Research On Control Strategy For Passively Enhanced Direct Drive Wind Turbine Connected Converter

Wind energy,as a renewable energy with the fastest development speed,the most mature technology and the largest development scale,plays an increasingly important role in the development and utilization of new energy.Among all kinds of wind power generation systems,permanent magnet direct drive wind power generation system has become the mainstream of wind power industry because of its unique advantages of high operation efficiency.The direct drive wind power is connected to the grid through the converter,but the control link of the converter and the interaction with the grid may lead to the instability of the whole system.Among them,the problem that the system is in an unstable state due to the mismatch between the output impedance of the converter and the grid impedance becomes more and more prominent.The research on the stability of grid connected converter has become a hot spot in the field of wind power generation.In this paper,the grid connected system of direct drive wind power generation is taken as the research object,in order to enhance the passivity of grid connected system,the stability of grid connected converter is mainly studied from the perspective of impedance analysis.The main tasks are as follows:Firstly,the structure and power generation principle of direct drive wind power generation system are introduced,and the mathematical model of direct drive permanent magnet synchronous wind power generation system（PMSG）,the mathematical model of full power wind power converter and the control structure of generator side and grid side are given,which lays the foundation for the control system parameter design and stability analysis.Then,the small signal modeling method is used to establish the impedance model of the wind power grid connected converter under the current and DC voltage double closed-loop control mode,which lays the foundation for the impedance stability analysis and passivity optimization.Then,the traditional controller design method is used to design the regulator parameters of the generator side converter and the grid side converter,and the simulation model is built to verify the correctness of the parameter design.Then,the impedance stability criterion and passivity theory are used to analyze the impedance stability of the direct drive wind power grid connected system.It is pointed out that the traditional controller parameter design method does not consider the impedance interaction between the converter and the power grid,which leads to the harmonic resonance problem caused by the safe and stable operation of the converter under the ideal power grid conditions in the weak current grid environment with high value reactance or the mutual coupling of multiple power electronic devices in parallel,which damages the stability and robustness of the system.Finally,the impedance model and passivity theory are used to analyze the influence of the control parameters:current inner loop proportional coefficient(k_pac),current inner loop integral coefficient(k_{ia c}),DC voltage loop proportional coefficient(k_pdc),DC voltage loop integral coefficient(k_idc)on the stability of grid connected converter system.It is pointed out that the risk of instability in the frequency range below50Hz and above1666Hz will be reduced with the decrease of the control parametersk_{pa c}.k_{ia c},k_pdc and k_idc have little influence on the interaction stability between the converter and the grid.This makes up for the shortcomings of the traditional controller parameter design,and provides a reference for the control parameter tuning of grid connected converter.In order to enhance the passivity of the system,the PCC point voltage feedforward compensation strategy is adopted,and the influence of feedforward compensation on the impedance model of the converter is analyzed.