Medium Frequency High Power Transformers for All-DC Wind Parks
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- Institutt for elkraftteknikk 
There is an increasing interest during recent decades for high power converters due to their capabilities in addressing different challenges in various applications such as renewable energies and smart grids and also at different voltage and power levels. Magnetic component is the bulkiest part in such converters. Increasing the operating frequency, on one hand decreases the weight and size of the transformer and leads to a compact design, and on the other hand the losses and especially the loss densities will be higher. Consequently, loss density rises and this escalation in the loss density results in thermal problems that can limit the efficiency and size of the transformer. Considering these facts, a comprehensive design methodology is presented in details in this work. It includes accurate loss calculation and thermal modeling. All the required expressions and analytical solutions are developed and given. These analytical solutions are provided for designers who prefer simple and accurate methods. In addition, Nanocrystalline from Vacuumschmelze is selected as the main magnetic material and characterized by doing several tests at various frequencies and inductions. A 3-phase prototype is also built to verify the presented and developed loss calculation methods. This prototype is used in a converter topology which is suitable for all DC conversion system for offshore wind farms. The experimental setup includes a Matrix transformer, the 3-phase medium frequency transformer, a diode bridge rectifier and an RL load. Designing the Matrix converter was a part of another PhD thesis. The measured and calculated losses using presented methods are in a very good agreement showing the accuracy of these approaches. Using the developed design methodology, an optimization procedure is presented by sweeping all the defined free parameters such as magnetic induction, conductor thickness, number of primary winding turns and current densities. Considering the limitations on maximum operating temperatures of insulating materials and also magnetic materials, the maximum obtained power densities and also efficiencies for medium frequency high power application are derived. This work is done at different operating frequencies and for two magnetic materials which are generally the most efficient materials for medium frequency high power applications; Nanocrystalline and Amorphous. For Nanocrystalline, the maximum achieved power densities for 1, 5 and 10 kHz are 13.43, 35.59 and 37.96 MW/m3, respectively. The relative values for amorphous are 14.96, 15.92 and 15.5 MW/m3. Accordingly, with Nanocrystalline, increasing frequency from 1 to 5 kHz results in a significant increase in power density, but having 10 kHz does not give a considerable change. However, for amorphous, increasing frequency does not lead to any substantial changes. Besides, at lower frequency e.g. 1 kHz, using amorphous gives a higher power density and for higher frequencies, e.g. 5 and 10 kHz, Nanocrystalline shows a better functionality and higher power densities.