Unipolar Double-Star Submodule for Modular Multilevel Converter With DC Fault Blocking Capability

Adopting overhead lines is an effective way to reduce the cost of flexible high voltage direct current (HVDC) systems. However, the dc short-circuit fault is easily occurred and the development of the overhead lines based HVDC system is hampered. This paper proposes a unipolar double-star submodule (UDSSM) with the dc fault blocking capability. Under the dc fault condition, the UDSSM based modular multilevel converter (MMC) could block the fault current by switching all power switching devices off to protect the MMC system. In each proposed submodule, two capacitors work under almost the same condition and could be in series or parallel connection to output different voltage levels. Then two voltage sensors of the two capacitors are redundant each other, which improves the reliability of the MMC system. Additionally, voltage balancing methods for the capacitors under both normal operation and sensor fault conditions of the MMC submodule are developed in this paper. The effectiveness of the proposed submodule and the capacitor voltage balancing strategy are validated by the simulation and experiment results.