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)
TITLE OF THE INVENTION
A natural convection cryogenic cooling system for a superconductor transformer
APPLICANTS
EMCO Limited, Plot No F-5, Road No 28, Wagle Industrial Estate, Thane - 400 604, Maharashtra, India, an Indian company and Central Power Research Institute (CPRI), Prof Sir C V Raman Road, Sadashivanagar P O, Bangalore - 560080, Karnataka, India, a Government of India Society, Ministry of Power
INVENTORS
Khadatkar Rajendra Moreshwarrao and Dixit Manglesh Gyanshankar, both of EMCO Limited, Plot No F-5, Road No 28, Wagle Industrial Estate, Thane - 400 604, Maharashtra, India, both Indian nationals
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
This invention, relates to a natural convection cryogenic cooling system for a superconductor
transformer.
BACKGROUND OF THE INVENTION
Superconductor devices like superconductor transformers, motors, generators, fault current limiters, superconductor quantum interference devices (SQID) or superconductor magnetic energy storage devices (SMES) are characterized by their superconductivity because of their very low electrical resistance. A superconductor transformer includes a superconductor coil inserted over a magnetic materiai core and comprising, a high voltage (HV) winding and a low voltage (LV) winding wound with superconducting material tape or wire and located concentrically in spaced apart relationship with each other. The superconductor coil with or without the core is maintained at the saturation temperature or at the subcooling temperature (below the saturation temperature) with a cryogenic cooling system employing a cryogenic fluid, which is normally liquid nitrogen. The saturation temperature of liquid nitrogen is 77.4 k (Kelvin) at one atmospheric pressure and the freezing point of liquid nitrogen is 63.2k (Kelvin) at one atmospheric pressure ("NIST Thermophysical Properties of Pure Fluids", NIST, 1992).
A batch cryogenic cooling system for a superconductor transformer comprises a cryostat, which is a double walled vessel with the space between the walls evacuated. The core coil assembly is located in the cryostat immersed in cryogenic fluid contained in the cryostat. (Van Sciver SW, "Cryogenic system for superconducting devices," Physica C, 354, 129-135, 2001 and Zueger H, "630 kVA high temperature superconducting transformer," Cryogenics, 38, 1169-1172, 1998).

Because of the batchwise operation of the cooling system, the application areas of the superconductor transformer are limited. Further, a batch cryogenic cooling system requires considerable storage capacity for the cryogenic fluid for refilling the cryostat.
A forced circulation cryogenic cooling system for a superconductor transformer comprises a primary cryostat of double walled construction with the space between the walls evacuated. The core coil assembly is located in the cryostat immersed in cryogenic fluid contained in the cryostat. A forced circulation cryogenic cooling system further comprises a secondary cryostat of double walled construction (storage tank) with the space between the walls evacuated and containing the cryogenic fluid. A cryocooler is mounted at the top of the secondary cryostat with the coldhead thereof extending into the cryogenic fluid contained in the secondary cryostat . through the vapour region above the cryogenic fluid level in the secondary cryostat. The primary and secondary cryostats are interconnected and cryogenic fluid is recirculated in the primary cryostat with the help of a cryogenic recirculation pump connected to the primary and secondary cryostats (Funaki K, Iwakuma M, Kajikawa K, et al., "Development of a 500 kVA-class oxide superconducting power transformer operated at liquid-nitrogen temperature," Cryogenics, 38, 211-220, 1998; Funaki K and Iwakuma M, "Recent activities for applications to HTS transformers in Japan," Supercond Sci. Technol, 13, 60-67, 2000; Funaki K, Iwakuma M, Kajikawa K, et al., "Development of a 22kV/6.9kV Single-phase Model for a 3 MVA HTS Power Transformer," IEEE Trans Appl Supercond, 11, 1578-1581, 2001; and Iwakuma M, Funaki K, Kajikawa K, et al., "Ac Loss Properties of a 1 MVA Single-phase HTS Power Transformer," IEEE Trans Appl Supercond, 11, 1482-1485, 2001). A forced recirculation cryogenic cooling system comprises a large number of component parts and is bulky and heavy

