At present, the research on five-level inverters mainly involves the modulation algorithm and topology, and few articles study the five-level inverter from the control strategy. In this paper, the nonlinear passivity-based control (PBC) method is proposed for single-phase uninterruptible power supply inverters. The proposed PBC method is based on an energy shaping and damping injection idea, which is performed to regulate the energy flow of an inverter to a desired level and to assure global asymptotic stability, respectively. Furthermore, this paper presents a control algorithm based on the theory of passivity that gives an inverter in a photovoltaic system additional functions: power factor correction, harmonic currents compensation, and the ability to stabilize the system under varying injection damping. Finally, the effectiveness of the PBC method in terms of both stability and harmonic distortion is verified by the simulation and experiments under resistive and inductive loads.

Summary Modular multilevel converter (MMC) is easy to realize more voltage levels, and its structure is easy to connect back to back. It is suitable for high‐voltage direct current (HVDC) transmission system. However, the unbalance of grid voltage will seriously affect performance of HVDC transmission. The Lyapunov‐function‐based control strategy proposed in this paper can well solve the existing problems of MMC under unbalanced power grid. First of all, according to the topological structure of MMC, this paper establishes the mathematic model under the condition of grid unbalance and balance. Secondly, aiming at three different control objectives: (1) eliminate negative sequence current components and keep AC currents balance, (2) eliminate the active power double base frequency ripples, and (3) eliminate the reactive power double base frequency ripples. Then the Lyapunov‐function‐based control strategy is designed, and the circulating current control and capacitors voltage balance control are added to make the overall control strategy. Finally, the simulation of Lyapunov‐function‐based control strategy and traditional PI control strategy based on Matlab/Simulink platform is given. The validity and superiority of the Lyapunov‐function‐based control strategy are proved. And experiments on the hardware platform further verify the feasibility and correctness of the control strategy.