Title of Invention

A METHOD OF PRODUCING SUPERCONDUCTING COIL PARTICULARLY ADAPTABLE TO HIGH TEMPERATURE SUPERCONDUCTING TRANSFORMERS

Abstract

A method of producing supercondueting coil particularly adaptable to high temperature supercondueting transformers, in the form of disk winding with superconducting tape, comprising the steps of providing a top circular ring (2) and a bottom circular ring (1) and joining the rings (1,2) with a plurality of stiffeners (3) by means of several non-magnetic insulated screws to form a circular configuration, the stiffeners (3) having grooves (5) with a pitch corresponding to the voltage in between the turns; forming a first layer (4) of the winding in a first disk (01) by providing a supercondueting tape (6) having a width equal to that of the grooves (5) and passing the tape (6) through the .grooves (5) of the stiffeners (3) in a circular form; forming a second layer (4) in a second Disk (01) by forming the stiffeners (3) in a circle, the top and bottom rings (1,2) being concentric to the first layer (4); forming further layers by repeating the steps of forming the first and/or second layer, and stacking all the formed disks (01) one above the other such that a tape end (6) of the first disk (01) is sandwitched along with the tape end (6) of the consecutive disk (01) thereby producing a LV-winding stack (8); configurating a HV-winding stack (9) by forming a plurality of disks (02) and stacking the disks (02) one above another, the HV-winding stack (9) being concentric to the LV- winding stack (8); and immersing the winding stacks (8,9) in a container filled with liquid nitrogen so that the supercondueting tape (6) of the coil (8,9) remains in direct contact with the liquid nitrogen.

