Broadband methods for stability assessment of double-star MMC based grid-forming STATCOM

Static Synchronous Compensators (STATCOMs) are key components in modern power grids, providing fast and accurate reactive power support to maintain voltage stability. When implemented using a double-star modular multilevel converter (MMC) in a grid-forming configuration, a STATCOM can also exchange active power, enhancing both voltage and frequency regulation. Reliable assessment of its dynamic behavior is essential for ensuring stable operation, as control-loop interactions between the converter and the grid can significantly influence overall system stability. Frequency-response measurements offer a powerful means of evaluating these dynamics, but conventional sine-sweep methods are too slow for real-time use due to their sequential nature. To address this limitation, this paper presents a broadband perturbation approach enabling fast, continuous, and comprehensive frequency-response characterization of a double-star MMC-based grid-forming STATCOM. Two excitation signals, Pseudorandom Binary Sequence (PRBS) and Discrete Interval Binary Sequence (DIBS), are employed, each providing benefits under different noise and disturbance conditions. Experimental validation on a three-phase prototype demonstrates that the proposed method achieves accurate and time-efficient dynamic characterization, establishing a practical foundation for real-time stability assessment of grid-forming STATCOMs.