Field experience has shown that sub-synchronous oscillation (SSO) and harmonic resonance can happen due to the interaction of wind energy conversion system's (WECS) converter controller, HVDC converter controller and impact of system impedance. This paper investigates the operation and control of WECS employing full-scale converter connected to the electrical network through voltage source converter (VSC)-based high voltage dc (HVDC) transmission system. SSO and harmonic resonance are observed depending on the control implementation of the ac collection bus side HVDC converter. The origin of these oscillations can be attributed to the propagation of the WECS 15 Hz resonance through the WECS full converter DC link and the interaction between the WECS and the dc link dynamics of the HVDC system. An active damping scheme is proposed to improve the system performance. The active damping is implemented to an offshore HVDC system installed with the purpose of integrating two wind farms. An analysis and time domain simulation results and its spectral analysis are presented to show how effective the application of the proposed active damping is.