Special Problems of Rotor Dynamics in the Design of Superconducting Generators with Multi-Shell Rotors

Compared with turbine generators of conventional design, which already have very high power-weight ratios, superconducting generators offer big advantages in the way of the required capacity range and improved efficiency. Now that the fundamental theoretical studies have been completed and the first lest rotors are being proof-tested by a number of manufacturers, work is in progress worldwide on the construction of prototypes intended for operation in power plants. The completely different method of cooling the rotor at temperatures of only a few Kelvin demands effective thermal shielding, best achieved by using a construction consisting of a number of concentrically arranged hollow cylinders. An unconventional design of this nature gives rise to totally new problems regarding bending and torsional vibration for which established design practice presents only partial solutions or even none at all. The flexural and torsional vibration behaviour of a K W U (Kraftwerk Union) superconducting 1000 MVA rotor is investigated in this paper. First, the tuning to resonance and the steady state behaviour are analysed; the transient behaviour of the machine after electrical disturbances is of decisive importance for grid operation, and an exhaustive account of this topic follows. It is demonstrated that the rotor, in spite of its complex and elaborate construction and the attendant multitude of natural frequencies, can be built and operated reliably on the grid. In contrast to the hitherto prevailing view, it may be assumed that the characteristic of such great importance to the owner/operator, namely that of performance on the grid, is at least just as good as that of a conventional turbine generator.