Stability Analysis And Control Strategy Research Of Multi-Terminal Convergence Of All-DC Offshore Wind Power

With the rapid development of power electronics and wind turbine technology,offshore wind power systems with all-DC have become the preferred solution for increasing offshore distances,and it is important to accelerate the technical research of full DC offshore wind power systems to promote clean energy transformation and achieve the "double carbon" goal in China.This paper studies the control strategy and stability of multi-terminal convergence and delivery systems based on ultra-high boost ratio DC-DC converters and other components,aiming to break through the technical bottlenecks faced by high-voltage convergence and delivery systems for new energy generation and provide theoretical basis and technical support for the field application of new offshore wind power converters,as follows:1)A topology of an isolated DC-DC converter with high boost ratio based on resonant network is introduced,based on isolation transformer,resonant network and multiplier rectifier for three-stage boost,and the equivalent circuit model method is used to derive the equivalent small signal model of switching network,rectifier network and resonant network in the high boost ratio DC-DC converter system;thus the small signal model of the high boost ratio DCDC converter is constructed.The accuracy of the constructed small signal model is verified through simulation,which provides theoretical support for the subsequent proposed closed-loop control strategy,the derivation of the impedance model,the construction of the all-DC offshore wind power multi-terminal convergence and delivery system,and the stability analysis.2)For the resonant network-based multi-terminal convergence control of high-boost-ratio DC-DC converters,a multi-terminal convergence feeding system based on high-boost-ratio DC-DC converters is proposed,and its control strategy is analyzed.Firstly,the structure of the multi-terminal convergence and delivery system is introduced,and the control strategy of the whole system level is proposed to follow the network;the VSC converter and the high boost ratio DC-DC converter are modeled and the control strategy is designed,and the closed-loop control of the input voltage is proposed through the frequency conversion control,and the onshore VSC adopts the voltage and current double closed-loop control,taking into account the output characteristics of the high boost ratio DC-DC converter.Finally,the effectiveness of the proposed control strategy is verified in the steady state and dynamics of the proposed multiterminal convergent feeding system through multiple operation scenarios.3)The stability and impedance characteristics of the proposed multi-terminal aggregation and delivery system for offshore wind power are studied,and the influencing factors are analyzed.According to the small signal model derived in Chapters 2 and 3,the output impedance expression of the high boost ratio DC-DC converter and the input impedance expression of the VSC converter station are derived,and the accuracy of the impedance model is verified by comparing the theoretical and simulated model Byrd diagrams;the influence of the circuit parameters and the variation of the regulator parameters of each control link on the input and output impedance of the subsystem is analyzed;the impedance analysis method is established according to the Based on the impedance analysis method,a mathematical model is established to analyze the influence of the circuit parameters and the regulator parameters of each control link on the stable operation of the whole system based on the Nyquist criterion.