Transformer Question

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CIECO

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I have question for you guys and maybe I am over thinking the problem. First of all we are installing a machine from China with no UL or anyone else?s stamp which is not my problem it?s in the OEMS and Owners hands. Now for my question, the machine came with two 125 KVA 220/380 transformers. According to the prints they should have bin 220 Delta to 380 Y but they came in as 220 Y to 308 Y with both Y connected to the same XO and the machine is using a good bit of 220. Now I am concerned about a single phase condition on the 220 side will over load the XO ground to the building steel what is your opinion. I don?t know if I should leave the ground off of XO or run a full size ground from XO to the neutral buss were the service is grounded but looking at the transformer pad for the building the neutral is a #2 and the water pipe ground is a 3/0,(a 1940s service) I am looking for a safe way to go.

Thanks in advance. Happy Holidays
 
CIECO said:
Now I am concerned about a single phase condition on the 220 side will over load the XO ground to the building steel what is your opinion. I don?t know if I should leave the ground off of XO or run a full size ground from XO to the neutral buss were the service is grounded but looking at the transformer pad for the building the neutral is a #2 and the water pipe ground is a 3/0,(a 1940s service) I am looking for a safe way to go.

Thanks in advance. Happy Holidays
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Single phase or unbalanced loading should cause to flow in the neutral (star connection to the Y winding) not in the ground. Put otherwise, it should flow from the transformer back to the transformer.
 
CIECO said:
But the XO term is grounded
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Grounding gives a fixed voltage with respect to ground. It is not the normal current path.

Edit:
I don't think that I explained that very well.
The transformer secondary is Star (wye) connected.
Three phases and neutral.
If you load one phase only, the current will flow from that phase to the transformer neutral. Regardless of whether that neutral point is grounded.
It doesn't need to flow through any steelwork or anything else in normal operation.
 
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CIECO said:
But if the pri 220 single phases you will draw FLC threw the star/XO/ Ground.
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Yes, to the star point, the neutral.
The fact that it happens to be grounded should not change that.
For 125kVA and 380V the rated line current is about 190A. If you load one phase only at full rating, you will get that current flowing from that phase to neutral. Even though that point is grounded, none of the current should flow anywhere in the installation other than the transformer windings and the phase and neutral conductors in normal operation.
It should not flow through any building steelwork.

The same applies on the primary side.
 
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