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Thread: central inverters vs string inverters

  1. #1
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    central inverters vs string inverters

    I have worked on quite a few 500k to 2 meg systems the last several years. My role has only been installer and I have nothing to do with design and pricing. I always have a significant eye toward improvements in design to cut costs and installation time. In contrast, the people who design these systems presumably havent worked in the field much, or at all, so are often mostly clueless on what goes in to installing these things. My question is on the use of string vs central inverters and if anyone has compared them in price, either just flat cost or also taking into account labor differences. I have actually never worked on a system with a central inverter. I see a huge time savings using fewer larger inverters. Granted the number of DC connections will not be less, but considering the mechanics of having to mount dozens of inverters, AC conduit and connections, communications conduit and connections, I would really like to see a cost breakdown for larger inverters. We are starting a system Monday that has like 40 inverters - Ughhh. I talked to a designer for one company we have done several systems for, and he said that string inverters were just way cheaper because they are a "commodity" item, but again I dont think he realized the potential labor savings. Just curious if anyone has weighed the numbers on this.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  2. #2
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    Of course. There are a lot of variables and labor is definitely a consideration. A central inverter is a single point of failure, and that's another; if one of a cluster of inverters fails the rest keep working, and replacing a string inverter is no big deal. Central inverters are big and heavy as well and they need to be on a slab.

    There are a lot of high capacity light weight transformerless inverters coming into the marketplace now, and they provide a useful middle ground between small string inverters and central inverters.

  3. #3
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    Quote Originally Posted by ggunn View Post
    Of course. There are a lot of variables and labor is definitely a consideration. A central inverter is a single point of failure, and that's another; if one of a cluster of inverters fails the rest keep working, and replacing a string inverter is no big deal. Central inverters are big and heavy as well and they need to be on a slab.

    There are a lot of high capacity light weight transformerless inverters coming into the marketplace now, and they provide a useful middle ground between small string inverters and central inverters.
    I dont really propose only a couple inverters. I could see between 200-400 K being good sizes. 400k could feed 600A equipment. 6 inverters would be good as you could feed them from 6 service disconnects, and it seems like all the ones we do with string inverters, the inverters are divided into about 6 groups anyway.

    Last system we did had 60k string inverters - still annoyingly small IMO.

    larger inverters could be set on a precast slab. Done. probably eliminates combiner panelboards too.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  4. #4
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    Quote Originally Posted by electrofelon View Post
    I dont really propose only a couple inverters. I could see between 200-400 K being good sizes. 400k could feed 600A equipment. 6 inverters would be good as you could feed them from 6 service disconnects, and it seems like all the ones we do with string inverters, the inverters are divided into about 6 groups anyway.

    Last system we did had 60k string inverters - still annoyingly small IMO.

    larger inverters could be set on a precast slab. Done. probably eliminates combiner panelboards too.
    It also depends on your service configuration. If it's 240V single phase, for example, your options are limited. I have designed several PV systems on the order of 100kW that interconnect at 240V 1P3W, and the biggest inverters I can find are ~11kW. Central inverters (all I have found, anyway) are 3P; you can connect 1P inverters to a 3P service but not the other way round. A slew of 1P string inverters is your only option.

    BTW, I am told that for some reason SMA has discontinued their 10kW and 11kW 1P inverters. I have a 102kW 1P PV system on my desk right now; we need to order the 11kW inverters NOW while there is still some stock out there.
    Last edited by ggunn; 09-23-17 at 12:07 PM.

  5. #5
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    Yeah I am assuming 480 three phase. Ouch, 100kw at 240 single phase. FYI Fronius has a 15k in their primo line. I also like the primo line because they can do 1kv strings.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by electrofelon View Post
    Yeah I am assuming 480 three phase. Ouch, 100kw at 240 single phase. FYI Fronius has a 15k in their primo line. I also like the primo line because they can do 1kv strings.
    I have had a couple of bad experiences with Fronius inverters; perhaps I will take another look.

  7. #7
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    Quote Originally Posted by electrofelon View Post
    Yeah I am assuming 480 three phase. Ouch, 100kw at 240 single phase. FYI Fronius has a 15k in their primo line. I also like the primo line because they can do 1kv strings.
    Where are you seeing that? I'm looking at the Fronius Primo 15.0-1 data sheet, and it says 600VDC max.

  8. #8
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    Quote Originally Posted by ggunn View Post
    Where are you seeing that? I'm looking at the Fronius Primo 15.0-1 data sheet, and it says 600VDC max.
    Yeah its weird. Seems like a huge advantage to me yet they don't promote it. Most of their literature says 600v. Dataplate says 1kv. Tech support will confirm 1kv is fine.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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    Quote Originally Posted by electrofelon View Post
    Yeah its weird. Seems like a huge advantage to me yet they don't promote it. Most of their literature says 600v. Dataplate says 1kv. Tech support will confirm 1kv is fine.
    What is "dataplate"? Do you mean the physical plate on the inverter?

  10. #10
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    Quote Originally Posted by ggunn View Post
    What is "dataplate"? Do you mean the physical plate on the inverter?
    Yeah. I haven't actually used the 15, but the 11.4 was this way, and my understanding is the whole primo line is 1 kv.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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