Title of Invention

AN INTEGRATED PARTIAL DISCHARGE-FREE EPOXY TERMINAL BUSHING DEVICE AND A METHOD FOR MANUFACTURING

Abstract

The invention related to an integrated partial discharge free epoxy terminal bushing device capable of withstanding momentary high fault current, comprising a HT-sleeve (4) provided at a center of an withdrawable die, the withdrawable die forming an epoxy body (13) via pump-feeding of a homogeneous resin mix, the HT-sleeve (4) having a plurality of knurled surfaces (5) configured through providing brass tube; a HT-conductor (6) interposable in the HT-sleeve (4) to establish an electrical connection between the bushing (14) and an electrical appliance; an LT-sleeve (08) forming a part of the epoxy body (13) for accommodating the HT-sleeve (04) having a plurality of studs (11) welded thereon for joining the bushing (14) with the terminal box.The HT- conductor (06) is configured as a replaceable component being a non-integral part of the epoxy body (13), and in that the LT-sleeve (08) surface being provided with a plurality of cuts (09) and holes (10) to form a perforated knurled surface which improves the mechanical strength of the bushing (14).

Full Text

FIELD OF INVENTION The invention generally relates to terminal bushings adaptable to low and medium voltage electrical applications for example AC-motors. The invention more particularly relates to an integrated partial discharge free epoxy based terminal bushing device capable of withstanding momentary high fault current. The invention further relates to a process of manufacturing such an integrated epoxy based terminal bushing device. BACKGROUND OF THE INVENTION Terminal bushings are essential for low and medium voltage electrical appliances like AC motors. Conventional terminal bushings use a central high tension conductor integrated with a radial built-up insulation of epoxy. The length of the bushing on either side of the system commensurate with the medium like air. The bushing assembly is housed in a terminal box with suitable fixing arrangement. The reliability of the above system is governed by following parameters: • Quality of the metal conductor and connectors during service. • Void free preparation of the epoxy insulation. • Heat conduction and thermal expansion based incompatibilities. • Life of epoxy insulation. Since conventional terminal bushings are moulded as a single piece i.e., integrated in nature, it is difficult to replace or to maintain if there is a minor damage to the conductor or to the insulator. The conventional bushing can not withstand higher fault currents as the HT conductor is integrated with epoxy insulation. This is mainly due to difference in thermal expansion of epoxy and HT conductor. The differences in thermal expansion of these materials may become a critical problem at higher fault currents due to increase in dimensions of the conductor. Alternatively, performance of the terminal bushing at higher fault currents can be improved by isolating HT conductor from the epoxy insulation. However, the arrangement may not be able to withstand for medium voltage systems due to discharges in service. This is due to an air gap between conductor and epoxy body. In terminal bushings, the LT insert can be part of epoxy body. The LT insert is designed by using studs, which are placed, in position by means of a high conductive wire with suitable number of turns. However, this design may not be suitable for high fault current applications. This may be due to the fact that the collar of the insulator undergoes for severe Electro-dynamic forces during faults. In practice, there is a possibility of cracks in ellipse collar due to a bit of over- tightening or unexpected mechanical forces. Thus, for mechanical integrity, more number of studs is required for fastening to the terminal box. However, the economical bushings like using ellipse collars may not allow for such option. Alternatively, the LT insert is provided on the outer surface of the epoxy body. The outer surface of the LT insert is provided with a locking arrangement to ensure fastening to connecting modules like terminal box or mounting flanges. However, the arrangement may not be reliable at higher system voltage. The interface between LT flange and insulator becomes critical and may lead to discharges at higher system voltage. To overcome these problems along with improving the design for dielectric/mechanical integrity, a new LT insert has been proposed. US patent publication US 2005/0199418 discloses a dielectric bushing in particular a high voltage bushing for a high-voltage apparatus . According to this invention, in order to realize the field control in the field-stressed zone (7; 7a, 7b), at least one screening electrode (6; 6a, 6b) arranged in the interior (20) of the insulator part (2; 2a, 2b; 2c) is eliminated and replaced with a non-linear electric and/or dielectric field control element (9; 9a, 9b; 9i, 9o; 9s) on the insulator part (2; 2a, 2b; 2c) in the region of the first installation flange (4; 8). Among other things, embodiments refer to: design criteria for the geometric arrangement and for the specific materials of the field control element (9; 9a, 9b; 9i, 9o; 9s), in particular various axial and radial arrangements of field control elements (9; 9a, 9b; 9i, 9o; 9s), as well as the realization of the field control element (9; 9a, 9b; 9i, 9o; 9s) in the form of a coating or of a massive element that absorbs mechanical forces. US Patent no. US 4965407 describes a bushing system for high voltage electrical equipment which includes a weathershed with integral annular shirts molded around an electrical conductor. The weathershed has a cavity at its base for mating with the opening in the housing for the high voltage electrical equipment. A dielectric gas in the housing communicates with the cavity in the weathershed. A flange and seal are provided at the base of the weathershed for sealingly connecting the bushing system to the housing. A secondary ground may also be provided. OBJECTS OF THE INVENTION An object of the present invention is to provide an integrated partial discharge free epoxy based terminal bushing