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In the cascaded power system, instability may occur because of the impedance interaction. The system can be stabilised by passive damping or control methods to reduce the output impedance of the source converter and improve the damping characteristics. This study focuses on the source converter in the cascaded system and presents a new active damping control to reduce the output impedance of the converter by a proper active damping controller. The method to compute the corresponding active damping controller is proposed in this study. By the application of the active damping method, a virtual resistor in parallel with the output of the converter's small signal model can be realised. Therefore the output impedance can be reduced. Because the virtual resistor exists only in the small signal model, the steady-state operating point is not affected and the value of the virtual resistor can be changed. The experiment results of a Buck converter with the active damping controller under different conditions are proposed to verify the effectiveness of the active damping control. Besides, an active damping controller is applied to an unstable cascaded system in simulation to stabilise the cascaded system so the effectiveness of this method is also verified.