The present invention relates to cryogenic refrigeration systems for cooling a superconducting device, such as a synchronous machine having a rotor with a high temperature superconducting component.
Cryogenic refrigerators are often used to cool thermal loads, such as a high-temperature superconducting field winding of a rotor in a synchronous electrical generator (HTSG). The field winding is cooled to cryogenic temperatures through an external cryogenic refrigerator that circulates cold helium gas through a fluid circuit to the field winding in the rotor.
Cryogenic cooling is necessary for a superconducting generator. The rotor field winding loses its superconducting capacity when heated above cryogenic temperatures. To ensure continuous generator operation, cryogenic cooling fluid should be constantly supplied to the super-conducting field winding. If the refrigerator fails, the temperature of the cooling fluid rises and the field winding warms enough to quench and cease to be superconducting. A backup refrigeration system is typically used to provide a constant source of cooling fluid for the field winding, especially in situations where the main cooling system fails or requires maintenance.
Conventional cryogenic refrigeration systems include Gifford-McMahon, Pulse Tube, Stirling and reverse Brayton refrigeration systems. FIGS. 4 and 5 schematically show a HTS generator rotor coil winding 102 being cooled by representative cryocooler refrigeration systems. FIG. 4 shows a cryocooler system 100 that uses coldheads 114 of a Gifford-McMahon (GM), pulse tube (PT), or Stirling system to cool the cooling fluid (typically helium gas at 20° Kelvin) circulated through the high temperature, super conducting (HTS) rotor coil 102. The refrigeration system 100 includes a circulating compressor(s) 104 that moves refrigeration fluid through the pipe lines 106 in the system 100 and between the system and the rotor 102. The refrigeration system includes a circulation heat exchanger 108, a bypass valve 110, a plurality of coldhead compressors 112 and coldheads 114 for a Gifford-McMahon or Pulse tubes system, and a coldhead heat exchanger 116.
FIG. 5 shows an alternative cryocooler system 120 that uses a Reverse-Brayton type refrigerator 120 to cool the fluid circulated through the rotor. Cryogenic cooling fluid cools a superconducting winding in a HTS rotor 102. The cooling fluid flows through a circuit 106 having feed and return lines to and from the rotor. The refrigerator 120 includes a compressor and oil removal device 122 that filters and compresses the cooling fluid, e.g., helium gas, and passes the compressed fluid to a circulating heat exchanger(s) 124 in a cold box 125. A turbo expander 126 causes the fluid to cool before it is fed to the rotor 102.
In both conventional cryogen cooling systems 100, 120, there are multiple components that can individually cause the refrigeration system to fail by not working. These components require redundancy, and special systems and procedures so that they can be removed temporarily without adding to the heat load of the refrigeration system.
The main cryogenic cooling system tends to be an expensive component in a high-temperature super-conducting generator (HTSG). A conventional cooling system with redundant components or a redundant cooling system further increases the cost of the cryogenic cooling system. Redundant components in a conventional cooling system may include compressors and coldheads. Alternatively, a redundant main cooling system may been provided to a conventional cooling system. In addition, conventional cooling systems tend to employ elaborate devices to facilitate the removal of redundant cooling components, e.g., the coldheads, for refurbishment while the generator remains on-line. Even so, there are some cooling components that are traditionally serviced by taking the cooling system and generator off-line, e.g., filters and turbines, which negatively affect generator availability and reliability.
There is a long-felt need for simple, inexpensive and reliable cryogen cooling systems that enable all components (or a large portion of components) of the main refrigeration system 100, 120 to be serviced without disrupting the generator operation. Further, there is a need for a system that reduces the redundancy of components in the main refrigeration system and that enables relatively simple means for removal of refrigeration components for refurbishment while the generator is on-line. Moreover, there is a need for a refrigeration system that enables rapid cooldown of the rotor coil during generator startup procedures.