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In order to improve the performance of the transformerless modular multilevel converters (MMC) under AC and DC grid fault conditions, this paper proposes an improved four‐layer capacitor voltage control strategy combined with arm current control. The proposed control strategy consists of overall submodule capacitor voltage averaging control, leg‐balancing control with flexible weight factor W, arm‐balancing control with zero‐sequence suppression, and inner‐arm individual balancing control. The new concept of weight factor W for leg‐balancing control is also originated, considering negative‐sequence current and DC circulating current jointly from power point of view. The proposed approach effectively realizes the capacitor voltage balance, ac‐side current tracking, and unwanted circulating current elimination. The practical implementation of the proposed control strategy and the experimental results from a downscale three‐phase transformerless MMC prototype are presented to validate the performance of the proposed four‐layer capacitor voltage control strategy under grid faults. [ABSTRACT FROM AUTHOR]

The quality of output current of L C L -type voltage source inverter (VSI) is degraded by the grid voltage distortion, the dead time effect, and the parameter mismatches. This paper proposes a novel fractional order harmonic disturbance observer-based control (FOHDO) for the three-phase L C L -type inverter to suppress all the dc- and ac-component periodic disturbances. Firstly, a harmonic disturbance observer based control for L C L -type inverter is proposed with the internal model knowledge of the periodic disturbances signals. Subsequently, the FOHDO based on Lagrange interpolating polynomial fractional order delay approximation is put forward to enhance the adaptability of the grid frequency. Then, the stability and robustness performance of the proposed FOHDO for L C L -type inverter are evaluated, which also involves a detailed parameter design process. Finally, the effectiveness and performance of the proposed FOHDO method are verified by both experimental and simulation results. • We propose a harmonic disturbance observer-based control to suppress disturbances. • A fractional order HDO (FOHDO) strategy at a fixed sampling rate is also proposed. • The stability, robustness analysis and the parameters design guideline are given. • Prototype experiment and on-field results of LCL-type inverter are presented. [ABSTRACT FROM AUTHOR]