Re: Series-ing Potential Transformers

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

Original poster: Tesla729-at-cs-dot-com 

"Tesla list" <tesla-at-pupman-dot-com> wrote:

 >>Dan and all,
 >
 >There are a couple of common styles of PT's used in the USA, and these will
 >have different insulation ratings on the HV terminals. The first style has
 >full insulation on both HV terminals, and is intended for line-line
 >connection. Connecting this type in series can be done with no potential
 >(no pun intended) problem since both HV outputs are rated at full Basic
 >Impulse Level (BIL).
 >
 >However, the second style, used mainly in a sheltered substation or
 >underground applications, may have only one HV terminal that is insulated
 >for the full Basic Impulse Limit (BIL), since the PT is intended to be
 >connected from line to ground. While this "ground" side HV terminal is
 >still isolated from the core, it has a much reduced BIL rating (~10 kVRMS
 >for a 14.4 kV PT).
 >
 >If you plan on using the second type of PT you MUST connect the terminals
 >with the low insulation rating to the center ground point to avoid
 >overvolting the HV winding.
 >
 >Best regards,
 >
 >-- Bert --

Hi Bert, all,

Bert is correct. I have (2) butyl rubber 7200 volt PTs.
One of them only has one HV bushing terminal while the
other has two. As I mentioned in an earlier post re-
garding this issue, I series the two together for 14.4
kVAC out to my HV rectifier for up to 20 kVDC to charge
my 10 kJ can crusher/quarter shrinker capacitors w/in
20 seconds. I run the single bushing one from ground and
take the "floating" 7200 volts off of the single bushing
of it and run it "piggie-back" to one side of the double
bushing one, making sure it's properly phased and get 14,400
volts out of the second PT's second bushing. I have had
no problems with this setup as I know the BIL rating of
both of the HV terminals of the second PT should easily
hold off up to 14,400 volts.

David Rieben