Grid-Forming Control Based on MMC-UPFC and its Multi-Mode Switching Strategy

When using traditional Modular Multilevel Converter-Unified Power Flow Controller (MMC-UPFC) for power flow control in existing high-proportion renewable energy (RE) systems, line voltage and frequency are prone to fluctuations due to the uncertainty of RE generation and its low inertia and low damping characteristics. This paper studies the control strategy of the grid-forming MMC-UPFC, which leverages its inertia and damping characteristics to enhance the system’ s ability to operate safely and stably, thereby avoiding instability issues. The MMC-UPFC equipped with both grid-forming and traditional control strategies can switch between traditional operating mode and grid-forming operating mode. It effectively fulfills the functions of flexible power flow control and stable system operation, addressing the drawbacks of renewable energy generation’ s uncertainty, low inertia, and low damping characteristics. Finally, the effectiveness of the proposed strategy is verified through simulations.