INTRA-TANK UNDER-OIL VACUUM PRIMARY SWITCHES FOR MEDIUM VOLTAGE TRANSFORMER APPLICATIONS

Abstract

A controllable primary switch for isolating a transformer from a power grid or network, wherein the controllable primary switch is mountable within and integral to the transformer and is electrically connected to high voltage feeder cables to allow the transformer to be disconnected from the power grid or network. The controllable primary switch includes one or more vacuum interrupters having first and second electrical switch contacts mounted inside the casing, actuator means for moving the second switch contact relative to the first switch contact in each of the one or more vacuum interrupters; and a handle connected to the actuator means, wherein said handle is capable of engaging the actuator means to move the second switch contact relative to the first switch contact.

Claims

1. A controllable primary switch for isolating a transformer from a power grid or network, wherein the controllable primary switch is mountable within and integral to the transformer and is electrically connected to high voltage feeder cables to allow the transformer to be disconnected from the power grid or network, the controllable primary switch comprising: a) one or more vacuum interrupters, wherein each of the plurality of vacuum interrupters comprises: i) a sealed casing, the sealed casing comprising a pair of end caps closing the ends of the casing; and ii) first and second electrical switch contacts mounted inside the casing, wherein the first switch contact is stationary and the second switch contact is movable relative to the first switch contact; b) actuator means for moving the second switch contact relative to the first switch contact in each of the one or more vacuum interrupters; and c) a handle connected to the actuator means, wherein said handle is capable of engaging the actuator means to move the second switch contact relative to the first switch contact; wherein, when the one or more vacuum interrupters are in an open position, the first and second electrical switch contacts are separated from one another, wherein the feeder cables are isolated and no power is conveyed through the transformer and the transformer is isolated from the power grid or network; and wherein, when the one or more vacuum interrupters are in a closed position, the first and second electrical switch contacts are in contact with each other, wherein the feeder cables are connected to the transformer and power is conveyed through the transformer.

2. The controllable primary switch according to claim 1, wherein the transformer is a three-phase power system, and wherein the one or more vacuum interrupters comprises three vacuum interrupters, one for each phase of the three-phase power system.

3. The controllable primary switch according to claim 2, wherein the transformer is a three-phase power system, wherein the one or more vacuum interrupters comprises another component of the switch that is capable of moving the feeder circuit from a closed position to a grounded position, wherein, when the additional vacuum interrupters are in an open position, the first and second electrical switch contacts are separated from one another, wherein the feeder cables are isolated and no power is conveyed through the transformer and the transformer is isolated from the power grid or network; and wherein, when the additional vacuum interrupters are in a closed position, the first and second electrical switch contacts are in contact with each other, wherein the feeder cables are grounded.

4. The controllable primary switch according to claim 3, comprising a second actuator means for moving the second switch contact relative to the first switch contact in the three additional vacuum interrupters, and a second handle connected to the second actuator means, wherein the second handle is capable of engaging the second actuator means to move the second switch contact relative to the first switch contact in each of the one or more additional vacuum interrupters.

5. The primary switch according to claim 1, wherein the casing is a tubular casing.

6. The primary switch according to claim 1, wherein the controllable primary switch is enclosed within a housing.

7. The primary switch according to claim 1, wherein the actuator means comprises a spring-loaded actuator, whereby the one or more vacuum interrupters can be variously actuated to the open position or the closed position.

8. The primary switch according to claim 3, wherein the actuator means comprises a spring-loaded actuator, whereby the one or more vacuum interrupters can be variously actuated to the open position, the closed position, or the ground position.

9. The primary switch according to claim 7, wherein the one or more vacuum interrupters cannot be actuated to a partially-open or partially-closed position.

10. The primary switch according to claim 8, wherein the one or more vacuum interrupters cannot be actuated to a partially-open or partially-closed position.

11. The primary switch according to claim 1, wherein the casing comprises an insulating material.

12. The primary switch according to claim 1, wherein the primary switch may be remotely activated.

13. The primary switch according to claim 1, comprising a security feature to prevent inadvertent activation of the primary switch.