and expensive and occupies a large area. Energy consumption and losses of such a cooling system are also high. Maintenance of the cryogenic recirculation pump is also troublesome and expensive.
A natural convection cryogenic cooling system for a superconductor transformer comprises a cryostat of double walled construction with the space between the walls evacuated. The core coil assembly is located in the cryostat immersed in cryogenic fluid contained in the tank. The cooling system also comprises a cryocooler mounted at the top of the cryostat with its coldhead extending into the cryogenic fluid in the cryostat through the vapour region above the cryogenic fluid level in the cryostat. The cooling effect of the coldhead of the cryocooler getting transmitted into the cryogenic fluid in the cryostat forms convection current so as to keep the cryogenic fluid cooled. (S. Yoshida et al, " 1 ATM subcooled liquid nitrogen cryogenic system with GM-Refrigerator for a HTS power transformer", CP 613, Advances in Cryogenic Engineering: proceedings of the Cryogenic Engineering Conference, vol 47.) In such a cooling system, the cooling effect is not well dispersed and distributed in the entire body of the cryogenic fluid because of the limited surface area of contact between the coldhead and the fluid. As a result, the cooling efficiency of the system is reduced. Rise in temperature may cause evaporation of the cryogenic fluid and bubble formation and damage the coil. As only a limited portion of the coldhead is exposed to the vapour region in the cryostat, condensation of the vapour into cryogenic fluid due to contact with the coldhead is limited. This may increase vapour pressure within the cryostat. Vapour pressure upto 2 to 5 bars does improve the dielectric strength of the cryogenic fluid but in case of excess pressure build up, part of the vapour may have to be vented out to regulate the vapour pressure within the cryostat. This may result in

wastage of the valuable cryogenic fluid. Besides, as the entire core coil assembly is immersed in the cryogenic fluid, part of the cooling effect is absorbed by the core. Therefore, the cooling effect is reduced and this can increase the temperature in the cryostat and reduce the cooling efficiency of the cooling system and damage the coil. Due to reduced cooling efficiency of the system, the dielectric strength of the cryogenic fluid and the critical current density of the superconductor device are also reduced.
Another natural convection cryogenic cooling system for a superconductor transformer comprises a cryostat of double walled construction with the space between the walls evacuated. The core coil assembly is located in the cryostat and supported on an annular copper flange disposed above the core coil assembly. The copper flange is mounted to the coldhead of the cryocooler which is mounted at the top of the cryostat. The cryostat also comprises a copper thermal shield extending over the entire core coil assembly upto the bottom end thereof and fixed to the copper flange. (P.Tixador et al, "First tests of a Bi/Y transformer", www.3-cs.co.uk/papers/EUCAS03Transfo.pdf) In such a cooling system, the core is also cooled by the cryogenic fluid and hence the cooling system requires very high cooling capacity to regulate the temperature and pressure rise in the cryostat. This in turn increases the cost of the cooling system. Also, the thermal shield may cause flash overs across the coil. Further the vibrations of the coldhead will be transmitted to the copper flange and the core coil assembly and the thermal shield and damage these components.
Another version of the natural convection cryogenic cooling system for a superconductor transformer comprises a cryostat of double walled construction describing a central bore. The

space between the walls is evacuated and the coil is located in the cryostat immersed in cryogenic fluid contained in the cryostat. It also comprises a cryocooler mounted at the top of the cryostat with the coldhead of the cryocooler extending into the cryogenic fluid through the vapour region in the cryostat above the cryogenic fluid level. A copper shield is disposed around the coil and connected to the coldhead of the cryocooler. The copper shield is also fixed to the top of the cryostat and is extending upto the bottom of the coil. The core is located in the central bore and air cooled. (Ho-Myung Chang et al, "Cryogenic cooling system of HTS transformer by natural convection of subcooled liquid nitrogen", .Cryogenics, 43 (2003), 589-596). In this construction of the cooling system, the copper thermal shield may cause flash over across the coil. The vibrations of the coldhead also will be transmitted to the copper thermal shield and damage the copper thermal shield.
OBJECTS OF THE INVENTION
An object of the invention is to provide a natural convection cryogenic cooling system for a superconductor transformer, which cooling system has improved cooling efficiency and flexibility to operate over a wide range of saturation and subcooling temperatures.
Another object of the invention is to provide a natural convection cryogenic cooling system for a superconductor transformer, which cooling system maintains vapour pressure in the cryostat and improves the dielectric strength of the cryogenic fluid and prevents loss of cryogenic fluid by way of venting out excess vapour pressure.

Another object of the invention is to provide a natural convection cryogenic cooling system for a superconductor transformer, which cooling system prevents temperature rise in the cryostat and improves dielectric strength of the cryogenic fluid and critical current density of the superconductor transformer.
Another object of the invention is to provide a natural convection cryogenic cooling system for a superconductor transformer, which cooling system prevents vibrations of the coldhead of the cryocooler getting transmitted into the heat exchanger of the cooling system so as to prevent damage to the heat exchanger.
Another object of the invention is to provide a natural convection cryogenic cooling system for a superconductor transformer, which cooling system prevents flash over across the coil and reduces eddy currents and heat losses to further improve the cooling efficiency of the cooling system.
Another object of the invention is to provide a natural convection cryogenic cooling system for a superconductor transformer, which cooling system facilitates solid cryogen cooling which has high storage capacity and high density of cold storage.
Another object of the invention is to provide a natural convection cryogenic cooling system for a superconductor transformer, which cooling system can operate in any direction without concern for gravitational effects.

