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A. The stationary
contact and piston assembly (containing the moving contact
and nozzle) are housed in clear cylindrical tubes. These are
mounted in a modular three-phase assembly which is independent
of the switch tank. The stationary contacts are supported
independent of the cable entrance bushings, eliminating possible
misalignment resulting from tank deflections. Tank deflections
are caused by normal tank pressure variance due to ambient
temperature fluctuations. This construction eliminates contact
alignment difficulties caused by deflections of the switch
tank walls. The modular construction also allows testing of
the module during assembly and complete flexibility in switch
design and configuration. The nozzle which directs the flow
of SF6 has a converging/diverging geometry (see photo) which
improves the arc interruption capability over designs using
straight throated nozzles. The converging portion of the nozzle
has a constantly decreasing flow area up to the nozzle throat
minimizing velocity changes in the flow of SF6 gas, while
improving arc interruption and dielectric recovery.
B. As the contacts
separate, the SF6 is compressed by the piston assembly and
directed into the arc zone by the nozzle. The compressed SF6
flows across the contacts and around the arc established by
the separating contacts. The cooling action of the gas is
increased by the higher pressure (due to compression) and
the flow which constantly provides a supply of cool SF6 into
the arc zone.
C. At current
zero the temperature of the arc is reduced to the point of
deionization, ceasing the flow of current. The SF6 rapidly
recovers dielectric strength withstanding the system recovery
voltage across the contacts.
D. As the contacts
are closing, the piston assembly compresses the SF6 between
the contacts. This increases the dielectric strength of the
gap, minimizing prestrike. The contacts are designed using
a tulip bayonet construction (see photo). The sliding action
of the contacts on engagement provides a self cleaning action
of the main current carrying surfaces. The contact fingers
are designed for increasing contact pressure with increasing
current for proper operation during momentary or close-into-fault
conditions. The contacts have arc resistant copper tungsten
tips to minimize erosion of material during load switching
and prevent damage to the main current transfer area of the
contacts.
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