Full Text

FIELD OF INVENTION The invention relates to a method of manufacturing superconducting coils in general and in particular for high temperature superconducting transformer/ more particularly, the invention relate to a method of producing superconducting coils, particularly adaptable to high temperature superconducting transformers. BACKGROUND OF INVENTION Electrical machines having field coil windings generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and one or more coil winding mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor. Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings. Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known super-conductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in weight and size of the machine. Air-core super- conductive rotors require large amounts of super-conducting wire. The large amounts of SC wire add to the number of coils required, the complexity of the coil supports, and the cost of the SC coil windings and rotor. High temperature SC coil windings are formed of super-conducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27.degree. K, to achieve and maintain super-conductivity. The SC windings may be formed of a high temperature super-conducting material, such as a BSCCO (Bi.Sub.x Sr.sub.x Ca.sub.x Cu.sub.x O.sub.x) based conductor. According to prior art, super-conducting coils can be cooled by liquid helium. After passing through the windings of the rotor, the hot, used helium is returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power. Prior SC coil cooling techniques include cooling an epoxy-impregnated SC coil through a solid conduction path from a cryocooler. Alternatively, cooling tubes in the rotor may convey a liquid and/or gaseous cryogen to a porous SC coil winding that is immersed in the flow of the liquid and/or gaseous cryogen. However, immersion cooling requires the entire field winding and rotor structure to be at cryogenic temperature, as a result no iron can be used in the rotor magnetic circuit because of the brittle nature of iron at cryogenic temperatures. In addition, high temperature super-conducting (HTS) coils are sensitive to degradation from high bending and tensile strains. These coils must undergo substantial centrifugal forces that stress and strain the coil windings. Normal operation of electrical machines involves thousands of start up and shut down cycles over the course of several years that result in low cycle fatigue loading of the rotor. Furthermore, the HTS rotor winding should be capable of withstanding 25% over-speed operation during rotor balancing procedures at ambient temperature and notwithstanding occasional over-speed conditions at cryogenic temperatures during power generation operation. These over-speed conditions substantially increase the centrifugal force loading on the windings over normal operating conditions. High strains can damage HTS super-conductor wire. To withstand high strains, HTS wire have in the past been protected by massive and complex coil winding and coil support structures. However, massive, complex super-conducting windings and supports are costly, especially in state-of-the-art air core electrical machines. Moreover, these massive windings have to be cooled to cryogenic temperatures, and thus require large refrigeration systems. The coil windings also are isolated from the hot coil supports and rotor. To isolate the coil windings, large thermal insulators have been used to separate the coils from their support systems. Because the insulators are between the coils and their support systems, prior thermal insulators are large structures that can support the high centrifugal loading of coils. Because these large thermal insulators are in contact with the cold coils, the insulators are a large heat source to the coils. While the isolators are designed to minimized heat conduction to the coils, the insulators result in large cryogenic heat loads and expensive cryorefrigerators. Earlier to developing the high temperature superconducting coils, Low Temperature Super Conductors (LTSC) required were in use, which liquid helium at 4.2 K for cooling. Liquid helium is expensive gas difficult to handle. With the reason it could not find wider acceptance for commercialization. High temperature superconducting transformers are one of the future areas for application of high temperature superconductors for commercialization. High temperature superconducting transformer is smaller in size, weight and volume and more efficient. They are commercially viable for higher rating where high voltage in variably used for bulk transmission of power. Disk winding is preferred over layer winding for high voltage transformer as it distributes the voltage requiring less insulation etc. With the lesser capacitance in high voltage transformer, it is vulnerable to voltage spikes requiring use of disk winding. High temperature superconductors made of ceramic material is available in tape form typical size 0.40mm X 0.30 mm. It does not have strength on its own; it requires mechanical support when formed in a in coil form. Coils have to be invariably immersed in liquid nitrogen in order to get superconducting properties. The superconducting coils recently been developed, mostly formed with a layer winding. Layer winding reduces the effect of perpendicular linkage of flux with less coil will be less. Superconductor current carrying capacity reduces with the presence of perpendicular flux over its surface. In layer type of superconducting winding, superconducting tape is wound over an insulated spool or bobbin like Glass fibre reinforced Plastic. Grooves are made over the hollow GFRP spool in which HTSC tape runs. Number of concentric spools are used depending upon the number, of coils. GFRP spools or bobbins are immersed