A two-stage active current limiting control coordinated protection scheme for multi-terminal MMC-HVDC grid

Abstract

The inherent flexible and controllable characteristics of the multi-terminal flexible DC grid based on modular multilevel converters (MMCs) have opened up new avenues for the research on fault current limitation and line fault identification. Existing active current limitation control strategies and fault identification methods operate independently, without considering their interplay and mutual influence, leading to an inability to fully integrate them. To enhance the synergy between active current limitation control and protection, this paper introduces a two-stage active current limiting control coordinated protection scheme for half-bridge MMCs. Firstly, the interdependence between current limitation control and fault identification is established; building on this, to prevent rapid blocking of MMCs post short-circuit faults and the impact of fault characteristic changes on the reliability of fault identification, the first stage of current limitation control is implemented using AC virtual impedance control. This approach limits the arm current without modifying the DC-side fault characteristics, thus creating favorable conditions for existing protection. Secondly, line boundary properties are leveraged to delineate the scope for the second stage of DC-side current limitation control, enabling selective deployment of current limitation measures and curbing the propagation of fault impacts. Finally, by integrating the current limitation areas with directional elements, reliable fault identification is accomplished. Simulation outcomes indicate that the proposed scheme effectively limits both AC and DC currents, providing a novel perspective for the convergence of current limitation control and fault identification.

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