floated wye/floated wye

[resqcapt19]

I recently worked on clearing a ground fault on a 1200 amp 3 phase 480 volt service. After the ground fault was cleared I had 490 volts phase to phase and about 900 volts phase to ground on all three phases. It was discovered that when the utility built the transformer bank they did not connect the primary wye point to ground as required by their standards. I understand that with both sides of the transformer bank floated that the voltage to ground can be just about any value and I understand that there is very little current to ground, but my question is, can this voltage to ground damage the conductor insulation over time?
The utility did connect the primary wye point to ground and the voltage to ground on the secondary went down to ~300 volts to ground.
Don

 

[busbar]

Extreme caution concerning ungrounded low-voltage systems have been around for well over 50 years. See

http://6L6.net/elm/etip.beeman.IPS6.doc

 

[alehman]

Yes. It need not take a lot of time either.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[waross]

The normal voltage to ground on a 480 volt wye system should be 277 volts. with 490 volts phase to phase, ~300 Volts to ground is ~correct. The secondary should have the wye point grounded, either solidly or high impedance. We ungrounded or high impedance you should have a ground detection/monitoring system.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[resqcapt19]

Bill,
The customer wanted an ungrounded system so the utiltiy provided a floated wye secondary. Not sure why they didn't supply the customer with a delta on the secondary, but they didn't. They provided a floated wye/floated wye, when their standard called for a grounded wye/floated wye for this installation. My concern was the ~900 volts to ground on all three phases when the primary of the bank was floated.

 

[resqcapt19]

busbar,
My understanding of the Beeman reference is that the high voltages were produced as a result of a ground fault on the 480 volt side of the ungrounded system. I had the high voltages to ground with no faults...the result of the capacitance of the system, I think. It was just the fact that I had ~900 volts phase to ground on all three phases before the utility connected the primary wye point to ground. After they made that connection I had about 300 volts to ground on all 3 phases.
I don't think that there is any current behind the 900 volts, but my question is if that voltage can damage the insulation on the conductors and the equipment.

 

[waross]

The high voltage was probably as a result of capacitive coupling between two ungrounded windings (primary and secondary).
We regularly megger test 600 Volt circuits with 1000 Volt meggers. A standard H-Pot test voltage standard for years was {2 X rated voltage + 1000 Volts} (1960 volts for 480 volt circuits).
Some equipment, mostly electronic, will not withstand 900 Volts to ground and will fail quickly.
Summary: Anything that would be damaged by the over voltage has probably already failed. Anything that has not failed already will probably not fail.
I had an experience some years back where the utility changed out a transformer bank and punched 416 Volts into a 208 Volt system. We had previously installed over voltage and reverse phase protection on the critical refrigeration equipment, and all the protected equipment survived. Almost all the unprotected equipment failed immediately but we had no "late" failures.
Note, we had double phase to phase voltage and you had a very high common mode voltage. But I surmise that the condition persisted for at least several hours if not several days. After such an exposure I will be surprised at any late failures.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[busbar]

{Sorry for the delayed response} resqcapt19, yes indeed one can expect higher-voltage [more importantly] transients on a floating secondary! That you measured ~300V to ground on the secondary phases indicates that (1) primary-to-secondary winding capacitive coupling, and (2) secondary-side phase-to-ground capacitive coupling were (fortunately, and at that point in time) balanced. BUT, you can’t rely on the balanced condition to remain stable.

One solution to maintaining “service continuity” where there are NO ø-neutral loads is by installing high-resistance grounding. See

http://www.neiengineering.com/pdfs/paper1JN.pdf

A search on Eng-Tips should return lots of hits.