Research On High Step-up Ratio DC/DC Converter Based On Matrix Transformer

In offshore wind farms,the medium-voltage DC collection system can eliminate bulky industrial frequency transformers compared with the medium-voltage AC collection system,effectively reducing the volume and weight of the system,and reducing the construction cost of the offshore wind farm.However,the high step-up ratio DC/DC converter in the medium voltage DC collection system has more switches,higher switching losses,and lower efficiency,and due to the higher frequency of the medium frequency transformer,the loss is larger than that of the traditional industrial frequency transformer.The heat dissipation design is more difficult.In response to this problem,the high step-up ratio DC/DC converter topology,soft-switching process and power control strategy,and high-efficiency and high-power-density optimization design methods for medium frequency transformers are studied in this paper.A high step-up ratio DC/DC converter topology suitable for offshore medium-voltage DC collection systems is proposed in this paper.The converter adopts a matrix transformer to reduce the difficulty of transformer heat dissipation design and a soft switching process is proposed which can realize the soft switching of all switches in the full load range.The mathematical relationship between converter power and frequency based on the principle of charge conservation is further derived in this paper,and power control strategy of the converter is proposed.The soft-switching process and power control strategy are verified by simulation.In the design of medium frequency transformers,the core loss and winding loss are analyzed in this paper.The analysis of core loss mainly includes the selection of core material and the calculation of core loss,core material is determined by considering the core loss per unit volume and the saturation magnetic density.And core loss calculation not only considers the influence of the main magnetic flux on the core loss,but also the influence of the leakage flux on the core loss.Then the winding loss of the litz wire is calculated,and the size of the strand diameter is determined by analyzing the influence of the litz wire strand diameter on the winding loss.Subsequently,the parameters(leakage inductance and magnetizing inductance)of the medium frequency transformer are modeled in this paper as the basis of the transformer parameter design.On the basis of loss analysis and parameter modeling,an overall design algorithm based on the parameter scanning method is proposed in this paper.Based on the restrictive conditions of temperature rise and insulation,the corresponding size of the optimal results of different transform er structures is obtained.The influence of magnetic leakage on the eddy current loss of nanocrystalline core s is further considered,the eddy current loss caused by magnetic leakage of different transformer structures is compared,and finally the structure and final size of the medium frequency transformer are determined.Finally,an experimental prototype of a high step-up ratio DC/DC converter based on matrix transformer with a power of 100 k W is builded in this paper,which the proposed soft-switching process and power control strategy are verified.Based on the matrix transformer(including two 50 k W medium frequency transformers),the effectiveness of the medium frequency transformer design is verified.