Third-order harmonic currents suppression method based on selective component compensation for cascaded H-bridge STATCOM under asymmetric grid

To achieve low cost and compactness, small dc capacitance in dc-link is preferred in the cascaded H-bridge (CHB) static synchronous compensator (STATCOM). However, small dc capacitance will definitely result in high dc-link ripples among three clusters, and thus cause severe third-order harmonic currents under the asymmetric grid. In this study, to mitigate the third-order harmonic currents, a selective component compensation approach for the CHB STATCOM is proposed. To study the effect of the dc-link ripples on the phase currents, sequence component decomposition is employed to derive the relationship between the dc-link ripples and the converter voltages. The relationship reveals that the converter voltages contain positive, negative, and zero sequence components in both fundamental and third-order under the asymmetric grid, while only the positive and negative sequence components in third-order converter voltages result in third-order harmonic currents due to the three-wire system. Therefore, the third-order harmonic currents suppression method is proposed by only compensating the positive and negative components in third-order converter voltages. A 380 V/15 kVA STATCOM prototype is built to verify the proposed control method.