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An Enhanced Damping Control Strategy for Circulating Current Suppression in Modular Multilevel Converters

This paper proposes a new feedforward control strategy to enhance the damping of the circulating current suppressing control. The proposed feedforward can be used as a stand-alone suppressing control, or together with other circulating current suppressing strategies. When used with another control strategy, the proposed feedforward control enhances the damping of the control. The proposed feedforward control is based on the off-line calculation of the amplitude of the second harmonic component of the arm voltage given only by the ac-side terminal current, i.e., assuming zero circulating current. Then, this value is used as a control action (feedforward), to cancel the second harmonic of the circulating current. Simulation results show that the proposed feedforward is capable of suppressing more than 95% of the circulating current when it is used alone. The simulations results also show the enhanced damping action of the proposed feedforward when it is used together with two circulating current suppressing strategies, tested in this paper.