14. A transformer tank system comprising: a) a sealed tank, said sealed tank comprising core and coil assemblies immersed in a dielectric fluid or gas within the tank; and b) a controllable primary switch for isolating a transformer from a power grid or network, wherein the controllable primary switch is mountable within the sealed tank and is electrically connected to high voltage feeder cables to allow the transformer to be disconnected from the power grid or network, the controllable primary switch comprising: i) one or more vacuum interrupters, wherein each of the plurality of vacuum interrupters comprises: (a) a sealed casing, the sealed casing comprising a pair of end caps closing the ends of the casing; and (b) first and second electrical switch contacts mounted inside the casing, wherein the first switch contact is stationary and the second switch contact is movable relative to the first switch contact; ii) actuator means for moving the second switch contact relative to the first switch contact in each of the one or more vacuum interrupters; and iii) a handle connected to the actuator means, wherein said handle is capable of engaging the actuator means to move the second switch contact relative to the first switch contact; wherein, when the one or more vacuum interrupters are in an open position, the first and second electrical switch contacts are separated from one another, wherein the feeder cables are isolated and no power is conveyed through the transformer and the transformer is isolated from the power grid or network; and wherein, when the one or more vacuum interrupters are in a closed position, the first and second electrical switch contacts are in contact with each other, wherein the feeder cables are connected to the transformer and power is conveyed through the transformer.

15. The transformer tank system according to claim 14, comprising means for verifying the position of the at least one primary switch in either the open or the closed position.

16. The transformer tank system according to claim 15, wherein the means for verifying the position of the at least one primary switch comprises a transparent viewing window.

17. The transformer tank system according to claim 14, wherein the controllable primary switch may be manually-actuated or remotely-actuated.

18. The transformer tank system according to claim 16, further comprising a plurality of sensors to monitor conditions in the transformer tank system.

19. A kit for retrofitting a transformer tank system to provide a controllable primary switch in a sealed tank of the transformer tank system, the kit comprising: the controllable primary switch for isolating a transformer from a power grid or network, the controllable primary switch comprising: a) one or more vacuum interrupters, wherein each of the plurality of vacuum interrupters comprises: i) a sealed casing, the sealed casing comprising a pair of end caps closing the ends of the casing; and first and second electrical switch contacts mounted inside the casing, wherein the first switch contact is stationary and the second switch contact is movable relative to the first switch contact; b) actuator means for moving the second switch contact relative to the first switch contact in each of the one or more vacuum interrupters; and c) a handle connected to the actuator means, wherein said handle is capable of engaging the actuator means to move the second switch contact relative to the first switch contact; wherein the controllable primary switch is enclosed in a housing and is mountable within the sealed tank and is electrically connectable to high voltage feeder cables to allow the transformer to be disconnected from the power grid or network.

20. A method of isolating a transformer from a power grid or network using a remotely-controllable primary switch, wherein the primary switch is mounted within and integral to a sealed transformer tank, wherein the remotely-controllable primary switch comprises (a) one or more vacuum interrupters, wherein each of the plurality of vacuum interrupters comprises (i) a sealed casing, the sealed casing comprising a pair of end caps closing the ends of the casing; and (ii) first and second electrical switch contacts mounted inside the casing, wherein the first switch contact is stationary and the second switch contact is movable relative to the first switch contact; (b) actuator means for moving the second switch contact relative to the first switch contact in each of the one or more vacuum interrupters; and c) a handle connected to the actuator means, wherein said handle is capable of engaging the actuator means to move the second switch contact relative to the first switch contact; the method comprising the steps of: engaging the handle to engage the actuator means to move the second switch contact relative to the first switch contact; wherein: in an open position, the first and second electrical switch contacts are separated from one another, wherein the feeder cables are isolated and no power is conveyed through the transformer and the transformer is isolated from the power grid or network; and in a closed position, the first and second electrical switch contacts are in contact with each other, wherein the feeder cables are connected to the transformer and power is conveyed through the transformer.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0050] FIG. 1 depicts a photograph of a network transformer comprising separate compartments for the primary cable connections and primary switch.

[0051] FIG. 2 depicts a photograph of a top-down view of a vault with a network transformer configured therein.

[0052] FIG. 3 depicts a photograph showing a separate primary switch compartment mounted to a network transformer.

[0053] FIG. 4 depicts a photograph of a close-up view of internal components within a separate primary switch compartment mounted to a network transformer.

[0054] FIG. 5 depicts a photograph of a network transformer showing the location of the handle for the primary switch, as well as the connections for the incoming high voltage cables.

[0055] FIG. 6 depicts a photograph of a primary switch and related contacts.

[0056] FIG. 7 depicts a photograph of a manually-operated switch showing considerable arcing and erosion damage to the fixed and moving contacts.

[0057] FIG. 8 depicts a photograph of a ruptured cover of a transformer tank.

