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Grid feed system advice.


MartinCoetzee

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I have done a lot of low voltage systems with 48V Mppts, and last HV system was at least 6 years ago. I am prop going for a 15Kw 3 phase fronious to have compatibility with Victron later when the client ads storage.

My question is what disconnect to you add on the roof and at the invertor and what protection do you add ?

I am busy reading nersa and sans docs and putting at all together,  maybe the people with more experience can help me ?

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Hi Martin

I can only provide some guidelines according to SANS and a little bit of logic/experience, and if you want to have proper (sometimes overdone) safety (in my terminology, read invertor as any solar charging circuit):

  • Each solar string should have a cutoff before merging into a feed to invertor. This should be a DC fuse & switch, or a DC circuit breaker before a junction that merges the supply into an array that feeds the invertor.
  • At the invertor, or as close as feasible, you need to have a similar arrangement (obviously at max voltage/current expected from PV) to isolate the feed from the invertor.
  • There is a specification in the SANS code regarding a single string of PV panels, so if you exceed the voltage/current, you may need to provide additional protections on your PV array between panels. Apologies, I don't remember what those values are.
  • The obvious electrical considerations need to be taken into account for your incoming AC supply (breakers, fuses, etc), and duplicating the same on your outgoing supply from your invertor.
  • Pay attention to Earthing (note I don't use the word Grounding here) and ensure that the Earth on each side of the invertor matches the Earth on the incoming supply (especially in regards to each phase). Test this in all stages that the invertor can get to (Grid-Tie, Offline, Online, Hybrid - Whatever it can do), seen a lot of and experienced plenty of shocks and damaged kit due to changeovers, so do test this and look for spikes.

 

  • Grounding - This is a very touchy subject, and some items I disagree with SANS, but you can comply with both my ideas and SANS at the same time with a bit of thought..
    • Ground fricking everything possible with the shortest possible route and with the biggest cable available (my thinking). Your route to ground should be direct and as short as possible. This is where SANS and I disagree, but is still compliant if you properly ground your array, but up to you to decide: I personally don't believe in linking your array ground supply to your incoming supply (I can provide arguments in separate messages), and if far away from your array then even your invertor should be separately grounded.
    • Depending on where you live, the shortest possible route to ground (any valid ground) can save a lot of equipment in a strike. And here is some of my reason for disagreeing with SANS: providing an extra path for high power (eg: lightning, inducted) energy to flow is going to damage something. Copper, even if temporary before breaking, is still an easier path than Ground!

Hope this at least answers some thoughts. If you need further detail, I work closely with a Senior Electrical Engineer who works multiple MW Solar projects and would be happy to pose the same question to him, although we would need to discuss offline.

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