| With the gradual depletion of traditional energy sources such as oil and coal and the intensification of environmental pollution,the efficient development and utilization of wind energy has become increasingly important.Compared to onshore wind power,offshore wind energy resources are abundant,and the power quality of offshore wind turbines is higher.Offshore wind farms have become an important direction of future wind power development.Therefore,it is of great significance to study the grid connection of large-scale deep-sea wind farms.Traditional high-voltage AC transmission and low-level flexible DC transmission have shortcomings such as high power loss,small transmission capacity,and low power quality.High voltage DC transmission technology based on modular multi-level converters(MMC-HVDC)can effectively so]ve the grid connection problem of deep sea wind farms.In this paper,MMC-HVDC is applied to the grid connection of offshore wind farms,and the working principle and control model of doubly-fed asynchronous wind turbines are studied.According to the hierarchical control principle of MMC-HVDC system,the control strategy are analyzed and studied respectively according to the order of valve-level control,converter station level control and system-level control.The specific research contents include:(1)The working principle and mathematical model of doubly-fed asynchronous wind turbine are analyzed.The control models of rotor-side converter and grid-side converter are established according to the stator flux orientation and grid voltage orientation principles.The wind turbine model is built on the Matlab/Simulink simulation platform,and the maximum tracking wind energy characteristics of the wind turbine and the decoupling control of the stator output active power and reactive power are realized.(2)The working principle and modulation strategy of modular multilevel converters are studied,and the adverse effects of capacitor voltage fluctuations and inter phase circulation on the normal operation of MMC are analyzed.The necessity of balancing the capacitor voltage of sub modules and suppressing inter phase circulation is pointed out.Based on the carrier phase shift modulation method,an improved sub module capacitor voltage balance control strategy with a loop suppression module is proposed.An MMC model was built on the Matlab/Simulink simulation platform to verify the feasibility and effectiveness.(3)The operation principle and mathematical model of the MMC-HVDC system in the dq coordinate system are studied,and the converter station level control strategy is designed based on the dual closed-loop vector control principle.According to the application scenarios of MMC-HVDC systems at both ends,they are divided into active systems at both ends and power supply systems to passive networks.Corresponding control strategies are designed based on their operational characteristics.The MMC-HVDC system model at both ends is built on the Matlab/Simulink simulation platform,and the stability and dynamic response ability of the system are verified based on simulation results under various scenarios.(4)Taking the loop parallel four-terminal MMC-MTDC system connected to offshore wind farms as the research object,an improved droop control strategy is proposed to address the problem that frequent fluctuations in the output power of offshore wind farms can easily cause instability in the DC voltage or even exceed its safety margin under traditional droop control strategies.Considering the DC voltage deviation and power deviation caused by power fluctuations in offshore wind farms,the droop coefficient is modified,and the structure of the droop controller is further optimized.Finally,a four terminal MMC-MTDC model connected to an offshore wind farm is built on the Matlab/Simulink simulation platform,verifying the effectiveness and superiority of the improved droop control strategy in stabilizing DC voltage and improving the dynamic response ability of the system. |
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