Phase Rotation with 300KVA XFMR

[Wessling30]

A real head-scratcher:

We cutover a feeder to a new breaker serving a 300 KVA xfmr. The original breaker was a 600A but the engineers downsized the new breaker to a 400A after reviewing the metering and realizing the load was very low. When we reenergized the xfmr from its new source we found rotation was backwards. We then swapped rotation and tried closing the breaker but it tripped instantly.
Thinking we may have shorted a wire or crossed phasing, we took apart our wiring and tested everything. All was good until we landed the wires back on the primary and tried to reenergize again. The breaker still tripped instantly. So we put the rotation back to the original way we had it (counter-clockwise) and the breaker would hold?? No one could explain it. I went back to the new breaker and turned the instantaneous setting all the way up and this time the breaker would hold with the correct rotation. What I don't understand is why the breaker would hold with counter-clockwise rotation, but trip with clockwise rotation before adjusting the instantaneous trip setting? There was no load on the secondary side of the xmfr either. Would the inrush from the transformer change based on rotation?

 

[kwired]

I'm going to say mostly is a coincidence of closing the circuit at the right moment in time to have higher inrush current when it did trip?

Especially if you had not turned up the instantaneous trip setting until about the very end there.

The current may been very near the IT threshold and likely would not have tripped every time had you tried multiple times.

 

[winnie]

I somewhat agree with kwired.

Inrush current is somewhat random, depending upon the residual flux in the core and the specific phase timing of when the supply breaker closes.

If you close the breaker perfectly synchronized with the residual flux in the core, you get _no_ inrush.

One could argue that by reversing the rotation you made it _impossible_ to synchronize the applied voltage with the core residual flux, since with the reversal when one phase is synchronized the other two phases have to be wrong. To demonstrate this, now that you have the phase rotation correct you would dial back the IT setting, and see if it holds when you energize, but trips when you reverse the rotation again.

But given that you have the system working now, I'd leave it alone!

-Jon

 

[Wessling30]

I'm going to say mostly is a coincidence of closing the circuit at the right moment in time to have higher inrush current when it did trip?

Especially if you had not turned up the instantaneous trip setting until about the very end there.

The current may been very near the IT threshold and likely would not have tripped every time had you tried multiple times.

We tried exercising the breaker several times, both counter and clockwise, and it would only hold on counter. Very strange

 

[Wessling30]

I somewhat agree with kwired.

Inrush current is somewhat random, depending upon the residual flux in the core and the specific phase timing of when the supply breaker closes.

If you close the breaker perfectly synchronized with the residual flux in the core, you get _no_ inrush.

One could argue that by reversing the rotation you made it _impossible_ to synchronize the applied voltage with the core residual flux, since with the reversal when one phase is synchronized the other two phases have to be wrong. To demonstrate this, now that you have the phase rotation correct you would dial back the IT setting, and see if it holds when you energize, but trips when you reverse the rotation again.

But given that you have the system working now, I'd leave it alone!

-Jon

lol yes we're not touching it again. Thanks Jon

 

[LarryFine]

I might have tried reversing the secondary rotation.

 

[Besoeker3]

I might have tried reversing the secondary rotation.

Not quite sure why?

 

[LarryFine]

Not quite sure why?

Practicality: So the breaker would hold and you would still have the load rotation correct.

Curiosity: To see whether the issue was with the transformer or with the supplied load(s).

 

[Besoeker3]

Practicality: So the breaker would hold and you would still have the load rotation correct.

Curiosity: To see whether the issue was with the transformer or with the supplied load(s).

Yes, I can understand if the loading was reversed. But simply reversing it would surely fix that?

 

[Hv&Lv]

Almost sounds like there some kind of neg seq protection in play.
Rotation swap on the primary side of a transformer (depending on configuration) changes the displacement, but if you aren’t paralleling anything that shouldn’t matter.

 

[LarryFine]

Yes, I can understand if the loading was reversed. But simply reversing it would surely fix that?

Unless I completely misunderstood the OP, he said that the primary breaker would only hold when the rotation was in the wrong direction for the loads. When he tried reversing the primary to correct that, the breaker would trip.

I'm suggesting that, by reversing the rotation direction of the secondary instead of that of the primary, it would have allowed him to correct the rotation direction to suit the loads, while also allowing the primary breaker to be reset.

 

[Besoeker3]

Unless I completely misunderstood the OP, he said that the primary breaker would only hold when the rotation was in the wrong direction for the loads. When he tried reversing the primary to correct that, the breaker would trip.

I'm suggesting that, by reversing the rotation direction of the secondary instead of that of the primary, it would have allowed him to correct the rotation direction to suit the loads, while also allowing the primary breaker to be reset.

Probably so. On the systems I have worked on, mostly industrial, power systems were all ABC or RYB. If the 3-phase loads needed reversing they were reversed.

 

[Little Bill]

I might have tried reversing the secondary rotation.

OP said this

There was no load on the secondary side of the xmfr either.

 

[texie]

The OP says this is a 300 KVA xformer. A 400 amp primary breaker is is way too small and is likely to trip on initial inrush, especially if unloaded. The fact that it does not trip on "reverse" polarity is just coincidence.

 

[Hv&Lv]

The OP says this is a 300 KVA xformer. A 400 amp primary breaker is is way too small and is likely to trip on initial inrush, especially if unloaded. The fact that it does not trip on "reverse" polarity is just coincidence.

Agreed. calculated inrush at about 1600 amps.
Probably going to nuisance trip on power outages or blinks.

 

[LarryFine]

OP said this

Then how did he know the rotation was wrong?

 

[Hv&Lv]

Then how did he know the rotation was wrong?

I was sort of wondering that myself.
I assumed he must have had a rotation meter but still curious.. I don’t see many of those in an electricians arsenal..

 

[LarryFine]

I was merely pointing out that reversing the secondary would have been as effective as reversing the primary to correct the rotation error, while allowing the primary to remain in the functioning configuration.

 

[Little Bill]

Then how did he know the rotation was wrong?

You'll have to ask him.

 

[Wessling30]

Then how did he know the rotation was wrong?

I had a rotation meter, when the breaker held on counter rotation we were able to test at a disconnect on the secondary side of the xfmr