Re: Why PT's?

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Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 


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From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, November 25, 2003 9:13 PM
Subject: Why PT's?


 > Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com>
 >
 > Here's a question I've always wondered about.  As I understand it,
 > potential transformers are devices designed to step down a 14.4kV or
 > whatever transmission line to a more manageable 120V signal that can be
 > more easily read by the utility companies, so they can see what's
 > happening on their HV lines.  Basically a step-down transformer with a
 > precise turns ratio and a minimal power handling capability, but happily
 > enough so that it's useful in "our" application.
 >
 > These are big, heavy devices.  I would think that a solid state
 > optically isolated equivalent could be easily produced.  Why are these
 > dinosaurs still used by the utility companies?

Economics!  Utility equipment is amortized over 40 years.

Tradition and a huge installed base.  Much easier to replace a failed PT
with another similar PT rather than go through the analysis and system
design process.  Most utilities are traceability and record keeping fiends
(you'd have to be with the size of the installed plant).  It would be
irresponsible to willy-nilly change out kinds of equipment without doing the
analysis (which could be quite far reaching, because you have to deal with
all the possible fault conditions) required, so changing the type of
equipment is an expensive process.

Reliability - PTs are pretty rugged devices without much that can go wrong.
They can take overloads(200-300%), overvoltage transients (90+kV BIL),
survive (for 40 yrs) in truly evil weather and environments(-40C to 60C),
etc.  And, they're cheap (<$1K, brand new, in quantity).  I doubt you could
build a optoisolated unit for measuring, say, 14.4kV, that would meet the
environmental requirements, life and reliability requirements for as little
as the PT costs. (Take the parts cost of your proposed opto design and
multiply by 10, for a rough selling price of the completed unit).

You can get small amounts of usable power from a PT  - Typically you want to
measure the voltage and then telemeter it somewhere, requiring power for a
transmitter.  Maybe you have some remote control relays/recloser/overcurrent
devices you want to power, etc.

And why a 120V output?

Because the entire civilized AC world (meaning North America, of course)
uses 120V as the nominal voltage.  It's enshrined as being "safe" in
electrical codes, etc. so measurement instruments all use 120V movements
(with different scales depending on the PT or voltage divider connected) (or
5A full scale for current measuring devices... why donut CTs have ratings
like 200:5 and not 40:1).  If you wanted merely to monitor the status of a
HV line, you could use a PT and a lightbulb, for instance.

I will note, though, that for much higher voltages (transmission levels,
100kV and up), fiber optics are becoming very common. Conventional PT
technology gets very expensive to handle those levels, there's fewer units
needed, so the economic justification is there for doing the analysis and
design.  And, at that voltage, they use capacitive voltage dividers to get
the voltage down to a level where a conventional PT can turn it into 120V.
 > I'm grateful that these devices work so well for our application but...
 >
 > Gary Lau
 > MA, USA
 >
 >