Patent Application
20050082260
The present invention pertains to current interrupting devices for power distribution systems. More particularly, the present invention relates to encapsulated vacuum interrupting devices for shielded power distribution systems.
Now more than ever, electric utility power distribution systems are being constructed underground due to public outcry about esthetics of aerial (i.e., above-ground) distribution systems in what is now known as the Not In My Backyard (NIMBY) phenomenon. To appease the NIMBY contingent, power distribution systems formerly constructed of poles, wires, and pole-mounted switches and transformers are being superceded and even replaced by underground systems constructed of conduits or duct-banks, underground vaults, cables, and ground level or sub-ground level switchgear and transformers. Underground systems pose new operational and maintenance challenges by virtue of being largely unseen. In response to these challenges, organizations such as the Institute of Electrical and Electronics Engineers (IEEE) and American National Standards Institute (ANSI) have implemented standards and codes to insure operating personnel safety and proper system performance. However, at times, personnel safety may conflict with system performance. One such standard recommends the grounding (i.e., shielding) of individual underground distribution system components at multiple system points (e.g., cable splices, transformers, switches). By grounding system components (or their enclosures), a system operator seeks to eliminate accessibility to hazardous voltages by operating personnel.
Vacuum interrupting switches are well known for use in power distribution systems for reliable interruption of fault current and load breaking, and have become effective substitutes for air, oil, and SF6 filled switches. When used in underground applications such as vaults or switchgear where there is a high probability of submersion, vacuum interrupting switches are enclosed or encapsulated in electrically insulating material. To ground a submersible vacuum interrupting switch in order to protect personnel from hazardous voltages, the entire switch exterior must be conductive. However, if the switch is grounded, the electric fields inside the device become distorted and reduce the dielectric withstand capability of the open gap during a switch “break” operation. Mitigation of this electric field distortion has so far been elusive to those knowledgeable in the art.
U.S. Pat. No. 4,618,749 to Bohme et al. discloses a vacuum switching apparatus inserted into an insulating material such as epoxy resin. The Bohme et al. switch also has a metallic cover which can be grounded for personnel safety. The disclosed switching apparatus is not integrally molded into the insulating material and a space exists between the apparatus and insulating material. Bohme et al. recognize that the space is susceptible to capacitive discharge due to breakdown of the insulating material (e.g., corona effect) especially during times when the switch contacts are open. Control electrodes embedded in the insulating material attempt to minimize corona effect inside the space by placing voltage stress in the insulating material. It is readily apparent to one knowledgeable in the art that the Bohme et al. device will still suffer from insulating material breakdown. Furthermore, as the switching apparatus is inserted in the pre-formed insulating housing, the device is expensive and complicated to manufacture.
Thomas & Betts Elastimold® MVI Molded Vacuum Fault Interrupter attempts to overcome the deficiencies of the aforementioned Bohme et al. patent by directly encapsulating the vacuum switch chamber in a molded insulating housing. The voltage stress is now present in the insulating housing which has a much higher breakdown strength. However, since the MVI device is shielded, the presence of a grounded surface in close proximity to the vacuum chamber causes an electric field distortion inside the device which decreases the withstand capability of the open gap. Thus, the device is prevented from operating to its full potential.
The present invention provides a device that overcomes the disadvantages of the prior art. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
The invention provides a shielded encapsulated vacuum interrupter. A ceramic vacuum chamber includes opposing conductive end caps. One end cap is electrically connected to a fixed contact, while an opposing end cap is connected to a moving contact. The moving contact is actuatable co-axially with the fixed contact for opening or closing an electric circuit. A floating shield inside the vacuum chamber, connected to the vacuum chamber ceramic wall and spaced from the fixed and moving contacts, is isolated from the contacts and ground and has a floating voltage potential. A portion of the vacuum chamber exterior ceramic wall is coated with a semi-conductive material. Conductive voltage screens are electrically connected to each conductive end cap of the vacuum chamber, and the entire vacuum interrupter including the chamber and connected screens is then encapsulated in a molded dielectric housing.
Referring now to the drawings,
Vacuum chamber 110 also includes an floating shield 105 which is a metallic generally cylindrical-shaped member. Floating shield 105 is supported in vacuum chamber 110 at a fixed coaxial distance from the fixed contact 120 and movable contact 130 by exposed ring 115. The ceramic housing of vacuum chamber 110 includes two generally cylindrical ceramic portions which sandwich exposed ring 115 and retain floating shield 105 at a spaced distance from the contacts. Since floating shield 105 is retained at a spaced distance from the contacts, it is electrically isolated and has a floating voltage potential. During switching operation of the contacts, floating shield 105 prevents metallic ions released from the contacts when arcing occurs from collecting on the interior of the ceramic housing, thereby preventing performance degradation of the interrupter 100.
Conductive leads electrically connected to the conductive end caps serve as a connecting means for power distribution conductors such as underground cables to interface with the interrupter 100. To ensure that the vacuum interrupter 100 will operate reliably and safely in wet environments, such as underground vaults or switchgear prone to flooding, the interrupter is encapsulated in a molded dielectric material such as epoxy or the like. As shown in
Bushings 170 are formed by encapsulating the conductive leads in the dielectric encapsulation 190. As shown in
To counteract the electrical field distortion, voltage screens are attached to the vacuum chamber 110 and are embedded in the dielectric encapsulation 190 to place the voltage stress in the encapsulation. Fixed voltage screen 210 (
The two opposing voltage screens substantially enclose vacuum chamber 110, but leave a central portion exposed. As shown in
Semiconductive material 240 preferably extends into the areas encompassed by fixed voltage screen 210 and movable voltage screen 220. In this way, each voltage screen overlaps a portion of the applied semiconductive material 240. One knowledgeable in the art will understand that the semiconductive material 240 on the vacuum chamber exterior will assume the same potential as the floating shield 105 inside the vacuum interrupter 110 since they are linked by exposed ring 115. Therefore, when the contacts are separated, the semiconductive material 240 eliminates the voltage stress on the ends of the floating shield 105. Voltage screens electrically coupled to the fixed contact 120 and movable contact 130 drive the potential on the semiconductive coating 240 to 50% of the difference between the conductive end caps of the vacuum chamber 110 thereby achieving a balanced voltage potential distribution.
Referring now to
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
