Re: PT Questions

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

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

Hi Steve,

Answers interspersed below...

Tesla list wrote:
> 
> Original poster: "S & J Young by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<youngs-at-konnections-dot-net>
> 
> Hi,
> 
> I have questions about HV Potential/Instrument Transformers of the type some
> use to power their TCs:
> 
> 1) Why are they so "beefy" so that they are at least 1 KVA?  Are they used
> to power a few lights or other things as well as a voltmeter that only
> requires miniscule power?

Yes. And to provide low voltage to operate various instrument and
control relays.

> 
> 2) Are most PT primaries "two-horn" style with neither primary side
> grounded?

It depends. There are two and one horn (outdoor styles) and "no horn"
(indoor substation) style. 

> 
> 3) Are typical PTs dry, oil immersed, or potted?

Up to 14.4 kV virtually all are dry, typically potted in a black rubbery
material (silicone rubber). 

> 
> 3) Does anyone in the US have one for sale at a reasonable price?  I am
> interested in a two-horn ungrounded configuration, 14 KV at around 2-3 KVA
> intermittent duty,  120 or 240 volt 60 Hz secondary, preferably dry style.
> I prefer an ungrounded HV winding so it can drive a bridge rectifier with
> one side grounded.

These are scarce - can't help you here, unfortunately!
 
> 
> 4) What commercial source has the best deals for them?  T&R?  Austin?

Most of the sources for refurbished transformers don't carry these since
they are not particularly rebuildable like power and distribution
transformers. The "new" price is too high (they are in fact instrument
grade items). Surprisingly, the best place seems to be junkyards that
accept electrical switchgear. That's where I got mine...

> 
> 5) Do utility companies take them out of service?  Seems like they would
> last forever and not need replacement.

They do have an expected service life. They're often replaced during
substation upgrades/rebuilds. Power companies or industrial (privately
owned) substations are both fair game. Try to develop a relationship
with electricians and EE's in electrical contracting houses, Also owners
of local junk yards - and take a picture of what you're looking for. One
will turn up eventually! And if you're lucky, they'll turn up in groups
of three... :^)

> 
> Thanks,
> --Steve

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Coins Shrunk Electromagnetically!
http://www.teslamania-dot-com