FORM 2
THE PATENTS ACT, 1970
(39 Of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TrTLE OF THE INVENTION
A resistive type superconducting fault current limiter
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTOR
Vaithiyanathan Ramakichenan, Crompton Greaves Limited, Product Technology Centre, CG Global R&D Centre, Kanjur Marg (East), Mumbai 400042, Maharashtra, India, an Indian national
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:


FIELD OF THE INVENTION
The invention relates to a resistive type superconducting fault current limiter.
BACKGROUND OF THE INVENTION
Superconducting fault current limiters are electric devices used for limiting fault current under short circuit conditions in power supply and distribution systems and are generally resistive and inductive types. A resistive type superconducting fault current limiter comprises an air core superconducting coil immersed in a cryogenic fluid contained in a cryostat. The number of turns of the superconducting coil will depend upon the current rating of the fault current limiter. Usually the cryogenic fluid is liquid nitrogen maintained at cryogenic temperatures of 65K-77K. Due to thermal loading on the cryostat, the temperatures of the cryogenic fluid increases. Unless the cryogenic fluid is maintained at the superconducting state ie at the superconducting temperatures the cooling and performance efficiency of the fault current limiter reduces. The superconducting coil may also go into quenching state and get damaged. The cryogenic fluid is maintained at the superconducting state by a cryocooler whose rating will depend upon the heat load on the cryogenic fluid contained in the cryostat. The capital and running costs of the fault current limiter will increase depending upon the rating of the cryocooler. The cryostats are generally made of fiber glass reinforced plastics (FRP) and stainless steel and are double walled construction with the space between the walls being evacuated. In the case of a FRP cryostat the thermal loading is more as compared to the stainless steel cryostat. Therefore, the cooling efficiency of a stainless steel cryostat is comparatively more. However, the stray losses in a stainless steel cryostat are more due
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to the magnetic flux generated by the excited superconducting air core coil finding its way to the inner and outer walls of the cryostat and heating it up. Also, due to the perpendicular flux penetration or concentration at the top and bottom edges of the superconducting air core coil, ac losses occur in the stainless steel cryostat.
OBJECTS OF THE INVENTION
An object of the invention is to provide a resistive type superconducting fault current limiter which reduces ac losses and improves the cooling efficiency of the fault current limiter.
Another object of the invention is to provide a resistive type superconducting fault current limiter which reduces stray losses and improves the cooling efficiency of the fault current limiter.
According to the invention there is provided a resistive type superconducting fault current limiter comprising a superconducting air core coil immersed in a cryogenic fluid contained in a cryostat, wherein the cryostat is made of stainless steel and is double walled construction with the space between walls being evacuated and the superconducting air core coil is surrounded by a plurality of magnetic material shunt elements supported in the cryostat electrically insulated therefrom.
The following is a detailed description of the invention with reference to the accompanying drawings, in which :
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Fig 1 is a schematic view of the resistive type superconducting fault current limiter according to an embodiment of the invention; and
Fig 2 is a schematic view of the resistive type superconducting fault current limiter according to another embodiment of the invention.
The fault current limiter 1A as illustrated in Fig 1 of the accompanying drawings comprises a superconducting air core coil 2 immersed in a cryogenic fluid 3 contained in a cryostat 4. The cryostat is made of stainless steel and is double waJJed construction (walls marked 4a and 4b) with the space 5 between the walls being evacuated. A plurality of magnetic material C-shaped shunt elements 6a are provided around the coil. The shunt elements are electrically insulated from the cryostat by mounting them to insulator members 7 which are fixed to the inner wall of the cryostat. The coil is electrically insulated from the cryostat by insulator members 8 and 9. The cryostat is electrically insulated by insulator members 10. Current leads of the coil are marked 11. 12 are the connecting wires. The shunt elements provide a path for the magnetic flux generated by the super conducting air core coil which would have otherwise travelled to the inner and outer walls of the cryostat and heated it up. As a result stray losses of the fault current limiter are reduced. Further the vertical flux concentration at the top and bottom edges of the coil is prevented by the ends of the shunt elements extending to the edges of the coil. The ends of the shunt elements provide a path for the vertical flux thereby preventing concentration of the vertical flux at the edges of the coil. As a result,
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the ac losses of the fault current limiter are reduced. In the case of the resistive type superconducting fault current limiter 1B as illustrated in Fig 2 of the accompanying drawings, the shunt elements 6b are straight sections. The shunt elements provide a path for the magnetic flux generated by the super conducting air core coil so as to reduce the stray losses of the fault current limiter. The shunt elements are for example, laminates of silicon or amorphous steel or ferrite material.
Stray losses and ac losses of a typical resistive type superconducting fault current limiter of Fig 1 with coil rating 125A with and without the shunt elements were simulated in FEMM (Finite element method magnetic) V4.0. The cryostat was made of stainless steel and the shunt elements were made of CRGO (Cold rolled grain oriented) steel. The results were as shown below :

Fault current limiter type Stray losses in the inner wall (watts) Stray losses in the outer wall (watts) Perpendicular flux across the coil (mT)
with shunt elements 0.00506663 0.00274942 3.5
without shunt elements 34.4884 18.3556 10.5
It is quite clear from the above simulation results that stray losses and ac losses in the fault current limiter of the invention were considerably reduced.
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We claim:
1. A resistive type superconducting fault current Iimiter comprising a
superconducting air core coil immersed in a cryogenic fluid contained in a cryostat,
wherein the cryostat is made of stainless steel and is double walled construction with the
space between walls being evacuated and the superconducting air core coil is surrounded
by a plurality of magnetic material shunt elements supported in the cryostat electrically
insulated therefrom.
2. The fault current Iimiter as claimed in claim 1, wherein the shunt elements are
straight sections.
3. The fault current Iimiter as claimed in claim 1, wherein the shunt elements are C-
shaped.
4. The fault current Iimiter as claimed in any one of claims 1 to 3, wherein shunt elements are laminates of silicon or amorphous steel or ferrite material.
5. The fault current Iimiter as claimed in any one of claims 1 to 3, wherein the shunt elements are made of laminates of cold rolled grain oriented steel.
Dated this 12th day of August 2008.
Crompton Greaves Limited By their Agent & Attorney

(Jose M A) of Khaitan & Co
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