Effects of torsional dynamics on nonlinear generator control

The performance of a feedback-linearizing control for excitation control of a synchronous generator is investigated with respect to unmodeled dynamics of both the turbine generator unit and the transmission network. It is found that certain types of dynamics that were not modeled during the design of the control enter in a manner that does affect the performance of the control, but that preserves the linearity of the closed-loop system. Moreover, the control acts to decouple the dynamics associated with the machine from the dynamics of the transmission grid, thus preventing subsynchronous resonance between the two subsystems when a series capacitor is used to compensate the transmission line. The stability robustness of the feedback-linearizing control is investigated with respect to a structured uncertainty. The uncertainty considered corresponds to the spring modes of the generator shaft and enters in such a way that analysis by Kharitonov's theorem is feasible. It is shown that the control remains stable over a wide range of values of the shaft parameters. A sliding control is designed and compared to the feedback-linearizing control with respect to performance degradation for this type of uncertainty, and it is found that, because of the tight saturation limits on the control signal, the sliding control offers no discernable performance advantage for this type of structured uncertainty.