240 Delta High Leg Interconnections

[SunFish]

I am looking at installing a PV system onto a 240 Delta High-Leg (center ground) service. I have found a single phase inverter that can be connected to two of three phases with this type of service and I'm planning on using three inverters to do a balanced backfeed. I need to calculate out my max line currents to size the equipment between the inverter combiner panel and the utility tie-in.

Are all three phases for this type of service 120 degrees out of phase, similar to a 208/120 or 480/277 where my max line currents would be the inverter current multiplied by the square root of three? This was my assumption until I started googleing the subject and read somewhere that the two "bottom" legs that have the neutral center grounded are 120 degrees out of phase similar to a 240 split phase.

Can anyone clarify on the phase angles measured between L1 & L2, L2 & L3, and L1 & L3? And how to calc out the max line currents when I have three single phase inverters tied to this service in a delta formation? Note this type of tie-in requires the inverter to operate WITHOUT the neutral, which is an acceptable application for the inverter according to the manufacturer.

 

[GoldDigger]

Since you are connecting the three inverters line to line (and the manufacturer has OK'd this) you will treat this just like a simple delta for calculation purposes. Phase difference of 120 degrees all around.
The line current in each if L1, L2, and L3 will be root three times the output current of a single inverter because the currents of the two inverters connected to that leg will add as vectors, just as you originally thought.
The 180 degree shifts only come in for two of the line to neutral currents. The high leg to neutral has a 90 degree phase shift.

 

[ggunn]

Since you are connecting the three inverters line to line (and the manufacturer has OK'd this) you will treat this just like a simple delta for calculation purposes. Phase difference of 120 degrees all around.
The line current in each if L1, L2, and L3 will be root three times the output current of a single inverter because the currents of the two inverters connected to that leg will add as vectors, just as you originally thought.
The 180 degree shifts only come in for two of the line to neutral currents. The high leg to neutral has a 90 degree phase shift.

FWIW, sometimes the utility would prefer/allow you to just interconnect with the two legs with the center tap and treat it as a 240V split phase connection. Often a high leg service is open delta with a much smaller transformer supplying the high leg, so imbalance is not an issue. You could save significantly with this approach as you wouldn't need a three phase AC combiner panel and disco.

 

[SolarPro]

[Wrong thread.]

 

[Birken Vogt]

FWIW, sometimes the utility would prefer/allow you to just interconnect with the two legs with the center tap and treat it as a 240V split phase connection. Often a high leg service is open delta with a much smaller transformer supplying the high leg, so imbalance is not an issue. You could save significantly with this approach as you wouldn't need a three phase AC combiner panel and disco.

A high leg service is always imbalanced to some degree across the lighting pot and usually very much so. I second the idea of installing the solar system as single phase across the lighting pot and ignoring the other terminal. It would be a lot of effort for no real gain to try and do it as three phase unless somebody is requiring it.