3 Phase Transformer Neutral Current

Location
Dominican Republic
Occupation
Electrical Engineer
Greetings,

I have tried to find information about this issue that we have presented several time and still does not have a clear explanation. We tipically do SOLAR PV installations and in a lot of cases need to install a step down Dry type transformer in order to interconnect 480V (Delta) PV inverters to 208 Wye electrical systems. The problem that we face is that when we connect the neutral conductor between the existing distribution transformer and the new dry type transformer a high current >200A runs between the 2 transformers in the neutral conductor, even with no load. I have read some things about coupling impedance on parallel transformers, but really don't now what a practical solution for this cases could be. The result is that we have had in a couple of cases a burned neutral that have affected power lines on the same conduit and caused short circuit and damage.

A solution proposed have been to use a Delta 208V / Delta 480V transformer and avoid the neutral current, but truth is i don't know how a 208V Delta would behave connected to a Wye system.

Anyways i would like to understand why is this happening and what is the typical engineering solution to this.

If anyone have faced this or know a practical solution please let me know.

Best regards,
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
It sounds like you are connecting the neutral on a 208/120V wye - 480V delta transformer to a 208/120V system. If so, it's not surprising that you would draw a high current when the neutral is connected. On such a transformer the 120V L-N voltages are stepped up a factor of four to 480V, and then the three secondary windings are connected in series to form a delta. If the three L-N input voltages are equal, then the open circuit voltages across the three secondary windings will add up to zero volts, and so no current will flow in the closed delta of the secondary. But if the L-N voltages from the utility are not equal, then this voltage mismatch can cause a large current to circulate in the secondary windings and therefore high currents on the primary side.

You could leave the neutral on the 208 wye of your transformer disconnected to prevent these unwanted currents. But a 480V delta secondary may cause problems with your inverters unless it's allowed by the manufacturer. Even if the inverters don't need a neutral, with a 480V delta secondary the line-to-ground voltages could easily get unbalanced (even as high as 480V L-G) and that may cause problems or failures in the inverters.

I believe a 208V delta - 480/277V wye transformer would be best in your case. A 208/120V wye - 480/277V wye should also be OK.
The thread at the following link discusses the situation you have:
https://forums.mikeholt.com/threads/is-it-possible-to-change-delta-to-wye.2560403/#post-2662586
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I agree with syncro.

Another way of putting this is that a wye:delta transformer is trying to 'derive a neutral' and hold the center of the wye at the neutral of the three phase conductors.

When you connect a delta:wye transformer 'backward' as you've done, and connect the transformer wye to the system neutral point, then the transformer does its upmost best to force the system into balance, with large circulating currents on the delta side and large currents on the primary side neutral.

I also agree that a 208V delta - 480/277V wye transformer will be the best solution, unless the utility has an 'effective grounding' requirement that necessitates a wye:wye transformer.

-Jon
 
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