Self-excitation of dual-excited synchronous machines

The paper deals with the study of self-excitation phenomenon of dual-excited synchronous machines connected to highly series-compensated transmission lines having low resistance, particularly in the case of e.h.v. long transmission lines using bundled conductors. Analysis for self-excitation for a typical system is carried out through the Routh-Hurwitz and a simpler approach for the unregulated case, whereas the parameter-plane approach is effectively used for the regulated case. The effect of the transformer (p¿) terms in the mathematical model on self-excitation has been highlighted. A comparison between the performance of dual-excited and equivalent conventional machines has been brought out. Dual-excited turbogenerators exhibit instability due to fast self-excitation only, whereas dual-excited hydroelectric generators exhibit instability due to both slow and fast self excitation. While the performances of both dual-excited and equivalent conventional turbogenerators are found to be nearly therome, the performance of a dual-excited hydroelectric generator is found to be decidedly better than that of an equivalent conventional one. In general, slow self excitation can be completely suppressed, but not fast self excitation, even with dual excitation. Complete elimination of self excitation seems to be feasible with a suitable resistance Rc in parallel with the capacitor. Another interesting conclusion is that the omission of the p¿ terms for the regulated case leads to highly optimistic results.