Title of Invention

A CURRENT LIMITING DEVICE FOR SWITCHING AN INDUCTIVE OR CAPACITIVE LOAD IN A POWER DISTRIBUTION SYSTEM

Abstract

A current limiting device for switching an inductive or capacitive load in a power distribution system. The device (1) comprises a vacuum interrupter (2) comprising a fixed contact system (3) and a movable contact system (4) located in a vacuum sealed ceramic body (5). The vacuum interrupter further comprises an auxiliary contact comprising a contact sleeve (13) disposed within the center shield (8) around the fixed electrode (6) of the fixed contact system in spaced apart relationship with the fixed electrode. The lower end of the contact sleeve is fixed to the fixed end plate (11) of the fixed contact system and the upper end of the contact sleeve protrudes out of the fixed contact (7). The contact sleeve is isolated from the fixed electrode and fixed contact. The auxiliary contact further comprises a wound resistor (26) disposed around the fixed electrode in spaced apart relationship with it and connected to the outer end of the fixed electrode and the fixed end plate. The movable contact (16) is inverted frustoconical shaped. The broad base of the inverted frustoconical shaped movable contact corresponds to the inner periphery of the contact sleeve and the movable contact is adapted to close against the contact sleeve prior to closing on the fixed contact.

Full Text

FIELD OF INVENTION This invention relates to a current limiting device. Current limiting devices are used to switch inductive or capacitive loads in power distribution systems. BACKGROUND OF INVENTION A current limiting device generally used in a power distribution system comprises a vacuum interrupter and an auxiliary circuit connected to a power supply and an inductive or capacitive load in the power distribution system. The vacuum interrupter comprises a fixed contact system and a movable contact system located in a vacuum sealed ceramic body. The auxiliary circuit comprises a mechanical switch and a resistor connected in series. During switching operation, there will be initially a high inrush current flow in the system. This inrush current not only reduces the life of the current limiting device but also generates stresses in the entire system. The sequence of operation of the current limiting device is controlled such that initially the auxiliary circuit will be closed to provide a high resistive path for the high inrush current. Following this, the vacuum interrupter will be closed to provide a low resistive path and a continuous flow of current through the low resistive path. The time delay between the operations of the auxiliary circuit and vacuum interrupter is set for a few cycles such that when the current reduces and stabilizes, it will flow through the low resistive path provided by the vacuum interrupter. The mechanical switch has limited life in that it can perform only a few contact making and breaking operations. This calls for periodic replacement of the auxiliary contact thereby increasing the maintenance cost of the auxiliary circuit. Besides, the vacuum 2 interrupter and auxiliary circuit are external to each other and comprise a large number of components because of which the system is bulky and costly. OBJECTS OF INVENTION An object of the invention is to provide a current limiting device which comprises reduced number of components and is compact and cost effective. Another object of the invention is to provide a current limiting device which has increased life. DETAILED DESCRIPTION OF INVENTION According to the invention there is provided a current limiting device for switching an inductive or capacitive load in a power distribution system, the device comprising a vacuum interrupter comprising a fixed contact system and a movable contact system located in a vacuum sealed ceramic body, the vacuum interrupter further comprising an auxiliary contact comprising a contact sleeve disposed within the center shield around the fixed electrode of the fixed contact system in spaced apart relationship with the fixed electrode, the lower end of the contact sleeve being fixed to the fixed end plate of the fixed contact system and the upper end of the contact sleeve protruding out of the fixed contact, the contact sleeve being isolated from the fixed electrode and fixed contact, the auxiliary contact further comprising a wound resistor disposed around the fixed electrode in spaced apart relationship with it and connected to the outer end of the fixed electrode and the fixed end plate, the movable contact 3 being inverted frustoconical shaped, the broad base of the inverted frustoconical shaped movable contact corresponding to the inner periphery of the contact sleeve, the movable contact being adapted to close against the contact sleeve prior to closing on the fixed contact. The following is a detailed description of the invention with reference to the accompanying drawings, in which Figs 1, 2 and 3 show the current limiting device of the invention according to an exemplary embodiment thereof in the fully open, auxiliary contact closed and fully closed positions. The device 1 as illustrated in Figs 1 to 3 of the accompanying drawings comprises a vacuum interrupter 2 comprising a fixed contact system 3 and a movable contact system 4 located in a vacuum sealed ceramic body 5. The fixed electrode, fixed contact and center shield of the fixed contact system are marked 6, 7 and 8 respectively. The center shield minimises metal vapour deposition on the ceramic body due to arcing during the switching operation of the vacuum interrupter. 