Feeding PV power in, not directly into the main CU

[solexious]

I'm trying to find the relevant part of the 18th edition about where in a property the AC of a PV inverter can be fed into.

As a smallholding we have a brick built stable that has a roof that is great for solar panels, but it is currently at the end of a chain of CU's. Currently it goes:
Smart Meter -> Main CU -> Laundry room CU (external building) -> Stable CU (different external building).

Obviously the rating of the breakers and cables between the CU's will be a factor, but for sake of argument assume they exceed the peak PV internet output.

Will the internet be able to feed power into the Stable's CU, or will we need to have a cable run back to the Main CU? Running it back to the main cu will be a pain as it will need drilling out channels in the concrete yard, going under/over sewage/storm water pipes, oil pipes and water pipes.

If anyone knows the answer and can point me to the relevent part of the regs that would be fab.

 

[ericmark]

Most Solar panel inverters are designed to auto disconnect if the voltage goes over 253 volts, so the loop impedance becomes important. Also in most cases you need certificates raising so even if you had the skill, you could not DIY the work.

So no option but to get profectionals to fit it.

 

[solexious]

I'm not asking to DIY it, I'm trying to work out if I'm going to need to do ground works.

 

[Murdochcat]

The regs won’t give you an answer but PV needs a dedicated circuit and if the installation method requires RCD protection it needs a dedicated RCD

 

[solexious]

Righto, thanks. Wasn't sure if there was a rule in the regs I couldn't find that required it to feed into the main CU regardless of anything else.

 

[Murdochcat]

Could you post a photo of the closest CU?

And where that is fed from ?

 

[JohnW2]

Most Solar panel inverters are designed to auto disconnect if the voltage goes over 253 volts, so the loop impedance becomes important.

Could you possibly explain that?

 

[Murdochcat]

What output are you hoping for?

 

[StephenStephen]

I wouldn't employ profectionals to do it...

... professionals maybe...

FWIW Our PV was professionally connected to a subsidiary CU...

 

[ericmark]

Could you possibly explain that?

Solar inverters must be designed so with a power failure they will auto disconnect from the DNO supply. Often a lot goes on, including connecting neutral to earth with a power failure, and the inverters often have dip switches which need setting for the country used in, in the UK it also depends on the certificate issued. Which is why I said not a DIY job.

However, the way the inverter detects loss of DNO supply includes monitoring the voltage, in UK it is 230 volt +10%/-6% so 216.2 to 253 volts. I am not sure how much the frequency can vary, that can also be used to detect islanding, where many arrays in the street can maintain the supply when the DNO supply has been lost, but in the main it used voltage to detect loss of DNO supply.

My house now,

well over 230 volts, but under the 253 volt cut off. But at moment not exporting, by around 1 pm my batteries will be full, and as export starts the voltage will raise, it does depend on the DNO equipment, so is auto adjusting, but also the prospective short circuit current or line - neutral loop impedance will change the voltage range of the supply, if my inverter and battery had been fitted in the loft (now banned) then the voltage to inverter would have varied more, as it is the battery and inverter are in the old garage, right by the incoming supply.

But if the loop impedance is saying 0.35Ω, and we are supplying/using 20 amps, then we will have 14 volts between the two extremes, so if that was my house today, that would mean 253.7 volts, so my system would shut down, lucky my loop impedance is lower than that, so I have no problem.

In my last house the voltage never changed when the country went from 240 to 230 volts, but when solar panels were put on a load of OAP's bungalows, my supply volts went from 245 to 235 volts, this caused the old 65 watt fluorescent fitting with 58 watt tubes to stop working, had to convert to LED.

I can measure my loop impedance, but only to 1 decimal place, I can monitor voltage, and power, the latter 5 readings PV, Battery, grid, consumption, and not really power but state of charge, plus what my smart meter tells me, but until solar is fitted, most people have no idea how much power used and when, the smart meter helps, but working out volt drop, easy now solar fitted, but not so easy before it was fitted.

Lucky I did not have to, a firm came in, and did all that for me, and I must admit, they got it right, they said better to have all panels one side of roof, the side which gets afternoon and evening sun, and my display now shows me, never run out of battery in the morning, if I do, always in the evening, so they got it right.

 

[JohnW2]

My house now, well over 230 volts, but under the 253 volt cut off [indeed, actually 246.7 V ]. ....

I may be wrong, but it seems to me as if the problem you go on to describe probably results from the fact that although the (upper) 'cut off voltage' should relate to the voltage at the DNO's transformer, all that the inverter can see any measure is the voltage at the origin of your installation, which will rise when you are exporting (because of the external L-N loop impedance). Is that correct?

.... But if the loop impedance is saying 0.35Ω, and we are supplying/using 20 amps, then we will have 14 volts between the two extremes, so if that was my house today, that would mean 253.7 volts, so my system would shut down, lucky my loop impedance is lower than that, so I have no problem.

Thanks. I agree with your 253.7 figure (246.7 + 7), and therefore take your point, but I think your mention of "14 volts" is a bit confusing. With a total L-N impedance of 0.35Ω, the total VD across that impedance would be 7 V (0.35 x 20), hence 3.5 V in each of L and N.

Kind Regards, John

 

[Murdochcat]

FWIW Our PV was professionally connected to a subsidiary CU...

That’s ok in most cases …

 

[ericmark]

Sorry, 14 volt = charge and discharge?

'cut off voltage' should relate to the voltage at the DNO's transformer

No, that will not tell you if the PEN is lost. Could be measured at the DNO head, however.