Positive-Negative Sequence Current Controller for LVRT Improvement of Wind Farms Integrated MMC-HVDC Network

Positive and negative sequence based current control strategy is developed for the modular multilevel converter (MMC), which complies with the grid code requirements during the balanced and unbalanced faults at the point of common coupling of AC grids. This research proposes an efficient and simplified current limiting technique to keep the inverter individual phase current at a predefined threshold during faults. In this work, the current controller of the MMC converter is designed based on stationary reference frame while positive sequence synchronous reference frame is used to design the current limiter. The proposed work considers the complete dynamics of permanent magnet synchronous generator (PMSG) and doubly fed induction generator (DFIG) based wind energy. The MMC aggregate model is developed and simulated in Matlab/Simulink. The MMC model is also developed using real-time digital simulation (RTDS). A comparative performance analysis between the proposed and the conventional current limiter is provided where the proposed sequence-based current controller with the current limiter shows promising performance under severe disturbances. Besides grid compliance, the results with the proposed control scheme demonstrate the improvement of low voltage ride-through capability without any violation of converter current and HVDC link voltage limits.