in liquid nitrogen. In this case superconducting tape is in direct contact with liquid nitrogen only on one side, moreover liquid nitrogen is not circulated freely in the container carrying bobbins along with winding. In case of fault in the one spool winding, one full spool winding has to be replaced with costly superconducting tape. In the disk winding, stacked disks are used one above the other affecting the cooling efficiency though voltage distribution is better than layer winding. OBJECTS OF INVENTION It is therefore an object of the invention is to propose a method of producing superconducting coils, particularly adaptable to high temperature superconducting transformers, which eliminates the disadvantages of prior art. Another object of the invention is to propose a method of producing superconducting coils, particularly adaptable to high temperature superconducting transformers, which requires a smaller capacity of cooling means. A still another object of the invention is to propose a method of producing superconducting coils, particularly adaptable to high temperature superconducting transformers, which provides an improved voltage distribution so as to require less insulation and reduced electrical stresses. A further object of the invention is to propose a method of producing superconducting coils, particularly adaptable to high temperature superconducting transformers, which is cheap to manufacture and easy for maintenance. Accordingly there is provided a method of producing superconducting coil particularly adaptable to high temperature superconducting transformer, in the form of disk winding with superconducting tape, comprising the steps: of providing a top circular ring and a bottom circular ring and joining the rings with a plurality of stiffeners by means of several non-magnetic insulated screws to form a circular configuration, the stiffeners have grooves with a pitch corresponding to the voltage in between the turns; forming a first layer of the winding in a first disk by providing a superconducting tape having a width equal to that of the grooves and passing the tape through the grooves of the stiffeners in a circular form; forming a second layer in a second Disk by forming the stiffeners in a circle, the top and bottom rings being concentric to the first layer; forming further layers by repeating the steps of forming the first and/or second; layer, and stacking all the formed disks one above the other such that a tape end of the first disk is sandwitched along with the tape end of the consecutive disk thereby producing a LV-wingding stack; configurating a HV-winding stack by forming a plurality of disks and stacking the disks one above another, the HV- winding stack being concentric to the LV - winding stack; and immersing the winding stacks in a container filled with liquid nitrogen so that the superconducting tape of the coil remains in direct contact with the liquid nitrogen. Thus, the disk winding with superconducting tape according to the invention is efficiently cooled and reduce the installed capacity of the existing cooling means. Disk winding with superconducting tape ensures better voltage distribution for high voltage transformer requiring less insulation and reduced electrical stresses. Disk winding is a continuous winding in a disk starting from one end layer to another without joints. Disks are stacked one above the other. Connection from one disk to another disk will be with copper strips. One stack of disk is used for LV winding and other stack for HV winding which makes it a compact system. In the present method of disk winding, superconducting tape is surrounded by liquid nitrogen except for the small part sitting in the groove of stiffener thereby achieving better and faster cooling. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 shows the top view of top/bottom ring of a disk made of insulating material. Fig. 2 shows the insulated stiffeners having grooves along the height, equal to the width of superconducting tape. Fig. 3 shows the stacked disks of LV layer and HV layers concentric with LV layers. DETAIL DESCRIPTION OF INVENTION Two insulated rings (1,2) with a plurality of stiffeners (3) forms a single disk (Dl). For making a first layer (4) of winding in a disk (Dl), the stiffeners (3) are joined to the top and bottom circular rings (1,2) with several non magnetic insulated screws (not shown) to form a circle. The stiffeners (3) have several grooves (5), each equal to the width of a superconducting tape (6) with a pitch to be decided by the voltage in between the turns. The superconducting tape (6) runs through the grooves (5) of the stiffeners (3) around the stiffeners (3) in a circular form. After completing the first layer (4), the stiffeners (3) of the second layer (7), are joined in a circle to top and bottom rings (1,2) concentric to the first layer (4) and second layer (7) is laid in the grooves (5) and carried out and so on for the remaining of the HV layers. Distance between the layers (4,7) is predetermined considering the insulation between the layers (4,7) and dielectric strength of the liquid nitrogen. The above disks (Dl) are stacked one above the other to form a LV-winding stack (8). A tape end (6) the (Dl) of the (Dl) disk is sandwiched along with the tape end (6) of a consecutive disk (Dl) in copper strip connected to the body of the disk. Similarly for HV winding, the disks (D2) are produced, stacked one above the other, and joined concentric to the LV disk stack winding to form a HV-winding stack (9). The LV and HV winding tack (8, 9) are immersed in a liquid nitrogen in a container. The liquid nitrogen is in direct contact with the superconducting tape (6) of the whole winding (8, 9). Circulation of the liquid nitrogen is much better in the invented case as there is no barrier