Study on coordinated control of multi-terminal DC system based on joint control of grid-following and grid-forming

The coordinated control of multi-terminal DC (MTDC) systems is a critical issue for ensuring reliable operation and stability. Traditional control methods relying on DC bus voltage signals often lead to voltage deviations under varying operating modes, negatively impacting constant power loads and potentially causing system instability. To address these challenges, a novel coordinated control strategy combining grid-forming and grid-following control is proposed. This approach leverages the dyadic instability law in power systems and employs voltage-building and frequency-following mechanisms. The method includes a detailed analysis of the coordinated control strategies for MTDC converter stations in both grid-connected and islanded modes, along with power distribution controls for distributed energy converters at each port. Additionally, the conditions for seamless switching between grid-connected and off-grid modes are thoroughly examined to ensure global system stability. These strategies effectively mitigate DC voltage fluctuations, enhancing power quality and maintaining system stability across diverse operational scenarios. Simulation results validate the proposed method, demonstrating its ability to eliminate voltage deviations and optimize system performance. By integrating advanced control techniques, this approach offers a robust solution for managing MTDC systems, paving the way for more stable and efficient power system operations in the context of increasing renewable energy penetrations.