SSR due to series capacitors is the most usual situation that may need torsional protection. An SSR study should be performed if transmission line series capacitors (fixed or TCSC) are being added, the amount of compensation is being changed, or if other system (or shaft) changes will be done that may affect a turbine-generator's exposure to SSR. The study evaluates the risk of damage due to SSR for normal and contingency system conditions.
Depending on the level of risk it may be prudent to add torsional protection (TSR). In some cases the risk is high enough to justify redundant protection. Large rated power electronic converters near to turbine generators are the next most common situation that may need torsional protection. HVDC is especially of concern, due to its typically very large power rating.
Large variable speed motor drives can also be of concern, particularly when smaller turbine-generators are nearby or operate in islanded conditions. In such cases a Sub-synchronous Torsional Interaction (SSTI) screening study should be performed. If screening determines risk, torsional protection and more detailed studies including control effects may be prudent. In rare cases exposure to SSR or SSTI can be so severe that active damping or even resonance blocking filters may be needed, in addition to torsional protection. Other cases such as nearby steel mill arc furnaces or high-speed reclosing typically have a lower risk. GE can determine the need for a study in such cases. New units being installed into electrical environments of concern should also be evaluated for risk.