Analysis of Ride-Through Capability of Unidirectional-Current MMC with Arm Current Unidirectionality Disrupted

Unidirectional-current full-bridge submodule (UC-FBSM) based modular multilevel converter (UC-FB-MMC) has high power-density, dc fault-clearing capability and dynamic dc voltage adjustability, and is therefore suitable for many new dc applications. The normal operation of UC-FB-MMC requires the arm currents being always positive. This requirement can be met in steady states by designing a sufficiently high ac voltage to decrease the ac components of the arm currents. However, some transient states may cause the arm currents to reach 0 and tend to be negative and cause the UC-FB-MMC to enter an abnormal state with non-positive arm currents. The feasibility of UC-FB-MMCs in practical applications is impacted by whether or not the UC-FB-MMCs can ride through these states. This article first analyzes the current paths and intuitively verifies that the ac- and dc-side currents are not interrupted in the states with non-positive arm currents. Then, the mathematical models of a single arm with non-positive current reveals that the ac- and dc-side currents are still under control, but the control effects become poor. After that, a modified controller is proposed to enhance the control effects. Simulation and experimental results verify the analysis.