Enhancing system stability: robust control strategies for modular multilevel converters in weak grid and load conditions

Abstract This paper focuses on operating a modular multilevel converter (MMC) under diverse operating conditions, particularly during weak grid scenarios. The system may approach instability as a result of changes in the resonance frequency and, subsequently, the grid impedance. To address the significant impedance variations between the converter and grid as outlined in IEEE standards 519 and DIN EN 61000-4-15, this paper proposes a novel hybrid damping approach to manage the converter under such conditions. This approach is used to reduce the extra power loss from passive damping and simultaneously increase the robustness of active damping. Various passive damping and hybrid damping methods are implemented to the proposed MMC model and their comparative analysis is presented in order to identify the stability attributes. Also, this paper provides a straightforward proportional resonant (PR) controller along with biquad filter-based approach that ensures system stability and improves system robustness. A detailed sensitivity evaluation of the cascaded control loops is conducted, with an emphasis on the effect of filter parameter variations on stability. Furthermore, the paper explores the contemporary circulating current references for MMCs. This approach offers a superior alternative approach to inject specific harmonics into circulating currents. Employing these schemes, this paper demonstrates the successful achievement of low voltage ripples. Simulation and experimental data are used to validate the proposed strategies’ enhanced stability.