Another object of the invention is to provide a natural convection cryogenic cooling system for a superconductor transformer, which cooling system comprises few components and is compact, light weight, energy efficient and economical and reduces space requirements and maintenance.
DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a natural convection cryogenic cooling system for a natural convection cryogenic cooling system for a superconductor transformer, the cooling system comprising a cryostat of double walled construction describing a central bore and having a top flange closing the top thereof, the space between the walls of the cryostat being evacuated, the superconductor coil of the transformer being immersed in cryogenic fluid contained in the cryostat describing an electrical clearance with the inner wall of the cryostat, the superconductor coil being mounted to the top flange electrically insulated therefrom, a part of the core of the transformer being disposed in the central bore of the cryostat and the remaining part of the core being disposed outside the cryostat, the cooling system further comprising a cryocooler mounted on the top flange with the coldhead thereof extending into the Cryostat through the top flange and vapour region of the cryostat and the cooling system further comprising a heat exchanger made of a non-ferrous thermal conductor material and having a semi-circular plate horizontally disposed in the cryostat above the superconductor coil describing an electrical clearance with the superconductor coil and the inner wall of the cryostat, the superconductor coil also being mounted to the semi-circular plate electrically insulated therefrom, the semicircular plate being mounted to the cold head of the cryocooler through a non-ferrous thermal conductor material flexible joint, the heat exchanger further comprising an inner row of spaced fins projecting down from the inner periphery thereof and an outer row of spaced fins projecting down from the outer

periphery thereof describing an electrical clearance with the inner wall of the cryostat, the fins describing an electrical clearance with the superconductor coil, the lower parts of the fins being disposed in the cryogenic fluid and the semicircular plate and upper parts of the fins being disposed in the vapour region of the cryostat, the cryostat and the said remaining part of the core being located in a protective tank.
According to the invention there is also provided a natural convection cryogenic cooling system for a superconductor transformer, the cooling system comprising a cryostat of double walled construction describing a central bore and having a top flange closing the top thereof, the space between the walls of the cryostat being evacuated, the superconductor coil of the transformer being immersed in cryogenic fluid contained in the cryostat describing an electrical clearance with the inner wall of the cryostat, the superconductor coil being mounted to the top flange electrically insulated therefrom, a part of the core of the transformer being disposed in the central bore of the cryostat and the remaining part of the core being disposed outside the cryostat, the cooling system further comprising a cryocooler mounted on the top flange with the coldhead thereof extending into the cryostat through the top flange and vapour region of the cryostat and the cooling system further comprising a heat exchanger made of a non-ferrous thermal conductor materia] and having a semi-circular plate horizontally disposed in the cryostat above the superconductor coil describing an electrical clearance with the superconductor coil and the inner wall of the cryostat, the superconductor coil also being mounted to the semi-circular plate electrically insulated therefrom, the semicircular plate being mounted to the cold head of the cryocooler through a non-ferrous thermal conductor material flexible joint, the heat exchanger further comprising an inner row of spaced fins projecting down from the inner periphery thereof

and an outer row of spaced fins projecting down from the outer periphery thereof describing an electrical clearance with the inner wall of the cryostat, the fins describing an electrical clearance with the superconductor coil, the lower parts of the fins being disposed in the cryogenic fluid and the semicircular plate and upper parts of the fins being disposed in the vapour region of the cryostat, the cryostat and the said remaining part of the core being located in a protective tank and a heater wire wrapped around the fins, the heater wire being made of non-ferromagnetic material and having a voltage rating of 5 to 50 V and a current rating of 25 to 100 watts.
The following is a detailed description of the invention with reference to the accompanying drawings, in which:
Fig I is a crosssectional view of a core coil assembly of a superconductor transformer and a natural convection cryogenic cooling system of the transformer according to an embodiment of the invention;
Figs 2 is an isometric view of the heat exchanger and coldhead of the cryocooler of the cooling system of Fig 1;
Fig 3 is an enlarged view at X in Fig 2; and
Fig 4 is a heat exchanger and coldhead of the cryocooler of the natural convection cryogenic cooling system of a superconductor transformer according to another embodiment of the invention.

