Abstract

Among all flexible DC system grounding methods, converter's AC side grounding is an important one. Modular multilevel converter (MMC) with the third harmonic injection modulation strategy can improve the utilization of DC voltage. The third harmonic current will be generated when MMC with neutral point grounded on AC side adopts the third harmonic injection modulation, because of the third harmonic potential. This paper describes the characteristics of the common mode voltage produced by the AC side grounded modular multilevel converter under the third harmonic modulation. Taking the flexible DC system with MMC at both ends as an example, the common mode equivalent network of the system with AC side neutral grounded through impedance is established. This paper analyzes the influence of neutral grounding impedance of connecting transformer and voltage angle difference of AC system on common mode voltage and current of DC system, considers the power loss of the grounding impedance and proposes a common mode current suppression method based on series grounding of resistance and reactance, so as to realize the third harmonic modulation of MMC flexible DC system with AC side grounding. Finally, it is verified on PSCAD simulation platform.

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