Design and Optimization of HF Transformers for High Power DC-DC Applications

Licentiate thesis, 2014

Increasing the operational frequency is the most common solution to achieve higher power densities, since the weight and volume of the magnetic part, the bulkiest element in power electronics converters, are then decreased. This solution is well established in low power high frequency applications, while in the recent decade, the possibility of utilizing high frequency at higher power and voltage levels has generated wide interest as well. This work proposes a design and optimization methodology of a high power high frequency transformer accounting for the tuned leakage inductance of the transformer, as well as high isolation requirements, particularly in DC offshore application where a converter module should withstand the MVDC or HVDC link voltage. To achieve this goal, several models were proposed and developed in order to accurately characterize such a transformer. One of these models is a so called pseudo-empirical expression derived from a rigourous regression algorithm based on an extensive 2D finite element simulation scenario, resulting in an accurate analytical expression with an average unsigned deviation of 0.51% and the extreme deviations not higher than 9%. Moreover, using the energy method, an analytical expression to precisely calculate the leakage inductance of high power density magnetic components is proposed. In addition, using the proposed modification of the Steinmetz equation for core loss calculations, general expressions are derived and presented for a rectangular waveform with its associated duty cycle and rise time. Applying the proposed design methodology, in which all the aforementioned models are implemented on a 1 MW case study transformer, indicates that such a transformer can achieve a power density of about 22 kW/L and the efficiencies as high as 99.74%. Moreover, with respect to the isolation requirements, desired leakage inductance and the magnetic material used, a critical operating frequency can be found above which the transformer does not benefit from volume reduction anymore.