Analysis On Sub/Super Synchronous Oscillation Mechanism Of Pmsg-based Wind Farm

With the expansion scale of new energy power generation bases such as wind farms and photovoltaic power stations,the equivalent short-circuit ratio of feeding points is decreasing,which causes frequent oscillation accidents and seriously threatens the safe and stable operation of China's power grid.At present,scholars have conducted extensive and intensive research on the stability problems brought about by power electronization in power systems.In order to enrich the influence mechanism of the control loop on sub/super synchronous oscillation in wind power system,this paper takes the weak AC grid system with direct-driven wind farm as research object,and carries out research and simulation verification on the oscillation caused by control interaction from three aspects: impedance modeling,stability analysis and oscillation suppression.Firstly,the wind turbine model is simplified appropriately,and the rationality,accuracy and rapidity of the average model are pointed out,which lays a theoretical and simulation foundation for the subsequent research on the impedance model and control loop optimization of voltage source inverters.Furthermore,by introducing the sequence impedance theory and taking the output impedance of one single grid-connected inverter as the minimum modeling unit,the sequence impedance model of voltage source converter under PI and PR control strategy are derived,and the equivalent impedance model of clustered inverters for stability research is proposed.The results show that when the current inner loop has the same stability margin,the positive sequence impedance of grid-connected inverters under PR control can be increased in 0~100Hz frequency band to reduce the risk of sub-synchronous oscillation.Then,based on the stability analysis method of impedance ratio,the interaction mechanism between power electronic control system and high-impedance grid is revealed in frequency domain.The results show that the control system of power electronic converters is equivalent to a "virtual impedance" in the circuit network,which exhibits the capacitive impedance characteristic of low negative resistance at a certain frequency.It can form a "RLC series resonant circuit" with the actual inductive power grid and cause diverging oscillation due to the negative resistance effect;Finally,by exploring the influence of current controller parameters and phase-locked loop(PLL)bandwidth on the stability of positive sequence network of clustered inverters,an impedance remodeling method and an optimization method of PLL parameters are proposed to enhance the system stability.The simulation results show that the proposed strategies can significantly reduce the harmonic components between the clustered inverters and the weak grid.