device capable of withstanding momentary high fault current which eliminates the disadvantages of the prior art. Another object of the present invention is to provide an integrated partial discharge free epoxy based terminal bushing device capable of withstanding momentary high fault current which can meet the power requirement in the order of 600 MVA to 800 MVA of the electrical system. A further object of the present invention is to provide an integrated partial discharge free epoxy based terminal bushing device capable of withstanding momentary high fault current which can withstand high input requirement for electrical appliances having system voltage of 11 KV and above. A still further object of the present invention is to provide an integrated partial discharge free epoxy based terminal bushing device capable of withstanding momentary high fault current which is cost-effective and easy for maintenance. An yet another object of the present invention is to provide an integrated partial discharge free epoxy based terminal bushing device capable of withstanding momentary high fault current which is configured to have a mechanically strong L.T insert. An yet further object of the present invention is to provide an integrated partial discharge free epoxy based terminal bushing device capable of withstanding momentary high fault current which is easy to manufacture. SUMMARY OF THE INVENTION Accordingly in a first aspect of the invention, there is provided an integrated partial discharge free epoxy based terminal bushing device capable of withstanding momentary high fault current, comprising a HT-sleeve provided at a center of an withdrawable die, the withdrawable die forming an epoxy body via pump-feeding of a homogeneous resin mix, the HT-sleeve having a plurality of knurled surfaces configured through providing brass tube; a HT-conductor interposable in the HT-sleeve to establish an electrical connection between the bushing and an electrical appliance; an LT-sleeve forming a part of the epoxy body for accommodating the LT-sleeve having a plurality of studs welded thereon for joining the bushing with the terminal box; the HT-conductor is configured as a replaceable component being a non-integral part of the epoxy body, and the LT-sleeve surface being provided with a plurality of cuts and holes to configure as a perforated knurled surface to improve the mechanical strength of the bushing. In a second aspect of the invention, there is provided a method of manufacturing an integrated partial discharge free epoxy terminal bushing comprising the steps of mixing bisphenol A epoxy resin and filler silica at a ratio of 100 parts by weight and 375 parts by weight for 7 to 10 hours and maintain under vacuum of 3 to 5 Torr at a temperature about 55 to 75°C; maintaining a hardener, carbolic acid anhydride 100 parts by weight separately for 7 to 10 hours at a temperature about 55 to 75°C under a vacuum of about 3 to 5 Torr; blending the mixture and the hardener with carbolic acid anhydride to produce a homogeneous compound; providing a die corresponding to the dimension of the bushing for moulding of the compound and arranging a non-ferrous metal tube at the center of the die, to form a HT-sleeve with an epoxy body, and forming a LT-sleeve (08) as a part of the epoxy body (13) and providing knurling and perforation via cuts and holes to configure an edge of the sleeve (08). BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1- shows a conventional terminal bushing Fig. 2- shows a HT Sleeve of a terminal bushing according to the present invention. Fig. 3- shows a HT conductor of a terminal bushing of the invention Fig. 4- shows a perforated LT insert of a terminal bushing of the present invention. Fig. 5- shows an epoxy body of a terminal bushing of the invention. Fig. 6- shows a complete assembly of a terminal bushing for high power applications according to the present invention. DETAIL DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION The present invention will now be described with the help of the accompanying drawings. Fig. 1 shows a conventional terminal bushings which use a central high tension (HT) conductor integrated with a radial built-up insulation of epoxy. The bushing assembly is housed in a terminal box with suitable fixing arrangement. Figs. 2 & 3 shows a high tension (HT) sleeve [04] made of brass or stainless steel arranged at the center of the mould. The periphery of the sleeve [04] is provided with plurality of step cut surface and knurled surface [05] to enhance the mechanical strength of the bushing [14] at the interface of epoxy [13] and the sleeve [04]. A HT conductor [06] designed to have provision for tightening of a lock nut [07] at either end is inserted via the sleeve [04]. Figs 4 & 5 describe a LT sleeve [08] proposed as a part of the epoxy body (13). The surface of the LT sleeve [08] is provided with a knurling [09] to enhance the mechanical strength of the bushing [14] at the interface of epoxy [13] and the LT insert [08]. To improve the mechanical strength of the insulator (13) further, small dia holes (10) are made on the sleeve (08). These holes (10) will be helpful for holding of epoxy to the sleeve (08). The dimensions of the LT sleeve (08) is based on the proof voltage as well as test voltage required for the device proposed to be Partial Discharge (PD) free. The sleeve (08) can be even in ellipse profile depending on the space availability for fastening the bushing (09) to the terminal box. Suitable studs (11) have been welded to the LT sleeve (08) and the whole arrangement will be part of an epoxy body (13). The projected studs (11) commensurate with the fastening means provided in the terminal box for necessary fixing arrangements. The LT insert provides enough mechanical strength and arrests the cracks at collar portions (12). Fig. 5 shows that the HT conductor (06) which is not an integral part of the epoxy body (13) of the proposed terminal bushing (14) is inserted through the epoxy body (13) to serve as an electrical connection between the electrical equipment and the incoming/outgoing of the power supply. After inserting the HT conductor (06) through