| With the continuous construction of offshore wind farms,the development of wind power energy in coastal areas tends to be saturated,and the offshore wind power generation projects are gradually advancing to the far-reaching sea.Therefore,the modular multilevel converter-based HVDC(MMC-HVDC)has become the best technical scheme for the integration of large-scale and long-distance offshore wind power generation.However,since offshore wind farms and MMC-HVDC systems both contain many power electronic devices and a variety of control links,it is easy to produce interactive coupling effects and lead to system oscillation.According to relevant reports,there have been many wind farms connected to the grid by MMC-HVDC actual engineering at home and abroad in the system commissioning or production operation stage of the wide-band oscillation instability phenomenon,resulting in difficulties in the consumption of offshore wind power energy while posing a serious threat to the security and stable operation of the regional power grid.Therefore,the following work is carried out in this thesis to solve the wide-band oscillation problem caused by offshore wind farm through MMC-HVDC system:(1)The small signal model of offshore wind farm through MMC-HVDC system is constructed.Firstly,the whole offshore wind farm is characterized by a single machine equivalent model,and the dynamic mathematical model of the offshore wind farm is constructed based on the unit modular modeling method.Then,by introducing the concepts of differential-mode and common-mode,the dynamic mathematical model of the MMC system based on the "Sigma-Delta" variable is deduced,which avoids the frequency coupling problem in the state variables of the MMC system.In addition,considering the difference in the synchronous rotating reference frame between the wind farm and the MMC system,the interface model between the wind farm and the MMC system is constructed.Finally,the small signal model of the system is constructed by combining the dynamic models and interface models and linearizing them at the steady-state equilibrium point.The correctness and effectiveness of the small signal model are verified by the simulation results in the time domain.(2)The wide-band oscillation dynamic characteristics of offshore wind farm through MMC-HVDC system are analyzed.Firstly,two kinds of oscillation instability modes of the system are identified by the eigenvalue analysis method,one is caused by the internal MMC system,and the other is caused by the interactive coupling between the wind farm and the MMC system.Then,the dynamic characteristics of the two types of oscillation instability modes are analyzed based on the sensitivity of eigenvalues.The results show that the system structural parameters have a great influence on the first type of oscillation instability mode,and the damping ratio of the second type of oscillation instability mode can be effectively improved by adjusting the wind farm controller parameters.Finally,the time domain simulation method is adopted to verify the correctness of the theoretical analysis results.(3)The wide-band oscillation suppression strategy of offshore wind farm through MMC-HVDC transmission system is studied.To eliminate the internal oscillation instability mode of the MMC system,a zero-sequence current controller based on MMC capacitor energy is proposed in this thesis.The eigenvalue analysis method and time domain simulation method are used to verify that the controller has a good oscillation suppression effect.Under the premise of eliminating the internal oscillation of the MMC system,a control parameter optimization algorithm based on eigenvalue sensitivity is proposed for the grid-side converter of the wind farm,and an additional damping controller is designed for the MMC system,which makes up for the lack of robustness of single optimization control parameters and greatly improves the system security and stability margin. |
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