Loss Analvsis And Design Of A High-Power Medium-Frequency Transformer In Pure DC Offshore Wind Farm

The exploitation and utilization of renewable resources have attracted much attention in energy research and development to reduce greenhouse effect and carbon emission during the past decade,such as wind power,biomass,solar energy and so on.Wind power generation has gone from small groups to large wind parks,inland region to offshore seas,and high voltage AC transmission to high voltage DC(HVDC)transmission periods.Due to the advantages of abundant and steady wind energy,no land use and available to large-scale development,along with the development of power electronics and HVDC,the research of offshore wind farms has gained rapid progress.The offshore wind farms which have been already built all adopt AC collection-DC transmission construction.In order to lower wind farm construction costs,reduce volume and weight of electrical energy conversion platform of the generators,decrease power integration and transmission loss and increase working efficiency,pure DC offshore wind farm has become new research focus.At the start stage of DC wind farm research,key equipment of high voltage high power and system loss analysis make great sense.A topology of DC offshore wind farm based on modular isolated DC/DC converter(MIDC)is proposed in this thesis.Considering the actual working conditions,a detailed analysis and design of the medium-frequency transformer is made.As the output voltage and input voltage of the transformer are relatively low while the voltage between primary and secondary windings is very high,a novel coaxial transformer is designed which uses high voltage insulated cable as one winding.Then an optimization model is established and the optimization process of the transformer is put forward.Moreover,a 250 kVA medium-frequency coaxial transformer is designed as an example and the volume-power density profile is depicted with different value of the transformer parameters.Simulations performed by Ansys software have verified the feasibility of the transformer design.Finally,supplementary instruction of the transformer design method is made in consideration of the minimum bending radius of the cable applied in engineering project.In order to analyze and compare the system loss of different wind farms,the thesis sets an example of traditional wind farm based on MMC-HVDC and DC wind farm based on MIDC to calculate the average loss and the number of electronic devices of the two wind farm systems.Due to randomness and instability of the wind speed in actual offshore wind farm,the average loss of the wind farm is defined to make the loss calculation meaningful.Then the system loss is classified based on the structures of the two wind farms.Afterwards,parameter design,device selection and loss calculation of the equipment applied in the system is done.Results show that the average loss in the traditional offshore wind farm is 4.0%,while 2.12%in the DC wind farm.Though the number of rectifier diodes increases,the DC wind farm reduces the number of IGBT used in the system effectively and modular structure lowers the voltage and current level of IGBT,which both contribute to cut down IGBT cost.The thesis mainly researches two aspects of pure DC offshore wind farm to satisfy the requirements in practical applications.On the one hand,a medium-frequency coaxial transformer applied in MIDC is analyzed and designed.Simulation results have verified the effectiveness of the proposed transformer design.On the other hand,system loss and device quantity have been analyzed and compared of the two typical wind farms.These researches lay further informative theoretical foundation for prototype manufacture and engineering applications in DC offshore wind farm.