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RE: 110KV BIL? (3 more questions)

Original poster: "Loudner, Godfrey by way of Terry Fritz <twftesla-at-uswest-dot-net>" <gloudner-at-SINTE.EDU>

Hi Ray

In the case that one end of the secondary is connected to the core, the
design engineers reduce the amount of insulation between the core and the
windings. This feature reduces manufacturing costs. I am not saying that you
cannot series such transformers, but you run the risk of insulation
breakdown. MOTs (microwave oven transformers) have one end of the HV winding
connected to the core, and you can series two of them employing the center
grounding thing. In the case of insulation breakdown, the financial loss is
small. But in the case of expensive PTs, I would not run the risk. I cannot
say for sure, but the picture seems to indicate that the PT has two HV
terminals and that the case is made of plastic. I would expect that the HV
winding is totally isolated from the core. It looks like there is an
information label that should show the configuration of the windings---a
look at this will tell for sure.

All that 110KV BIL means is all parts of the transformer which are insulated
for HV can withstand a brief impulse of 110KV. 

If you applied 240V to the primary with the secondary terminals open
circuited, the risk of insulation breakdown is extremely high. Recall the
economics of transformer design. The engineers put in enough insulation for
continuous 14.4KV stress with some margin for more. They certainly would not
put in enough for continuous 28.8KV stress. The amount of iron in the core
is probably a little above minimal. Without a lot more iron, the core would
saturate. If you pulled an arc, the primary would draw a huge current. In
short, you would smoke the primary. I would not do this to any of my
precious transformers.

It seems to me that you are putting too much stock into the BIL thing. At
the transformer factory, they subjected the PTs to a couple brief pulses of
110KV DC. If the insulation held, the PTs were shipped to customers.

Godfrey Loudner

> -----Original Message-----
> From:	Tesla list [SMTP:tesla-at-pupman-dot-com]
> Sent:	Wednesday, May 16, 2001 7:29 PM
> To:	tesla-at-pupman-dot-com
> Subject:	RE: 110KV BIL? (3 more questions)
> Original poster: "Ray Haynes by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> <ray.haynes-at-home-dot-com>
> 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)?
> 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.
> 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?
> 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
> -----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>