Research On Subsynchronous Oscillation Of DFIG Grid-connected System

In the context of environmental and energy crises,wind power generation technology is gradually developing in the power industry.Wind power resources and load centers are distributed inversely,and series compensation capacitance technology is one of the important methods of wind power transmission.However,the interaction between series compensation capacitors and wind turbines may cause sub-synchronous oscillation,which will damage the security and stability of power system operation.Therefore,the research and suppression of sub-synchronous oscillation in wind power grid-connected system is one of the keys to improve wind power grid-connected technology.In this dissertation,the sub-synchronous oscillation in doubly-fed induction generator(DFIG)wind turbines connected to series-compensated lines is studied.The main contents are as follows:(1)The topological structure and operation principle of DFIG are analyzed,and the mathematical model of DFIG series-compensated grid-connected system is established.The aerodynamic model of DFIG capturing wind energy,the pitch angle control model for maximum power point tracking,the mechanical transmission system model of two mass blocks including wind turbine mass block and generator mass block,the dynamic model of the generator,the rotor-side converter and the grid-side converter and their double closed-loop control system model,and the series compensation circuit model are studied respectively.(2)Combined with modal analysis and time domain simulation,the mechanism and influencing factors of sub-synchronous oscillation in DFIG series-compensated grid-connected system are studied.The linearized model of the system at steady-state operation point is deduced,the eigenvalues of the system state matrix and the participation factors of each state variable are calculated,and the oscillation modes are identified.The mechanism and influencing factors of sub-synchronous oscillation are analyzed according to the eigenvalue locus of the system when different variables change.The simulation results show that the main reason for the sub-synchronous oscillation of the DFIG series-compensated grid-connected system is sub-synchronous control interaction between the control system and the series compensation line.The interaction mechanism of sub-synchronous control is analyzed by theoretical deduction.The effects of wind speed,series compensation and controller parameters on sub-synchronous oscillation are analyzed by time domain simulation.(3)An additional sub-synchronous damper controller based on MIMO structure is designed to suppress sub-synchronous oscillation.It consists of a state observer,a state feedback controller and a high-pass filter.The system model is simplified and the input signal of the controller is selected based on HSV value.The gain matrix of state observer and state feedback controller is calculated by using quadratic optimal regulator.The inner loop of RSC control system and the inner loop of GSC control system are selected as the installation positions of the controllers.The robustness of the controllers and the performance of restraining sub-synchronous oscillation are compared by small interference analysis and time domain simulation analysis.The simulation results show that the additional sub-synchronous damper controller designed in this dissertation has good performance in suppressing sub-synchronous oscillation: when installed in the same position,the controller with d-axis component and q-axis component of stator side and rotor side current as input signal has better performance than only using stator side component as input signal;when the input signals of the controller are the same,the performance of the controller on the both side are higher than that on the rotor side only and the performance of the controller on the rotor side are better than that on the net side only.