Study On Sub-Synchronous Oscillation Of The Grid-Connected Inverter For Direct-Drive Wind Turbine

The penetration of powe grids has also increased dramatically due to the continuous advancement of renewable energy sources(such as wind and solar energy),the penetration of power grids has also increased dramatically,resulting in power systems undergoing big changes in the planning and operation of large-scale power systems.The change in this aspect is that wind power generation,solar power generation,and battery energy storage systems are increasingly connected to the power grid.These systems usually use a large number of power electronic interfaces to connect to the power grid asynchronously,resulting in a weaker and more unstable power grid.Especially in the weak grid environment,the direct-drive wind turbine based on inverter is easy to cause sub-synchronous oscillation.First,this paper introduces the background and significance of subsynchronous oscillation in permanent magnet direct-drive wind turbine,and summarizes its mechanism.Secondly,according to the control principle of permanent magnet direct drive fan,the mathematical models of machine side inverter and grid side inverter are built,and their control strategies are analyzed.The process and mechanism of sub-synchronous oscillation of permanent magnet directdrive wind turbine are analyzed.Subsequently,verification is performed from the direct-drive wind turbine side.Integrating the characteristics of the static and motionless subsynchronous compensator,the static and the motionless sub-synchronous compensator(STATCOM)is used for the operation system of the directdrive centrifugal wind turbine.The static data synchronous compensator is used to improve the vibration reduction of the system and suppress the secondary synchronous oscillation caused by the direct drive centrifugal wind turbine putting into operation.In the end,according to the previous interpretation of the subsynchronous oscillation of the grid-connected inverter of the direct-drive centrifugal wind turbine and the model establishment and deduction,it can be known that the sub-synchronous oscillation is mainly caused by the change of the system vibration reduction under the weak current grid,Moreover,the stability of grid-connected inverter is affected by phaselocked loop to a certain extent.Therefore,the phase-locked loop of the integrator based on the second-order basic theory is improved,and the stability of the grid-connected inverter is verified when the improved phaselocked loop is selected.The established mathematical characteristic impedance model and MATLAB/SIMULINK are used to carry out frequency domain simulation,and the characteristic impedance of PLL system at the same operating frequency again and the changes of the left and right three-phase before and after improvement are compared.