| In recent years,the Chinese electricity market has witnessed vigorous growth.With the continuous increase in electricity demand,the development speed of the power industry has been accelerating.At the same time,China has consistently upheld the ideology of green development in the field of energy.Therefore,effectively suppressing the fluctuations in the wind power market and improving its market penetration rate have become urgent and significant challenges.Flexible direct current transmission technology has the capability to connect with weak grids or passive networks without the need for filters and reactive power compensation.It exhibits features such as active and reactive power decoupling.For the integration of large-scale wind power into the grid,the application of voltage-source converters in flexible DC transmission technology can achieve excellent voltage regulation.However,during grid integration,issues may arise such as power fluctuations caused by unstable power and voltage instability.Therefore,this thesis aims to investigate the issues of voltage instability and frequency variation associated with the integration of doubly-fed wind power into flexible DC transmission systems.The research will focus on the theoretical and applied aspects of stability and coordinated control in doubly-fed wind power generation.The findings of this study will provide valuable guidance for engineering practices.The main contributions of this research are as follows:(1)Firstly,starting from the composition of flexible direct current transmission systems,a comprehensive analysis of the types of grid-connected converters is conducted.Based on the operation mechanism of the grid-connected converter,modeling is performed in the abc three-phase static and dq two-axis synchronous rotating coordinates.Addressing the issue of application mode selection in flexible DC systems for wind power generation,the control modes of the converter are studied,providing insights into the control modes of the transmission system.(2)Secondly,to address the problem of voltage instability when integrating wind power into flexible DC grids,a mathematical model of wind power integration into flexible DC grids is established.The fundamental operational characteristics and power characteristics of the doubly fed induction generator are investigated.Furthermore,a thorough discussion on the control methods of the converter stations at the sending and receiving ends in the grid integration system is conducted.A coordinated control method suitable for wind power generation in flexible DC transmission systems is proposed based on master-slave control,voltage margin control,and DC voltage droop control.To validate the effectiveness of the proposed control strategies,simulation models are constructed using a two-terminal flexible DC grid as an example.Through simulation analysis,the proposed control methods can reduce the fluctuations in wind power and stabilize the DC voltage on the AC side,achieving excellent control results.(3)Finally,to improve the accuracy of the control system,a control parameter optimization strategy is designed for wind power integrated with flexible DC transmission systems.A small-signal model of the voltage source converter based high voltage direct current system is developed,and an improved adaptive differential evolution algorithm is proposed based on the traditional differential evolution algorithm.The control parameters are optimized by adjusting them using this algorithm.Simultaneously,the small-signal model is validated through simulation experiments,and the results demonstrate that the optimization strategy effectively improves the overall optimization of the controller parameters and achieves high control accuracy. |
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