for the liquid nitrogen to circulate thereby achieving an efficient and uniform cooling. Holes in the rings are used for connecting the stiffeners to the plate. Holes of adjacent layers of LV and HV windings are segregated. Stiffeners give the mechanical support to the weak superconducting tape. Depth of groove can be changed depending upon the no. of parallel superconducting tape. Distance between the LV and HV layer is as the calculated values of design for getting design parameters of the transformers. Disk winding helps in reducing the electrical stresses and level of insulation. The innovated disk winding is lighter in weight, less in volume, and reducing initial cooling of coils. The voltage distribution will be better than the prior art layer winding, and reduces the insulation and electrical stresses. It is comparatively cheap to manufacture and easy for maintenance to replace faulty coil. Cooling is uniform and efficient reducing the cost of installed cooling capacity. Each disk winding is made from continuous length of a superconducting tape with joint reducing the unnecessary joint losses. For passing large currents parallel conductors can be used by making grooves, having double the thickness of the superconducting tape. WE CLAIM 1. A method of producing superconducting coil particularly adaptable to high temperature superconducting transformers, in the form of disk winding with superconducting tape, comprising the steps of: - providing a top circular ring (2) and a bottom circular ring (1) and joining the rings (1,2) with a plurality of stiffeners (3) by means of several non-magnetic insulated screws to form a circular configuration, the stiffeners (3) having grooves (5) with a pitch corresponding to the voltage in between the turns; - forming a first layer (4) of the winding in a first disk (Dl) by providing a superconducting tape (6) having a width equal to that of the grooves (5) and passing the tape (6) through the grooves (5) of the stiffeners (3) in a circular form; - forming a second layer (4) in a second Disk (Dl) by forming the stiffeners (3) in a circle, the top and bottom rings (1,2) being concentric to the first layer (4); - forming further layers by repeating the steps of forming the first and/or second layer, and stacking all the formed disks (Dl) one above the other such that a tape end (6) of the first disk (Dl) is sandwitched along with the tape end (6) of the consecutive disk (Dl) thereby producing a LV-winding stack (8); - configurating a HV-winding stack (9) by forming a plurality of disks (D2) and stacking the disks (D2) one above another, the HV- winding stack (9) being concentric to the LV- winding stack (8); and - immersing the winding stacks (8,9) in a container filled with liquid nitrogen so that the superconducting tape (6) of the coil (8,9) remains in direct contact with the liquid nitrogen. 2. The method as claimed in claim 1, wherein the distance between the layers (4, 7) is determined corresponding to the insulation between the layers (4, 7), and a dielectric strength of the liquid nitrogen. 3. The method as claimed in claim 1, wherein each disk winding is formed from continuous length of a superconducting tape (6). 4. The method as claimed in any of the preceding claims, wherein the portion of the superconducting tape (6) sitting in the grooves (5) of the stiffeners (3) does not come in direct contact with the liquid nitrogen. 5. A method of producing superconducting coil particularly adaptable to high temperature superconducting transformers as substantially described and illustrated herein with reference to the accompanying drawings. A method of producing superconducting coil particularly adaptable to high temperature superconducting transformers, in the form of disk winding with superconducting tape, comprising the steps of providing a top circular ring (2) and a bottom circular ring (1) and joining the rings (1,2) with a plurality of stiffeners (3) by means of several non-magnetic insulated screws to form a circular configuration, the stiffeners (3) having grooves (5) with a pitch corresponding to the voltage in between the turns; forming a first layer (4) of the winding in a first disk (D1) by providing a superconducting tape (6) having a width equal to that of the grooves (5) and passing the tape (6) through the grooves (5) of the stiffeners (3) in a circular form; forming a second layer (4) in a second Disk (D1) by forming the stiffeners (3) in a circle, the top and bottom rings (1,2) being concentric to the first layer (4); forming further layers by repeating the steps of forming the first and/or second layer, and stacking all the formed disks (D1) one above the other such that a tape end (6) of the first disk (D1) is sandwitched along with the tape end (6) of the consecutive disk (D1) thereby producing a LV-winding stack (8); configurating a HV-winding stack (9) by forming a plurality of disks (D2) and stacking the disks (D2) one above another, the HV-winding stack (9) being concentric to the LV- winding stack (8); and immersing the winding stacks (8,9) in a container filled with liquid nitrogen so that the superconducting tape (6) of the coil (8,9) remains in direct contact with the liquid nitrogen.

Documents:

1945-KOL-2008-(15-05-2014)-ABSTRACT.pdf

1945-KOL-2008-(15-05-2014)-CLAIMS.pdf

1945-KOL-2008-(15-05-2014)-CORRESPONDENCE.pdf

1945-KOL-2008-(15-05-2014)-DESCRIPTION PAGES.pdf

1945-KOL-2008-(15-05-2014)-DRAWINGS.pdf

1945-KOL-2008-(15-05-2014)-FORM-1.pdf

1945-KOL-2008-(15-05-2014)-FORM-2.pdf

1945-kol-2008-abstract.pdf

1945-kol-2008-claims.pdf

1945-kol-2008-correspondence.pdf

1945-kol-2008-description (complete).pdf

1945-kol-2008-drawings.pdf

1945-kol-2008-form 1.pdf

1945-kol-2008-form 2.pdf

1945-kol-2008-form 3.pdf

1945-kol-2008-gpa.pdf

1945-kol-2008-specification.pdf