Extension of the Under-Excited Stable Region of the Dual-Excited Synchronous Machine

The capacitive power, which a properly regulated conventional synchronous machine can supply is severly limited at no load as well as at light loading. Such a machine, when operating at these loading conditions, cannot be fully utilized. In this paper, it will be shown that these limitations can be overcome by the dual-excitation of the synchronous machine. Such a dual-excited synchronous machine, when provided with appropriate schemes of excitation control, has extended dynamic stable region at any loading condition. Two modes of operation are found possible in this case. In the first, the two field windings are always equally excited, while in the second their excitation currents are adjusted so as to keep the rotor-angle fixed at a certain value independent of the loading condition. Although the first mode of operation is of advantage from the rotor heating point of view, it suffers from the drawback that the sign of the control signal for stable operation depends on the operating condition. To overcome this drawback, the second mode of operation is suggested. With this mode, the dynamic stability of the machine can be greatly improved over the whole loading range. However, since fixing the rotor-angle at a certain value necessitates unequal excitation currents in the two field windings, this mode of operation would result in unbalanced distribution of rotor heating.