The natural convection cryogenic cooling system 1 for a superconductor transformer (not shown) as illustrated in Figs 1 to 3 of the accompanying drawings comprises a cryostat 2 of double walled construction (inner wall and outer wall of the cryostat marked 2a and 2b, respectively) describing a central bore 3 and having a top flange 4 closing the top thereof. The space 5 between the walls of the cryostat is evacuated through port 6. The superconductor coil 7 of the transformer is immersed in cryogenic fluid (not shown) contained in the cryostat describing an electrical clearance with the inner wall of the cryostat. The cryogenic fluid level is marked 8. The superconductor coil is mounted to the top flange electrically insulated therefrom with insulator material studs 8a. 9 is the core of the superconductor transformer comprising a middle limb 9a disposed in the central bore of the cryostat. The remaining part 9b of the core is disposed outside the cryostat. The cooling system further comprises a cryocooler (not shown) mounted on the top flange 4 with the coldhead 10 thereof extending into the cryostat through the top flange and vapour region 8b of the cryostat above the cryogenic fluid level. The cooling system further comprises a heat exchanger 11 made of a non-ferrous thermal conductor material and having a semi-circular plate 11a horizontally disposed in the cryostat above the superconductor coil describing an electrical clearance with the superconductor coil and the inner wall of the cryostat. The superconductor coil is also mounted to the semi-circular plate electrically insulated therefrom with studs 12, The semi-circular plate is mounted to the cold head 10 of the cryocooler with a non-ferrous thermal conductor material flexible joint 13. The flexible joint 13 is held to the semicircular plate Ha with a holding plate 13a fixed to the plate 11a against the flexible joint. The heat exchanger further comprises an inner row of spaced fins lib projecting down from the inner periphery thereof and an outer row of spaced fins 1 lc projecting down from the

outer periphery thereof describing an electrical clearance with the inner wall of the cryostat. The fins describe an electrical clearance with the superconductor coil. The heat exchanger is so disposed in the cryostat that the lower parts of the fins are located in the cryogenic fluid and the semi-circular plate and upper parts of the fins are located in the vapour region of the cryostat. The cryostat and the remaining part of the core are located in a protective tank 14. The heat exchanger and flexible joint are preferably made of copper. The flexible joint is, preferably, a flexible braid. Pressure relief valve of the cryostat and HV winding and LV winding leads of the superconductor coil are marked 15, 16 and 17 respectively.
The lower parts of the fins disposed in the cryogenic fluid have a large area and transmit and distribute the cold effect of the coldhead of the cryocooler over a large area of the cryogenic fluid. The cold effect being transmitted through the fins forms a convection current and keeps the cryogen in the cryostat cooled. The semicircular plate and upper parts of the fins located in the vapour region also provide a larger area. The cold effect of the coldhead at the semi-circular plate and upper parts of the fins located within the vapour region of the cryostat condenses the vapour. The condensate of cryogenic fluid falls down into the cryogenic fluid in the cryostat. As a result of all this, the cooling efficiency of the cooling system is substantially increased and it is possible to maintain the cryogenic fluid over a wide range of saturation and subcooling temperatures. Vapour pressure is well maintained in the cryostat and loss of the cryogenic fluid by way of release of vapour pressure from the cryostat is avoided. Temperature rise in the cryostat is avoided and dielectric strength of the cryogenic fluid and critical current density of the transformer are improved. Because of the heat exchanger being mounted to the coldhead with the flexible joint, vibrations of the coldhead are absorbed by the flexible joint and are not

transmitted to the heat exchanger. As a result, damage to the heat exchanger is avoided. Because of the non-ferrous nature of the heat exchanger, the heat exchanger also acts as a flux diverter and repels the magnetic flux around the coil. Therefore, the radial flux cutting the superconductor coil is reduced. There is reduction of eddy currents generated by the alternating magnetic field in the core and heat losses in the cryogenic fluid due to the eddy current. This. further improves the cooling efficiency of the cooling system. The cooling system can be operated over a wide temperature range from 4 to 80 k. Therefore, it is possible to achieve solid cryogenic cooling ie cooling below freezing point of the cryogen. The solid cryogen cooling improves the critical current density of the superconductor. Moreover, the solid has a high storage capability and therefore a high density of cold storage is possible. Furthermore, a solid system can operate in any orientation without concern for gravitational effects. Such a cooling system may be used in superconductor magnetic energy storage (SMES). However, other superconductor devices like motors, generators, fault current limiters, superconductor quantum interference device (SQID) or superconductor magnetic energy storage device (SMES) are also to be construed and understood to be within the scope of the invention. The natural convection cryogenic cooling system of the invention also will additionally have the general advantageous associated with natural convection cryogenic cooling system. For instance, they comprise few components and are compact, light weight, energy efficient and economical and occupy reduced space and require low maintenance.
The heat exchanger as illustrated in Fig 4 of the accompanying drawings includes a heater wire 19 wrapped around the fins. The heater wire is made of a non-ferromagnetic material and has a

