G99 knocked back by DNO - trying understand proposed "reinforcemen"

Qyburn

So our DNO has knocked back the G99 application. Our supply is from 11kV overhead, with a 64A (16kVA) transformer serving just our property. The application was for 5kW inverter and AC coupled battery also with 5kW inverter. They've assessed on the basis of the battery not exporting but for some reason they still take account of the battery rather than just export ..

Thanks for getting in contact regarding an application for a 5kW battery and 5kW kW solis inverter, the battery will be limited to 0 export therefore this will be a total capacity of 10kW limited to 5kW export 

Unfortunately the volt rise is too high for the proposed capacity at 3.37%, the g99 limit is 1.3% and G100 limit is 2.3%.

Would that voltage be the LV, ie the rise in our house mains voltage? Or the effect on the 11kV?

Their proposed solution is what they call "reinforcement"

"SSEN will look to tap the 15kVA transformer to bring the volt rise rise within limits ... If the transformer does not have a tap changer however, further reinforcements would be needed"

How would that work? Changing tapping on the transformer changes the voltage, but surely the voltage rise will remain more or less the same. And if they are reducing our LV voltage then the effect on the 11kV will actually be greater.

I'm trying to understand all this because I would be really disappointed to have to can our project or pare it right back. But equally I content to write a blank cheque to SEEN for works that might not do the trick or have undesired side effects.

 

markin

Yes the LV, Your house mains voltage would be to high, And you being the only house i think means you have no one to export to, transformers are 1 way streets.

Forum members with high voltage didn't have to pay for the transformer to be tapped lower but as this is a new install and serving only you im sure you may have to pay something, depends on who your DNO is.

Qyburn

markin said:

Yes the LV, Your house mains voltage would be to high, And you being the only house i think means you have no one to export to, transformers are 1 way streets.

Thanks. I don't believe that last bit is correct though. Electricity generally behaves symmetrically, connecting 250V to the LV side will create 11kV on the HV side. If it didn't then I wouldn't be able to export at all. 

The comments on voltage rise are concerning, if exporting 5kW causes a 3.37% voltage rise that implies similar voltage drops when importing. Can it really be the case that using the 10kW shower results in 6.74% voltage drop (around 15.5V)?

I guess I'm really trying to decide if this "tapping" would be the right thing to do, or is it a bodge which will cause harmful side effects, needed only because our supply is of a low standard.

1961Nick

How about using a hybrid inverter so that you remove 50% of the potential export?

Qyburn

Thats one of the options, going for a DC coupled battery and 3.6kW inverter. The trouble is that really limits the amount of power we can use. Even if there's full sun and power in the battery we can only use 3.6kW and any load over that will be imported.

Whereas our ideal with AC coupled battery would give 5kW from the battery plus whatever the solar inverter is suppyling. AC coupled battery also provides whole house backup and solar operation during power cuts. 

I didn't realise DNO permission is needed for the battery, even if it won't export. Hence they treat our application as 10kW even though export is limited to 5kW.

ABrass

Why not a DC coupled battery on a 5kW inverter?

Reed_Richards

In principle my hybrid inverter can software limit the export whilst allowing full (panels+battery) power to the house.  I've never explored this feature because I am being paid for deemed export and it seemed unfair to make the actual export any smaller than it could be. 

Qyburn

I've had some clarification from SSEN, firstly they assessed our neighbour and not our property at all! However assessing the correct supply, it is still outside limits although not quite so bad. They now say they will accept 7kW total size with 4kW export (up from 6kW with 3.68kW export.

The reason given is the length and size of service cable from transformer to house. Not quite stated outright, but interpreting what they say a 4kW export is estimated to create a 2.3% voltage rise at the house. They've clarified the reinforcement..

"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%"

That would allow for our original plan, but at the expense of lower voltage under load. At full whack (16kVA) voltage would drop to roughly 211V. And their quoted 4.5% drop would be exceeded if we draw 8kW.

Qyburn

ABrass said:

Why not a DC coupled battery on a 5kW inverter?

They won't allow a 5kW export without retapping.

There seem to be two limits and we mustn't breach either.
(1) export limit which is related to the allowed voltage rise
(2) total system size in which they count the battery even if not set to export. It way be that they're including the risk that a grid tied inverter my briefly export even when not supposed to, for example when loads rapidly reduce. Or something. 

Qyburn

Reed_Richards said:

In principle my hybrid inverter can software limit the export whilst allowing full (panels+battery) power to the house.  

That's interesting. Using that configuration we would be allowed up to a 7kW inverter, with export limited to 4kW. We would lose the benefits of AC coupled battery, but am coming round to thinking we're just not going to be allowed that anyway.

What type of inverter is yours?

Reed_Richards

Qyburn said:

What type of inverter is yours?

It's made by Goodwe, specifically a 93648 EM.  But this model is hard-limited to 3680 W of export power (or less set by software, I think).