Research On The Topology And Control Strategy Of High-voltage High-volume DCDC Converter For Offshore Dc Wind Farm

With the development of clean energy,offshore wind farm has gradually become the focus hotspot,especially for the offshore All-DC wind farm.Because of the superior performance,the offshore All-DC wind farm has become more and more popular.In the study of offshore all-DC wind farms,research on high-voltage high-volume DCDC converters is particularly important.For system safety and other considerations,the DC transmission grid often adopts the bipolar DC system,while the output DC voltages of the wind turbines are unipolar.Therefore,this paper proposes a new DCDC converter that can be applied to the offshore All-DC wind farm.The bipolar output DCDC converter can interconnect the unipolar DC system with bipolar DC system.In addition,the proposed DCDC converter also has the following advantages:(1)This topology is suitable for high-voltage high-volume condition and can achieve a large step-up ratio;(2)Without isolation transformer,the converter has a smaller size which is suitable for offshore station;(3)It doesn't have DC-AC-DC conversion process,so it can achieve high efficiency;(4)Lower switching frequency results in lower loss;(5)The number of submodules is easy to change which can realize better redundancy and fault tolerance.Firstly,this paper discusses the design of the bipolar-output direct-coupled transformerless DCDC converter including topology design,mathematical modeling analysis and parameter selection based on the traditional three-level DCDC converter.Secondly,this paper designs a corresponding modulation strategy and control strategy for this proposed DCDC converter.With these strategies,the converter can realize the stable output voltage and can balance the capacitor voltages of all submodules.Thirdly,in order to deal with the fault conditions,this paper also considers the corresponding fault handling method for various short-circuit faults to improve the converter's fault tolerance.Based on the theoretical analysis,this paper builds a simulation model which consists 12 submodules based on the software MATLAB/Simulink.Not only the normal working conditions but also the load changing and fault conditions are also simulated.The simulation verifies the correctness of theoretical analysis and the effectiveness of the control strategies.At the same time,this paper builds an experimental platform consisting of 12 half-bridge submodules and carries out the experiments.The correctness of the theory is further verified by open-loop and closed-loop experiments.