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Re: 110KV BIL? (3 more questions)

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>


My responses are interspersed below...

-- Bert --
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> Original poster: "Ray Haynes by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> If one side of the HV winding is grounded (see G.L.'s text from previous
> posting below) then why can't I tie the cases together and pull the HV from
> the 2 non-grounded HV bushings (assuming proper phasing)?

You can. As you suspect, it's similar to tying two MOT's or two "one eared"
pigs in series where one end of the HV winding has minimal insulation to
the core.

> Also how does a 110KV BIL rating apply to a PT with only one HV terminal. I
> assume that would be the max impulse in or out of that terminal.

Exactly - to the fused HV terminal.

> These transformers are 120:1 types. With a BIL rating of 110KV it seems I
> could run the LV side off 240V and get 28.8KV out and not stress it too
> much. True?

False - the core will likely begin to saturate at 140-150 volts - the added
primary current will simply cause the primary winding to start
> Could some in-the-know take a look at a picture and offer any comments. They
> don't look like I expected a PT to look (they have built in fuse holders and
> no apparent HV bushings).
> http://www.soap-lotion-dot-com/images/pt.jpg
> Thanks All,
> Ray

Ray, you have a fairly standard "indoor" PT, similar to a "2-eared pig" but
smaller, fused on each HV terminal. Your particular PT does not have the
ribbed insulators that are often seen on many "outdoor" PT's. In your
particular PT, both HV terminals are fused and are intended to "float" for
phase-to-phase measurements on three phase delta circuits. 

Other style PT's come with only one "hot" leg and only one fuse - in these
the other end of the HV winding is meant to be connected to the common
neutral point (near ground potential) in a three phase Wye configuration.
Connecting the pair in series will work in the fashion you described
although you may not want to use this in a Tesla Coil application for other
reasons (see Bill Wysock's previous post). Nice find!! 

> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Wednesday, May 16, 2001 7:00 AM
> To: tesla-at-pupman-dot-com
> Subject: RE: 110KV BIL?
> <SNIP>
> If your PT has only one HV terminal, then it might be the case that one of
> the HV leads is connected to the core of the transformer. You cannot series
> such PTs as you are sure to have an insulation failure.
> Godfrey Loudner
> <SNIP>