Third Harmonic Circulating Current Generation Mechanism in Delta-CHB STATCOM

Static synchronous compensators (STATCOMs) are increasingly present in the power system scenario. The Delta-connected Cascaded H-bridge (Delta-CHB) is a widespread option for medium-and high-voltage STATCOM systems. However, the non-negligible second-harmonic ripple in the capacitor voltages directly affects the submodule voltages and, consequently, leads to an inherent third harmonic circulating current, which can increase power losses. This paper analytically describes this phenomenon and proposes a simple strategy based on a proportional resonant controller in the circulating current loop. So far, the strategy has been validated through both simulation and experimental results. The proposed model presents an error of 1.6% between the calculated and simulated circulating current amplitude. The proposed mitigation strategy provides a reduction in the circulating current ripple by 95.45% in the experiment, compared to the conventional control strategies found in the literature.