What is subsynchronous resonance and how is it mitigated?

Subsynchronous resonance (SSR) occurs when series capacitors create electrical resonances below synchronous frequency that interact with turbine-generator mechanical resonances, potentially causing shaft damage or instability. SSR mechanisms include: torsional interaction (electrical damping becomes negative), induction generator effect, and transient torque amplification. Mitigation includes: blocking filters, static synchronous series compensators (SSSC), thyristor-controlled series capacitors, and supplementary damping controls. SSR studies are mandatory for series-compensated lines near large generators.