Analysis of self-dual excited synchronous machine. II. Smallperturbation model and dynamic behaviour

For Part I, see ibid., vol.3, no.2, June 1988, pp.305-314. Having developed the general mathematical model of an isolated self-dual excited synchronous machine in part I, the author derives the small displacement equations for the two alternative excitation systems. A novel analytical method is developed for the small displacement model of the self-dual excited synchronous machine that requires less CPU time than previously. This method is general and take into account the variation of the firing angle and the automatic feedback control circuit during small disturbances. The coefficients of the characteristic equation and the eigenvalues are calculated to study the dynamic stability of the system. The effects of the ratio of the two field currents, the inertia constant, the power factor, and the gain and time constant of the feedback control circuit on the dynamic stability of the system are investigated. The results obtained show that the self-dual excited synchronous machine with the two field windings having MMF ratios between 0.75 and 1.5 presume better dynamic stability bounds than the conventional self-excited synchronous machine. In the range where the other proposed excitation system can be compared with the conventional machine, it has been found that the developed system has the same dynamic stability response with the advantage that the effect of automatic voltage regulator is implicitly included with a smaller time constant. Including the change of the firing angle during the small disturbance, a pronounced effect on the dynamic stability of the system results     

Analysis of self-dual excited synchronous machine. I. Developmentof the general mathematical model and steady-state performanceexperimental verification

The use of the dual excitation system for improving the overall performance of a self-excited synchronous machines is considered, along with the replacement of the compound transformer and rectifier bridge by a potential transformer and thyristor bridge for the self-excitation system. The output DC voltage of the bridge is controlled over a wide range by an automatic feedback control circuit to vary the firing angle of the thyristors in such a way that the terminal voltage is sustained at a constant value. The mathematical models for two distinctive alternatives of the excitation system are derived. The mathematical model thus derived is suitable for transient, dynamic as well as steady-state analysis. However it should be modified to investigate the steady-state and dynamic performance. Exact steady-state operating points are achieved by solving the steady-state equations obtained from the general model. Charts describing the performance of the self-dual excited synchronous machine under steady-state operation for the two alternatives of the excitation system have been calculated at different values of the power factor, i.e., the turns ratio of the transformer and the ratio of field currents. The experimental results obtained on a 7.6 kVA induction machine converted to a d-q synchronous machine confirm the validity and accuracy of the analysis and mathematical models developed