Compensation of asymmetric transformers in high-power DC-DC converters

The three-phase dual-active bridge (DAB3) is a dc-dc converter, which is most suitable for high-power applications, as it features inherent soft switching and bidirectional power flow. The medium-frequency three-phase transformer in the ac link of the converter allows galvanic isolation and an arbitrary voltage conversion ratio. This work discusses asymmetric impedances in high-power medium-voltage transformers and their influence on the DAB3. Especially in high-power applications, asymmetries in the impedance occur, due to bigger dimensions of the transformer and holding on to conventional transformer designs. Considering the DAB3, due to unequal stray inductances in the transformer, a second harmonic component can be observed on the dc currents. Thereby, the ripples on the dc voltages and losses in the dc link capacitors are increased unnecessarily. This effect is demonstrated in this work using a laboratory prototype. A new approach is presented, which allows compensation of these effects and optimizes the performance of the DAB3 for asymmetric transformers. The concept of this balancing method is to control each of the interacting phases. The control includes solving an optimization problem, which is too complex to be calculated in real time on a digital signal processor (DSP). To ensure realizability on a DSP, a simplified approximation of this approach is presented as well. Finally, the added value of the proposed balancing control and the comparison with the simplified control are quantified through simulation and measurement.