Circulating Current Suppression Strategies for D-STATCOM Based on Modular Multilevel Converters

This work focuses on the circulating current measurement of a STAtic synchronous COMpensator (STATCOM) based on Modular Multilevel Converter (MMC). In a Delta (D) connected MMC-based STATCOM configuration the currents are divided in two parts: grid currents and circulating currents. The latest circulates between legs and has no direct impact on the output current, but it has a third harmonic component, which causes problems in the circulating current control. Thus, this work evaluates three circulating current measurement strategies, with focus on the converter characteristics improvement, such as losses reduction. A classical strategy is compared with other two strategies using a Second Order Generalized Integrator (SOGI) structure. The first proposed strategy eliminates the third harmonic from the circulating current and the second detects the fundamental component of the circulating current eliminating the other harmonic components. Simulated and experimental results show that this strategies achieve reduction in the circulating current, and consequently reduction in the STATCOM losses