[0058] FIG. 9 shows three parallel vacuum interrupters arranged as a three-phase switch.

[0059] FIG. 10 depicts a photograph of a typical vacuum interrupter with the external porcelain housing partially cut away.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] In one embodiment, the present invention relates generally to a controllable primary switch for isolating a transformer from a power grid or network, wherein the controllable primary switch is mountable within and integral to the transformer and is electrically connected to high voltage feeder cables to allow the transformer to be disconnected from the power grid or network, the controllable primary switch comprising:

[0061] a) one or more vacuum interrupters, wherein each of the plurality of vacuum interrupters comprises: [0062] i) a sealed casing, the sealed casing comprising a pair of end caps closing the ends of the casing; and [0063] ii) first and second electrical switch contacts mounted inside the casing, wherein the first switch contact is stationary and the second switch contact is movable relative to the first switch contact;

[0064] b) actuator means for moving the second switch contact relative to the first switch contact in each of the one or more vacuum interrupters; and

[0065] c) a handle connected to the actuator means, wherein said handle is capable of engaging the actuator means to move the second switch contact relative to the first switch contact;

[0066] wherein, when the one or more vacuum interrupters are in an open position, the first and second electrical switch contacts are separated from one another, wherein the feeder cables are isolated and no power is conveyed through the transformer and the transformer is isolated from the power grid or network;

[0067] wherein, when the one or more vacuum interrupters are in a closed position, the first and second electrical switch contacts are in contact with each other, wherein the feeder cables are connected to the transformer and power is conveyed through the transformer;

[0068] and wherein, another one or more vacuum interrupters, when closed, connect the feeder cables to ground.

[0069] The vacuum interrupter portion of the switch of the invention comprises a tubular casing of an insulating material, such as ceramic or a suitable glass, and a pair of metallic end caps that close the ends of casing, which are disposed within a cylindrical bore of the housing. The casing is generally cylindrical and forms an airtight vacuum chamber. The first and second end caps are sealed to the casing to render the enclosed chamber vacuum-tight. The vacuum interrupter switch of the preferred embodiment is a high-voltage vacuum-type current interrupter. The vacuum interrupter switch includes an enclosure having a generally cylindrical shape. The term generally cylindrical is used to refer to a housing that is substantially cylindrical but not necessarily a circular cross-section. Other cross-sections may be employed, if desired. The cylindrical enclosure includes a cylindrical bore having a vertical axis.

[0070] Furthermore, each vacuum interrupter that makes up the primary switch comprises an outer case formed from an electrically insulating porcelain material, such as a glazed aluminum oxide. This outer case is fully evacuated and permanently sealed during manufacture.

[0071] The end caps are preferably stainless steel and are affixed to the cylindrical casing using suitable means, such as by brazing with a high temperature alloy. In this instance, the alloy is melted at the interface of the parts (like a solder) to form a vacuum-tight, hermetic seal. First and second switch contacts are disposed within the chamber of the casing. The contacts may have faces formed from tungsten-containing copper, pure copper, chromium-modified copper, or another suitable material.

[0072] The first switch contact is disposed on the terminal end of a conductor, which passes through an aperture in the first end cap and is brazed or welded thereto. The other terminal end conductor is affixed to an upper extension conductor that is in electrical engagement with a terminal. As described herein, the first contact is the stationary contact.

[0073] The second contact is the movable contact and is mounted on one terminal end of a conductor. The conductor comprises a movable contact stem or rod extending into a current interchange. The other terminal end of the movable contact rod is attached by a threaded bushing to an actuator rod mounted to an actuator means. The actuator rod is threaded onto the threaded end of the threaded bushing. The actuator rod is preferably made of an insulating or dielectric material that will not conduct the electricity passing through the conductor rod to the actuator means. Suitable materials include, but are not limited to, cellulose-filled or mineral/glass-filled phenolic, melamine, polyester, diallyl iso-phthalate, polycarbonate, or epoxy resins. In a preferred embodiment, the material is a glass-filled epoxy resin.

[0074] The present invention replaces a conventional intra-tank oil-filled primary switch with a controllable primary switch that comprises one or more vacuum interrupters. As described herein, the under-oil open contacts of the primary switch are replaced with electrical contacts that are sealed within a vacuum environment within a porcelain housing which constitutes the vacuum interrupter. Thus, the primary switch described herein provides a fully shielded set of electrical contacts that prevents any chance for an under-oil arc fault event due to a switch problem. By combination with a mechanical actuator device that does not allow the contacts to be partially or incompletely opened or closed, under-oil arcing is eliminated.

