DC Grid-connected Technology For Large-capacity Offshore Wind Turbine Based On Dual Phase-shift Power Conversion

In the context of energy crisis and environmental problems are becoming more and more serious,the ambitious goal of "carbon peaking and carbon neutral" is born.Offshore wind energy resources are abundant,and the development of offshore wind power is of great significance for the "double carbon" goal.With the expansion of wind farm capacity and the increase of offshore distance,all-DC wind farm with DC collection and DC transmission avoid reactive voltage and frequency problems,which has received more extensive attention in recent years.It is important to study the DC output transformation of large-capacity direct-driven wind turbines,so as to meet the demand of all-DC wind farm.In this paper,a large-capacity DC directly-driven wind power generator(DD-WPG)is selected as the research object.The optimized topology of DC/DC converter and the grid-connected control technology of the turbine are studied in systematic,the main work of this paper is as follows:Firstly,the overall design scheme of DD-WPG is given,and it is pointed out that the biggest difference from the traditional AC direct-driven wind turbine lies in the design of DC/DC converter.The mathematical model of the direct-driven wind turbine is established,which provides the theoretical basis for the design of the control strategy of the turbine.The technical requirements of the DC/DC converter used in DD-WPG are summarized to provide guidance for the design of the DC/DC converter and its control strategy.Second,a dual three-level soft-switching DC/DC converter is designed to meet the requirements of large capacity,high ratio and electrical isolation while reducing the size of the converter.The inverter adopts a dual duty cycle optimized modulation strategy to achieve the comprehensive optimization goals of reducing switching losses,suppressing midpoint potential shift,and reducing harmonic content.The converter operates in discontinuous conduction mode(DCM).With the help of resonant elements and optimized design modulation strategy,the switching devices achieve softswitching,which can effectively reduce the switching losses of the converter.Based on the detailed analysis of the sub-converter operation principle,the transfer power equation of the sub-converter and the design method of the important parameters of the sub-converter are given.Then,an input voltage and output voltage decoupling control strategy for DC/DC converter is presented,which is simple to implement and can achieve the purpose of improving the transmission efficiency in steady state.It can ensure the voltage stability at the input side of the converter and the voltage equalization at the output side of the sub-converter.The fault blocking unit of DC/DC converter and fault ride-through control strategy are designed for collection line short-circuit faults,which ensure the turbine to ride-through the fault period smoothly.Finally,the simulation model of DD-WPG is built in Matlab/Simulink.The effectiveness of the topology and grid connection technology of the DD-WPG is verified under unbalanced converter parameters condition,variable wind speed condition,voltage fluctuation of DC grid condition and short-circuit fault at DC collection line condition.