Re: Series-ing Potential Transformers

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

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 



Tesla list wrote:
>Original poster: "Mccauley, Daniel H" <daniel.h.mccauley-at-lmco-dot-com>
>
>
>
>I plan on series connecting two 14.4kV (1.5kVA) potential transformers
>for an output voltage o 28.8kV.
>The series pair will run off 240VAC and the midpoint of the outputs will
>be grounded.
>
>http://users.snip-dot-net/~liche/pt01.jpg
>
>I currently don't see any reason this will not work, but if anyone has
>any experience otherwise with these things,
>let me know.

>Thanks
>Dan
>
>
>

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 --
-- 
-