The propagation of surge voltages through high-speed turbo-alternators with single-conductor windings

摘要

After a brief review of previous work on the propagation of surge voltages through machine windings, the fundamental differential equations for the propagation of waves through a winding with two coil sides per stator slot are derived. These show that all component frequencies of an incoming wave below a certain critical frequency are propagated through the winding with velocities which are dependent on the frequency. For super-critical frequencies the winding behaves as a condenser whose capacitance diminishes with increasing frequency. Various modes of winding are discussed, together with their effects on the impulse-voltage distribution. Test results on a typical machine are described and the voltage distribution explained with the aid of the fundamental theoretical deductions. The surge impedance of a large 11-kV alternator is about 80 ohms. Evaluation of the inductance for one of the machines tested shows that the majority of the flux produced by the surge current surrounds both conductors in the slot and must therefore be carried by the iron. This is also borne out by the fact that the removal of the rotor decreases both the self and mutual inductances of the windings, owing to the increased reluctance of the flux path.