The constant current controllers at high voltage direct current (HVDC) rectifier stations and the outer loops which set the reference to the current controller have potential to introduce negative damping on the nearby generating units. It is therefore vital to investigate the torsional interaction behavior between turbine-generator (TG) units and the HVDC systems over a frequency range of interest. In this paper, subsynchronous torsional interaction (SSTI) phenomenon for hydroelectric TG units with small generator-to-turbine inertia (GTI) ratios interconnected to an HVDC system has been investigated. The sensitivity of different system parameters, such as generating capacity and loading, DC power flow, AC network strength and current controller gains, towards electrical damping contribution is also examined. Hydro units with different shaft systems are examined for SSTI proneness by conducting perturbation analysis. The impact of hydroelectric TG units with different GTI ratios has also been examined by applying a three-phase to ground fault at the rectifier station. Simulation studies have been carried out in PSCAD ®/EMTDC © environment. Due to lack of modal damping with relatively low decrement factor and logarithmic decrement, sustained and growing TG torque oscillations are noted for certain operating conditions of hydro units with small GTI ratios interconnected with an HVDC system..