Three-Level/-Phase PWM Converters Dc-Link Voltages Ripple Reduction Technique in the αβ Reference Frame

This work is part of a research effort to evaluate and develop power converter systems that are to be used in future bipolar dc active distribution networks. A characteristic of such networks is that the power levels across each of the dc-buses can be highly unbalanced at steady-state. Therefore, the front-end bidirectional rectifier system is required to provide well regulated, balanced and low ripple dc voltages to the bipolar dc bus. A straightforward modulation technique is introduced here for three-level/-phase bidirectional PWM rectifiers. The method is based in the analysis of three-level converters in the alpha beta 0 stationary reference frame, where the ac currents and duty-cycle alpha and beta components are used to compute the required 0-axis duty-cycle that guarantees that the dc-link center point low frequency current components are nullified. The technique is implemented in a carrier-based PWM modulation scheme. Implementation in a digital processor is used in a three-level bidirectional converter lab prototype. The prototype is tested to present experimental results of the proposed technique. The main consequences of the proposed technique are that both bipolar dc bus voltages present negligible voltage ripple even under highly unbalanced bus power levels and variable ac-side power factor, which is appropriate to interface a dc bipolar distribution network to an ac system with controlable reactive power injection. An important secondary effect is that the rms current value is greatly reduced in the converter dc bus capacitor.