Patent Application
20160163484
1. Field
The disclosed concept pertains generally to electrical disconnects for interrupting or breaking an electrical connection. More particularly, the disclosed concept pertains to electrical disconnects related to medium voltage auxiliary devices.
2. Background Information
In an electric power system, switchgear is the combination of electrical disconnect switches, fuses or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is directly linked to the reliability of the electricity supply.
Typically, switchgear in substations are located on both the high-voltage and low-voltage side of large power transformers. The switchgear on the low-voltage side of the transformers may be located in a building, with medium-voltage circuit breakers for distribution circuits, along with metering, control, and protection equipment. Switchgear commonly includes medium voltage disconnects for electrically connecting (and disconnecting) auxiliary devices (e.g., without limitation, voltage transformers, control power transformers) to a main power feed. Such disconnects include separable contacts which are movable between a closed position, in which the contacts are electrically engaged and thus current is provided to the auxiliary device, and an open position in which the contacts are spaced a suitable separation distance apart, thus isolating the auxiliary device. Movement of the contacts is typically carried out by a racking mechanism which moves one of the contacts a required separation distance from the other fixed contact. In known arrangements, the contacts may require separation distances in the range of about five to twenty inches (depending on the voltage) in order to safely separate the contacts and avoid partial discharge and arcflash. Such generally large separation distances, as well as the racking mechanism(s) required to move the contact such distances, require the use of generally large housings which thus limit the quantity of such arrangements which may be used in a particular facility.
There is thus room for improvement in electrical disconnects, and particularly in electrical disconnects for use in electrically connecting an auxiliary device to a medium voltage power source.
These needs and others are met by embodiments of the disclosed concept which are directed to an electrical disconnect. The electrical disconnect comprising: a housing having a first end and an opposite second end, the housing including a recess defined therein extending from an opening disposed at or about the second end of the housing toward a base defined near the first end of the housing; a vacuum envelope defined within the housing about the first end; a fixed contact assembly including a fixed contact disposed partially within the vacuum envelope, the fixed contact assembly structured to be in electrical communication with a voltage source; and a movable contact assembly including a movable contact having a first end disposed within the vacuum envelope and an opposite second end disposed in the recess near the base thereof, the movable contact being movable between a closed position in electrical contact with the fixed contact and an open position spaced apart from the fixed contact a separation distance, wherein the housing comprises a dynamic shield electrically connected to the movable contact, the dynamic shield being disposed about the recess within the housing and extending from the base toward the opening thereof.
The separation distance may be equal to or less than one inch.
The housing may be formed from an insulating material. The insulating material comprises an epoxy material.
The dynamic shield may comprise a conductive mesh material disposed about the recess.
The housing may further comprise a number of fins extending from the second end of the housing about the opening of the recess.
The housing may further comprise an outward extending flange, the flange being structured to couple the housing to a panel member.
The housing may further include a ground shield disposed on or near the outer periphery thereof.
The ground shield may comprise a conductive coating disposed on the outer surface of the housing.
The movable contact assembly may comprise a current limiting fuse electrically connected to the second end of the movable contact.
The current limiting fuse may be electrically coupled to the second end of the movable contact via a fuse contact which is biased outward from the current limiting fuse and toward the second end by a contact pressure spring.
Such needs and others are also met by embodiments of the disclosed concept which are directed to a system comprising: a voltage source; an electrical device; and an electrical disconnect as described herein.
A full understanding of the disclosed concept can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:
As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality).
As employed herein, the term “about” shall mean at or near a further specified location.
As employed herein, the statement that two or more parts are “connected” or “coupled” together shall mean that the parts are joined together either directly or joined through one or more intermediate parts. Further, as employed herein, the statement that two or more parts are “attached” shall mean that the parts are joined together directly.
As employed herein, the statement that two or more parts are “electrically connected” shall mean that the parts are disposed in electrical contact with each other either directly or through one or more intermediate parts such that a flow of current may pass from one part to the other.
As employed herein, the term “vacuum envelope” means an envelope employing a partial vacuum therein.
