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Torsional interaction is an important phenomenon of concern when a turbine-generator (TG) unit is connected to a series compensated line or a high voltage direct current (HVDC) system. The subsynchronous torsional interaction (SSTI) may have significant impact on the hydraulic system due to the negative damping introduced by HVDC converter controls. This paper investigates the SSTI behaviour of hydro TG units interconnected with a HVDC system for a varying generator-to-turbine inertia ratio. Case studies are carried out by changing the value of firing angle, applying a three-phase fault at the inverter bus and line switching at the rectifier ac system.