9 is a shield disc provided on the fixed electrode. The fixed seal cup is three piece construction comprising pieces 10a, 10b and 10c. 11 is the fixed end plate and 12 is a ceramic disc fixed to the fixed electrode. An auxiliary contact comprising a contact sleeve 13 is supported on the fixed end plate 11. The upper end of the contact sleeve protrudes out of the fixed contact. The contact sleeve is isolated from the fixed electrode and fixed contact by the gap 14 between the fixed end plate 11 and fixed electrode and by the ceramic disc 12. The movable electrode, movable contact, bellows and bellows shield of the moving contact system are marked 15, 16, 17 and 18 respectively. The bellows shield protects the bellows from metallic vapours generated during switching operations of the vacuum 4 interrupter. The stacking ring, movable seal cup and end plate, guide bush and arresting cap are marked 19, 20, 21, 22 and 23 respectively. The guide bush guides the axial movement of the movable electrode. The outer ends of the movable electrode and fixed electrode are provided with threaded holes 24 and 25 respectively for connecting them to a power supply (not shown) and an inductive or capacitive load (not shown) by fitting flexible conducting cables (not shown) from the power supply and the load in the threaded holes 24 and 25 respectively or vice versa. 26 is a wound resistor (cylindrical or spring type) disposed around the fixed electrode in spaced apart relationship with it and connected to the outer fixed electrode and the fixed end plate. The movable contact is inverted frusto conical shaped with the broad base corresponding to the inner periphery of the contact sleeve. The switching device in the fully open position is as shown in Fig 1. During switching when the movable contact moves towards the fixed contact, it first closes against the contact sleeve which provides a large surface area of contact and a high resistive path for the initial inrush current (Fig 2). The arcing between the movable contact and the contact sleeve will be considerably reduced because of the large surface area of contact between the two and the high electric resistance offered by the contact sleeve and the wound resistor and the high dielectric property of the vacuum in the ceramic body. The movable contact slides forward against the contact sleeve and closes against the fixed contact by the time the inrush current reduces and stabilises thereby fully closing the contact and ensuring low resistive path for the current to flow to the load (Fig 3). 5 Typically, the fixed and movable electrodes and stacking ring are made of copper, fixed and movable contacts and auxiliary contact sleeve are made of copper chromium alloy, fixed and movable end plates, bellows, bellows shield, arresting cap, center shield and shield disc are made of stainless steel, fixed and movable side seal cups are made of copper nickel alloy, ceramic body and ceramic disc are made of high alumina and bushing is made of nylon. It is to be understood that the above examples of the materials of the components are only preferences and that the components can be made of other vacuum compatible materials. The auxiliary circuit configuration and construction can vary. Such variations are to be construed and understood to be within the scope of the invention. According to the invention, the auxiliary circuit comprising mechanical switch has been eliminated and the auxiliary circuit is built into the vacuum interrupter to provide both the high resistive path and low resistive path within the same switching device. The number of components of the switching device is reduced and it is rendered compact and cost effective. The life of the switching device is also increased. 6 We claim 1. A current limiting device for switching an inductive or capacitive load in a power distribution system, the device comprising a vacuum interrupter comprising a fixed contact system and a movable contact system located in a vacuum sealed ceramic body, the vacuum interrupter further comprising an auxiliary contact comprising a contact sleeve disposed within the center shield around the fixed electrode of the fixed contact system in spaced apart relationship with the fixed electrode, the lower end of the contact sleeve being fixed to the fixed end plate of the fixed contact system and the upper end of the contact sleeve protruding out of the fixed contact, the contact sleeve being isolated from the fixed electrode and fixed contact, the auxiliary contact further comprising a wound resistor disposed around the fixed electrode in spaced apart relationship with it and connected to the outer end of the fixed electrode and the fixed end plate, the movable contact being inverted frustoconical shaped, the broad base of the inverted frustoconical shaped movable contact corresponding to the inner periphery of the contact sleeve, the movable contact being adapted to close against the contact sleeve prior to closing on the fixed contact. Dated this 4th day of January 2007. 7 Abstract A current limiting device for switching an inductive or capacitive load in a power distribution system. The device (1) comprises a vacuum interrupter (2) comprising a fixed contact system (3) and a movable contact system (4) located in a vacuum sealed ceramic body (5). The vacuum interrupter further comprises an auxiliary contact comprising a contact sleeve (13) disposed within the center shield (8) around the fixed electrode (6) of the fixed contact system in spaced apart relationship with the fixed electrode. The lower end of the contact sleeve is fixed to the fixed end plate (11) of the fixed contact system and the upper end of the contact sleeve protrudes out of the fixed contact (7). The contact sleeve is isolated from the fixed electrode and fixed contact. The auxiliary contact further comprises a wound resistor (26) disposed around the fixed electrode in spaced apart relationship with it and connected to the outer end of the fixed electrode and the fixed end plate. The movable contact (16) is inverted frustoconical shaped. The broad base of the inverted frustoconical shaped movable contact corresponds to the inner periphery of the contact sleeve and the movable contact is adapted to close against the contact sleeve prior to closing on the fixed contact (Fig 1).