As employed herein, the term “partial vacuum” means a space (e.g., within a vacuum envelope) partially exhausted (e.g., to the highest degree practicable; to a relatively high degree; to a degree suitable for use in a vacuum switching apparatus application) by a suitable mechanism (e.g., without limitation, an air pump; a vacuum furnace).
Referring to
As shown in
Continuing to refer to
A movable contact assembly 20 is also disposed partially within the vacuum envelope 12, and partially in the recess 10 near the end 10B thereof, and includes a movable contact 22, having a first end 22A and a second end 22B, which is movable between an open position (such as shown in
In order to overcome the effects of atmospheric pressure acting on the movable contact 22 (as a result of the vacuum envelope 12), a biasing mechanism, such as coil spring 24 is employed to bias movable contact 22 toward the open position, away from the fixed contact 15.
In order to prevent partial discharge phenomena when the contacts 16, 22 are in any position, the electrical disconnect 2 may include one or more shields. In the example embodiment illustrated in the FIGS, electrical disconnect 2 includes a dynamic shield 26 disposed about the recess 10 within the housing 4 and extending from the base 10B toward the opening 10A thereof. More particularly, dynamic shield 26 is embedded within the wall which defines the recess 10 and is formed from a conductive mesh or other suitable material. Dynamic shield 26 is electrically connected to the movable contact 22 such that when the movable contact 22 is disposed in the closed position (i.e., in contact with the fixed contact assembly 14), voltage is applied to the dynamic shield 26. Such dynamic shield 26 acts generally as a Faraday Cage which serves to reduce partial discharge phenomena while the movable contact 22 is in the closed position as the auxiliary device passes through a barrier of ground potential. When the movable contact is in the open position, voltage is removed from the dynamic shield and it becomes a floating potential. At this time the auxiliary device is considered disconnected and therefore ground potential. If the dynamic shield was connected to the fixed contact end of the vacuum envelope then the dynamic shield would remain energized and a lot of partial discharge would be created between the shield and a conductor disposed within the recess 10 of housing 4.
In addition to the dynamic shield 26, electrical disconnect 2 further includes a ground shield 28 disposed on or near the outer periphery of the housing 4 which is electrically connected to the panel member 100. In the illustrated embodiment, ground shield 28 is formed from a conductive coating applied (e.g., without limitation, via spraying) to the outer surface of the housing 4. However, it is to be appreciated that ground shield 28 may be formed from other suitable materials or and/or via other suitable process without varying from the scope of the disclosed concept.
In order to provide a sufficient creepage distance from the exposed portion of the movable contact 22 to the panel member 104 while also minimizing size, housing 4 may include a number of cylindrical fins 30 extending from the second end 8 of the housing 4 about the opening 10A of the recess 10. Although four of such fins 30 are shown in the illustrated example, it is to be appreciated that one or more of the quantity, thickness or length of the fins 30 may be varied without varying from the scope of the disclosed concept.
In order to protect the device 102 from a current surge/fault, movable contact assembly 20 may further include a current limiting fuse 32 disposed between, and electrically connected with the second end 22B of movable contact 22 and the device 102. Fuse 32 is sized such that in the event of a surge/fault fuse 32 will blow before any damage is done to the device 102 or the electrical disconnect 2. By minimizing the let-through current, fuse 32 reduces the force required to hold the movable contact closed during fault conditions. Although shown as extending from the device 102 (shown schematically) it is to be appreciated device 102 may be mounted remotely from fuse 32 and electrically connected thereto via one or more intermediary elements (e.g., without limitation, wires) without varying from the scope of the disclosed concept. In the illustrated embodiment, fuse 32 includes a contact pressure spring 34 which biases a fuse contact 36 outward from fuse 32 toward movable contact 22. Such contact pressure spring 34 serves to account for any dimensional misalignment(s) while maintaining constant pressure for the fuse contact 36.
While specific embodiments of the disclosed concept have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the disclosed concept which is to be given the full breadth of the claims appended and any and all equivalents thereof.
