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This paper proposes a novel STATCOM by cascading packed U-cell (CPUC) and using low capacitance. Compared to cascaded H-bridge, the CPUC-STATCOM can largely reduce switch counts and their related gate driver circuits. Then, skillfully utilizing the principle that the second-order harmonic component of capacitor voltage is in phase with the absolute value of grid voltage in capacitive mode, low capacitance is used in CPUC-STATCOM. It can reduce cost, weight and volume of system compared to usually using large capacitance to limit voltage ripple within 10% of nominal capacitor voltage. However, it has limited compensation capability in inductive mode by using low capacitance. Aiming at overcome the aforementioned drawback, a flexible third harmonic control method is proposed, which can reduce capacitor peak voltage and averaged voltage. Finally, the simulation results verified the performance of the proposed topology and control method.1

With the number of installed high voltage direct current (HVDC) transmission systems increasing, there is a growing requirement to develop high voltage and power DC-DC converter for interconnecting the HVDC systems with different voltage levels, which can improve the flexibility and reliability of power system. This paper proposes a multiport DC-DC modular multilevel converter (MMC) for multiple HVDC systems interconnection. Besides inheriting the advantages of conventional MMC, the proposed multiport DC-DC MMC also presents several other advantages of fewer submodules amount, more attractive investment cost, and smaller volume. The topology and mathematical models are firstly presented in this paper. Next, the operation principle analyses including the required submodules amount and capacitor voltage balancing are also elaborately described. Moreover, the control scheme is put forward in this paper as well. Finally, the simulation and experimental results are displayed to verify the feasibility and performance of the proposed multiport DC-DC MMC and corresponding control strategy.