Direct voltage fluctuations due to the presence of relatively large DC reactors (as an essential part of HVDC breakers), lack of inertia, and unwanted frequency fluctuations in the AC side of HVDC grids, have major consequences on the stability of HVAC/HVDC grids. The use of the DC Power System Stabilizer (DC-PSS) can damp and eliminate voltage oscillations caused by the presence of the DC reactors. However, DC-PSS cannot address the issues of inertia and unwanted frequency fluctuations. A

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... d to improve inertia is proposed here that can operate well with the droop controller, and DC-PSS does not interfere with power-sharing and does not interact with any of these elements. Since the presence of a droop controller in HVAC/HVDC grids associates with power and direct voltage, the method proposed here can improve direct voltage fluctuations by eliminating severe power peaks. Moreover, this method does not change the voltage level of the entire system, so there is no need to change the set-points of controllers. In addition, all parameters of the controllers are tuned by an intelligent algorithm, and the Participation factor (PF) scheme is used to find the proper placement of the proposed controller.