Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2003-03-25
2004-07-13
Le, Dang (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S054000, C310S064000, C029S598000
Reexamination Certificate
active
06762517
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a super-conductive rotor in a synchronous rotating machine. More particularly, the present invention relates to an electromagnetic shield and vacuum vessel for super-conducting field windings in the rotor of a synchronous machine.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core, and one or more coil windings 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 (HTS) coil field 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° 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
x
Sr
x
Ca
x
Cu
x
O
x
) based conductor.
Super-conducting coils have been cooled to cryogenic temperatures, such as by liquid helium. After passing through the windings of the rotor, the warmed, used helium is returned as 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.
In addition, 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 operational 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.
SC coils generally must be thermally insulated by a vacuum to yield super-conducting characteristics. The vacuum prevents heat from the warm rotor core from being transferred by convection to the SC coils. The SC field coil has to be completely enclosed by vacuum. The vacuum requires that a vacuum vessel and associated air-tight seals be maintained on the rotor.
SC coils used as the HTS rotor field winding of an electrical machine are subjected to stresses and strains during cool-down and normal operation. They are subjected to centrifugal loading, torque transmission, and transient fault conditions. To withstand the forces, stresses, strains and cyclical loading, the SC coils should be properly supported in the rotor by a coil support system and shielded against dynamic and transient magnetic fields. These support systems hold the SC coil(s) in the HTS rotor and secure the coils against the tremendous centrifugal forces due to the rotation of the rotor. Moreover, the coil support system protects the SC coils, and ensures that the coils do not prematurely crack, fatigue or otherwise break.
Developing shields and coil support systems for HTS coil has been a difficult challenge in adapting SC coils to HTS rotors. Examples of coil support systems for HTS rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. The need also exists for a coil support system made with low cost and easy to fabricate components.
SUMMARY OF THE INVENTION
Structural supports for the HTS field coil windings have been one of the primary challenges to incorporating SC coils into rotors. The structure must support the SC coil winding without conducting substantial heat into the winding. In the disclosed novel concepts the structure of the coil support has been minimized so as to reduce the mass that conducts heat from the rotor core into the cooled SC windings. However, minimizing the coil supports also limits the level of forces that can be withstood by the supports. If the forces that act on the rotor exceed the force carrying ability of the coil supports, then there is a substantial risk that the coil support will fail or that the coil windings will be damaged.
A potential source of forces that act on a rotor is torque due to grid faults. A high temperature super-conducting (HTS) generator having a field winding SC coil is susceptible to electrical grid faults. A grid fault is a current spike in the power system grid to which is coupled the stator of the machine. Under grid fault conditions, excessive current flows in the stator. This current causes an electrical disturbance in the stator winding that induces a strong magnetic flux that can penetrate into the rotor field winding coils.
The potential penetration of a magnetic field into the rotor field winding coil creates significant torque on the rotor coil winding. This torque can damage a SC coil and a weak coil support structure. In addition to this mechanical effect, magnetic field penetrations of the rotor can cause alternating current (AC) losses in the rotor structure, especially in the HTS wire. It would be advantageous to minimize the penetration of the rotor by grid fault induced and other magnetic fields. Reducing the rotor torque due to grid faults allows the coil support structures to be minimized. Minimizing magnetic field penetrations of the rotor should also reduce AC current losses in the HTS rotor.
Shielding the rotor prevents stator alternating and time-varying magnetic fields from penetrating the rotor. If a rotor field winding coil is not well shielded, the magnetic flux from the stator penetrates the rotor and causes torque in the magnetic rotor and SC coil. Such torques may damage a brittle SC coil, even though such stator flux induced torque has not generally damaged prior ductile copper rotor coils. If a rotor having SC coils is not properly shielded, then coil support must be reinforced to withstand fault-induced torque. However, a drawback of reinforcing the coil support is that it also increases the mass of the support, and leads to potential problems with increased heat transfer to the cold SC coil.
Instead of increasing the mass of the coil support, it is preferable to have an electromagnetic (EM) shield that prevents alternating magnetic flux from penetrating the rotor and inducing torque on the SC coils. Cylindrical EM shields and vacuum vessels that cover the entire rotor core are difficult to fabricate for large SC machines because of their size. Forming
Huang Xianrui
Nygard Robert John
Sivasubramaniam Kiruba
Wang Yu
Weeber Konrad Roman
General Electric Company
Le Dang
Nixon & Vanderhye P.C.
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