An improved topology of isolated MMC with fault ride-through capability under MVDC fault.

• An improved hybrid Isolated modular multilevel converter (IM2C) and a medium-voltage DC (MVDC) fault ride-through (FRT) control method for three-port modular multilevel converters are proposed. • With the utilization of isolated matrix submodules, the improved IM2C can regulate its MVDC voltage much lower than the rate value, even negative. • During MVDC fault, the proposed FRT control method protect the normal operation of IM2C by the suppression of the surging arm currents and the LVDC voltage fluctuation. • The fault characteristic of split capacitor grounding is improved by an energy dissipation branch. Modular solid-state transformers (SSTs) are the core devices of the medium-voltage DC (MVDC) distribution network, interlinking the MVDC system with medium-voltage AC (MVAC) system and low-voltage DC (LVDC) system. With the adoption of the modular multilevel converter (MMC) topology, the isolated MMC (IM2C) is a three-port SST sharing one common LVDC-link capacitor. The IM2C saves the cost of sub-converter capacitors. However, the existing IM2C topology is vulnerable under MVDC fault, with the overcurrent risk of power switches and the malfunction danger of LVDC and MVAC ports. In this paper, an improved hybrid IM2C topology is proposed to provide the MVDC fault ride-through (FRT) capability. Using matrix converter topology for sub-converters, the improved IM2C is able to generate negative MVDC voltage. An improved FRT control method for three-port modular multilevel converters is also proposed. With the proposed method, the hybrid IM2C can control the arm currents and maintain the operation of the LVDC and LVAC ports during the MVDC fault. The simulation results with different MVDC fault locations, fault types and system parameters prove the validity of the proposed topology. [ABSTRACT FROM AUTHOR]