Subsynchronous resonance studies and mitigation methods for series capacitor applications

Turbine-generators have torsional natural frequencies due to physical properties of their long multi-element shafts. Series capacitor compensation in ac transmission networks has a tendency to reduce damping of torsional vibrations of nearby turbine generators. The phenomenon is called subsynchronous resonance (SSR), and it affects turbine-generators at subsynchronous frequencies that are specific to torsional oscillation modes of individual units. Series capacitors also have a tendency to amplify the shaft stress during major network transient events. The mechanism of the subsynchronous resonance is well understood. Numerous mitigation schemes have been developed and successfully implemented, and many series capacitors have been installed and operated without incidents since the first two shaft failures of the Mohave unit in 1970 and 1971. This paper first provides an overview of subsynchronous resonance (SSR) and the impact of series compensation on SSR. It then describes subsynchronous resonance (SSR) studies that should be performed for each series capacitor application. Analysis methods to evaluate SSR risk are described, including SSR stability analysis and transient torque analysis. The paper also provides descriptions of technical methods for mitigating SSR problems. The solution methods range from simple techniques to avoid SSR to complex solutions involving sophisticated combinations of mitigation equipment. Lastly, the paper explains a philosophy for torsional protection schemes, and describes methods for SSR protection and monitoring. Technical descriptions and implementation examples are provided for each type of study, mitigation method, and protection scheme