central inverters vs string inverters

[electrofelon]

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.

 

[ggunn]

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.

 

[electrofelon]

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.

 

[ggunn]

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.

 

[electrofelon]

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.

 

[ggunn]

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.

 

[ggunn]

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.

 

[electrofelon]

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.

 

[ggunn]

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?

 

[electrofelon]

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.

 

[GoldDigger]

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.

One factor in why they do not promote it may be that for residential use the upper limit on array voltage is still only 600V, regardless of the inverter used.

 

[ggunn]

One factor in why they do not promote it may be that for residential use the upper limit on array voltage is still only 600V, regardless of the inverter used.

I haven't verified it for myself, but if it's true it sounds pretty stupid to me. Why would anyone publish an erroneous data sheet which will put them out of the running for many designs?

 

[electrofelon]

One factor in why they do not promote it may be that for residential use the upper limit on array voltage is still only 600V, regardless of the inverter used.

I haven't verified it for myself, but if it's true it sounds pretty stupid to me. Why would anyone publish an erroneous data sheet which will put them out of the running for many designs?

This came up a while back, and I dont remember if fronius told me this, or I heard it from someone else, but they seem to think that since the primo are generally targeted for residential, no one cares about 1 KV strings, so they dont even bother to say it on the literature. The first time I used a primo, I designed for 600v. When I got the inverter, I saw 1000 on the dataplate. After some confusion and calls to fronius, I ended up stringing for 1000V, as this system was on a detached residential structure.

 

[GoldDigger]

This came up a while back, and I dont remember if fronius told me this, or I heard it from someone else, but they seem to think that since the primo are generally targeted for residential, no one cares about 1 KV strings, so they dont even bother to say it on the literature. The first time I used a primo, I designed for 600v. When I got the inverter, I saw 1000 on the dataplate. After some confusion and calls to fronius, I ended up stringing for 1000V, as this system was on a detached residential structure.

If I were the inspector, I am not sure I would call that non-residential unless the detached structure was demonstrably and by zoning a commercial structure.

 

[electrofelon]

If I were the inspector, I am not sure I would call that non-residential unless the detached structure was demonstrably and by zoning a commercial structure.

Note that the restriction for 1kv is narrower than just "residential".

 

[Carultch]

Note that the restriction for 1kv is narrower than just "residential".

Elaborate, please

 

[jaggedben]

The wording is:

'In one and two family dwellings, PV source circuits and PV
output circuits that do not include lampholders, fixtures,
or receptacles shall be permitted to have a maximum PV
system voltage up to 600 volts.'

690.7(C)

That leaves open apartment buildings. Also leaves open ground mounted or detached structures structures where the PV source or output circuits aren't in the building.

 

[electrofelon]

The wording is:



690.7(C)

That leaves open apartment buildings. Also leaves open ground mounted or detached structures structures where the PV source or output circuits aren't in the building.

:thumbsup: thanks