Please note this extract from : Power System Dynamics Stability and control -- By K.R Padiyar
The application of series capacitors for long distance power transmission helps in improving power transfer and is economical compared to addition of new lines.
While it has been known that series capacitors can cause self excited oscillations at low frequencies ( due to low X/R ratio) or subsynchronous frequencies ( due to induction generator effect), the problem of self excited frequency oscillations ( due to torsional interactions) was experienced at Mohave power station in USA in december 1970 and october 1971.
So the general problem of oscillations at subsynchronous frequencies has been termed as Subsynchronous Resonance (SSR)..
Hope this may suffice. But there are much more details which can be explored and can be analysed in detail. ( Eigen value analysis will be used to evaluate stability)
The book which i have refered previously gives a complete inlook in to SSR.
-nag400
Subsynchronous resonance happen more often in AC compensated lines and also in HVDC system were negative damping signal is introduced on the turbine-generator voltage control system.
These phenomena could be mitigated or corrected installing in the generator control system a power system stabilizer (PSS) with the proper tuning to damp wanted signals and keep the system with adequate stability margin.
Enclosed are few sites with the topic. If you should interested in additional info search in the internet for key words such as: Subsynchronous Resonance (SSR), Power System Stabilizer (PSS), Torsional Interaction (TI), damping, stability
