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A study of a floating-capacitor (FC) multilevel-converter-based VSC transmission system operating under unbalanced AC conditions is presented. The control strategy is based on the use of two controllers, i.e. a main controller, which is implemented in the synchronous d-q frame without involving positive and negative sequence decomposition, and an auxiliary controller, which is implemented in the negative sequence d-q frame with the negative sequence current extracted. Automatic power balancing during AC fault is achieved without communication between the two converters by automatic power modulation on the detection of abnormal DC voltages. The impact of unbalanced floating capacitor voltages of the FC converter on power devices is described. A software-based method, which adds square waves whose amplitudes vary with the capacitor voltage errors to the nominal modulation signals for fast capacitor voltage balancing during faults, is proposed. Simulations on a 300 kV DC, 300 MW VSC transmission system based on a four-level FC converter show good performance of the proposed control strategy during unbalanced conditions caused by single-phase to ground fault.