The invention relates to superconducting synchronous electrical machines with a reverse flow ventilation system to cool the stator of the machine.
In the process of producing electricity, power generators create heat that must be dissipated from the generator. Heat occurs in generators due primarily to windage and friction, electric current flow, and time-varying magnetic fields in magnetic structures. Frictional heating occurs as the rotor spins at high speed in the generator. Similarly, heating also occurs as current flows through the rotor and stator coils, as these coils rotate relative to one another in the magnetic fields of the generator. Losses in the magnetic circuit occur as the magnetic fields change with time in permeable materials, such as for example in the stator core and the rotor poles of a synchronous generator. Generators are conventionally equipped with cooling systems to transfer heat from the stator and rotor away from the generator.
Gas ventilation cooling systems have been used in conventional synchronous machines, such as generators and motors, that do not employ superconducting materials. These gas ventilation systems tightly couple the cooling of stator and rotor. The ventilation system cools both the rotor and stator by forcing cooling gas through gas passages in the rotor and stator. Conventional ventilation systems have employed forward flow and reverse flows of cooling gases through the stator and rotor.
In conventional synchronous machines, such as synchronous generators and motors, the ventilation system of gas-cooled machines tightly couples the cooling of stator and rotor. In the forward flow ventilation scheme (FIG. 1) the cooling gas flows through sections of the rotor and stator in series which creates a tight coupling between rotor and stator cooling systems. In the reverse flow ventilation scheme (FIG. 2) the cooling gas flows through stator and rotor in parallel, but then mixes in the machine air gap, also leading to a coupling of the stator and rotor cooling.
Because of the coupling of the cooling of the rotor and stator, the ventilation system must be configured to provide adequate cooling for both the stator and rotor. To provide cooling for the rotor, some compromises may have to be made in a conventional ventilation system with respect to cooling the stator and vice versa. It may be difficult to optimize the cooling of either the stator or rotor with a ventilation system that must provide cooling for both the rotor and stator. Nevertheless, ventilation systems have conventionally provided cooling for both the stator and rotor in large industrial and utility power generators.
In a superconducting synchronous machine the rotor field winding is operated at cryogenic temperatures through a cryorefrigeration system that has its own self-contained cooling circuit. A cold, cryogenic coolant is supplied to the rotor through a transfer coupling, from where it is circulated through a cooling circuit on the rotor where it picks up heat to be removed, and then returns to a stationary cooling system through the transfer coupling. This cryogenic cooling system provides effective cooling of the rotor in a superconducting machine.
The cryogenic cooling system for a superconducting rotor does not cool the stator. The stator of such a superconducting synchronous machine requires a stator cooling system. Contrary to conventional machines where stator and rotor cooling systems are coupled in a single ventilation system, the cooling system of the cryogenic rotor and the gas-cooled stator may be completely independent. Thus, a stator cooling system is needed to cool the stator in a synchronous machine having a superconducting rotor.
A stator ventilation system has been developed for a superconducting synchronous machine. The stator of a superconducting synchronous machine is cooled by a reverse ventilation system in which a cooling gas, such as air or hydrogen, is drawn from the air gap and pumped through a diffuser, heat exchanger and through the stator core back to the air gap. In addition, a conventional synchronous machine may be retrofit with a superconducting rotor and a conventional ventilation system modified to embody the ventilation system disclosed here. An alternate stator ventilation configuration follows the principle of forward flow, in which the air flows through the stator in the opposite direction to the reverse flow stator cooling system.
In one embodiment, the invention is a synchronous machine comprising: a rotor coupled to a rotor cooling system; a stator around the rotor and separated from the rotor by an annular gap between the rotor and an inner surface of the stator, and a stator ventilation system separate and independent of the rotor cooling system.
In another embodiment, the invention is a superconducting electromagnetic machine comprising: a solid core rotor having a cryogenically cooled superconducting rotor coil winding; a stator coaxial with said rotor and having stator coils magnetically coupled with said superconducting rotor coil winding, said stator coils arranged around said rotor, and said stator having cooling passages extending from an outer periphery of the stator to an inner periphery of the stator, said inner periphery separated from the rotor by an annular air gap; said rotor having cooling passages for cryogenic cooling fluid; a stator ventilation system providing cooling gas to said outer periphery of the stator and said passages of the stator.
In a further embodiment, the invention is a method for cooling a superconducting electromagnetic machine having a solid core rotor including a superconducting rotor coil winding and a stator and a stator ventilation system, said method comprising the steps of: cryogenically cooling the rotor coil winding independently of cooling the stator; cooling the stator with a cooling gas flowing through the stator, and drawing the cooling gas out of the stator into an air gap between the stator and rotor core, where the cooling gas is isolated from any rotor cooling system.
The proposed stator cooling systems are independent of the type of superconducting rotor configurations, and can be equally applied to iron-core and air-core superconducting rotors.