Delta High Leg

Zyb

Zyb

Member
Location
Maine
Occupation
Design Engineer
I have one SolarEdge SE30KUS Inverter (30,000 Max. Power). going to a transformer Delta-Wye
1.) Do I need neutral on both sides of the transformer?
2.) Is my calculation correct for the current on the other side (30,000 / (240 x 1.732)) = 72A ?


delta.png
 
You cannot connect a neutral to the 240V delta primary side of the transformer; there is no lug for it, but that's a good thing because you would not want one. The primary of the transformer is 240V 3P3W and the secondary side is 480/277V 3P4W. The secondary neutral must be grounded but you do not need to run a neutral conductor to the inverter as long as the N to G strap in the inverter is installed; I believe they ship that way from SE.

You must use the manufacturer's published Imax number rather than calculating it; the max current for the inverter is 36.25A, so the current on the primary side is simply (36.25A)(480V/240V) = 72.5A
 
tortuga said:
Is this a standard utility open delta bank / service?
Or a closed delta?
Click to expand...
Obviously, the POCO must approve the connection (some will not allow a PV system to connect to the B phase), but it is a small-ish PV system that would load the transformer bank with only 5.8kVA per phase if there are no loads.
 
ggunn said:
Obviously, the POCO must approve the connection (some will not allow a PV system to connect to the B phase), but it is a small-ish PV system that would load the transformer bank with only 5.8kVA per phase if there are no loads.
Click to expand...
Sorry, I do not know where my math went south, but obviously a 30kW inverter even through a transformer will load the utility transformer bank by a maximum of 10kVA per phase. Pesky sqrt(3)!
 
  • Like
Reactions: Zyb
ggunn said:
Where is that 0.867 coming from?
Click to expand...
It is used with open delta configurations.

The math is such that a delta made up of 3 equally sized transformers is simply 3X any one transformer; 3 x 10 = 30kVA effective
But because there are only two transformers the effective bank kVA is 57.7% of what it would have been using 3 transformers; 3 x10 x.577 = 17.31kVA
But the total of the transformers is 2 x 10 = 20kVA and playing with the math 17.31/20 = .866.
 
ggunn said:
Where is that 0.867 coming from?
Click to expand...
It's sqrt(3)/2, and applies to balanced 3 phase loading on an open delta.

If you connect three 2-wire 1 amp loads in a delta configuration across a closed delta, you get 1 amp in each coil of the closed delta. If you now delete one coil, that coil's current has to travel through the other two coils. In those coils the current is 60 degrees out of phase with the current from the load across that coil, so the current in each of the two coils is sqrt(3) amps.

So in the closed delta, for 3 amps of L-L load, you are using 3 amps of coil capacity, a ratio of 1. While for 3 amps of L-L load, you are using 2 * sqrt(3) amps of coil capacity, a ratio of 3/(2*sqrt(3)) = sqrt(3)/2.

Cheers, Wayne
 
You must log in or register to reply here.
Top