Research On Impedance-Shaping Strategy Of Three-Phase Grid-Connected Converters

Increasing the proportion of renewable energy power generation is becoming the adjustment direction of China’s energy strategy.As a key device for the grid connection of renewable energy power generation,the stability of three-phase grid-connected converters have attracted more and more attention.Due to the renewable energy power generation is far away from load centers,the grid impedance cannot be ignored.The system oscillation caused by the dynamic process of each control loop of three-phase grid-connected converters interacting with the grid impedance seriously harms the reliability of new energy power generation.Therefore,this paper analyzes the factors affecting the stability of three-phase grid-connected converters by impedance analysis,and proposes the corresponding impedance-shaping strategy to improve the system stability.In this paper,the dq impedance model under the synchronous rotating coordinate system is established by using small-signal modeling method.The main circuit and various control loops,including current loop,dc voltage loop and Phase-Locked Loop(PLL),are considered in the model.Since the dq impedance model cannot quantitatively evaluate the stability of the system,the dq impedance model in the synchronous rotating coordinate system is transformed into the impedance model in the form of complex signals in the static coordinate system by the method of uniform impedance.After that,two kinds of impedance models verification methods are introduced,and the accuracy of the two impedance models is proved by frequency scanning verification.Based on the impedance model of three-phase grid-connected converters,the factors affecting system stability are analyzed from the perspective of impedance,and the validity of the theoretical analysis is verified by simulation method.Increased grid impedance will reduce the system stability margin.If the system stability margin is less than zero,the system will oscillate.Reactive power will also affect the system stability.When the inverter absorbs reactive power,the system stability margin will increase,which will help to improve the system stability,while emitting reactive power will further reduce the system stability margin and worsen the system stability.The PLL and the dc voltage loop introduce negative resistance effect into the q-q channel and d-d channel of dq impedance respectively,which will worsen the system stability.Based on the above stability analysis,the mechanism of system stability caused by PLL and dc voltage loop is analyzed by dq impedance model,and the impedance-shaping strategy of adding control branches to the controller is proposed.To targeted to solve the PLL and dc voltage loop stability problem,and minimize the impact on the original control system,the implementation method is introduced based on the second order bandpass filter design method.Theoretical analysis shows that the proposed impedance-shaping strategy can effectively solve the problem caused by PLL and dc voltage loop.The correctness of the theoretical analysis is verified by simulation and the validity of impedance-shaping strategy.Aiming at the sub/super synchronous frequency oscillation phenomenon of direct-driven wind turbine under the condition of weak ac system connection,the equivalent simulation model of 2MW direct-driven wind turbine was built at first,and the stability problem caused by weak ac system was verified through the simulation.The two impedance-shaping strategies are applied to suppress the sub/super synchronous frequency oscillations of direct-driven fans.Theoretical analysis and simulation verify that the proposed method can effectively solve the sub/super synchronous frequency oscillation of direct-driven wind turbine in the connection of weak ac systems,and further prove the effectiveness of the impedance-shaping strategy.