Investigating the Impact of HVDC on the Damping Subsynchronous Resonance

Turbine generators have natural torsional frequencies due to the physical properties of their long multielement shafts. Conventional series capacitor compensation in transmission networks tends to reduce the damping of torsional vibrations of neighboring turbine generators. This phenomenon is called subsynchronous resonance (SSR). It affects the turbogenerator at subsynchronous frequencies specific to the torsional oscillation modes of individual units. Series capacitors also tend to amplify the shaft stress during major transient events on the network. This paper investigates the impact of adding a High Voltage Direct Current (HVDC) line in parallel with an existing series compensated Alternating Current (AC) line on torsional interaction. The main objective of this work is to investigate the capability of HVDC line to damp out unstable torsional oscillations due to the capacitive series compensated AC transmission line. Detailed simulation studies are presented using Power System Computer Aided Design (PSCAD). The study system is a modified IEEE First Benchmark Model (FBM) for subsynchronous resonance analysis to include an HVDC transmission line in parallel.