[0075] The present invention thus eliminates the need for any type of external primary switch, as well as any added connections, and reduces the risk for arcing potential associated with open switch contacts that directly contact the oil or insulating fluid inside the transformer. The vacuum interrupters are fully self-contained within a vacuum housing in the transformer tank and thus any arcing resulting from the opening and closing of the vacuum interrupters within the controllable primary switch can be fully isolated within the sealed vacuum housing.

[0076] By operation of the vacuum interrupters, power is conveyed from the incoming supply to the transformer when the pair of electrical contacts is in mutual contact, and no power is conveyed to the load from the incoming power supply when the pair of electrical contacts are separated from one another. In a three-position embodiment, the feeder circuit can be grounded through another one or more vacuum interrupters.

[0077] By primary switch what is meant is a controllable switch that is capable of isolating a transformer from a power grid or network so that the transformer may be disconnected from the power grid or network for maintenance and testing operations or other reason.

[0078] The present invention can be designed for two-position or three-position operation, depending on customer needs. Thus, in one embodiment, the primary object of the invention is a two-position switch having a clear and a ground position. In another embodiment, the primary object of the invention is a three-position switch having an open, closed, and ground positions. In addition, the primary switch can be manually operated or, alternatively, can be motor-operated. If the primary switch is motor-operated, the motor-operators can be applied externally to the transformer tank.

[0079] As described herein, in one embodiment, the primary switch is a three-phase power system device, and the one or more vacuum interrupters comprise three vacuum interrupters, one for each phase of the three-phase power system.

[0080] In another embodiment, instead of individual vacuum interrupters for grounding each phase, the present invention also contemplates the use of a single vacuum interrupter that is capable of grounding all three phases in parallel. Thus, in this instance, the additional vacuum interrupter would be designed to simultaneously ground all three phases.

[0081] By open, what is meant is a position in which the feeder cables are isolated for de-energizing the transformer. Transformers are operated in this condition, for example, to allow testing of the feeder cables for fault-locating purposes. By closed, what is meant is a position in which the feeder cables are connected to the transformer. By ground, what is meant is a position in which the feeder cables are grounded for safety reasons.

[0082] In this embodiment, the vacuum switch may comprise a second actuator means for moving the second switch contact relative to the first switch contact in the one or more additional vacuum interrupters, and a second handle connected to the second actuator means, wherein the second handle is capable of engaging the second actuator means to move the second switch contact relative to the first switch contact in each of the one or more additional vacuum interrupters.

[0083] In another embodiment, the primary switch described herein includes a visible break feature that allows the operator to verify the status of the switch.

[0084] In one embodiment, the actuator means comprises a spring-loaded actuator, whereby the one or more vacuum interrupters can be actuated to the open position or the closed position. It is also critical to the present invention that the one or more vacuum interrupters cannot be actuated to a partially-open or partially-closed position.

[0085] The actuator means is preferably a quick-close/quick-open, motor-driven or manually-driven, over-toggle spring type device, which provides the necessary speeds and forces to adequately interrupt the load current and provides close and latch capability. Other similar actuator means would also be usable in the present invention. Appropriate linkage is included to accommodate an external manual operating handle for both manual opening and closing, and positive contact position indication. The components of the mechanism are also chosen for their ability to work without lubricants. For corrosion resistance and long-maintenance-free life, it is preferable that the components are stainless steel or bronze. The components within the tank may be made with various carbon and alloy steels, bronze, or stainless steel.

[0086] The actuator means reciprocates the vacuum interrupter connecting rods and thus moves the movable contact away from the stationary contact, creating a circuit-interrupting or arcing gap between the contacts. The resulting arc, although quickly extinguished, vaporizes some of the metal on the contacts. In order to prevent this metallic vapor from condensing on the internal insulating surfaces within the chamber, a generally cylindrical, central metallic shield can be mounted within the chamber and extending along an interior axial length of tubular casing, as described, for example, in U.S. Pat. No. 5,597,992 to Walker, the subject matter of which is herein incorporated by reference in its entirety.

[0087] The controllable primary switch described herein may also comprise various security features to prevent inadvertent activation of the primary switch.

[0088] In another embodiment, the primary switch includes design features that enable the primary switch to be compatible with smart grid electronic technology and for the primary switch to be remotely activated and operated. Thus, a sensor can be installed to remotely confirm that the switch is open or closed or grounded. Such sensors may be interfaced through a fiber optic network using protocols developed by various control manufactures. One such control system manufacturer is Schweitzer Engineering Laboratories.

