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A shunt active dc filter (ADF) has been applied in high-voltage dc transmission systems for harmonic suppression. Some key problems in the design of ADF control systems have not been investigated thoroughly due to high harmonic frequency and the complexity of system modeling. In this paper, a high-gain feedback control strategy is proposed, which is independent of circuit parameters. The authors draw a comparison between the harmonic suppression effects of ADF and that of a passive filter and prove that the former can increase the series impedance of the hybrid filter, while the latter can decrease the equivalent voltage. Based on circuit analyses, solutions, such as selection of the dc-side voltage, regulation of the modulation index, system stability conditions, filter design, and switching noise are presented. The consistency between theory analyses and experimental results verifies the feasibility of the proposed control strategy.