum, making 8.68kW (maximum) of microgeneration in total.

    Not an attractive offer.
    • CommentAuthorEd Davies
    • CommentTimeNov 16th 2015
     
    Posted By: moulesnfritesCommon sense says a 100A incoming supply is capable of dealing with the load both ways.
    Well, yes, the cable isn't going to burst into flames. But it might well go over the maximum allowed voltage in some circumstances - e.g., a sunny day when everybody on the line is out and they're all exporting nearly 4 kW.
    • CommentAuthorskyewright
    • CommentTimeNov 16th 2015 edited
     
    Posted By: Ed DaviesWell, yes, the cable isn't going to burst into flames. But it might well go over the maximum allowed voltage in some circumstances - e.g., a sunny day when everybody on the line is out and they're all exporting nearly 4 kW.

    My emphasis.
    I think the all is a very relevant point. Given the increase uptake of micro-generation (only likely to increase further over time, even if FITs drop) DNO's must be considering the possibility that all other connected properties might install a G83 system?

    However, based on our experience it can't be the whole story. Initially SSE though we lived in a disused ruin on the other side of the road (~50m from the transformer & would be 'first connection' if it were connected). A connection there could have had up to 10kW total PV for £1500 (for 'overlay part of the service cable with mains cable'). It would be the same tapping to the same transformer in each case, so the difference between status quo (3.68kW), extra pole (8.68kW) & connection to the ruin (10kW) looks to be mainly down to distances, ~150m, ~100m & ~50m respectively?

    Edit: Correction of a unit.
    • CommentAuthorEd Davies
    • CommentTimeNov 16th 2015
     
    Yes, I think it's right that the DNOs are considering what happens if further customers add 16 A systems. Basically, if a certain number of people do it the DNO have to upgrade the cable at their own expense. If they let one person put in a 10 kW system now they're reducing the capacity for future people to add systems before they spend money.

    The position along the line from the transformer does matter, though. They have to keep the voltage at everybody's meter in the range 230 V +10%/-6% so between 216.2 V and 253 V. Adding somebody near the transformer will have some effect but it will not “see” the resistance of the line further along so will have less effect on the voltage for everybody else than somebody at the far end.
    • CommentAuthorEd Davies
    • CommentTimeNov 16th 2015
     
    Worked example: suppose there are three subscribers, A, B and C in a line with the transformer at one end with 0.1 Ω of resistance in each line segment (transformer(T)-A, A-B, B-C).

    If A is sourcing 40 amps (9.6 kW) and B and C both 16 amps then the currents and voltage drops in each segment will be:

    T-A: 40 + 16 + 16 = 72 amps, 7.2 volts
    A-B:16 + 16 = 32 amps, 3.2 volts
    B-C: 16 amps = 1.6 volts.

    If the nominal voltage at the transformer is 240 V then C will see 240 + 7.2 + 3.2 + 1.6 = 252 volts which is just legal (by one volt).

    If, instead, it's C that's sourcing 40 amps and A and B both doing 16 amps then it's:

    T-A: 40 + 16 + 16 = 72 amps, 7.2 volts (unchanged)
    A-B: 40 + 16 = 56 amps, 5.2 volts
    B-C: 40 amps = 4 volts.

    C will now see 240 + 7.2 + 5.2 + 4 = 256.8 V and their inverter will sense a fault and trip out leading to much wailing and gnashing of teeth until the DNO puts in a thicker bit of wire.

    0.1 Ω is probably a bit high: it doesn't allow for everybody to draw 100 amps for example but on a practical rural line it wouldn't surprise me if that was actually what happens. If everybody's heat pumps start at the exact same moment while their immersions are on and the sun's not shining then, yes, the lights will likely dim a bit.
    •  
      CommentAuthorSteamyTea
    • CommentTimeNov 16th 2015
     
    The DNO down here have had problems. Too many rural properties, long cables and small transformers, willy nilly installations of PV and wind (I was in Buckinghamshire last week and only saw a few PV modules on roofs).
    May be different in more built up areas like Bournemouth (my first University town), they can spread loads better.
    There may well be a higher day time load that is much more local to the micro generators.

    But I think there is validity in Ed's statement about the costs to the DNO.
    •  
      CommentAuthordjh
    • CommentTimeNov 17th 2015
     
    Posted By: ActivePassiveI am also aware that DECC are looking to install Smart Meters in everyone's homes and potentially use them to remove the deemed export mechanism on renewable installations.

    Posted By: ActivePassiveYes. the installer is likely to go for deemed, but the recent DECC FIT consultation asked the question about switching all new installs, and perhaps retrospectively to a non-deemed smart meter system.

    I can see they might want to move new installs to metered export (although as I understand it they typically benefit from deeming) but existing installs have contracts so how would they change them?

    Any links to actual proposals?
    • CommentAuthorskyewright
    • CommentTimeNov 17th 2015
     
    Posted By: djh
    I can see they might want to move new installs to metered export (although as I understand it they typically benefit from deeming) but existing installs have contracts so how would they change them?

    Our contact (from 2010) covers both deemed & measured export, whichever applies, i.e. measured if there is a meter, else deemed. There was no change of contract when an export meter was fitted (it was an option at the time of application, but not fitted for several months - beyond the end of the 1st quarter). I can't see anything to indicate that the contract is a special one for people who ticked the export meter option. Unless other contracts are different I can't see anything that says once deemed, always deemed.

