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Control-system errors and imbalances in the AC system may generate and magnify abnormal harmonics in the AC-bus voltages and the DC current. In this paper, a technique based on harmonic impedance matrices has been used to simulate the steady-state response of AC/DC converters. The technique allows the computation of converter impedances or admittances viewed from the DC or AC bus. At any operating point, a linear circuit may represent the converter and harmonic magnification factors are calculated. High harmonic magnification indicates the possibility of composite resonance. The impact of a small negative-sequence imbalance in the AC system voltages is shown for a six-pulse current-controlled converter.