[0089] In another embodiment, and as described herein, the present invention also relates generally to a transformer tank system comprising:

[0090] a) a sealed tank, said sealed tank comprising core and coil assemblies immersed in a dielectric fluid or air or inert gas within the tank; and

[0091] b) a controllable primary switch for isolating a transformer from a power grid or network, wherein the controllable primary switch is mountable within the sealed tank and is electrically connected to high voltage feeder cables to allow the transformer to be disconnected from the power grid or network, or for grounding the primary connection, the controllable primary switch comprising: [0092] i) one or more vacuum interrupters, wherein each of the plurality of vacuum interrupters comprises: [0093] (a) a sealed casing, the sealed casing comprising a pair of end caps closing the ends of the casing; and [0094] (b) first and second electrical switch contacts mounted inside the casing, wherein the first switch contact is stationary and the second switch contact is movable relative to the first switch contact; [0095] ii) actuator means for moving the second switch contact relative to the first switch contact in each of the one or more vacuum interrupters; and [0096] iii) a handle connected to the actuator means, wherein said handle is capable of engaging the actuator means to move the second switch contact relative to the first switch contact;

[0097] wherein, when the one or more vacuum interrupters are in an open position, the first and second electrical switch contacts are separated from one another, wherein the feeder cables are isolated and no power is conveyed through the transformer and the transformer is isolated from the power grid or network;

[0098] wherein, when the one or more vacuum interrupters are in a closed position, the first and second electrical switch contacts are in contact with each other, wherein the feeder cables are connected to the transformer and power is conveyed through the transformer and [0099] wherein one or more vacuum interrupters, when in a closed position, connect the feeder circuit to ground.

[0100] By integrating the switch and transformer, a fully integrated product can be designed, from the outset, to provide maximum protection against an under-oil arcing fault or arcing fault within a dry type transformer tank. This fully integrated design can be used in combination with a fault-tolerant transformer design, as described, for example in co-pending application Ser. No. 14/931,144 to Groeger et al., the subject matter of which is herein incorporated by reference in its entirety. This fully integrated design is also compatible with standard designs.

[0101] The switch described herein does not have any protective response in the sense of a circuit breaker, for example. It is a device intended only for establishing, isolating, and/or grounding the feeder circuit.

[0102] The mechanism must be robust, to allow simultaneous switching of each phase, and to make the switching action as fast and as smooth as possible to prevent arc-carryover due to slow opening or closing of the contacts.

[0103] The transformer tank optionally, but preferably, comprises means for verifying the position of the at least one primary switch in either the open or the closed or ground position. In one embodiment, the means for verifying the position of the at least one primary switch may comprise a transparent viewing window. In another embodiment, the means for verifying the position of the at least one primary switch may comprises a sensor or other device that is capable of providing feedback to a user to confirm the position of the at least one primary switch.

[0104] The controllable primary switch may be manually-actuated or remotely-actuated. In one embodiment the primary switch is configured to allow for remote switching capability, which enables the primary switch to be fully smart grid compliant.

[0105] The transformer tank system described herein may also comprise one or more sensors to monitor conditions in the transformer tank system. Various visible and audible alarms and control actions may be operatively connected to the one or more sensors to provide feedback if operating conditions exceed prescribed parameters. These one or more sensors may monitor, for example, temperature, ground current, seismic/vibration magnitude, oil level, oil pressure, and oil temperature, among others. In one embodiment, the seismic/vibration sensor may include, for example, an accelerometer, such as a microelectromechanical systems (MEMS) accelerometer, or a seismometer. In one aspect, if seismic activity is determined to exceed a threshold level, then the utility service can be disconnected.

[0106] Various security features may also be included in the transformer tank system to provide remote access and/or to prevent unauthorized intrusion.

[0107] In still another embodiment, it may be desirable to retrofit a transformer tank system to replace an existing primary switch with the controllable primary switch described herein. In this embodiment, the present invention also relates generally to a kit for retrofitting a transformer tank system to provide a controllable primary switch in a sealed tank of the transformer tank system, the kit comprising:

[0108] the controllable primary switch for isolating a transformer from a power grid or network, the controllable primary switch comprising:

[0109] a) one or more vacuum interrupters, wherein each of the plurality of vacuum interrupters comprises: [0110] i) a sealed casing, the sealed casing comprising a pair of end caps closing the ends of the casing; and [0111] ii) first and second electrical switch contacts mounted inside the casing, wherein the first switch contact is stationary and the second switch contact is movable relative to the first switch contact;

[0112] b) actuator means for moving the second switch contact relative to the first switch contact in each of the one or more vacuum interrupters; and

[0113] c) a handle connected to the actuator means, wherein said handle is capable of engaging the actuator means to move the second switch contact relative to the first switch contact; [0114] wherein the controllable primary switch is enclosed within a housing and is mountable within the sealed tank and is electrically connectable to high voltage feeder cables to allow the transformer to be disconnected from the power grid or network.

