Delta wye transformer

Location
Norfolk va
Occupation
Electrician
I am new to pv. So please excuse me . i have a delta wye transformer . So I need a neutral on the high and low side of transformer. The transformer only has one neutral connection point. Can I land both high and low neutrals on the xo connection poi t of the transformer . As I’ve looked into this it seems to me the transformers orderd for this job might be incorrect. Any help would be appreciated
 
Location
Norfolk va
Occupation
Electrician
The inverters need neutral I know that I don’t see why the grid side would need it. So would I need to wire the Xfm backwards


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Carultch

Senior Member
Location
Massachusetts
The inverters need neutral I know that I don’t see why the grid side would need it. So would I need to wire the Xfm backwards
If you wire it backwards, the turns ratio and voltage ratio will be inverted. A 208V to 480V transformer, will turn into a 208V to 90V transformer. Unless this started as a 1:1 isolation transformer, you can't swap the sides to fix the problem of the topology being backwards.
 
Location
Norfolk va
Occupation
Electrician
Or is there a way to have a neutral on the primary side ... all the equipment was orderd before I got here I just inherited this job so I’m trying to figure out what to do


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GoldDigger

Moderator
Staff member
Unless the primary (POCO) side winding of the transformer is configured as a wye there is no place to connect a neutral on the primary side.
If the primary side winding is a wye and the secondary is a delta, the neutral still should not be connected to the wye point. Nor bonded to EGC or ground.
 

Carultch

Senior Member
Location
Massachusetts
Or is there a way to have a neutral on the primary side ... all the equipment was orderd before I got here I just inherited this job so I’m trying to figure out what to do
There is, but it is complicated. It involves a using an auxiliary transformer to generate the mid-point among the three phases, so you get a neutral point for grounding, and sustain the three phases so they are "equidistant" from ground. One topology for this purpose is a zig-zag transformer. Another topology is a grounded wye:delta transformer, where the grounded wye is on the side of the grid you desire to ground, and the delta lines are dead ends.

Take a closer look at the inverter manual, and see if there is a possibility of running off 480V delta, either corner-grounded or ungrounded. Some inverters can use this topology, while other inverters strictly require a wye grid, that is grounded at its neutral.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Step back. Please describe what the utility service is, the voltage needed by the pv system, and the transformer voltages.

I am guessing you have a 208/120V utility service, 480V PV system, and a 480V delta : 208/120V wye transformer. But making this explicit will help get you a better answer.

Jon
 

kwired

Electron manager
Location
NE Nebraska
Step back. Please describe what the utility service is, the voltage needed by the pv system, and the transformer voltages.

I am guessing you have a 208/120V utility service, 480V PV system, and a 480V delta : 208/120V wye transformer. But making this explicit will help get you a better answer.

Jon
sounds likely to me as well. Couldn't one connect about any imaginable 277 volt load(s) in a wye configuration parallel to the PV output along with the 480 delta transformer winding and it would probably work?

That said this transformer possibly not intended to be "back fed" though it very well may still work that way.
 

Carultch

Senior Member
Location
Massachusetts
sounds likely to me as well. Couldn't one connect about any imaginable 277 volt load(s) in a wye configuration parallel to the PV output along with the 480 delta transformer winding and it would probably work?

That said this transformer possibly not intended to be "back fed" though it very well may still work that way.
I don't think you'll get a stable grid, just by connecting three identical loads, from each phase to neutral, in order to define the neutral. Otherwise, we'd do that, instead of using grounding transformers or zigzag transformers. Eventually those loads won't be so identical. It might work initially, but if there is any asymmetry in the voltage, it would eventually supply a different amount of current to one of the loads, and possibly exacerbate the asymmetry in the voltage.

If anything that transformer is being fed in the direction it was meant to be fed in the first place. I.e. with power travelling from the delta side to the wye side. It just isn't being initially energized in that same direction, because PV production is a subordinate source to the service voltage/frequency being within specification. This topology is common for the application of generating 120/208V grids from a 277/480V service, meant for feeding the (probably) minority of loads that require 120V instead of 277/480V.
 

kwired

Electron manager
Location
NE Nebraska
I don't think you'll get a stable grid, just by connecting three identical loads, from each phase to neutral, in order to define the neutral. Otherwise, we'd do that, instead of using grounding transformers or zigzag transformers. Eventually those loads won't be so identical. It might work initially, but if there is any asymmetry in the voltage, it would eventually supply a different amount of current to one of the loads, and possibly exacerbate the asymmetry in the voltage.
If the loads are identical and line voltages remain stable why is the neutral going to drift? this is in parallel to the intended load being supplied, not the same thing as running three lines to random loads and hoping it will balance out and have a stable neutral point.

That said I don't know what the PV equipment is looking for in neutral stability, but as long as the three lines are stable that neutral point should remain stable. If load conditions cause voltage imbalance, the PV equipment possibly goes off line, if that is a persistent problem utility supply and or separately derived system transformer is possible undersized or has problems?
 
If the loads are identical and line voltages remain stable why is the neutral going to drift? this is in parallel to the intended load being supplied, not the same thing as running three lines to random loads and hoping it will balance out and have a stable neutral point.

That said I don't know what the PV equipment is looking for in neutral stability, but as long as the three lines are stable that neutral point should remain stable. If load conditions cause voltage imbalance, the PV equipment possibly goes off line, if that is a persistent problem utility supply and or separately derived system transformer is possible undersized or has problems?
An interesting idea. Could be even just the smallest 277 control transformer you could find connected in a wye, indicator light, etc. I assume these inverters only use a few milliamps.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If you want to synthesize a neutral, you specifically want to add something that has lower impedance than the neutral loads to be served.

If the neutral load is just an inverter voltage sense, then you might get away with a balanced resistor wye.

But to really do the job you need something designed to be low impedance without being a huge parasitic load itself. This is where a zig-zag transformer or the wye of a wye-delta transformer comes into play.

You can construct a zig-zag with 240:240V transformers.

Jon
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
Inverters have a significant amount of common mode noise on their outputs because of the limitations of 2-level or even 3-level PWM in synthesizing a sine wave. So I'm wondering how tolerant the inverter line outputs would be in driving a transformer delta winding without a ground reference. In such a case the common-mode impedance looking into the transformer would be very high, and so I'd expect larger noise voltages could be developed on the inverter's line outputs. The inverter IGBT ouput devices may not care, but perhaps internal sensing circuitry for the output voltage and current could be less tolerant. I don't know if this issue is really going to be a problem, but I'm bringing it up because the non-standard configuration being discussed creates additional unknowns.
This is really a separate issue from having a neutral for the inverter.

I noticed that SolarEdge 3-phase inverters for 208V can be connected as either a 4-wire wye or 3-wire delta. Others require a 4-wire wye only. However, I believe the expectation is that a 3-wire connection would be made to a 208V system that's sourced by wye connected secondaries on the POCO side that would be ground referenced and not completely floating. SolarEdge does not mention a requirement for a grid connection that is ground referenced but perhaps that hasn't been a issue to merit their attention.

Ultimately, the inverter manufacturer would need to verify whether it would work into delta transformer windings without a ground reference.
 
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