Research On Stability And Control Of DFIG-based Wind Turbines During Weak Power Grid Condition

With the vigorous development of new energy technology,the penetration of wind power continues to increase.This trend has stabilized the position of wind power as the third largest energy source in China after thermal power and hydropower.In China,large-scale wind farms are mostly distributed in North China,Northwest China,Northeast China,which are far away from load centers,located in Southeastern coastal areas.Due to the limitation of the small short-circuit capacity of the system,the short circuit ratio(SCR)has become the main form of wind turbine access to the grid.With regards to wind turbine,it is connecting to impedance power grid that cause the oscillation and divergence of the grid-tied current during the period of fault ride-through.Therefore,it is necessary to clarify the mechanism of triggering system instability,so as to lay a foundation for improving the uninterrupted operation ability of wind turbine.At the same time,in order to further guarantee the steady operation of the power system and settle down all kinds of asymmetric power grid failure scenarios,the Technical Committee of the Standardization Administration of Electric Power of the People’s Republic of China has put forward increasingly stringent requirements for the grid codes of wind farms,that is,negative sequence reactive current response,which is to suppress the rise of negative sequence voltage under the condition of asymmetric power grid.Under the above dual background,it is of great significance and engineering application value to carry out the research on stability and control of DFIG-based wind turbines during weak power grid condition.In this paper,a DFIG-based wind generator with high grid-connection friendliness is taken as the research object,multiple steady-state operation points are taken as the research scene,and theoretical analysis,simulation research and experimental design and verification are taken as the means to deeply study its small-signal stability under weak power grid conditions.Furthermore,the instability mechanism is clarified from the aspect of impedance characteristics,and the key factors affecting the stability of machine-grid cascade system are analyzed.Meanwhile,a coordinated control strategy of RSC and GSC which is more suitable to the latest grid codes is proposed.The main research contents of this paper are as follows:The first part of this paper refers to adopting impedance analysis method to study and analyze the small signal stability of DFIG-based wind turbine during grid-connected operation under weak grid condition.First of all,a small signal complex vector impedance model for the grid-connected DFIG-based wind turbine system is established,which fully considers the key factors affecting the operation and control during grid-connected period,such as stator-and rotor flux,current inner loop and phase-locked loop of RSC and GSC,etc.Secondly,the accuracy of the model was verified by the phase plan.Then,the influence of the control parameters on the impedance characteristics and the stability of the source-charge cascade system is analyzed by means of Bode diagram.Then,all the theoretical analysis results above are verified by simulation.The second part of this paper comes down to analyze in detail the operation requirements of the newly revised grid-code for DFIG-based wind turbine under the condition of asymmetric power grid.On this basis,considering the modulation constraints and capacity constraints of RSC and GSC,the current distribution of the converters is clarified.Based on the above research,a control strategy for RSC and GSC of DFIG-based wind turbine under asymmetric power grid conditions is proposed.Finally,simulation and experiment are designed to verify the effectiveness of the strategy.