[0115] In still another embodiment, the present invention relates generally to a method of isolating a transformer from a power grid or network using a controllable primary switch, wherein the primary switch is mounted within and integral to a sealed transformer tank, wherein the controllable primary switch comprises (a) one or more vacuum interrupters, wherein each of the plurality of vacuum interrupters comprises (i) a sealed casing, the sealed casing comprising a pair of end caps closing the ends of the casing; and (ii) first and second electrical switch contacts mounted inside the casing, wherein the first switch contact is stationary and the second switch contact is movable relative to the first switch contact; (b) actuator means for moving the second switch contact relative to the first switch contact in each of the one or more vacuum interrupters; and c) a handle connected to the actuator means, wherein said handle is capable of engaging the actuator means to move the second switch contact relative to the first switch contact; the method comprising the steps of:

[0116] engaging the handle to engage the actuator means to move the second switch contact relative to the first switch contact;

[0117] wherein in an open position, the first and second electrical switch contacts are separated from one another, wherein the feeder cables are isolated and no power is conveyed through the transformer and the transformer is isolated from the power grid or network; and

[0118] wherein in a closed position, the first and second electrical switch contacts are in contact with each other, wherein the feeder cables are connected to the transformer and power is conveyed through the transformer; and

[0119] wherein in a ground position, the feeder circuit is connected through the corresponding one or more vacuum interrupters to ground.

[0120] The primary switch described herein can be used with a variety of transformer types, including network transformers and pad-mounted transformers, using insulating fluid or with dry type insulation systems. In a preferred embodiment, the primary switch is used with a network transformer, insulated with oil or other suitable fluid.

[0121] The tank system of the invention may include a viewing window that is incorporated into the outer wall that allows personnel to visually inspect the position of the vacuum interrupter within the sealed case and determine if the particular vacuum interrupter is in an open, closed or grounded position within the sealed case. Personnel may also be able to manually change the position of the switch, such as from open to close or close to open, if necessary. Thus, it is desirable that the primary switch includes a visible break feature that allows for visual confirmation of the switch position.

[0122] In one embodiment, the switch comprises a lock-out relay to prevent grounding in the event that the transformer is back-fed from the secondary side. In the ground position, the under-oil or intra-tank switch further comprises a manual locking feature that would prevent any inadvertent remote operation thereof.

[0123] FIG. 5 depicts a top-down view of a network transformer showing one location of the handle for the primary switch, as well as the connections for the incoming high voltage cables.

[0124] FIG. 6 depicts a traditional primary switch and related contacts.

[0125] FIG. 7 depicts a switch with considerable arcing damage to the fixed and moving contacts. This switch was operated incorrectly, resulting in the contacts not being fully closed. Arcing across the gap caused major contact erosion and localized welding. This is a good example of the benefits of the invention described herein. Because the contacts are protected inside a vacuum environment and have a positive-close and positive-open spring-loaded switch action, it is not possible to hang the switch in a partially-open or partially-closed position.

[0126] FIG. 8 depicts a ruptured cover of a transformer tank and illustrates the magnitude of the concerns that we want to overcome by having a protected primary switch as described herein. As is readily apparent, it is highly desirable to have a primary switch that is not subject to premature failure and that does not contribute to a major internal pressurization event.

[0127] FIG. 9 shows three parallel vacuum interrupters arranged as a three-phase switch. The interrupters have electrical contacts that are enclosed by the white porcelain housings. The first contact is movable and the second contact is fixed. Vacuum is maintained by using a bellows seal around the moving contact shaft. A second set of one or more interrupters would be needed in order to have a grounding feature.

[0128] As can be seen, there is no open arc exposed to the oil in which the vacuum interrupters are immersed or to the gas space inside a dry type transformer.

[0129] FIG. 10 depicts a view of a typical vacuum interrupter with the external porcelain housing partially cut away. Both of the mating contact faces can be seen in this view.