A dc link capacitor minimization method through direct capacitor current control

If we make the converter side dc link current the same as the inverter side dc link current in a PWM converter-inverter system, no current will flow through the dc link capacitor and as a result, no dc link voltage variation occurs. This leads to the possibility of reducing extremely small the size of dc link capacitors which are expensive, bulky, and have a lifetime limit, if we can manage to match inverter and converter currents at the dc link. However, a converter inherently involves a delay in supplying the required current. Therefore, matching the converter current to the inverter current necessitates the use of differentiator to meet the requirements of a fast dynamic response. But in practice, differentiating the measured value is almost impossible. In this paper, we propose a new differentiation method that utilizes a one-step predictor developed from converter and inverter dynamics. In the model, the dc link voltage is regarded a variable. Since the inverter current compensation term is applied in terms of voltage, the response is fast. Simulation and experiments are performed with only 40μF dc link capacitance for a 9kW motor, and the results support the validity of the proposed scheme.