reasons for DNO decisions on limits, transformers and balanced phases?

[Gavin A]

Hi all,

I just wanted to pick some brains ahead of having a chat with a potential customer.

Essentially it's a farm with a single phase supply, but an 11kVA 3 phase line a few hundred yards away. There are actually 3 farms clustered together, and I'm thinking it's likely that they're all fed from a different phase from a 3 phase transformer.

My query basically is around the issues with maximum SSEG sizing, and how much of this is related to whether the 3 phases are balanced or not. ie if all 3 phases have the same level of generation on them is this more likely to mean the DNO would allow a greater level of SSEG on all 3 phases than if an SSEG was only installed on a single phase.

Reason being that we should be able to fit 50kWp on the barn at this farm, but I can't see any way the DNO is likely to allow this. The other 2 farms though also look like they'd be suitable, so I was thinking it could be worth asking about the potential to do all 3 together, to see if it made any difference to the total allowed.

There's also the potential to ask about upgrading the transformer, which would be more cost effective if spread between the 3 farms.

I'll obviously be doing a G59 application, just wondering if anyone had any experience of anything similar / think I'm on to something / barking up the wrong tree?

I'm not thinking we're going to get anything like 50kW on a single phase, but was thinking it might push up what they allow from 10kW to 17kW per phase or something - I've vague recollections of a DNO engineer saying something along these lines at some stage.

 

[Worcester]

Strange as it may seems - can you call them? I've got the DDI numbers of most the the network planners in our area and a quick phone call works wonders - I got a 30kWp G59 application turned round and approved in under 36 hours the other day, of which 20 minutes was on the phone talking about HIS problems

 

[Gavin A]

it's outside our usual area, and Northern PowerGrid don't let you get past the switch board, though I have got a couple of direct lines for a couple of engineers we've dealt with, and thinking about it, I could ask one of their engineers we've installed a system for for advice.

Besides, I was kinda wanting to check out that I'd got the right end of the stick on unbalanced generation between the phases being an issue before discussing it with them / the client.

 

[Worcester]

If i've got it right, you are / were looking at 3 x 50kW single phase systems... It's not just about the out of balance, it also about where the power would be used, the DNO don't like have to reverse feed the upstream transformers, so like to see it consumed locally.

It certainly goes against the normal process to have three seperate 50kW single phase systems as it would be difficult to gaurantee any sort of balance unless they all go through three phase inverters, you only need one system to develop a fault and you've suddenly got 50kW out of balance, you could introduce some kind of multiple phase monitoring and shut down the other two if it happened, - that would be a customised g59 switch with remote shutdown.

If you could convince them of the resilience you might get approval, it would all of course need witness testing etc. I can see that there is a fair chance that someone will be making regular visits to keep all the three plates spinning at the same speed. - We run numerous 50kW sites and have remote monitoring on all of them and sometimes plants less than a mile apart can have wildly different outputs as rain clouds etc pass over - in this case that would trigger each of the other two plants to stop / reduce grid export at that time.

It's doable, SMA for example have some extensive grid output control capabilities on their inverters, and at a much smaller output this is what the EMMA was originally designed to do, wether it would be cheaper to install a three phase hook up at each location is one for consideration - espicailly allowing for the reduced income from the reduction in output because the downtime or reduced output of one plant is multiplied three fold. -

 

[Gavin A]

I'm not expecting to be allowed to install 3 x 50kW systems, just wondering if it might be that it would allow us to install a little more on each phase than if we were to only be installing on the single phase at the one site.

as in, if they'd have allowed us to install 10kW on a single phase by itself, they might allow us to install 17kW on all 3 properties if each were on its own phase.

I know they don't generally like feeding stuff back into the grid through the transformer, but it's this point that I'm thinking is actually partly to do with unbalanced phases, which I'm also fairly sure is why we don't tend to have as much problem installing 50kW on 3 phase as we do getting permission to go above 10kW on a single phase line.

I could be wrong about the set up anyway, but just have a suspicion that this is how they'll have done it rather than running 3 fairly large farms of one single phase supply when the main feed comes from a 3 phase line in the first place.

I take your point about the reliability, variations across the phases etc, though they'd be within a couple of hundred yards of each other, so any variations should be relatively short lived. I suppose this could well be the major stumbling block for this for the DNO.

The alternative is obviously to put in a 3 phase supply to the farm to take a 50kW system... don't suppose you've got any example figures for that have you? I'm aware this is probably a bit of a how long's a piece of string type question.

cheers for your input anyway.

 

[Worcester]

On occasion Wetern Power have allowed us to install up to 17kWp on a single phase, (G59) - definitely case by case!