Optimization and experimental validation of medium-frequency high power transformers in solid-state transformer applications

Abstract

High power isolated DC-DC converters are likely to provide solutions for many technical challenges associated with power density, efficiency and reliability in potential applications such as offshore wind farms, inter-connection of DC grids, MVDC in data centers and in future solid state transformer applications. The high power medium frequency transformer (HPMFT) is one of the key elements of such a converter to realize the voltage adaption, isolation requirements, as well as high power density. This paper describes a design and optimization methodology taking into account the loss calculation, isolation requirements and thermal management. Incorporating this design methodology, an optimization process with a wide range of parameter variations is applied on a 50 kW, 1 / 3 kV, 5 kHz transformer to find the highest power density while the efficiency, isolation, thermal and leakage inductance requirements are all met. The optimized transformers are then manufactured and will be presented in this paper.

A Novel Oil-Immersed Medium Frequency Transformer for Offshore HVDC Wind Farms

“…A few types of core materials are popular among the researchers which work with medium frequency transformer designs. Ferrite and Nanocrystalline cores are previously used by the author to produce two 50 kW transformers [8]. The 'core losses -frequency' curves of Nanocrystalline cores from their datasheets are normally given at a very low operational working point (0.1-0.3 T for Nanocrystalline cores compared with 1-1.2 T as working point flux).…”

“…To minimize the number of sharp corners and to provide a better heat transfer between the windings and the core stacks, a polygonal design is considered (Figure 4-3) instead of the previously used planar designs [8]. This structure allows for installing OIP barriers to achieve a high voltage withstand level in a minimum space.…”

“…The suitability for the relatively high operating frequency is the first criterion for the selection of a core for a medium frequency transformer which experiences high frequency voltage harmonic components of the applied square wave from the DAB side (see Figure 2-1a) [8]. Another crucial criterion for the selection of a core for manufacturing a prototype HVDC MFT is a large window size.…”

“…It is noticeable that at room temperature, the losses are not dependent of the wave shape change from sinusoidal to square wave shape at 5 kHz if the flux density is kept under 0.3 T. This becomes clearer comparing the recorded B/H curves at different excitation voltage shapes. HVDC MFT core losses First to verify the calculation, the measurement procedure is applied to recalculate the already measured core losses of the previously manufactured dry type planar Ferrite design MFT [8]. The total volume of the cores is 0.0030576 m 3 and their total weight 14.83 kg.…”

A Medium Frequency Transformer Design Tool with Methodologies Adapted to Various Structures

“…Because of this complexity, a lot of work has been done to develop MFT design methodologies [6][7][8][9][10][11]. Most of these works have followed the strategy of computing a large number of designs in order to cover the entire design space and select the best one according to the specifications.…”