Control of modular multilevel converter based HVDC systems during asymmetrical grid faults

Modular multilevel converter (MMC) is a relatively new and promising topology for HVDC systems. HVDC systems should remain connected during grid faults and isolate the fault. This paper studies the dynamic performance of transformer-less MMC integrated HVDC systems during unbalanced conditions and asymmetrical grid faults. It proposes a new control technique to improve unbalanced system's performance. The objective of the proposed controller is eliminating negative and zero sequence currents and to improve the overall performance. The controller calculates zero and negative sequence reference voltages and eliminates zero and negative sequence currents without using any current regulator. Therefore the controller is very fast and robust. The effectiveness of the proposed control technique has been validated by EMTDC /PSCAD simulations.