Performance Analysis and Improvement of MMC-Based Active Signal-Injection Strategy

The active detection (or active injection) protection scheme based on the signal-injection strategy has been extensively researched in modular multilevel converter (MMC)-based transmission and distribution systems. However, existing DC-side MMC-based signal-injection strategies often introduce harmonic leakage into the AC grid, and generally exhibit insufficient injection accuracy. This study aims to analyze, improve, and quantitatively evaluate the performance of MMC-based DC-side injection strategies. Firstly, based on instantaneous power balance, a response model of the external circuit is supplemented to establish relationships between key variables reflecting injection performance. Secondly, a control-response block diagram incorporating the injection strategies is developed. The injection performance is then analyzed based on the structural characteristics of the block diagram, as well as the amplitude-frequency and phase-frequency characteristics of the closed-loop transfer functions between the reference and the measured values of the injected signal. In addition, an injection performance improvement method and two evaluation indictors are proposed to systematically enhance and evaluate the performance of MMC-based strategies. Finally, simulation and experimental tests are conducted to verify the effectiveness of the analysis results and the improved injection strategy.