Multilevel three-phase dual active bridge dc-dc converters - A study on converter modulation, capacitive energy requirement, ZVS boundaries, MFT's winding connections, and core losses

Doctoral thesis, 2023

The collection and transmission of power from offshore wind turbines using dc have advantages over the ac system. One of the enabling technologies for deploying dc collection is the dc-dc converter that boosts the dc output from the turbine to the medium-voltage dc level of the collection grid. This dc-dc converter should be able to transfer nominal power of at least 15 MW, boost the voltage from a few kilovolts to tens of kilovolts, have high power density and efficiency. The dual active bridge (DAB) converter concept is a suitable candidate for this application. It comprises two active inverters and an intermediate transformer which provides galvanic isolation and voltage matching between the LV and MV systems. Its soft-switching capabilities allow an increase in the switching frequency without deteriorating the efficiency of the converter. Therefore, the size of passive components can be reduced to achieve high power densities.

Using partial power processing DAB converters for this application has been studied extensively in the literature, while this document considers bulk power transmission using multilevel converter topologies focusing on three-phase topologies. Different aspects of the converters are studied in detail, including; desired leakage inductance of the transformer; capacitive energy storage requirement of the converters; the lifetime of semiconductors; the effect of winding configurations; the flux waveforms inside the transformer core; the transformer's core losses; the soft-switching boundaries; and harmonic and partial load performance. Mathematical models are developed for different studies and verified by simulations or measurements.

Several converter topologies and winding configurations are studied. As an example, it is shown that a three-phase DAB with YD winding configuration, controlled transition bridge converter, and quasi-two-level (Q2L) modulation is the best choice for this application. A modulation strategy is proposed to improve the partial load performance. A loss split calculation showed that the semiconductor losses decreased by 94% and 81% at 5% and 10% of the nominal power, respectively, compared to the Q2L modulation.