IEEE Power & Energy Magazine - May/June 2016 - 82

plain-break device filled with candidate gases. The gas exhibiting the most
promise was SF6, a gas that had been
used previously as an insulating gas in
a variety of applications, including Xray equipment and transformers.
In a patent application filed 19 July
1951 (and issued 31 July 1956), H.J.
Lingal, T.E. Browne, and A.P. Strom
described, in detail, a series of tests
that demonstrated the arc-interrupting
performance characteristics of SF6.
SF6 gas had been used for many
years as a dielectric medium. Westinghouse Research determined its unique
interrupting capability related to its
high-electronegative characteristic,
which describes its affinity for attachment of free electrons. This is identified
by an arc time constant, which is its
ability to recover its dielectric strength
after conducting an arc. The arc time
constant for SF6 is approximately 1 μs
compared to approximately 100 μs for
air. This characteristic, along with its
nonconductive arc decomposition products and its high stability over long periods of time without degradation, made
it ideal for circuit breaker applications.

Initial Challenges
Materials resistant to corrosive arc-decomposition products required extensive investigation. A special test fixture
consisting of a pressure chamber with
SF6 and arcing electrodes was used
to expose material samples to the arc
products. The samples were evaluated
by surface conductivity and voltage
breakdown. Hundreds of materials and
coatings were evaluated. The initial
selections were wood, paper, and cloth
phenolics with selected coatings.
Seals to contain the SF6 were critical. Silicone gasket material was initially selected because of its stability
over a wide temperature range. This
turned out to be a poor choice because
of its high permeability, resulting in
SF6 leakage and air migration into the
sealed equipment. This led to the use of
neoprene and later ethylene propylene
seals. Investigations of the shaft seal
designs concluded that rotating shaft
seals were the most reliable, and Teflon
82

ieee power & energy magazine

seal material was best for the expected
temperature range and seal life. (Teflon is a registered trademark of E. I. du
Pont de Nemours and Company or its
affiliates.) Initial designs used castings
for high-pressure gas containment.
Frequent leaks caused by porosity required impregnation of the castings.
Instrumentation was required for
monitoring the gas pressure system.
The conventional pressure switches
were not acceptable because the dielectric and interrupting performance
of SF6 is dependent on the density of
the gas. A joint development with the
United Electric Company resulted in
an SF6 density switch to provide alarms
and lock-out for low gas conditions.
To encourage potential customers to
accept this new technology, the tanks
were made to comply with the ASME
Pressure Vessel Code. This code requires
rupture disks, and the graphite type was
selected because of its long life.
During the initiation of the SF6 development, there were no environmental concerns and cost was low, resulting
in the rather casual release of the gas.
There were no storage facilities, and
the first storage equipment consisted of
gas tanks with a gas transfer compressor. This was followed by the design of
a gas reclaimer with a joint development with the Pall Corporation. This
was the forerunner of the current reclaimers consisting of a vacuum pump,
compressor, and liquid storage.

Interrupter Investigations
Investigations of various types of interrupters utilizing SF6 were made using a
series of prototypes. One of these was
an interrupter using liquid SF6 with an
operation similar to oil technology. This
was nicknamed the "Mighty Mouse"
because of its size and magnitude of interrupting capability. Another unique
interrupter was one that injected liquid
into the arc during interruption. Both of
these were rejected because of the high
pressure and maintenance requirements
of the liquid. A live-tank breaker pole
unit was built for 138 kV and 10,000
MVA using two puffer interrupters in
series. Because of the concern for the

mechanical energy required to operate
the puffers, a hydraulic mechanism was
designed for operation.

First Commercial HighVoltage SF6 Circuit
Breaker Design Criteria
A task force was assembled in 1957
to determine guidelines for the development of a commercial SF6 breaker.
Recognizing that the power industry
would be slow to adopt a radical design using SF6, the decision was made
to mimic, as much as possible, the
features of the reliable, long-accepted
oil breaker designs. The task force included input from a number of power
company engineers and listed the following must-have features for the first
SF6 breaker:
✔ "dead-tank" design
✔ overlapping current transformers
✔ all three poles tied together
✔ closed mechanically by a single
mechanism using compressed air
(present design with slight changes)
✔ opened by stored energy supplied by a charged spring (failsafe design)
✔ no opening resistor
✔ three-cycle interrupting time
✔ meet specified duty cycle requirements, including close-open-15-second-close-open and open-close-open
✔ operate, as specified in C37.04,
down to -30 °C (actually used
-40 °C for margin)
✔ meet all existing ANSI standards for its rating.

Early SF6 Applications

The first commercial high-voltage,
low power SF6 breaker, installed in
1956, was a porcelain-clad device
rated at 115 kV and 1,000 MVA. It
was initially applied as a capacitor
switch. The interrupter developed for
this breaker was based on experience
with oil interrupter designs using a
self-pressure generating principle. In
this SF6 design, pressure generated by
an arc between contacts in a closed
volume caused gas to exhaust through
an interrupting nozzle surrounding a
second arc. The 115-kV breaker used
may/june 2016



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