Subsynchronous Oscillation Suppression Of Grid-Connected Doubly-Fed Wind Power Generation System

Wind power generation system can convert clean and renewable wind energy into electricity that is easy to transmit.Technical research on its system com position and operation performance has always been a hot topic in the field of electrical engineering.With the continuous improvement of wind power penetration,the proportion of wind power in the power grid is gradually increasing,and the problems of wind power itself and the impact on the infinite grid are becoming more and more prominent.Due to the wide use of series compensation capacitors in long-distance transmission,the subsynchronous oscillation of doubly-fed wind generator(DFIG)is easily induced by the interaction between line inductance and series compensation capacitors,which poses a great threat to the safe and stable operation of power grid and generation system.In this paper,the causes and hazards of grid-connected sub-synchronous oscillation of doubly fed wind power generation system are analyzed,and an effective suppression scheme of sub-synchronous oscillation is proposed.Firstly,the model of DFIG is built.Through coordinate transformation and simplification,the DFIG mathematical model suitable for engineering practice is obtained.Secondly,the grid-side converter(GSC)is modeled,and the control objectives of GSC are pointed out.According to the control objectives,a control scheme with PI controller as the core is designed.Secondly,the rotor side converter(RSC)is modeled.The main goal of RSC is to decouple the output power of DFIG.In order to achieve this goal,power outer loop and current inner loop control schemes are designed.Finally,the simulation models of each part are built in the simulation tool,and the proposed control scheme is verified by simulation.In the research of this paper,the reasons for the sub-synchronous oscillation of doubly-fed machine caused by the low frequency components of power network line structure and parameters are analyzed in depth.It is revealed that the existence of RSC and the direct coupling between series compensated line and stator are the fundamental reasons for SSCI.Subsequently,the phase-locked performance of PLL in the sub-synchronous oscillation environment is studied.Both theoretical analysis and simulation results show that there will be AC fluctuation components in the output voltage and frequency of PLL in this environment.Finally,a strict formula for the oscillation divergence process of SSCI is deduced,and the effect of the sub-synchronous oscillation frequency on DFIG is emphatically analyzed.In order to suppress the subsynchronous oscillation of DFIG,a scheme of adding quasi-resonant control loop to RSC is proposed.A quasi-resonant controller is proposed to suppress the fluctuation of reactive power and DC bus voltage in GSC sub-synchronous environment.To solve the problem of poor frequency adaptability of quasi-resonant controller,an adaptive quasi-resonant controller is designed.Compared with quasi-resonant controller,the adaptive quasi-resonant controller can dynamically adjust the resonant frequency with the change of system frequency,which can effectively improve the suppression effect of sub-synchronous oscillation.In order to verify the performance of the adaptive quasi-resonant control scheme to suppress the sub-synchronous oscillation,a simulation model is built in the simulation tool,and a simulation platform of doubly-fed wind power system is built under the laboratory conditions,which is verified again by the simulation experiment platform.