Research On Sub-synchronous Oscillation Of PV Grid-connected Power Generation System

With its outstanding advantages of clean and sustainable,the solar energy has effectively alleviated the environmental degradation and the shortage of energy,and is gradually showing a rapid development trend.However,with the continuous expansion of the grid-connected scale of photovoltaic(PV),the intensity of its AC system at the receiving end will be relatively weakened,and the resulting sub-synchronous oscillation(SSO)problem will become more and more prominent.Therefore,aiming at the SSO problem of PV grid-connected power generation system,the following research is carried out in this paper:Based on the typical topological structure of a PV power station in Northwest China,the equivalence effect and applicability of the equivalent precisions of the controlled source equivalent method and the equivalent parameter equivalent method are compared and studied,and an equivalent method which is suitable for the study of SSO characteristics of PV grid-connected system is proposed.Based on the equivalent model of PV power station,the mathematical model is established,which includes PV array,inverter and its control system,phase-locked loop(PLL),LCL filter and transmission line.Based on the mathematical model of PV power station,the state space model of the whole system is derived by modular modeling method,and the SSO modes of PV grid-connected system are identified by participation factors and time-domain simulation.The effects of LCL filter parameters and PV output on the characteristics of PV grid-connected system in weak power grid are studied by eigenvalue analysis and time-domain simulation.The results show that the parameters of LCL filter and the output of PV have different effects on the damping ratio of the oscillatory modes of the system.Considering the interaction characteristics between PLL and power grid impedance in weak AC system,the dynamic characteristics of PLL are qualitatively explained by deducing PLL dynamic equation,the interaction mechanism between PLL and the intensity of power grid impedance is revealed by the system control loop,and the interaction characteristics of them are verified by eigenvalue and time domain simulation analysis.The analysis results show that when the PLL parameters are not set improperly in the weak power grid,when the PLL is weakly damped and the natural oscillation frequency is close to the disturbance frequency of the system,the participation of PLL to the system SSO is increased,which has a bad influence on the stability of the system.The SSO generation mechanism of PV power station integrated into weak AC power grid is analyzed.Based on the idea of parallel equivalent positive resistance consuming SSO energy,a static var generator(SVG)based supplementary control strategy method is proposed,and the mechanism of SVG suppression is expounded on the additional damping control strategy,and the parameter design method of additional damping controller is given.The simulation results in time domain verify the suppression effect of the control strategy on SSO of the PV grid-connected system under various working conditions.