Effect of SSEN "down tapping" our transformer

[aesmith]

Hi,
This relates to our planned PV installation. The G99 application came back outside SSEN's limit for voltage rise, which for property connected at 240V is 1.3%. They will, for an estimated £2K, "look to" change our transformer to drop voltage in normal operation.

"Alternatively we can look at tapping the transformer again. We tap the transformer by 2.5% reducing the voltage from 240v to 234V, this then increases the volt rise limit from 1.3% to 4%, and in turn this reduces the volt drop limit from 7% to 4.5%."

If they can do this, our proposed system would be within limits.

What concerns me, apart from cost and the fact they can't guarantee it will even prove possible, is the possible efect of this permanent voltage reduction. The characteristics that make this necessary are the length and size of the service cable. These also mean larger voltage drops under load, compared to better supplies.

Alternatively and with no change they would permit 4kW export limit as this would not exceed that 1.3%

Any thoughts?

 

[marconi]

What is the peak power of your solar array(s) and inverter(s)? Or, if your panels where completely insolated what power would they produce and could your inverter(s) then convert all this solar power to mains ac power for export? Single phase?

 

[aesmith]

Hi, thanks. The planned installation was to be (roughly) 6kWp panels with a 5kW inverter. And an AC coupled battery, 5kW output but configured not to export. Single phase. So overall a 10kW system export limited to 5kW.

SSEN assess that without dropping our voltage this would break their G99 limit of 240V+1.3% and their G100 limit of 240V+2.3%

 

[marconi]

Since the greatest return on the investment in this Solar PV and storage system is by self-consumption ie avoiding paying for imported electricity at peak/day rates, and that for most of the time during the year your panels will be producing far less than 5kW, your inverter's maximum power, you are rarely going to export 5 kW for any appreciable length of time. And when you do you will be paid pittance for the energy you feed in to the grid.

You will be better off maximising self-consumption when the sun shines and what you cannot use store in the battery to power loads during peak rate first and then during low rate if you have it.

In sum, the sizeable investment in altering your SSEN supply will not be paid back by what you export in a reasonable period when compared with the amount of interest you could earn on this money spent if you put it in a bank deposit/building society account.

A good load combination for solar pv is air conditioning if that is something you have or would like to have in your home or business. You can also use excess PV to heat water in a well insulated tank which could be useful if you use a lot of hot water. And power your white appliances with a heater inside albeit newer machines and low temperature detergents can use much less energy per cycle than older ones do.

The only time to export then is when you cannot consume and your battery is fully charged.

Finally a 4kW export per phase limit for your power of array and inverter is perfectly acceptable and typical for grid-tied pv installations connected to the street electricity distribution network.

 

[aesmith]

Thanks again. I was really looking for opinions on side effects od the proposed voltage reduction.

However as we're discussing the design there's no harm in getting comments on what I see as, the advantages of our original plan
(a) a separate AC battery helps self consumption because household loads can be served by its 5kW in addition to any solar generstion, rather than either/or
(b) it would provide backup during power cuts much more conveniently and effectively than the generator that we currently have to use
(c) 5kW export is self-evidently better than 4, and if we felt the proposed panels didnt justify 5 then the solution would be more panels rather than a smaller inverter. Although the installers quoting all seem to think 6 or 6.5kW of panels is about right for a 5kW inverter.

For context our supplier will pay a flat 15p/kWh for export. Iimport today is 18.75p, it's not a TOU tariff but it varies day by day. So there's a substantial difference, but I don't see the export as a pittance. Export will pay about twice what it would cost us to heat DHW with oil.

Any thoughts appreciated. I need to decide whether to go for the cost, uncertainty and possible side effects of retapping. Or downgrade our proposed installation and losing the benefits referred to.

 

[littlespark]

The "normal" supply voltage in theory is 230v.... and they are allowed to go a little less than that, so them reducing your supply isn't going to be a problem.

Conversely, there are issues with some EV charge points where they cut out on over voltage..... so a relatively high supply, plus the extra from a PV invertor can go over the EV's threshold.

Its a fine line

 

[marconi]

Who is your electricity supplier and what is the tariff's name you are on please?

 

[aesmith]

Who is your electricity supplier and what is the tariff's name you are on please?

Octopus. The import tariff is "Tracker" ..

Octopus Tracker

Britains' fairest energy tariff, with dynamic daily pricing and 100% renewable electricity.

octopus.energy

For export i will most likely choose Outgoing Fixed to get a flat 15p/kWh. But will depend on tariffs and prices when we finally go live.

 

[aesmith]

The "normal" supply voltage in theory is 230v.... and they are allowed to go a little less than that, so them reducing your supply isn't going to be a problem.

Its a fine line

Thanks. What I was concerned about is that the very things that make SSEN require this voltage reduction, also mean that we have larger voltage drop under load than we would get from a higher quality supply. For example 5V drop when using 2.7kW electric kettle. We've not been aware of any issues as a result, but I am concerned that we might if SSEN drop the voltage at the supply.

 

[aesmith]

I've had a good look at the transformer data plate, obviously not close up. From what i can make out it gives tap settings for +5% Normal and -5%. Furthermore it states LV as 250V at Normal, suggesting it may already be on the -5%. That would be consistent with our voltage reading 240 or a bit less.

I've sent the photo to SSEN to see if he will make any comment. But if they can't say from those whether it is or is not possible, the I presume they'd have to open the housing which would mean the HV shutdown and estimated £2k charge, just to find out.

 

[aesmith]

Response back from SSEN to the effect that downtapping impossible (lucky I hadn't accepted their offer and paid £2K to find out. They propose installing a pole in the middle of our yard with 95sq.mm (!) LV overhead cable which then connects to the existing 25sq.mm split concentric for the remainder of the route. Looking at cable spec it seems this will reduce voltage drop/rise by around 20%. I dont think its worth it for the inconvenience of having those obstructions across the yard.

Disappointing I was all excited when they phoned to say they proposed upgrading the service cable, and I took it as read that this would be underground.