Nonlinear control for modular multilevel converters with enhanced stability region and arbitrary closed loop dynamics

Abstract

This paper presents the nonlinear control design with formal stability proof for a Modular Multilevel Converter (MMC), which is critical for the interaction between AC and DC grids. Lyapunov theory is used to develop the proposed controller, which is based on Backstepping and Feedback Linearization techniques and to perform the stability analysis. The proposed controller allows to asymptotically stabilize all converter’s states, i.e., AC currents, circulating currents, and internal energy. The proposed algorithm allows to manage the converter in a wide range of operation points, by means of arbitrary tuning assignment. Robustness and performances of proposed control are verified by means of Matlab Simscape Electrical simulations, including active and reactive power reference variations and grid imbalance conditions. A detailed MMC switching model, of 450MVA is considered on Matlab validation. A phase-shift PWM is considered, with a sorting algorithm. Also, a comparison with a standard PI controller is performed. In addition, MMC’s internal energy is also controlled, which can contribute to the high-speed frequency control by using this energy for synthetic inertia.