NC7 - an epic saga

TerraSolis

Hi All,

In recent weeks I finally received positive word from ESB Networks that my proposed NC7 installation could go ahead (with some added caveats). I thought I would make a post on the experience and share some stuff that might be helpful to others considering going the same route.

So first of all: the proposed installation.

I live on a farm with an 11kW turbine that is grid-connected through two 6kW ABB inverters. The turbine was installed back in 2012 (pre-dating the NC7 scheme) via a very limited trial scheme in which ESB Networks were allowing people to connect more than 6kW via NC6 if they installed an early-days export limiting device manufactured by an Irish company called Coolpower (since gone under). Essentially, we could connect the 12kW inverters but had to maintain an MEC of 6kW.

Over the years, the export limiting device (ELD hereafter) took a hammering during a grid voltage event, and with Coolpower gone under, I had to repair the controller myself (thankfully successfully). However, when the NC7 process was announced, I saw my opportunity to rid ourselves of the ELD and install a bit of solar on a shed roof. Frankly, because the trial scheme was niche, I was concerned that under the later NC6 and NC7 policies, our turbine would be seen as illegitimately connected and that we'd run into trouble, so I really wanted to get it grandfathered in properly via NC7 with an MEC to match. Our MIC is 16 kVA single phase, and under the NC7, one can connect inverter capacity up to 17kVA (50 kVA for 3 phase) or their MIC, whichever is lesser. So, on top of the 12kW existing ABB inverters, I applied to connect a 4kW solar inverter.

Now, of course, there was further complication. The two ABB wind inverters do not have EN50549 certs as they are since discontinued and were connected under the old EN50438 standards. Luckily, ESBN will accept existing inverters that were installed under the relevant standard at the time, without forcing you to purchase new inverters certified to the EN50549 standard. This was something I really wasn't clear on when setting out with the application, but it seems they have sensible policy in this regard. I did, however, have to find the old EN50438 certs for the ABB inverters which wasn't easy (I'm a pro cert hunter these days 😎).

I filled out the application (straight forward) and attached the necessary certs for the existing and proposed inverter(s) and put together a single-line diagram as required using a handy app called SLD on my iPad. At the time, I ticked the boxes stating that I wanted be quoted for the nearest MEC to the 16kVA requested that didn't involve paying for significant grid reinforcement as that was out of budget, I also ticked the boxes stating that I didn't intend to install an export limiting scheme (ELS) but also to quote to the nearest MEC permissible with ELS if they couldn't offer the MEC requested.

This was all submitted in February with a very helpful lady who went back and forth via email to double check all the documentation was in order. Once payment was submitted (€1k) I was advised that the typical waiting time was 12 weeks, but in some unique cases (such as mine) it could take longer. Eventually, in May, a young fella with ESBN rocked up at the house to inspect the cable from the transformer to our meter. At the time, he gave me a heads up that it was smaller than they'd like these days for 16kVA but that he'd see what he could do for me. I gave him my number and he promised to keep me in the loop.

Skip forward a few weeks and I got a call from the ESBN lad telling me that, unfortunately, he's ran the numbers and the best MEC that they could offer me was 7kVA! Less than the 12 kVA already connected. He was also kind enough to run through it with me and told me that the limiting factor, according was Harmonics limits. He was very apologetic and very clearly wanted to do more for me. I explained our difficulty with the existing 12 kVA and my concerns that the old ELD wouldn't last forever etc. etc. and he said he'd talk to some higher ups to see what they could do with us. I want to stress, every experience I've had with ESBN was that they're very willing to put in the effort in spite of being a little short-staffed and having high standards to uphold. Generally, I must say I'm a fan.

Now, I'm an engineer by training, and a self-professed nerd. So I took a really deep dive into whatever docs I could find, paying particular attention to harmonics. The ESBN policy doc on Export Limiting Schemes outlines a brief assessment example including a harmonics calc. Essentially, in the absence of certified type-tested data indicating otherwise, they have to assume that an inverter being connected emits the maximum amount of harmonic current that is permitted under the applied EN61000-3-12 harmonics standard. Naturally, I did what any boards.ie renewable energies nerd would do and I purchased the EN61000-3-12 standard for some light bedtime reading. In the standard, I found that the applicable limit was 23% Total Harmonic Current Distortion (THDi) while most inverters emit far less than 3% THDi. I'll spare you the spiral of harmonics learning I went off on, but I did do a gigantic rambling twitter thread on a string of subsequent harmonics-related discoveries that you can read here if you hate free time: https://twitter.com/duncmat/status/1676227387485302793.

