Re: 3-phase question was: Silly question?

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

Original poster: "harvey norris by way of Terry Fritz <twftesla-at-qwest-dot-net>" <harvich-at-yahoo-dot-com>


--- Tesla list <tesla-at-pupman-dot-com> wrote:
> 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.
Thanks for this good explanation. I am somewhat at a
mystery as to how a delta could even be grounded.
Correct me if I'm wrong here, but in the 4 wire wye
delivery, the 4th wire is grounded, both at the
generator and the recieving end, and furthermore no
current will be in that wire if the loads on all 3
phases are balanced. We can then know that if the
voltage is obtained in wye, one side will be grounded.
In the above explanation concerning the dual voltages
available from the wye connection, when we choose the
higher voltage option, which is 1.7 times the wye
phase value, that then is actually a Wye to Delta
application, or conversion. Is a ground really
necessary at all in that situation? Doesnt the power
station itself having the ground help any? That would
mean or imply that ALL 3 phase power is obtained from
wye stator field arrangements, and not delta, thus
making every delta load application essentially a wye
to delta conversion, correct? If a delta ground were
necessary, what would you do, arbitrarily ground one
side? Thanks for any answers here.

It is also noteworthy here to say some complications
involved with a wye to wye transformation. Since it is
presumed that all conventional three phase
(emf)outputs themselves are generated in wye, I also
noticed that this is generally true with the stator
fields on a alternator, having 4 connections. Since
the application of loads in delta gives the higher
voltage, I was exclusively interested in obtaining
voltage in that manner. For getting higher voltages I
decided to try a 15.7 KVA 3 phase transformer, and the
one tried happened to be a Delta primary to Wye
secondary. That actually makes two conversions, wye to
delta on primary, and secondary output as wye, meaning
a total wye to wye conversion. The first thing I
noticed about the voltage secondary output on the
first phase being measured, that it seemed way higher
than it should be, where I could then obtain 600 volts
in 30 some volt primary stator input at 480 hz. But
the primary amperage consumption for that phase also
was higher than the proportionally increased amount of
current the primary should draw, based on the measured
secondary output. This transformer was made as a step
down tranformer, and not a step up, so that might
account for the higher than predicted primary
amperage. However it was later learned that the
voltages on the three phases were not equal! In fact I
was getting 600, 400, and 200 volts for all three
phases. So there is an example of the problems
involved with wye to wye transformations.

> 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
> 
> 
> 
> 


=====
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