Operational impedances of turbogenerators with solid rotors

It has long been known that the performance of a turbogenerator differs in certain respects from that predicted by the idealised theory based on Park´s equations. The reason is that the damping effects of eddy currents in the solid rotor are not correctly represented by damper coils acting on the two axes. The theory presented in the paper allows for the eddy currents in the determination of the direct- and quadrature-axis operational impedances Xd(p) and Xq(p) They are calculated as frequency-response functions Xd(j¿) and Xq(j¿) by considering sinusoidally varying fluxes on one or the other axis in the machine at standstill. Two alternatives are considered: one when the flux reverses and the other when there are only flux changes of limited magnitude. The impedance values are used to calculate the characteristics during asynchronous operation and the current after a 3-phase short circuit. Two large machines and one model machine were tested and showed good agreement with the theory. A new method is suggested for determining the value of damping torque for use in a stability study.