Thesis for The Degree of Licentiate of Engineering
Design and Optimization of HF Transformers for
High Power DC-DC Applications
Mohammadamin Bahmani
Division of Electric Power Engineering
Department of Energy and Environment
Chalmers University of Technology
Goteborg, Sweden 2014
Abstract
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.
Keywords
High Power High Frequency Transformer, Isolation Requirements, Leakage Inductance.
LINK ORIGINAL THESIS COMPLETE
http://publications.lib.chalmers.se/publication/195670-design-and-optimization-of-hf-transformers-for-high-power-dc-dc-applications
Direct download link
http://publications.lib.chalmers.se/records/fulltext/195670/195670.pdf
