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Abstract-This paper investigates the dynamic interactions of a Modular Multi-level Converter (MMC) -based HVDC grid and its host AC system during AC-side disturbances. The HVDC grid is composed of five MMC stations that are interconnected through a network of cables and over-head lines. The AC grid includes ten turbine-generators. The studies are conducted in the PSCAD platform, including the detailed models of the AC and DC subsystems.The reported case studies indicate that without the HVDC supplementary control functions and based on the conventional dq-frame control structure: 1) each MMC behaves as almost a fixed sink/source of power during the AC-side electromechanical transient oscillations; 2) the MMC-HVDC grid effectively improves dynamic interactions between AC subsystems that are connected only by the HVDC grid; 3) without appropriate provisions the embedded MMC-HVDC grid can aggravate the transients of the host AC system, e.g., inter-area oscillations. The observations also contradict the widely accepted notion that, even without supplementary controllers, the MMC-HVDC grid necessarily improves the dynamics of the AC system.The study results provide a comprehensive insight into the transient behavior of the MMC-based HVDC-AC grid. Such an understanding is essential to design the protection and control systems of the future HVDC-AC grids.Index Terms-Modular multi-level converter, HVDC grid, electromagnetic transients, power system dynamics.Firouz Badrkhani Ajaei (S'12-M'15) received the B.

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The mode-adaptive droop control (MADC) strategy enables bus voltage regulation and power sharing between the distributed energy resources (DERs) in the direct current (dc) microgrid without communication systems. The conventional MADC strategy may fail to provide acceptable voltage regulation and power sharing performance in large dc microgrids where the voltage drops across the dc lines are not negligible. This paper proposes an improved MADC strategy for the dc microgrid. The proposed control strategy minimizes the adverse effects of the aforementioned voltage drops on the bus voltage regulation and the power sharing between the DERs in the dc microgrid. The performance of the proposed control strategy is investigated under various operating conditions and disturbance scenarios, using a detailed and realistic dc microgrid study system that is modeled in the PSCAD/EMTDC software environment. The study results indicate that the proposed control strategy: 1) effectively maintains the power balance in the dc microgrid; 2) accurately regulates the dc bus voltages under various operating conditions; 3) improves power sharing between the DERs without using communication systems; 4) significantly reduces the circulating currents between the DERs in the islanded microgrid; and 5) enhances the dc microgrid reliability, flexibility, modularity, and scalability. INDEX TERMS DC microgrid, mode-adaptive droop control (MADC), power sharing, voltage regulation.

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