Delta wye?

[rathgar]

Please assist with the following situation. I am looking to see if anyone else has ever encountered this phenomenon.

Power logged secondary of a delta wye 480/208/500 KvA xformer for six hours. After about four hours, the RMS on one leg(L3) dropped to about 40 volts from 120 volts, with simultaneous jump in voltage from about 120 volts to 145 volts on the other two legs(L1&2). current on L3 at the time went from~170a to ~120a,L1 current went from ~260a to ~180a,L2 current had no appreciable change.Here's the kicker....

A neutral current jump occurred simultaneously, from about 160 amps to 760 amps,and remained that way till the logger stopped
(2 hours)---Following morning all systems were normal
xformer feeds office building tenant outlets mostly L-N loads

 

[waross]

Have you verified the voltages and currents independently of the logger? Did you get complaints from about one third of the occupants that their computers and coffee machines had stopped? If not, check the data logger for an intermittent fault.
respectfully

 

[jghrist]

My first guess was a loose neutral connection causing the neutral to be displaced with respect to the phase voltages. But this is not consistent with the increase in neutral current. But then, nothing is consistent with the measured neutral current. Even if L1, L2, and L3 currents were all in phase by some strange phenomenon, they would not add to 760A. That is assuming that L2 current was less than 460A (you don't say what it was).

 

[waross]

Where were the measurements taken? At the transformer, at the main panel or at a sub panel. What is the AWG size and length of the neutral conductor from the transformer to the point that the data logger was connected? Where in relation to the data logger is the grounding point?
Is the length and impedance of the neutral from the transformer to the data logger consistent with a voltage drop of about 80 volts at 760 amps?
Is this the only transformer in the building?
I see two possibilities, a heavy current on the neutral from another system, possibly a ground fault combined with improperly interconnected grounds and neutrals, or a problem with the data logger.
respectfully

 

[rathgar]

L2 current was about~160a.-Haven'nt heard about any tenants w/o power Current and voltage measurements were made right at the xformer secondary.grounding point is at 8" water main about 80' away --did put a clamp on ground the following morning showed 1.5a on xformer grounding wire.----there was lightning in the area on the day in question but the fault seemed to last too long for this to be the explaination.
There is one other 25kva emergency xtransformer --have not ruled out a bad logger---although it's new--
Thanks

 

[waross]

Are you somewhere cold? If you made your neutral voltage connection to the transformer case instead of to the X0 terminal, AND someone was using a high current to thaw water pipes, the high neutral current and the voltage shifts may be reconciled.
I am sure that if one phase actually went to 40 volts you would hear about it.
respectfully

 

[jghrist]

Barring a bad logger, the most likely explanation is stray ground currents as indicated by Waross. This could happen if there were multiple grounds both on the 120/208 neutral where the measurements were made and on some other neutral (not necessarily another 120/208 system), and an open neutral on the other system.

An open neutral on the other system could force neutral current through the ground and some could flow through the 120/208 neutral if it were grounded in two places. Voltage drop in the neutral could produce imbalanced voltages at the transformer but balanced voltages at the load.

This explanation only is possible if the neutral measurements were made on the load side of the transformer neutral ground.

It might even be utility primary neutral current flowing through the ground.