voltage rating of 5 to 50 V and a current rating of 25 to 100 watts. The heater wire is preferably made of Phosphor Bronze or Manganin. The heater wire can be used to heat the heat exchanger to temperatures of the order of 0.5 °C to 4°C so as to raise the cryogenic fluid vapour pressure to 2 to 5 bars and improve its dielectric strength. This can be advantageous in some applications where improvements in dielectric strength of cryogen are required at superconductor saturation temperatures.

We claim:
1- A natural convection cryogenic cooling system for a superconductor transformer, the
cooling system comprising a cryostat of double walled construction describing a central bore and having a top flange closing the top thereof, the space between the walls of the cryostat being evacuated, the superconductor coil of the transformer being immersed in cryogenic fluid contained in the cryostat describing an electrical clearance with the inner wall of the cryostat, the superconductor coil being mounted to the top flange electrically insulated therefrom, a part of the core of the transformer being disposed in the central bore of the cryostat and the remaining part of the core being disposed outside the cryostat, the cooling system further comprising a cryocooler mounted on the top flange with the coldhead thereof extending into the cryostat through the top flange and vapour region of the cryostat and the cooling system further comprising a heat exchanger made of a non-ferrous thermal conductor material and having a semi-circular plate horizontally disposed in the cryostat above the superconductor coil describing an electrical clearance with the superconductor Coil and the inner wall of the cryostat, the superconductor coil also being mounted to the semi-circular plate electrically insulated therefrom, the semicircular plate being mounted to the cold head of the cryocooler through a non-ferrous thermal conductor material flexible joint, the heat exchanger further comprising an inner row of spaced fins projecting down from the inner periphery thereof and an outer row of spaced fins projecting down from the outer periphery thereof describing an electrical clearance with the inner wall of the cryostat, the fins describing an electrical clearance with the Superconductor coil, the lower parts of the fins being disposed in the cryogenic fluid

and the semicircular plate and upper parts of the fins being disposed in the vapour region of the cryostat, the cryostat and the said remaining part of the core being located in a protective tank.
2. The cooling system as claimed in claim 1, wherein the heat exchanger and flexible joint are made of copper.
3. The cooling system as claimed in claim 1 or 2, wherein the flexible joint comprises a flexible braid.
4. A natural convection cryogenic cooling system for a superconductor transformer, the cooling system comprising a cryostat of double walled construction describing a central bore and having a top flange closing the top thereof, the space between the walls of the cryostat being evacuated, the superconductor coil of the transformer being immersed in cryogenic fluid contained in the cryostat describing an electrical clearance with the inner wall of the cryostat, the superconductor coil being mounted to the top flange electrically insulated therefrom, a part of the core of the transformer being disposed in the central bore of the cryostat and the remaining part of the core being disposed outside the cryostat, the cooling system further comprising a cryocooler mounted on the top flange with the coldhead thereof extending into the cryostat through the top flange and vapour region of the cryostat and the cooling system further comprising a heat exchanger made of a non-ferrous thermal conductor material and having a semi-circular plate horizontally disposed in the cryostat above

the superconductor coil describing an electrical clearance with the superconductor coil and the inner wail of the cryostat, the superconductor coil also being mounted to the semi-circular plate electrically insulated therefrom, the semicircular plate being mounted to the cold head of the cryocooler through a non-ferrous thermal conductor material flexible joint, the heat exchanger further comprising an inner row of spaced fins projecting down from the inner periphery thereof and an outer row of spaced fins projecting down from the outer periphery thereof describing an electrical clearance with the inner wall of the cryostat, the fins describing an electrical clearance with the superconductor coil, the lower parts of the fins being disposed in the cryogenic fluid and the semicircular plate and upper parts of the fins being disposed in the vapour region of the cryostat, the cryostat and the said remaining part of the core being located in a protective tank and a heater wire wrapped around the fins, the heater wire being made of non-ferromagnetic material and having a voltage rating of 5 to 50 V and a current rating of 25 to 100 watts.
5. The cooling system as claimed in claim 4, wherein the heater wire is made of Phosphor bronze or Manganin.
6. The cooling system as claimed in claim 4 or 5, wherein the heat exchanger and flexible joint are made of copper.

7. The cooling system as claimed in any one of claims 4 to 6 wherein the flexible joint
comprises a flexible braid.