Arm-current-limiting control for modular multilevel converters with combined AC and DC grid-forming control

Grid faults pose a particular challenge to grid-forming (GFM) converters: The voltage-source behavior of a GFM converter requires current-limiting control to protect the semiconductors. In modular multilevel converters (MMCs), current limits must be imposed on arm currents rather than output currents to reliably limit current through the semiconductors. In addition, currents during transients may deplete the energy of one or more arms. Arm energy depletion may impose voltage limits that differ between phases. Voltage limitation has adverse effects on output voltages and controller function. This work proposes a strategy for arm current and arm voltage limitation for a grid-forming MMC. The current limitation strategy allows for open-loop voltage control; the current controller is active only during limitation. Current and voltage limitations are designed to prioritize terminal behavior, reducing distortion of the AC and DC terminal voltages and improving grid support during faults. The effectiveness of the approach is demonstrated through simulations of phase-angle jumps and an unbalanced voltage sag.