Instability issues for control system in induction generator

Conventional field oriented (FO) induction machine drives are being actively pursued as a viable high power generating means for automobiles. Although such induction generators are dynamically stable under normal conditions, observations of possible drive instability during high-speed generation have been reported in the field. Simulation of the induction generators also showed possible instability. However, no theoretical foundation for indirect FO instability has even been found in the literature. To investigate mechanism of instability, this paper proposes an approach for stability analysis of a FO induction machine drive system. By assuming a constant speed and current command, the nonlinear FO drive is reduced to a linear system, and its system poles are examined for stability. The effect of current command variation is then taken into account, and a new concept of system poles migration with the current command variation is then proposed to provide a complete view of system dynamics under all possible conditions. The new concept was applied to a FO drive based induction starter-generator and its instability behavior at high speeds as observed in the field was confirmed. A comparison of stability between the mirrored motoring and generating points is also included in the paper to reveal some fundamental differences in system dynamics between the motoring and generating modes. The frequency domain analysis is also backed up by a time domain simulation.