| With the accelerated realization of the goal of "Double Carbon" emission reduction,it has become a development trend to increase the proportion of wind energy consumption and build a new power system mainly composed of new energy.Flexible DC transmission provides an ideal solution for realizing large-scale long-distance acceptance of new energy.With the putting into use of many wind power transmission projects at home and abroad,the system oscillation phenomenon caused by the interaction between the wind turbine and the flexible direct has appeared.The stability evaluation method and strategy optimization method for this phenomenon are in urgent need of breakthroughs.Focusing on the oscillation problem of wind power via flexible direct transmission system,this paper takes direct-drive wind turbines and flexible DC transmission systems as the research objects,and conducts in-depth research and simulation from three aspects: modeling of wind turbines and flexible DC transmission systems,wind turbine parameter tuning and stability analysis,wind turbine adaptation to flexible control strategy optimization and stability improvement.In the aspect of system modeling,the mathematical model of the direct-drive wind turbine composed of permanent magnet synchronous generator,converter and its control system is established.At the same time,the mathematical model of the flexible DC transmission system composed of the MMC flexible-to-DC converter electrical system and the control system is established.In terms of parameter tuning of direct-drive wind turbines,considering the delay of PWM converter,a method for tuning the inner loop parameters of grid-side converter based on internal model control is proposed.The frequency domain analysis method is used to adjust the parameters of the grid-side converter outer loop and phaselocked loop.From the perspective of time domain,variable load conditions or phase step tests are used to observe the relevant control variables under different key control parameters,and the effectiveness of the proposed parameter tuning method is verified.The frequency sweep method is used to extract the impedance of the wind turbine,and the influence of different key parameters on the frequency domain impedance characteristics of the wind turbine is analyzed,which lays a foundation for the wind turbine to adapt to the vibration stability of the flexible direct transmission system.In the aspect of control strategy optimization of direct-drive wind turbines,the Bode stability criterion of wind turbines via flexible DC transmission system is deduced,and impedance extraction and oscillation risk assessment are carried out for the case of wind turbines connected to flexible DC transmission system.The design scheme and parameter tuning method of additional damping controller for wind turbines,which are suitable for improving the stability of flexible DC transmission system,are proposed.The effect of the improvement strategy is analyzed from the perspective of frequency domain.The research results show that the oscillation suppression strategy of wind turbine output via flexible DC transmission based on additional damping control effectively eliminates the oscillation risk.Finally,the additional damping control parameters with good effect are selected for time domain simulation,and the vibration suppression effect of the additional damping control on the output system of the wind turbine via the flexible DC transmission system is verified. |