    Wasn't deemed always only meant to be a stop gap measure till SmartMeters arrived, i.e. "not worth bothering with special export meters as part of FIT as everyone will have new, export aware, meters within a few years anyway..."?
    •  
      CommentAuthordjh
    • CommentTimeNov 17th 2015
     
    Who's your contract with? The one SSE have sent me is a complete shambles and I'm still arguing with them about it because it doesn't actually state I will be paid for export. The relevant paragraph actually excludes itself from the contract by its first sentence! I'd be interested to see a sane contract if there is one.
    • CommentAuthorskyewright
    • CommentTimeNov 17th 2015
     
    Posted By: djhWho's your contract with?

    SSE

    Posted By: djhThe one SSE have sent me is a complete shambles and I'm still arguing with them about it because it doesn't actually state I will be paid for export. The relevant paragraph actually excludes itself from the contract by its first sentence! I'd be interested to see a sane contract if there is one.

    In ours, section 4 covers Export & section 5 covers Meters.

    4.6 is about the "opt in" for FIT Export Payments. FIT Export Payments are described as being an opt in "during the registration process".
    If I check our original FIT application there was indeed a question about that (Section Three), but it was only relevant if an export meter was chosen (no longer an option offered?). Section Three of our application says "If you do not want your export to be metered it will be deemed at 50% of your generation, as set by the Government."
  5.  
    In answer to djh's request for references to Smart Meter's and the DECC consultation. The consultation which is closed is here: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/469476/Consultation_on_a_Review_of_feed-in_tariff_scheme.pdf Chapter 4 is most relevant as its all on Smart Meters

    Snippets include (lacking some context):

    63. When smart meters have been rolled out, we could consider whether a dynamic export tariff, updated every half hour to reflect the current wholesale spot price, would be appropriate. There are various options for setting the value of smart-metered exports.

    108. It has been clear since the introduction of FITs that deemed export was intended as a temporary measure to be in place until smart meters are available, which will be able to record the amount of energy exported in each half-hour period.14 ,15,16 Building on our proposals set out below, we intend to end deemed exports for all FITs installations and see the entire scheme moved to export tariff payments based on actual meter reads on the completion of the smart meter roll-out.

    Option 1: New entrants and existing FITs generators with installations of 30kW or below would be required to accept a smart meter or, in the case of non-domestic generators in certain circumstances, an advanced meter, when offered one by their energy supplier
    •  
      CommentAuthordjh
    • CommentTimeNov 18th 2015
     
    Posted By: skyewright
    In ours, section 4 covers Export & section 5 covers Meters.

    Yes, same in mine.


    4.6 is about the "opt in" for FIT Export Payments. FIT Export Payments are described as being an opt in "during the registration process".
    If I check our original FIT application there was indeed a question about that (Section Three), but it was only relevant if an export meter was chosen (no longer an option offered?). Section Three of our application says "If you do not want your export to be metered it will be deemed at 50% of your generation, as set by the Government."

    But not the rest of it. Section 4.6 is about VAT registration. But section 4.1 says "This clause will only apply to You in the event that You request to register to export Your generated electricity". Which is an option on the application form that only applies to installations greater than 30 kW.

    The Statement of Terms doesn't refer to the application, which surprised and worries me. It seems to me that they've changed the wording and didn't proofread it so are asking me to sign nonsense. I don't want a contract full of loopholes.
    • CommentAuthorrhamdu
    • CommentTimeNov 19th 2015
     
    Posted By: Ed Davies
    C will now see 240 + 7.2 + 5.2 + 4 = 256.8 V and their inverter will sense a fault and trip out leading to much wailing and gnashing of teeth until the DNO puts in a thicker bit of wire.


    Why do inverters have to switch off completely when they sense maximum voltage across their terminals, rather than simply limiting the current they export while the condition persists?

    It would be nice to be able to export as much as the local grid can take at any particular moment.
    • CommentAuthorEd Davies
    • CommentTimeNov 19th 2015
     
    Good question. Dunno but maybe partly because it's a fault condition really and needs to flagged up and dealt with.

    The rules were changed to allow/require (not quite sure which) them to throttle as the frequency increases rather than just drop out. This was in response to German fears of grid instability if all their PV dropped out more-or-less at the same time. However, I think the main worry was under-frequency if the grid was struggling on a sunny day for some reason then all the PV suddenly went off giving it a bit of a terminal kick.

    It's common to allow up to 3680 W which limits the current to the legal maximum (without prior agreement) of 16 amps at 230 V. Below 230 V keeping to that power limit would actually result in more than 16 amps being supplied but that would be a good thing if the line is struggling with under voltage anyway.
    •  
      CommentAuthorted
    • CommentTimeNov 23rd 2015
     
    Inverters have to sense grid-voltage and then set their output a couple of V higher, that is how they work. They have a max V setting (requirement under G83) so if the grid V is too high and they need to go above that value then they must power off.

    For the OP - a 3-phase meter is required to register a reading that is the balance across its 3 phases. So if you have 2kW import on phase 1, 1kW import on phase 2 and 1 kW export on phase 3 then it should, if correctly configured, register a net import of 2kW. If you have a 3-phase inverter then it is required to balance its output across all 3 phases. So if you had a 3-phase supply, but only connected 1 phase to any loads, then at least 2/3 of on-site generation would be exported.
Add your comments

    Username Password
  • Format comments as
 
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press