voltages on floated wye/wye system

[don_resqcapt19]

What voltage would you normally expect to see on the secondary side of a wye/wye transformer bank where both wye points are "floated"? We had the service disconnect open and I was seeing ~900 volts on each phase to ground, and 495 phase to phase. The utility came out and connected the primary wye to their static ground and the phase to ground went down to more what I would expect on this type of system. 365, 316, 268.

 

[charlie b]

There is no number that I would "expect." The notion of floating is that any number is possible. It is a throw of a dart, and it can change from moment to moment.

 

[bob]

Don
More information
http://books.google.com/books?id=CT...lb2BDA&sa=X&oi=book_result&ct=result&resnum=2

 

[don_resqcapt19]

Charlie,
I just have never seen the line to ground voltage that much higher than the phase to phase voltages on other ungrounded systems, but I don't think I have ever worked on one with the transformer bank wired floated wye/floated wye before.

 

[don_resqcapt19]

Bob,
Thanks for the link. The page does say that a floating wye/floating wye connection should be avoided and the utility standard says that the primary wye should have been grounded, it just didn't get built that way when they installed the bank. They added the grounding to comply with their installation standards and the line to ground voltages are in the range I have seen on other ungrounded systems in the past.

 

[mull982]

I had a similar situation once on the secondary of a 120/240V transformer where the L-G voltage on the secondary was reading up to 600V which was much more than the 240V L-L reading. It turned out that the neutral bond to ground on the secondary of the transformer had been disconnected.

I belive someone had said that the higher L-G voltage was due to a ground reference.

 

[jim dungar]

Charlie,
I just have never seen the line to ground voltage that much higher than the phase to phase voltages on other ungrounded systems, but I don't think I have ever worked on one with the transformer bank wired floated wye/floated wye before.

I have seen some ungrounded systems with almost 2x normal voltages. Unbalanced coupling capacitance can do 'strange' things.

 

[don_resqcapt19]

I have seen some ungrounded systems with almost 2x normal voltages. Unbalanced coupling capacitance can do 'strange' things.

That is what I had before the utility grounded the primary wye.
Is this a "real" voltage? That is if it is high enough can it damage the conductor insulation?

 

[mcclary's electrical]

How can voltage damage conductor insulation? And theoretically, no it's not a real voltage, there is no true reference there for a meter to base it's "difference of voltage" between your two leads. Just go by the phase to phase reading, and trust that.

 

[wirenut1980]

How can voltage damage conductor insulation? And theoretically, no it's not a real voltage, there is no true reference there for a meter to base it's "difference of voltage" between your two leads. Just go by the phase to phase reading, and trust that.

Voltage that is higher than the conductor's insulation rating can and will damage the insulation, because current can now flow through the insulation causing it to break down.

Don, that is a good question and I am going to say that the insulation could be damaged, even though the voltage would decrease substantially if a load was connected phase to ground. I can't remember, but I think 600 V is the standard rating on that class of cable insulation...

 

[drbond24]

How can voltage damage conductor insulation?

Insulation is only rated for certain voltages. Above those voltages, it becomes less and less good at being an insulator as the voltage goes higher. An insulator isn't perfect, it is just a bad conductor. If it gets enough voltage, it will break down and decide to start conducting more and more. That is bad.

Also, the voltage on the line is constantly stressing the insulation. There are formulas for calculating voltage stress, but I believe the basic idea is that if voltage got too high then the insulation would be aging at a faster rate than normal. I'm willing to accept I might not be 100% right on that, but I'm at least 99.5%.