3 phase transformer help

[tjaite]

I am trying to troubleshoot a system with 3 single phase transformers bank.
480v delta on the high side and 120V wye on the low side.

on the low side....
L1-Nuetral=120v
L2-Nuetral=120v
L3-Nuetral=120v
L1-L3=208v
L1-L2=120v
L2-L3=120v

My question is, is this normal? shouldn't I have 208v across L1-L2, L2-L3, and L1-L3?

Any help is greatly appreciated
TJ8

 

[templdl]

It appears as though you may have lost your 'Y' connection between the X2 and X0.
Disconnect the transformer, disconnect all loads, and check for continuity between the X1 and X3, X1-X0, X3-X0 all of which should show continuity.
Now check the continuity between X1-X2, X2-X3 and X2-X0 which most likely will show open.

 

[StephenSDH]

L2 sounds fishy. Ohm on the secondary side (with power off) phase to neutral on all three phases. You should see the same low ohm resistance of the windings. A guess would be neutrals are not tied together or L2 is connected to neutral.

 

[Rick Christopherson]

You have your B-phase reversed, where its normal neutral point is the phase point, and vice versa. To make this easier to understand I have drawn it out. This is a scale drawing from SolidWorks and the dimensions shown are actual dimensions of the lines (all lines are 120 long, but I removed the dimensions so it wasn't so cluttered).

The Red Dotted line is where your B-phase is supposed to be, but the pink line is where it is actually located. This is the only position where this phase can have 120 volts to neutral and to the other two phases. It is also exactly 180 degrees from where it is supposed to be.

Sorry, I have tried to make the image larger, but it didn't work.

 

[StephenSDH]

Nice Drawing

 

[hillbilly1]

If the load is still connected, I would also look for L2 fuse on the primary to be open, this is assuming that it worked before, but stopped working. The load still being connected will make it appear to have 120 volts to neutral on all phases.

 

[Smart $]

tjaite,

I concur with Rick's assessment... but just in case you aren't quite sure what to do about it, simply swap either the primary line connections (H1-H2) or secondary line and neutral connections (X1-X2), but not both sets connections.

If the secondary connections are like the other two transformers, e.g. L1, L2, and L3 connected to X1 and N(eutral) to X2 of their respective transformers, I would leave the secondary as is and swap connection on the primary.

 

[don_resqcapt19]

Is this a new install being tested before use, or is this an existing system with a problem that you are trying to troubleshoot? If it is the second, then the windings are not reversed.

 

[tjaite]

Guys
Thanks for all the help.

I got some bad information from the guy who explained to me what his problem was.
Once I looked at it in person I found they did not have 120v from all phases to neutral.

I figured then that either a transformer in the bank went bad or there was a loose connection somewhere.

Sure enough there was a loose connection, fixed it up and we are good to go.

Again, Thanks for the help.