SUMMARYIt is well documented that an inappropriate HVDC controller will cause dynamic instability in an AC/DC system. In order to have a better analysis on the mechanism of this phenomenon, this paper proposes a generic theoretical equation of electric damping coefficient of a HVDC link using the complex torque coefficient (CTC) method for quantitatively analysis of the damping characteristics of different types of HVDC control systems. Furthermore, a theoretical design methodology will also be established to improve the performances of the HVDC primary and auxiliary control systems with an application example.