Study On Sub-synchronous Oscillation Detection And Suppression Strategy Of DFIG-based Wind Turbine Connected To Grid Via Series Compensated Lines

With the massive access of wind power generation,the stable operation of the power grid is affected and many new problems arise.The randomness of wind resources,the access of power electronic equipment and the series compensation device invested in long-distance transmission will cause the current and power oscillation of large-area wind turbines.Aiming at the problem of subsynchronous oscillation caused by grid connection of double-fed wind generator compensated by fixed series capacitor,the operation characteristics and mechanism of fan oscillation are analyzed,and the oscillation identification method of wind power grid connected power system is proposed.Finally,based on the operation mechanism of subsynchronous oscillation,the suppression strategy of subsynchronous oscillation of doublefed wind generator is formulatedFirstly,in order to analyze the subsynchronous oscillation caused by series compensation in double-fed wind generator system,the mathematical models of wind turbine system,doublefed induction motor,series capacitor compensated transmission line,stator / rotor side converter and corresponding control system are established.Based on the MATLAB simulation platform,the time-domain simulation of the operation state of the system under various working conditions is carried out,and the correlation between different series compensation and different operating conditions and the system subsynchronous oscillation fault is simulated.The causes and characteristics of continuous oscillation of output power are studied.In order to deeply understand the oscillation mechanism,study the influencing factors and fault characteristic information of subsynchronous oscillation,locally linearize each link of double-fed wind generator series compensation grid connected system,and deduce the state equation of the whole system.Finally,the correlation between different operating modes and subsynchronous oscillation is studied based on the stability theory of linear system.Through the stability analysis,five main modes causing the oscillation of the wind power grid connected system are obtained.According to the five main working modes,the strongly related variables causing the subsynchronous oscillation of the system are obtained,including the control parameters of the shafting,series compensated transmission line and stator / rotor side control system.The above stability analysis results are verified by simulation.Based on the deep-learning algorithm,the artificial intelligence fault detection method of double-fed wind generator series compensation grid connected system is proposed.Based on the on-line fluctuation of strongly related variables of subsynchronous low-frequency oscillation,the multi condition simulation of double-fed wind generator grid connected system is carried out.The main oscillation characteristic information data are extracted and normalized on-line,and the subsynchronous oscillation detection network based on Convolutional Neural Network is constructed.The normal operation data and oscillation operation data under different working conditions are grouped,and the grouped data is used to train the network.Finally,the oscillation faults caused by series compensation,wind speed and unreasonable control parameters caused by system operation nonlinearity are verified,and the recognition accuracy is more than 95%.Finally,the suppression scheme of subsynchronous oscillation is studied.Aiming at the negative damping characteristic of induction generator in subsynchronous state,the suppression strategy of virtual impedance is proposed.Considering the increasing characteristics of the rotor side converter control system to the subsynchronous frequency and current disturbance,combined with the frequency response characteristics of the quasi proportional resonant controller,the scheme of switching to the quasi proportional resonant controller in the oscillation state is adopted.The simulation results show that the virtual impedance scheme can effectively suppress the proportion of the low-frequency resonant current in the same time period after the oscillation.The quasi proportional resonant controller can stabilize the proportion content within5%.The two schemes maintain the stable operation of wind power series compensation grid connected system and reduce the risk of power oscillation.