The follow up from this, is that I found (miraculously) type-tested harmonics data for every single one of the inverters involved via the phenomenal resource that is the UK's Energy Networks Association (ENA) G59 and G99 connection procedure cert database: https://www.ena-eng.org/gen-ttr/. Chapeau brits, chapeau. I also found that ESBN is an ENA member, and that the ENA recently published a document on harmonics assessment recommendations (details on twitter thread above). Based on the G5 ENA harmonics policy, I did my own harmonics assessment that showed that I could in fact connect all the proposed gen without violating harmonics standards in a mammoth spreadsheet and email explanation and sent it to the ESBN fella. This, was passed on to his manager as it was above his paygrade. His manager, who was also extremely nice and willing actually passed this on to his higher up, the gent who wrote the Minigeneration & ELS policies. At this point, I'm in awe that they're putting up with my BS when their default calcs came back with "computer says no", particularly given that I was pushing back on (read attacking) their default policies.

Eventually (last week) I heard back from young fella's manager with a full response form the ESBN policy dude. Frankly, it was brilliant. They conceded many of the points I made but also schooled me on some other stuff that I had got slightly wrong. But ballpark, my assessment was correct and they would allow me to connect the full 16kVA but I had to limit the exports to 12kVA (convenient, as I don't have to limit the turbine, just the additional solar) due to voltage rise limits - remember the slightly undersized cable. However I will add a caveat that I still get the impression that ESBN policy dude has slightly misunderstood some aspects of the EN61000-3-12 standard , but given I achieved what I set out to do and already spent wayyy more time on this than I'd of liked (lies, I loved every minute of this), so I'm leaving it at that.

The final logistics to iron out related to the ELS. If you've read ESBN's Export Limiting Scheme policy doc, you'll know that manufacturer's must submit a particular form (Manufacturer's ELS declaration) to ESBN in order for them to accept it as is (IE your inverter's built-in export limiting function) OR you must install an export limiting relay (ELR) aka a reverse power relay. I did ask the equipment supplier if they happened to have a manufacturer's ELS declaration for the inverter in question (Sofar EP) but as I suspected, nobody else has needed one yet. They did send a request to Sofar, but I don't expect to hear back. As a result, I went on the hunt for an ELR. Not easy to find *at all* but I did hunt one down via Kempston Controls in Dundalk who have ordered an reverse power relay from the manufacturer for me.

All-in-all, my NC7 journey has been a bit of a mammoth undertaking and a boat-load of effort. However, I've also learned a crap load about harmonics standards, had a bit of craic, and garnered a lot of respect for ESBN and their willingness to engage and work stuff out. But the lesson is, if you do apply via NC7 DO NOT ACCEPT DEFAULT HARMONICS ASSESSMENTS, make sure you attach the type-tested harmonics data and if you have any hassle, drop me a line 😂😎

10-10-20

All I can say is: well done for wading into a harmonics discussion. I'd have pleaded the 5th (harmonic 😉) and gone down the ignorance route long before that.

So the turbine feeds into two 6kW ABB inverters, was there no option for a single unit at the time?

TerraSolis

https://www.boards.ie/discussion/comment/121020757#Comment_121020757

Ha - thanks. I would have done the same but for the fact that I kind of wanted to learn more about harmonics assessment anyway, the €1k dropped on the NC7 was just the push I needed.

Frankly, if I received an ultimate and unfair "No" I was going to press ahead, I just really wanted the turbine grandfathered in.

There was an option for a fancier single unit at the time but there was a long lead time and it was much more expensive. I'm glad of it now as the fact that there will be more small inverters rather than fewer large inverters actually went in my favor in ESBN's ultimate harmonics assessment funnily enough. If it was 1 12kW inverter on the wind I wouldn't be allowed to connect the full additional 4kVA!

ColemanY2K

ha that's mental. fair play to you and to ESBN for not fobbing you off.

Jonathan

https://www.boards.ie/discussion/2058310631/nc7-an-epic-saga

Glad to hear it paid off in the end. I did think at the time that the 7kVA MEC was mad.

Fair play to ESBN for entertaining you also. It sounds like both sides learnt a lot.

slave1

Well done, I suspect your setup is isolated enough (location wise), lots of us in housing estates would have no capacity on local transformers which is a stumbling block. In a roundabout fashion I've heard that if there is no spare capacity on your local transformer (to supply your house with more "power") you are essentially fuct with NC7 regardless of your MEC limitation capability.

TerraSolis

https://www.boards.ie/discussion/comment/121022204#Comment_121022204

Yup, middle of nowhere here, thankfully!

Regarding housing estates, you're shafted on MEC but not on installed inverter capacity. You will be allowed to install with export limitation (as that solves voltage rise and thermal capacity of the transformer). For harmonics, ESBN only consider the individual meter and lean on the overall grid code to provide "headroom" in so far as everybody else connected to the same transformer is involved. So possibly not as shafted as you might think.

slave1

I’ll let someone else be the guinea pig