1. Field of the Invention
This invention relates generally to a superconducting generator rotor, and more particularly to a generator rotor to which liquid helium is supplied to keep the superconducting field coil cold.
2. Description of the Prior Art
Superconducting generator rotors which use superconducting coils are being developed to raise the efficiency of the generator employing the rotors. The superconducting coils are soaked in liquid helium in the rotors and kept about 40.degree. K.
In order to restrict the heat flow from the atmosphere into the liquid helium in such a rotor and to reduce the evaporation rate of the liquid helium, the rotor has an inner rotor, an outer rotor and vacuum layers between them, as shown in FIG. 1 in page 27 of IEEE Transactions on Energy Conservation, Vol. EC-1, No. 3. Furthermore, the cold evaporated gas helium is used to cool the electric lead which leads to the superconducting coil through thr rotor shaft to reduce the heat conduction through the electric lead.
In the prior art, the electric lead to the superconducting coil terminates in a free end which faces the structure of the rotor, which is at the ground voltage level and directs the gas helium toward this structure, as shown in FIG. 2 (a) in page 218 of the above-mentioned reference and reproduced in FIG. 1 of this application. Since the electric lead voltage is as low as 10 V, there is no need for protection against discharging through the gas helium.
Referring to FIG. 1, the superconducting generator rotor according to the prior art will be described in more detail. This rotor has a superconducting coil (not shown) which is cooled by liquid helium. Evaporated gas helium returning from the coil is guided through a coolant path 10 which is a hole in a hollow electric lead 12. The electric lead 12 has a distal end which connects to the superconducting coil. A connector 14 connects the near end of lead 12 to a first collector ring 16. The first collector ring 16 is attached to the outer surface of a main shaft 18 which supports the whole generator rotor, and the first collector ring 16 is kept in contact with a brush (not shown) through which electricity is supplied. The electric lead 12 is cooled by the gas helium flowing through it, and the electric lead 12 is covered with an electric insulator 20.
A second collector ring 22 is attached to and electrically connected to the main shaft 18. The second collector ring 22 is kept in contact with a brush (not shown) which is connected to the ground level electrode, and the main shaft 18 is kept at the ground level voltage.
The gas helium flows out of the coolant path 10 in the electric lead 12, flows into a cavity 24 formed in the main shaft 18 and is guided axially away from the superconducting coil to a helium transfer coupling (not shown).
In the cavity 24, helium feeding/exhausting pipes 26 which are at ground level voltage, the main shaft 18, the electric lead 12, and the connector 14 are exposed to the gas helium, although part of the main shaft 18 in the cavity 24 is covered with an electric insulator 28.
In the prior art, excitation control is not applied to the superconducting generators because their internal reactance is low, and consequently, their stability is high.
If excitation control is applied to the superconducting generators, their stability increases further. However, in this case, electric current in the field coil changes rapidly, and the voltage at the first collector ring 16 reaches the kV range or may be as high as tens of kV. When the voltage reaches such a high value, electric discharge would occur through the gas helium in the cavity 24 in the main shaft 18. Such discharge is predicted by the discharge characteristic curve for 200.degree.-300.degree. K. shown in FIG. 2, which is reproduced from page 36 of Denki Gakai Gijyutsu Hoh, Part II, Vol. 93.