the bushing (01) the lock nuts (07) are tightened at either end and securely fitted. This unique arrangement arrests any movement of the copper conductor (06) during service. Since the HT conductor (06) is not an integral part of the bushing (14), this arrangement facilitates easy assembly, maintenance and replacement. The process of manufacturing the terminal bushing (14) incorporated with the unique design features described herein above using the described composition of material, results in a new bushing capable of meeting the requirement of withstanding higher voltage rating as well as higher fault currents of electrical appliances. Manufacturing process of the terminal Bushing The epoxy based terminal bushing consists of Bisphenol 'A' epoxy resin, carboxylic acid anhydride and silica powder. The composition and process are described below: Composition of epoxy system : SI. No. Material Parts by weight 1. Solvent free bisphenol A epoxy resin 100 2. Carboxylic acid anhydride 100 3. Silica Powder (filler) 375 Apart from the above raw materials, hardware like brass tube, perforated brass sheet of ellipse profile and brass studs, which are welded to brass sheet are also employed in processing these bushings. A current carrying brass/copper rod with suitable fasteners are used in the assembly of a terminal bushing. Bisphenol A epoxy resin and the filler silica powder are thoroughly mixed for 8 hours in a chamber maintained under vacuum of 5 Torr and at a temperature of about 65°c. Hardner, carbolic acid anhydride is kept in another chamber maintained at the same conditions. The operation of maintaining the vacuum and temperature in both the chambers for 8 hours is necessary to remove any dissolved gasses present in the materials. After this operation, both the filler mixed resin and the hardener are blended together by using a mechanical stirrer to get homogenous compound. For moulding this homogenous compound into the bushing, a mild steel die is fabricated to the required dimensions of the bushing. The brass tube described above is arranged at the center of the mild steel die. Knurling being made on the outer surface of the brass tube to enhance the mechanical bonding between the brass tube and the epoxy compound. Brass sheet is profiled into the ellipse shape and is also placed at the center of the mould. The brass sheet with diamond knurling and perforations is provided as a collar support to the bushing for better mechanical strength. The homogenous resin mix, which will be in semi-solid state is pumped into the mould under a pressure of 3 atmospheres. The temperature of the mould is maintained at about 140°c. Under these conditions, the epoxy mix is kept in the mould for 3 hours for curing. The cured bushing is then removed from the mould and kept in an air-circulating oven for 8 hours at 130C for post curing. The post curing helps in cohesive bonding of the resin and the hardener system, thereby resulting in improved mechanical strength. A brass / copper rod is inserted through the brass tube to serve as an electrical connection. Necessary locking nuts and fasteners are provided to the brass / copper rod for connecting bushing to other electric equipment. The terminal bushing with the following exemplary technical specifications can be developed by using the above design features: Rating : 11kV Proof voltage : 28kV Lightning Impulse Volatge : 70kV PD level : Tan o Level, Capacitance : Comparative Tracking Index : > 500 V Insulation resistance : > 10,000 Mega Ohms Short time current rating : > 44kA, 0.25 second Continuous current rating : 600 Amps We Claim: 1. An integrated partial discharge free epoxy terminal bushing device capable of withstanding momentary high fault current, comprising: - a HT-sleeve (4) provided at a center of an withdrawable die, the withdrawable die forming an epoxy body (13) via pump-feeding of a homogeneous resin mix, the HT-sleeve (4) having a plurality of knurled surfaces (5) configured through providing brass tube; - a HT-conductor (6) interposable in the HT-sleeve (4) to establish an electrical connection between the bushing (14) and an electrical appliance; - an LT-sleeve (08) forming a part of the epoxy body (13) for accommodating the HT-sleeve (04) having a plurality of studs (11) welded thereon for joining the bushing (14) with the terminal box; - characterized in that the HT-conductor (06) is configured as a replaceable component being a non-integral part of the epoxy body (13), and in that the LT-sleeve (08) surface being provided with a plurality of cuts (09) and holes (10) to form a perforated knurled surface which improves the mechanical strength of the bushing (14). 2. The device as claimed in claim 1, wherein the HT-sleeve (04) having lock nuts (07) at either end for tightening the HT-conductor (06) when interposed. 3. A method of manufacturing an integrated partial discharge free epoxy terminal bushing, comprising the steps of: - mixing bisphenol A epoxy resin and filler silica at a ratio of 100 parts by weight and 375 parts by weight for 7 to 10 hours and maintain under vacuum of 3 to 5 Torr at a temperature about 55 to 75°C; - maintaining a hardener, carbolic acid anhydride 100 parts by weight separately for 7 to 10 hours at a temperature about 55 to 75°C under a vacuum of about 3 to 5 Torr; - blending the mixture and the hardener with carbolic acid anhydride to produce a homogeneous compound; - providing a die corresponding to the dimension of the bushing for moulding of the compound and arranging a non-ferrous metal tube at the center of the die, to form a HT-sleeve with an epoxy body, and - forming a LT-sleeve (08) as a part of the epoxy body (13) and providing knurling and perforation via brass sheets and holes to configure an edge of the sleeve (08). 4. An integrated partial discharge free epoxy based terminal bushing device capable of withstanding momentary high fault current as substantially described herein with reference to the accompanying drawings. 5. A method of manufacturing an integrated partial discharge free epoxy based terminal bushing as substantially described herein with reference to the accompanying drawings.

