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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    Does anyone have experience of a G59/3 protected solar PV installation?

    I am about to apply for planning permission for an 8 kWp ground-mounted solar PV array and a 2.5 kWp roof mounted solar PV array - so a total of about 10.5 kWp. Its a rural location, I have a single phase overhead connection of about 300m back to a transformer mounted in a field, 3 phase is distributed from the same transformer to within 20m of the property. There are approximately 30 homes connected to the transformer split across the 3 phases with only 1 other generator, a 3.84 kWp G83 installation but I think on a different phase (we have had power-cuts at differing times when the overhead cables have come into contact with trees).

    Although I am getting mixed message from my DNO - 1 analyst says I will need (expensive) reinforcement back to the transformer to support anything above a G83 3.84 kWp connection and another says they will support up to 17 kW single phase with G59/3 protection, I am assuming for the moment a 17 kW G59/3 connection is feasible given my incoming fuse is 100A (24kW), so the cable at least should be able to support 10.5 kW of generation?

    On this basis could someone with knowledge confirm the following:
    1. Given I am likely to have separate inverters for the roof mounted system (2.5kWp) and the ground mounted system (8kWp) I assume it would be better (a requirement?) to have a single independent G59/3 relay connected near to the meter on the supply side of the inverters rather than individual G59/3 certified inverters?
    2. If so, do you know how much such a G59/3 relay might cost?
    • CommentAuthorEd Davies
    • CommentTimeSep 15th 2015
    Sorry, no idea about G59 relays.

    Posted By: ActivePassiveI am assuming for the moment a 17 kW G59/3 connection is feasible given my incoming fuse is 100A (24kW), so the cable at least should be able to support 10.5 kW of generation?
    That doesn't follow. It's not (just) the current carrying capacity of the cable but also a matter of keeping the voltage at each of the consumers in the legal range.

    The ESQR [¹] says that normally mains voltage as supplied to the premises should be 230 V +10%/-6% so 216.2 to 253 volts. In practice they seem to normally aim for about 240 V (the old standard up until 1994) presumably because a) it's what's already there so they won't get complaints if it doesn't change, b) they sell more energy for resistive devices (like most heaters), and c) it gives them a goodly margin for voltage sag on the days when all the consumers on that phase gang up to draw their full 100 amps.

    So there's a 23.8 volt margin for voltage sag but only 13 volts of headroom for over-voltage and therefore about half the current capacity in the opposite direction (e.g., on a sunny day when all your neighbours have had their 3.84 kW installed but gone out for the day leaving nothing switched on). And maybe there are also considerations of transforms being designed to work one way - in principle transformers work both ways but I think that in practice they're less efficient going the other way.

    Bear in mind that the DNO has to consider the case that everybody on the line eventually gets 16 A/phase PV on your line. If they don't charge you for any required upgrades now they might well have to pay for it themselves when the last person gets theirs installed and everybody's inverters overvoltage trip.

    [¹] The Electricity Safety, Quality and Continuity Regulations 2002
    2002 No. 2665 PART VII Regulation 27
    • CommentAuthorbarney
    • CommentTimeSep 15th 2015
    G59 protection is going to cost you an arm and a leg - you are into the territory of "proper" embedded generation

    You'd be better off getting a 3 phase supply brought in and use a G83 compliant invertor - that would allow you up to 16A per phase - so about 15kW in the old money

    Distribution transformers will work either way with equal efficiency, incidentally


    Ed, Barney thanks for your comments.

    Ed, do you know how they calculate the over voltage margin based on the max kW of the generators versus number of consumers (houses) on the same phase (on the 240V, house side of the transformer)? For example if the overnight supply voltage was 240V and every home had a 3.84kWp solar PV system facing south, what over-voltage would you get on a very sunny mid-day?
    • CommentAuthorEd Davies
    • CommentTimeSep 15th 2015
    I don't know. I'd assume they just work on a nominal resistance for the cable (ohms/metre or whatever) but there might be another complication.

    They can adjust the tappings on the transformer to adjust the voltage. E.g., if somebody complains that the voltage at their house is too high (perhaps because their inverter keeps tripping out) they'll put a logging device in by the meter tails for a week or so and if it turns out to be correct they'll then turn the voltage down at the transformer.
    When I spoke to them on the phone they talked about 'reinforcement using 300 wavecon' but didn't explain why. Would this be a way of reducing the nominal resistance of the cable?

    Given their initial estimate is £20K for reinforcement its something I would want to avoid. If this is the case switching to a different phase or bringing 3 phase on site might be the cheaper option? The existing overhead cable looks pretty meaty from the ground and all the local cabling was recabled about 5 years ago.
    • CommentTimeSep 16th 2015
    When we used to fit PV system the local DNO (Western Power) where very helpful. We did (before my time) connect up a couple of houses on 2 of the 3 local phases (the third phase went to a farm). The DNO came out and viewed it, did the calculations and passed if all off as OK.
    So I think you really need to find the right person to talk to at your DNO. We had a contact in the engineering office.

    I do know that Candlemaker and MikeL have hit on this issue.
    I thought I would try a little experiment - turn everything on in the house and see what impact it has on the voltage and then potentially imply what impact in the opposite direction a solar array might have?

    The voltage in the area as monitored by a plug-in appliance monitor is quite high - 235V to 240V during the day and 245V in the middle of the night. I switched everything on - the induction hob, oven, microwave, electric heater, HW immersion, kettle drawing about 16kW according to my Owl (not accurate if inductive or capacitive load) but it seemed about right for the appliances I switched on.

    The voltage at a mains socket in the kitchen dropped from 240V to 210V. A socket in the garage where the meter and incoming cable is, dropped from 240V to 225V.

    It does suggest there is some issue with load/generation in my house if 15 kW drops the incoming mains voltage by 15V - my gut feel is that this voltage drop is quite high, or is this normal? The experiment might be saying more about my connection (10m) from the garage underground to the telegraph pole than the onward connection to the transformer?

    If you assume symmetry between load and generation and the voltage on a sunny summers day could be as high as 245V, then you would have 8V leeway to the max 253V allowed, which approximates to about 7 kW of generation at the incoming supply?

    I would appreciate your thoughts on this experiment and whether it has any validity and/or whether anyone else might have tried this in a more urban environment rather than my more rural setting?

    What I also don't know is what impact this load had on the voltage difference between neutral and earth which I have seen as high as 35V in the house and whether the appliance monitor measures voltage to neutral (I presume it does) or to earth? The house has an earth spike which I believe is common in rural areas with overhead telegraph pole supplies?
    • CommentAuthorEd Davies
    • CommentTimeSep 16th 2015
    Well done on the experiment - I was going to suggest something like that but decided it would be a bit much.

    Pretty sure the energy meter will measure between phase and neutral rather than to earth.

    Static caravan on a rural caravan park: everything off except laptop and microwave clock the voltage was fluctuating around 233 to 234 volts; turn on a 2 kW heater, 232 V; add an additional 2.5 kW heater to total 4.5 kW, 230 V; add a 2 kW kettle for total 6.5 kW, 227 V. So approx 1 volt/kW which is cromulant with your garage readings.

    Something to be aware of is your resistance between the house and the garage which any inverter in the house will see. If, say, the 8 kW ground-mounted array pushes the voltage in the garage close to the limit, say to 252 volts, and the 2.5 kW roof-mounted array inverter is in the house then it'll see that 252 volts plus the voltage drop it's causing between the house and the garage (another 2.5 volts, perhaps) which could cause it to trip out.
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