Re: 12740Y/7200

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: David Dean <deano@xxxxxxxxxxxxxxxx>

On Saturday 30 December 2006 00:21, you wrote:
Hi
I thought that too and have seen it expressed that way on some supposedly
authoritative web sites.  I figured that I could get 24940 out of my 14400
pig if I could step up the low voltage high enough, so just to try I hooked
the variac to 240 and the wiper across the 120V windings in the pig
(paralleled as if they were to be used for 208/120Y output. The transformer
went into saturation soon after passing the 145V in, and took out the 30 amp
fuse on the variac before I got to 150V in. Being surprised and confused, I
ask Bobby. Bobby is the head HV guy for the city. ( I work for an electrical
contractor and we do the underground primary for the city, so I have an "in"
with these guys) He said the reason the older pole mount distribution
transformers had two high voltage bushings was simply for convenience when
they are installed. The installer has a choice as to which bushing gets
grounded and which goes to the primary. I have looked at thousands of poles
all over the area and never seen a polemount distribution transformer
connected phase to phase. The Only pole mount transformers I have Ever seen
with more than one bushing connected to primary are the buck/boost banks
every few miles to adjust primary voltage to compensate for voltage drop on
long lines. These are used often out in the oil fields.

Primaries are connected phase to ground. In the case of 208/120Y, the LV
windings inside the transformers are paralleled across X1, X3, and X2 which
is the grounded midpoint in 120/240 single phase is not used. This is a Y - Y
connection. In the case of 240 delta, the three transformers are connected Y
- Delta with neutral taken from the midpoint ground on the center
transformer. The "freak" or "high leg" is 208 phase to ground. In the case of
480/277Y, the secondaries of the transformers are 277V and connected Y - Y.
In the case of 480 Delta transformers are 240/480 on the secondary side.
Usually this is used with no neutral in industrial apps. or in the oilfields.
If a neutral were to be used, the "freak" would read 416 phase to ground.

To the best of my knowledge that is how it is. For what it is worth.

later

deano

> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi David,
>
> When a two bushing single phase PIG is marked 7200/12470Y,  are the
> two bushings suppose to be connected across two of the three phases
> and subsequently see 12470 across the two bushings, or are the  two
> bushings suppose to be connected from one phase to neutral and thus
> see only 7200V?? If the former connection is allowed, it would seem
> like there would be a common mode voltage on the two bushings and
> this would be a problem.  If only the latter connection is allowed,
> then why two HV bushings??
>
> Also, does the latter connection result in 120/240 on the LV side,
> and if so then would the 12470V connection result in 208/416 on the LV
> side??
>
> (just when I thought I understood it)
>
> Gerry R.
>
> >Original poster: David Dean <deano@xxxxxxxxxxxxxxxx>
> >
> >It means The primary voltage is 7200 phase to ground, 12470 phase to
> > phase. All single phase pole mount transformers are connected Y to Y in 3
> > phase, If a pad mount 3 phase TRANSFORMER IT WILL BE Connected Delta to
> > Y. In single phase all power distribution transformers are connected
> > from one phase primary to one phase secondary. All pole pigs rated
> > 12470Y/7200 Are connected to a single 7200V line. Only 3 phase
> > transformers are connected 12470 Delta on the primary side, and all of
> > them that I have seen in the real world are Pad Mount. Of the possible
> > ways transformers can be connected there are Y - Y, Delta Delta, Y delta.
> > Y - Y is a no no for a three phase transformer. Any imbalance in currents
> > will result in neutral currents that will destroy the transformer. Using
> > three single phase transformers in Y - Y is OK because the cores are not
> > connected and the neutral of the secondary can be connected to the
> > neutral of the primary and no neutral current will flow if there is an
> > imbalance.
> >
> >Anyway that is the theory.
> >later
> >
> >deano