说明：  This paper discusses the impact of HVDC on Power System Stability and proposes a new control mechanism based on Fuzzy set theory to augment dynamic performance of a multi-machine power system. To have good damping characteristics over a wide range of operating conditions, deviations in speed and acceleration of the machines are chosen as the input signals to the fuzzy controller. These input signals are first characterized by a set of linguistic variables using fuzzy set notations .The fuzzy relation matrix allows a set of fuzzy logic operations that are performed on controller inputs to obtain the desired output. The effectiveness of the proposed controller is demonstrated by a multi-machine system example. The performance of this fuzzy controller, in comparison to the conventional fixed gain controller, demonstrates the efficacy of this new fuzzy PID controller. Index Terms—HVDC, Eliminated variable