Design and Optimization Considerations of Medium-Frequency Power Transformers in High-Power DC-DC Applications

Doctoral thesis, 2016

Recently, power electronic converters are considered as one of the enabling technologies that can address many technical challenges in future power grids from the generation phase to the transmission and consequently distribution at different voltage levels. In contrast to the medium-power converters (5 to 100 kW) which have been essentially investigated by the automotive and traction applications, megawatt and medium-voltage range isolated converters with a several kilohertz isolation stage, also called solid-state transformers (SST), are still in an expansive research phase. Medium-frequency power transformers (MFPT) are considered as the key element of SSTs which can potentially replace the conventional low-frequency transformers. The main requirements of SSTs, i.e., high power density, lower specific losses, voltage adaptation and isolation requirements are to a great extent fulfilled through a careful design of MFPTs. This work proposes a design and optimization methodology of an MFPT accounting for a tuned leakage inductance of the transformer, core and winding losses mitigation, thermal management by means of a thermally conductive polymeric material as well as high isolation requirements. To achieve this goal, several frequency-dependent expressions were proposed and developed in order to accurately characterize such a transformer. These expressions are derived analytically, as in frequency-dependent leakage inductance expression, or based on finite element method (FEM) simulations, as in the proposed expression for high-frequency winding loss calculation. Both derived expressions are experimentally validated and compared with the conventional methods utilizing detailed FEM simulations. Utilizing the proposed design method, two down-scaled prototype transformers, 50 kW/5 kHz, have been designed, manufactured and measured. The nanocrystalline-based prototype reached an efficiency of 99.66%, whereas the ferrite-based transformer showed a measured efficiency of 99.58%, which are almost the same values as the theoretically predicted ones. Moreover, the targeted value of prototype’s leakage inductances were achieved through the proposed design method and were validated by measurements. Finally, using SiC MOSFETs and based on the contribution above, the efficiency and power density of a 1 / 30 kV, 10 MW turbine-based DC-DC converter with MFPT are quantified. It was found that, with respect to the isolation requirements, there is a critical operating frequency above which the transformer does not benefit from further volume reduction, due to an increased frequency.