Controller-in-the-Loop of a Transformer Saturation Control for High-Power Three-Phase Dual-Active Bridge DC-DC Converters

In recent years a worldwide trend towards renewable energy source emerges. The transition from fossil fuels to renewable energies confronts the grid with new challenges, especially from solar and wind power. The use of multi-terminal direct-current transmission grids promise a flexible and efficient way to transport energy over long distances. With suitable dc-dc converters, such as the three-phase dual-active bridge, it is possible to interconnect medium-voltage dc distribution grids. Here, a high-power three-phase dual-active bridge for a nominal voltage of 5 kV in the mega-watt class is introduced. During commissioning of this high-power dc-dc converter, a dc component in the magnetizing currents of the transformers was observed which forces those into saturation. This results in increased losses and elevated noise emissions.The present paper gives a brief overview on the proposed feed-back compensation controller based on the star-point voltage. The measured saturation effects are post-processed in offline simulation, modeled, optimized and embedded on a controller-in-the-loop systems to emulate the behavior of the experimental test-setup. The feedback controller is written in VHDL on a custom made controller platform and tuned using a controller-hardware-in-the-loop approach. Experimental test results of the high-power converter are presented, verifying the functionality of the controller.