Strong Resonance Effects in Normal Form Analysis and Subsynchronous Resonance

Power system normal form analysis has developed coefficients and indices in modal coordinates to quantify nonlinear model interactions. The coefficients can become very large near a strong, nondiagonalizable resonance occurring in the power system linearization. Moreover, the changes in the coefficients when the power system equations are expressed in different coordinates or units show that the coefficients are not intrinsic. On a different topic, the paper suggests an explanation for the modal interaction that causes the subsynchronous resonance instability in power systems. The modal interaction is associated with a pair of strong resonances which arise as a perturbation of a weak resonance of complex eigenvalues. This idea is supported using results from the IEEE first benchmark subsynchronous resonance model and perturbation theory.