3-phase question was: Silly question?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Eric Davidson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <edavidson-at-icva.gov>

David,

The wye/wye connection is indeed susceptible to unbalance and harmonics if
the midpiont is not solidly grounded.  If you look closely at the
connections on the 3-phase banks in you area, you will find that the
midpoint is not left floating, but connected to ground.  An advantage to the
wye connection is that each transformer in the bank sees 1/3sqrt of the
phase to phase voltage, lessening the insulation requirements.  On the
secondary side the wye connection provides for grounding and dual voltages
e.g. 120/208 or 277/480.  The delta connection is not as easily grounded.
Usually one of the midpoints is grounded and this provides for 120/240
volts.  Three phase service can also be obtained with 2 transformers using
the open delta or scott connections.  The open delta connection can sorta be
viewed as 2/3 of a wye connection with the midpoint grounded.  Hope this
helps.

Eric
edavidson-at-icva.gov


> Now, I have a question about utility three phase transformer
> service that is bothering me and I was hoping some of the re-
> sident engineers could answer it  for me. I've read that a wye/
> wye three phase transformer hookup is rarely used by the
> utility power companies because of harmonics and balancing
> problems. However, virtually all of the three phase pole pig
> banks that I see in my local area (Memphis, TN) are hooked
> up exactly this way (all three phases sharing a common
> ground return on both the primary and secondary sides of the
> transformer). I was wondering if anyone could shead light on
> this for me?
>
> Sparkin' in Memphis, TN, USA,
> David Rieben