| With the rapid development of photovoltaic industry,photovoltaic(PV)grid power capacity in China is growing rapidly in recent years.Since the large-scale PV power station in China are mainly distributed in remote areas with small local load,and the PV power generation itself has the characteristics of uncontrollable volatility,partial unpredictability and location dependence,constrained by grid transmission channel restrictions and system peak capacity,a serious abandonment phenomenon appeared,which leads to a large amount of waste of clean energy.In order to make better use of the PV energy,and realize the goal of clean energy replacement as soon as possible,it is of great significance to study the strategy and stability of the large-scale PV grid-connected system.Firstly,this paper analyzed the active power control process of PV power station,and introduced the functions and realization methods of each link.Then,the mathematical model of PV grid-connected system is analyzed under the stationary coordinate system considering the coupling effect between grid inverters and the grid impedance,combine with the quasi-proportional resonant(PR)control,the current inner loop control block diagram and the open-loop transfer function of the current inner ring are obtained,which provides the basis for the analysis of the control strategy and the stability analysis of the system considering grid impedance.Then,four control strategies applied to different working conditions are analyzed,including the maximum power tracking control,constant power control,low voltage ride through control,constant voltage/constant frequency control.Combined with the control block diagram of the current internal loop in the third chapter.The simulation models of the four control strategies are set up on the Matlab/simulink platform respectively.Through simulation we can know that the effectiveness of the four control models is verified,and the smooth switching between different control strategies is realized.Finally,based on the open loop transfer function of the current inner loop obtained in Chapter three,the influence of grid impedance on the system stability margin is analyzed,and the influence of the grid impedance is simulated based on the constant power control model,It is concluded that it will threaten the stability of the system when the inductive reactance of the grid impedance is too large.Then,three methods to improve the stability of the system are analyzed.Through the analysis,it can be seen that the stability of the system can be improved by selecting the appropriate control parameters,using the virtual impedance method to calibrate the output impedance of the inverter or the network impedance,and compensating the influence of the network impedance by feedback control. |
