Determining Taps for a Single Phase Inverter on a Three Phase Service

[pvgreeze]

Hi all, I have a small design where the customer has a wireway that feeds two meters/services; one is a small single phase DS and the other is a small three phase DS. This particular design has one single phase inverter interconnected on the single phase service, and another single phase inverter on the three phase service. My questions is, would there be a design preference for selecting conductors/phases to interconnect with on the line side of the three phase service? Is there any benefit to matching the same two phases that are feeding the single phase DS? Or varying them?

My initial thought was that it doesn't really matter as its utility side conductors, and the fact that they're physically connected by a wireway doesn't overrule the fact that they are technically separate services and could in theory be in two separate buildings. But, if there is a tangible benefit to selecting two specific conductors for the three phase DS over any combination of the other two, then might as well incorporate that into the design. Thanks!

 

[jaggedben]

My first question is what type of 3 phase. Wye or high leg delta? (I'm assuming not 3wire delta.)

 

[jaggedben]

Also it sounds like only one service, if it all comes out of a single wireway.

 

[pvgreeze]

Fortunately just a three phase wye 208/120V. Yes, one incoming set of service conductors, two meters, and I'm assuming two interconnection applications though I am not handling the interconnection paperwork, so I don't know for certain.

If the meter was above the wireway (electrically speaking) then this would be straightforward, but having two separate meters is unusual.

 

[jaggedben]

So if I understand correctly, the contract calls for backfeeding both meters with separate systems.

One question is: why not use a 3-phase inverter on the 3-phase meter? Also are these systems of comparable size?

Assuming you can stick with your plan, I would backfeed a different phase of the 3-phase meter than the 1-phase meter. So that output is more balanced.

But I would also make no assumptions as to what the utility will allow. They may want the aggregate backfeed on both meters to be more balanced, which might require using two single phase inverters on the two phases the 3-phase meter that aren't the same phase as the 1-phase meter. Not that I've ever heard of this, just saying I would make no assumptions about what they will say or ask for, especially not knowing the relative system sizes.

 

[pvgreeze]

Inverters are both Solaredge SE10000H-US, so both very small systems. The installer who gave me this design selected the equipment and sold the project as the the two separate interconnections. And yes, the meters sit between the wireway and the DS on each system.

Noted regarding the balancing, thanks!

 

[Joethemechanic]

If it was me, and PECO let me, I would connect to the phase that is most heavily loaded during peak solar

 

[jaggedben]

If it was me, and PECO let me, I would connect to the phase that is most heavily loaded during peak solar

Why, exactly?

 

[Joethemechanic]

Why, exactly?

To unload that phase in the service lateral and the transformer. It's a power source in parallel with the poco. Probably help balance the phase voltage by bring up the phase with the highest load. PECO energy is notorious for installing tiny service laterals

 

[pv_n00b]

With a 120/208 3ph service and multiple single-phase inverters just distribute them across the phases to balance out as well as you can. It's like adding single-phase loads, we don't put them all on the same phase we spread them around.