Research On Topology Of High Boost Ratio DC /DC Converter Based On MMC

With the development of many green energy,wind energy has become the most potential renewable energy.Offshore wind power will be the future of renewable energy power generation.Traditional offshore wind farms usually use large and expensive transformers and high-voltage AC-DC converters with unnecessary bidirectional power flow capacity,which increases the construction cost.Therefore,DC power grid technology will become an effective solution for large-scale offshore wind power system integration.This paper presents a new topology of non-isolated high voltage rise ratio DC converter for DC collection and transmission of offshore wind power system,which can connect high voltage difference DC grid at low cost.The research work of this paper always focuses on topology and control.Firstly,the traditional topology and working principle of interleaved boost converter are analyzed.According to the interleaved structure and MMC direct coupled DC converter,a new topology of HVDC converter is proposed.The working principle of the proposed topology is deeply studied,and the mathematical analysis of the key stress and the core parameters of the topology is completed.A phase-shifting PWM modulation strategy is proposed.The switching frequency and switching loss can be reduced by using the pulse signal generated by comparing the duty cycle modulation wave with the carrier wave to control the capacitor switching in the chain module.The main challenge of the proposed DC/DC converter is to balance the capacitor voltage of the sub modules on each bridge arm.Therefore,a closed-loop control strategy is proposed to balance the capacitor voltage of the sub modules.The unidirectional power flow control scheme is designed for DC side and sub module short circuit fault conditions,and the fault handling strategy further improves the reliability of DC converter operation.Based on the Simulink simulation environment of MATLAB software,the simulation model is built,and the steady-state and shortcircuit faults of the proposed topology are simulated and analyzed.Finally,an experimental platform is built to study the topology,modulation method and control strategy of DC-DC converter under steady-state conditions.The simulation and experiment verify the rationality of the theoretical analysis.This paper presents a novel two-phase interleaved DC converter topology based on hybrid cascading of sub modules,which provides a theoretical basis and solution for DC collection and transmission of offshore wind turbines.It can effectively improve the power density and power conversion efficiency of offshore wind power system,reduce the volume,weight and cost of equipment.