first commercial systems coming up, do I need 3ph inverters?

JoeNorm

Senior Member
Location
WA
We're getting a few commercial jobs in the pipes which will be new to me, I've been doing residential PV for 10+ years and know very little about 3ph power.

I am wondering if the best approach is typically to employ 3ph inverters or if there is a way to stack 1ph inverters? Probably depends on other factors. Is there a good reference online for commercial design?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
We're getting a few commercial jobs in the pipes which will be new to me, I've been doing residential PV for 10+ years and know very little about 3ph power.

I am wondering if the best approach is typically to employ 3ph inverters or if there is a way to stack 1ph inverters? Probably depends on other factors. Is there a good reference online for commercial design?
You can indeed combine single phase inverters onto three phase services, but you probably won't want to if a three phase inverter is an option, especially if you are generally not that savvy about three phase systems. Line currents, for example, are straight off the data sheet for three phase inverters but for combined single phase inverters on three phase it involves some calculation.
 
I agree, other than a few odd ball situations, you would want to use three phase inverters.

A common thing you may run into is a building that has a 208 or 240 service and you will find the selection of larger string inverters for this voltage to be quite limited. Last I checked, the only 240 three phase string inverter that does 1kv strings is the fronius symo, and it caps out at 15kw. Get into a hundred kw or so and you are going to want 1kv strings and to not have a bazzilion inverters. At some point it becomes more economical to use a transformer and 480 inverters.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Yeah, 480/277V inverter(s) through a transformer to a 208/120V service is fairly common for me as well. Wye on the 480 side, delta on the 208 side.
 

Carultch

Senior Member
Location
Massachusetts
You can indeed combine single phase inverters onto three phase services, but you probably won't want to if a three phase inverter is an option, especially if you are generally not that savvy about three phase systems. Line currents, for example, are straight off the data sheet for three phase inverters but for combined single phase inverters on three phase it involves some calculation.
To add onto factors in favor of using 3-phase inverters instead of compounding 1-phase inverters:

Some utilities either don't allow "stacking" single phase inverters to make three phase, or require your system to balance the total amps on each phase. If you use a three phase inverter, you don't even need to think about this being an issue.

When inverters used to be built with internal transformers, you had more options for setting up single phase inverters on a 3-phase grid. You could adjust taps on the internal transformer, and set the same unit for 208V, 240V, and 277V. The same inverter could connect to 240V split-phase, 120/208V 3-phase by connecting inverters across phases, or 277/480V 3-phase by connecting to individual phases. Current and breaker requirements would change upon configuration to compensate, but the same inverter model would have the same power rating regardless of grid voltage.

Now with transformerless inverters being the industry norm, this is no longer the case. Grid voltage is a firmware setting instead of a hardware setting, and maximum output current is what remains constant. Single phase units are built with 240V grids in mind. Some (not even all within the same product family) can configure for 208V grids, but few (if any) can be configured for 277V. Expect the inverter's maximum output power at 208V to be 13% less than nameplate, rather than expecting 15% more inverter output current to compensate.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Now with transformerless inverters being the industry norm, this is no longer the case. Grid voltage is a firmware setting instead of a hardware setting, and maximum output current is what remains constant. Single phase units are built with 240V grids in mind. Some (not even all within the same product family) can configure for 208V grids, but few (if any) can be configured for 277V. Expect the inverter's maximum output power at 208V to be 13% less than nameplate, rather than expecting 15% more inverter output current to compensate.
That is not universally true. If you look at the data sheet for the smaller SMA single phase inverters (SB3.0-7.7-US-DUS170619W) you will see that some of them produce the same output current at 208V as 240V but at lower power, and some deliver more current at the same power. Still others operate at different current and different power at the different voltages. One needs to consult the data sheet.
 
SMA and fronius single phase inverters do both 208 and 240. But again, I probably wouldn't be using single phase inverters for a three phase service. Only exception I can immediately think of is if it is an open Delta supplied service and POCO doesn't want three phase interconnection (I believe it would still work fine).
 

pv_n00b

Senior Member
Location
CA, USA
Make sure you check with your AHJ to see if they need PE sealed documents submitted for permits. I see folks making the transition from residential to commercial get caught at the last minute on this all the time. I agree about using 3-phase inverters. You are in the big time now, might as well use the best equipment for the job and not try to treat it like a big residential job.
 
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