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This paper proposes a new method of damping harmonic resonance at the DC link of a large-capacity rectifier-inverter system such as in rapid-transit railways. A voltage-source pulsewidth-modulation (PWM) converter is connected in series to the DC capacitor of the rectifier through a matching transformer, acting as a damping resistor to the dc capacitor current. No filters are needed to extract harmonic components from the DC capacitor current. This results in a quick response and highly stable damping. The required rating of the PWM converter is less than one-thousandth of that previously proposed. The basic principle of the proposed damping method is discussed in detail, then confirmed using digital computer simulation. Experimental results verify the validity and practicability of the proposed damping method.