The behavior of power systems with conventional turbine-generators and variable speed induction generators.
Electrical Resonance
In the case of power systems with conventional turbine-generators only, the issue related to SSR is one of torsional interactions and resonance. The Induction Generator Effect (IGE) (or self-excitation) has never been observed in real power systems with conventional synchronous generation. However, it was predicted as early as 2003 that variable speed induction generators used in doubly-fed induction generators (DFIG) would be highly susceptible to IGE self-excitation type SSR. This is due to the fact that a variable speed DFIG generator is an induction generator directly connected to the grid, which makes such an electrical resonance between the generator and series compensation possible. In this case, the self-excitation type SSR occurs when the series capacitor forms a resonant circuit, at sub-synchronous frequencies, with the effective inductance of the induction generator, and at these frequencies, the net apparent resistance of the circuit is negative.