Documents:

15-MUM-2007-ABSTRACT(23-5-2014).pdf

15-MUM-2007-ABSTRACT-100215.pdf

15-mum-2007-abstract.doc

15-mum-2007-abstract.pdf

15-MUM-2007-CLAIMS(23-5-2014).pdf

15-MUM-2007-CLAIMS(AMENDED)-100215.pdf

15-mum-2007-claims.doc

15-mum-2007-claims.pdf

15-MUM-2007-CORRESPONDENCE(10-4-2007).pdf

15-MUM-2007-CORRESPONDENCE(17-11-2008).pdf

15-MUM-2007-CORRESPONDENCE(21-10-2009).pdf

15-MUM-2007-CORRESPONDENCE(28-10-2014).pdf

15-MUM-2007-CORRESPONDENCE(30-6-2011).pdf

15-mum-2007-correspondence-received.pdf

15-mum-2007-description (complete).pdf

15-MUM-2007-DESCRIPTION(COMPLETE)-(23-5-2014).pdf

15-MUM-2007-DRAWING(23-5-2014).pdf

15-mum-2007-drawings.pdf

15-MUM-2007-FORM 1(10-4-2007).pdf

15-MUM-2007-FORM 1(23-5-2014).pdf

15-MUM-2007-FORM 1(30-6-2011).pdf

15-MUM-2007-FORM 1(4-1-2007).pdf

15-MUM-2007-FORM 13(30-6-2011).pdf

15-MUM-2007-FORM 18(17-11-2008).pdf

15-MUM-2007-FORM 2(TITLE PAGE)-(23-5-2014).pdf

15-MUM-2007-FORM 2(TITLE PAGE)-(4-1-2007).pdf

15-MUM-2007-FORM 2(TITLE PAGE)-100215.pdf

15-MUM-2007-FORM 26(23-5-2014).pdf

15-MUM-2007-FORM 26(30-6-2011).pdf

15-MUM-2007-FORM 26-100215.pdf

15-MUM-2007-FORM 3(23-5-2014).pdf

15-mum-2007-form-1.pdf

15-mum-2007-form-2.doc

15-mum-2007-form-2.pdf

15-mum-2007-form-26.pdf

15-mum-2007-form-3.pdf

15-MUM-2007-MARKED COPY(23-5-2014).pdf

15-MUM-2007-MARKED COPY-100215.pdf

15-MUM-2007-OTHERS-100215.pdf

15-MUM-2007-PUBLICATION REPORT(17-11-2008).pdf

15-MUM-2007-REPLY TO EXAMINATION REPORT(23-5-2014).pdf

15-MUM-2007-SPECIFICATION(AMENDED)-100215.pdf

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