Enhanced Short-Circuit Capacity of Grid-Forming Modular Multilevel Converters with Oval-Shaped Current Limiters

Grid-forming converters are a promising solution to providing grid support during faults, but their current handling capabilities are constrained by power semiconductors. A current limiter in grid-forming controllers protects the power semiconductor devices from overcurrents. Different from two-level converters, the current flowing through power semiconductors in modular multilevel converters (MMCs) is the arm current, which includes both fundamental output current and internal circulating current. This paper proposes an MMC-specific current limiting scheme, which constrains the dq-frame current reference according to the peak arm current in MMCs. The current limits are oval-shaped in the dq frame, because the MMC circulating current changes under different power factors. Compared with circular current limiters, the proposed limiter can help to provide more reactive power support during grid voltage sags, without exceeding the current limits of the power devices. The oval-shaped current limiter enhances the short-circuit capacity of the MMC, which distinguishes the fault current from normal operation and benefits fault detection. The effectiveness of the proposed current limiter is validated in simulations with different grid fault levels.