Impedance Modeling of Series-Connected Modular Multilevel Converter Considering Third Harmonic Suppression

The major harmonic component of the secondary side current of the series-connected modular multilevel converter (SC-MMC) is the third harmonic. The existence of the third harmonic current will impair the accuracy of impedance modeling at AC side of SC-MMC. In this paper, firstly, the small signal impedance model at AC side of SC-MMC is derived by using the method of multi-harmonic linearization without considering the influence of the third harmonic current. Results indicate that the accuracy of impedance modeling is affected by the third harmonic current. Secondly, in order to suppress the bad effects caused by the third harmonic current, a proportional-resonant (PR) controller is applied. The small signal AC impedance model of SC-MMC taking account of the aforementioned PR controller is established. Finally, the accuracy of the impedance model considering third harmonic suppression is verified by simulation results.