Improved Current Limiter for Expanding Grid-forming Modular Multilevel Converters Operation Area

Grid-forming modular multilevel converters (MMCs) require a higher overload capacity during grid faults. However, the traditional circular limiters are only designed to restrict the peak of the AC-side current without considering the arm current characteristics of MMC, leading the waste of arm current margin. In this paper, the regularity between the arm current and the AC voltage is analyzed, restricting the peak of the precise arm current limiting can enable the converter to output more active and reactive power. Furthermore, by injecting second-harmonic component to suppress the maximum of arm current, the proposed improved current limiter can further increase the reactive power generation of MMC, reducing the voltage drop of AC voltage and active power limit. The effectiveness of the improved current limiter is proved in MATLAB/Simulink.