Strangeness with SMA Sunny Design

[JohnP]

So, I have a system to design for a client.

Striving for roughly 50kW system. Currently the 'old' design I did from back in 2012 was for (3) arrays consisting of (72) 235W panels each. Each array would be connected to (2) SB8000 inverters. Everything worked great on the design. Project was put on hold.

Now flash forward to 2014, same client, still looking for a 50kW system. Instead of simply dusting off the old design, I opted to shake it up a bit and see how the new SMA online design tool works. I selected the Canadiansolar 235W panels, and opted for "Design Suggestions" with a system size of 50kW. I was quite surprised to see my arrangement of (6) SB8000's not listed. Instead, it came up with several different options, that IMHO, will not work nice with the 208/3-ph grid tie. A few scenarios it suggests are:
(4) SB10000's and (1) SB5000
(4) SB9000, (1) SB5000, (1) SB4000
(5) SB9000, (1) SB5000

I guess I am confused about this. Seems to me that with a 3-phase grid tie, you try and balance out your feeds as much as possible, which would mean using something in a combination of 3, 6, 9, etc. With multiple unequal sized inverters, I am not sure how we would connect that to the grid?

Any thoughts about this?

[Naptown]

Make your life easier and less expensive

http://www.fronius.com/cps/rde/xchg/SID-34288A8F-EFB18A5C/fronius_usa/hs.xsl/2714_1483.htm#.UzmsUqhdWBo

They make 3 phase inverters

[JohnP]

Sadly....

My boss is totally hooked on SMA and Canadiansolar. Of course, this does not say that I can not change his mind.

Still I find it odd that the SMA Sunny Design would come up with those options.

[Naptown]

Then run your own calculations.
Did you break it town into an array or 72 modules and 2 inverters and do a manual design choosing your inverter yourself?

[inetdog]

POCOs, especially in a residential environment, are used to unbalanced three phase loads, and can tolerate them pretty well.

In fact, if you have a particular legacy form of service called "120/240 three phase four wire" (or just high leg), they will probably prefer to have all of the load (or generation) on a particular one of the three phases instead of uniformly split.

How much you can or should unbalance the three or more inverters across the three available phases will depend very strongly on the distribution equipment the POCO is serving you from.
Only if POCO is giving you a symmetric full delta or wye service do you really need to worry too much about an equal split.
Under other conditions, getting the right level of inequality will be necessary.

For a symmetric three phase service, any of the SMA configurations that you listed will be close enough if wired up properly. With the second and third being better than the first one.

[JohnP]

Hmmmm

So, to Naptown, I always do my own calcs and selections. That said, I broke it down 1 inverter to 36 panels (3-strings of 12). But knowing that SMA has come out with some new products recently (I was away from Solar for a year), I decided to try the auto design feature as well as their design suggestion feature and was quite surprised with the results.

Thinking that I need to dig a bit deeper and see what the POCO is truly going to provide us for a tie-in.

[inetdog]

Thinking that I need to dig a bit deeper and see what the POCO is truly going to provide us for a tie-in.

If this is a 50kW system that is not directly associated with a load (commercial or residential) at the same location, then you do need to see what POCO is willing to offer you.
For a dedicated connection to small generation, they may give you a symmetric three phase service and include a penalty cost for imbalance. A 50kW single service may well get its own distribution transformer(s) installed just for that purpose.
That will be the information you need to make an informed decision.