Documents:

00440-kol-2005-claims.pdf

00440-kol-2005-description complete.pdf

00440-kol-2005-drawings.pdf

00440-kol-2005-form 1.pdf

00440-kol-2005-form 2.pdf

00440-kol-2005-form 3.pdf

440-KOL-2005-ABSTRACT.pdf

440-KOL-2005-CANCELLED PAGES.pdf

440-KOL-2005-CLAIMS.pdf

440-KOL-2005-CORRESPONDENCE.pdf

440-kol-2005-correspondence1.1.pdf

440-KOL-2005-DESCRIPTION (COMPLETE).pdf

440-KOL-2005-DRAWINGS.pdf

440-kol-2005-examination report.pdf

440-kol-2005-form 1.1.pdf

440-KOL-2005-FORM 1.pdf

440-kol-2005-form 13.1.pdf

440-KOL-2005-FORM 13.pdf

440-kol-2005-form 18.1.pdf

440-kol-2005-form 18.pdf

440-KOL-2005-FORM 2.pdf

440-kol-2005-form 3.pdf

440-kol-2005-gpa.pdf

440-kol-2005-granted-abstract.pdf

440-kol-2005-granted-claims.pdf

440-kol-2005-granted-description (complete).pdf

440-kol-2005-granted-drawings.pdf

440-kol-2005-granted-form 1.pdf

440-kol-2005-granted-form 2.pdf

440-kol-2005-granted-specification.pdf

440-KOL-2005-REPLY TO EXAMINATION REPORT.pdf

440-kol-2005-reply to examination report1.1.pdf