A DC-link capacitor minimization method through direct capacitor current control

If the converter side dc-link current is made equal to the inverter side dc-link current in a pulsewidth modulation (PWM) converter-inverter system, no current will flow through the dc-link capacitor. As a result, no dc-link voltage variation occurs. This leads to the possibility of making a large reduction in the size of dc-link capacitors, which are expensive, bulky, and have a lifetime limit. However, a converter current control loop inherently involves a delay in supplying the required current. In this paper, a capacitor current is nullified with a feedforward compensation term. To minimize the response delay, the authors calculate the compensation amount in terms of voltage and inject it at the voltage node. In calculating the compensation term, a differentiator is incorporated with a one-step predictor developed from converter and inverter dynamics. Simulation and experiments are performed with only 40 μF of dc-link capacitance for a 9-kW motor, and the results support the validity of the proposed scheme.