AC versus DC Cable

[kingpb]

Because I'm lazy and hope someone else has done the analysis I'll ask first;

What are the economics of utilizing a DC combiner box at the array, and running a 2 wire DC circuit (2//#4/0AWG) to the inverter from the combiner box using a larger Inverter say 166KW, versus utilizing smaller inverters (3 x 50KW) with built in string connection capability (SMA) and running ckt as #1AWG, 3ph AC circuit from each inverter to power panel?

There as equipment cost considerations but also installation and labor cost that need to be factored.

Thanks

 

[jaggedben]

My guess is that the DC combiner option gets better the longer the distance, as your material and labor savings for the pull and the AC combiner start to make up for the cost of the DC combiner. Also you don't have to worry about AC voltage range issues, and it's been argued that upsizing DC conductors for voltage drop doesn't pay for itself.

I think electrofelon, who does these size systems (I don't) will say AC is better, at least up to a point. Where that point is reached for your particular project is something only you can say given the prices and wages you've got to deal with.

 

[electrofelon]

My guess is that the DC combiner option gets better the longer the distance, as your material and labor savings for the pull and the AC combiner start to make up for the cost of the DC combiner. Also you don't have to worry about AC voltage range issues, and it's been argued that upsizing DC conductors for voltage drop doesn't pay for itself.

I think electrofelon, who does these size systems (I don't) will say AC is better, at least up to a point. Where that point is reached for your particular project is something only you can say given the prices and wages you've got to deal with.

I have said AC works out better than DC IMO, but that was for Resi systems in the up to 600 to 800 feet range. Sounds like the OP is talking about more of a commercial system so It may be different and I cant comment much on those economics. All of my utility scale ground mount work (in the 2-3 MW range) had string inverters with AC combiner panels scattered throughout the array. I wasnt involved with design or cost analysis on any of those. I have a friend who has been working on lots of systems that seem to be big on DC combiners and running hundreds of amps of DC back. I never worked on systems the employed that strategy.

 

[pv_n00b]

This shifts back and forth like water sloshing in a bathtub. The PV industry will be all hot on distributing inverters in the array to reduce DC run length, then it will shift to mounting all the inverters together in a virtual central inverter with longer DC runs and shorter AC. What I take away from this is that there is no clear winner. There are other reasons for inverter placement that will trump minimizing DC or AC runs.

 

[kingpb]

It certainly seems that cost of a larger inverter is cheaper on a $/KW basis. Perhaps is it more of a rule for ground mount systems, were rooftop or canopy is better for short DC and longer AC?

 

[jaggedben]

My takeaway from years of seeing these discussions is that there is no 'rule' and the best decision may vary according to the project details and the current fluctuations in prices.

Another factor to consider might be what the crew already has experience doing, especially if things otherwise pencil out about equal.