Adaptive Two-Step Arm Current MPC for Transformerless Hybrid MMC With Overmodulation Conditions in Unbalanced AC Grids

To save the volume and cost in medium-voltage distribution networks, transformerless hybrid modular multilevel converters (HMMCs) will work under overmodulation conditions. However, the traditional single-step model predictive control (MPC) has poor steady-state performance in this scenario, and cannot well balance capacitor voltages under unbalanced grid faults in medium-voltage distribution networks. Meanwhile, it is difficult to tradeoff between the low computation amount and the fast-dynamic performance. This article proposes an adaptive two-step arm current (ATSAC) MPC for transformerless HMMC under overmodulation conditions in unbalanced ac grids. Two-step MPC independently tracks each arm current based on the arm current model to increase the output voltage level to 2<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i>+1. This model integrates multiple objectives into one target without weighting factors to avoid the compromised control effect. The computation amount is reduced significantly without sacrificing the fast-dynamic performance by the adaptive search scope method. Capacitor voltages between phases and arms are balanced by comprehensive energy control, which generates the arm current reference. Moreover, the robustness is improved and the system delay is compensated by two-step prediction. Finally, the ATSAC-MPC is validated by simulations and experiments.