A Novel Symmetrical MMC Submodule Topology With DC Fault Ride-through Capability

The rapid isolation and ride-through of DC faults are required urgently for DC transmission system based on modular multilevel converter (MMC). Based on the induction and improvement of the existing submodules, this paper proposes a bidirectional switch clamp symmetrical submodule (BCSSM) with DC fault current symmetrical clearing capability. After blocking, the DC fault current is led to charge all capacitors on the arm in series, which ensures both fast fault current clearing speed and excellent capacitor voltage balance on the arms during blocking. Meanwhile, compared with other submodules with fault ride-through capability, BCSSM performs better in terms of economy because the bidirectional switch has the function of both clamping and bypass channel. Simulation results based on MATLAB/Simulink show that BCSSM-MMC is capable of fast DC fault current clearing speed and high capacitance voltage balance, which improves the reliability of DC fault ride-through while reducing the cost of converter construction and operation.