Power oscillation occurs in electrical network due to variety of phenomena. Subsynchronous resonance (SSR) and ferroresonance are the phenomena that cause power oscillation of rotary systems. Ferroresonance is likely to occur due to traversing capacitance line of the system across nonlinear area of transformer saturation curve due to several configurations like breaker failure, voltage transformer connected to grading capacitor circuit breaker, line and plant outage, etc. It causes misshaping the waveforms and frequency difference between two points in the network. Frequency difference (Δf) results in oscillation of power with a swing frequency which is equal to Δf. During SSR, electrical energy is exchanged between generators and transmission systems below power frequency. It happens due to interaction of a series compensated transmission line with a generator. It results in oscillation in the shaft and power oscillation. In addition, SSR causes the magnitudes of voltage and current to increase. Increasing the voltage causes saturation of iron core of transformer or reactor and consequently occurrence of ferroresonance in the presence of series capacitance.
Part of the book: Power System Stability