Reversed Transformer

kec said:
Does not seem right to me.
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This is a frequent wiring error. Your failure is more extreme than typical.

We install per the NEC, we are not utilities, we should not be bonding primary side X0 terminals, especially if they are not MV.
 
From what I can see the install needs evaluated and corrected.

(E) Primary OCPD sizing
(E) Primary conductors for damage and sizing.
(C) Equipment ground bar needs installed at transformer to its enclosure.
(C) A GEC need ran to a qualified electrode.
(C) XO need to float (attached to nothing).
(E) Secondary conductor sizing based on secondary OCPD ( looks like 4/0 in and out).
(C) Corner ground secondary with a SBJ or run as ungrounded delta.

Looks like ser cable ran on secondary with the bare taped green( SSBJ). Should be ok on size if the cable is 4/0 with 2/0 bare.
If so the secondary OCPD can no be larger than the cables ungrounded conductors 4/0 ampacity.

This is an example of the conductors withstand rating in IT² based on the EGC size.
In this case to much ampacity for to long based on the size of the conductors CM. The OCPD only seen this as load and no reason to open. By the time it did the damage to the insulation was done. The root cause is the tie to XO as many have mentioned. A way to look at it too small of wire for the load. An extreme example may be 100 amp load on a #12 with a 100 amp breaker. The #12 being the netural and the 4/0 being the ungrounded. When a phase is dropped on a three phase balanced load the #12 is used to carry the unbalanced load.


This is why we want the OCPD to open very quickly when a fault is detected on the EGC. In this case you used it as a normal current carrying conductor which was sized to small for the load condition by attaching to XO on a reverse feed configuration.

Thanks for sharing the pic.
 
herding_cats said:
You would if a neutral was fed through it?
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If you have a transformer installed as an SDS, the only voltage relationship between the primary and secondary voltage systems is defined by the grounding choices on each side. If the secondary coils have no neutral (are delta), you can't use a primary side neutral with the secondary side conductors to form a circuit. It would be a fault.

If the secondary is corner grounded, and the primary wye is grounded as always, you'd have secondary side L-L voltage between the primary neutral and two of 3 secondary conductors, and no voltage between the primary neutral and the third secondary conductors, as they are both grounded.

If the secondary is ungrounded, then the voltage from a secondary side conductor to the primary neutral would be unstable and not sufficient to power a load (absent a fault). If measured with a high impedance meter, it may often measure near what the L-N voltage of the secondary would be if it had a neutral conductor.

Cheers, Wayne
 
herding_cats said:
You would if a neutral was fed through it?
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Not if the transformer is delta. You would need the correct a star configuration on the windings.
Figure+5.png
 
wwhitney said:
If the secondary is ungrounded, then the voltage from a secondary side conductor to the primary neutral would be unstable and not sufficient to power a load (absent a fault). If measured with a high impedance meter, it may often measure near what the L-N voltage of the secondary would be if it had a neutral conductor.

Cheers, Wayne
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Right, trying to power a load from a ungrounded secondary side conductor and the primary neutral would be like trying to jump start a car from two other cars, A and B, using the negative of car A and the positive from car B. Damn u Kirchoff! 🤬
 
jim dungar said:
One problem with connecting the X0 of a reverse fed wye is when unbalanced currents flow, such as when the utility has a phase loss. By forcing the wye point to stay as a neutral point, rather than float around, the transformer draws increased current on the X0 to source neutral conductor.
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Agree with Jim 100% here. This is a common wiring mistake, and in "normal" conditions, bonding the X0 maybe won't lead to a disaster straight away - but it can cause unbalanced current to flow (in the neutral / EGC) and create unnecessary heat in the transformer.

There will always be small imbalances between the neutral to line voltages on the POCO mains. As these 3 slightly different voltages energize the primary side of the transformer (along with tiny differences in the transformer's 3 primary winding impedances) they will create a new transformer X0 that might not be exactly at the same potential as the POCO X0. So you don't want to tie them together. You don't need it - let the X0 float.

I like to think of X0 like the X=0, Y=0 origin on a piece of graph paper when you draw a nice symmetrical 3 phase set of Wye vectors. The POCO X0 and the new primary X0 in the transformer will never line up exactly. They will normally be close, but the "real world" will cause them to be offset and never perfectly aligned.

In "normal" conditions, if they are connected, the POCO source will be the bully, and fight hard (by creating current flow) to draw both X0 points together. And an important thing to remember, regardless if the X0 bond is via a neutral or EGC, there is typically no over-current protection on that conductor.

If the POCO dropped a phase, then those are certainly "abnormal" conditions and odd things can result. What I think would happen here is that the primary Wye windings in the transformer will try to create the dropped phase voltage. The Secondary will still see all its three windings fired up (because it's a closed Delta) and that can magnetize the winding back over in the primary that is NOT being fed by the POCO, energize it, AND reverse feed all the loads back in the mains (and maybe even back into the POCO world?). This would be a very unbalanced situation and the POCO and transformer midpoints (X0) will absolutely get pulled farther apart than normal, with objectionable current flow between these 2 midpoints. This will create abnormal current in a conductor (or the steel in an RGC/EMT if that is the EGC) that has no path thru a circuit breaker to stop it. And often times that conductor is undersized to boot.

I did an experiment a few years back in my shop to supply a Wye - Delta transformer only 2 of 3 phases and a neutral to see if it would generate the 3rd phase on the unconnected Wye winding . It absolutely did. Ran a little 3 phase motor just fine.

That's my 2 cents, feel free to poke holes if you see them.
 
MD Automation said:
Agree with Jim 100% here. This is a common wiring mistake, and in "normal" conditions, bonding the X0 maybe won't lead to a disaster straight away - but it can cause unbalanced current to flow (in the neutral / EGC) and create unnecessary heat in the transformer.
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about 8 years ago, I had client lose a phase and they had a miswired reverse fed transformer. I guess things got so imbalanced and hot that a primary conductor faulted to the 2"emt and blew a big hole in it.
 
herding_cats said:
I’ve never seen 480 delta. Is there a wild leg in this configuration?
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Mostly industrial. It will be corner grounded, where two of the phases will be 480 volts to ground, while the third will be zero volts to ground, 480 volts to the other phases. There is also a delta ungrounded system, where voltages to ground can fluctuate greatly, if any, to ground, but require a ground detector to indicate a phase has a fault to ground. OCP will not trip until a second phase goes to ground on that system.
 
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