Analysis Of Grid-connected Characteristics Of Large-Scale PV And Its Delivery System Stability

In order to protect the environment and conserve resources,on the one hand,all countries have been to increase the utilization rate of fossil fuels and to optimize the emission technology of coal-fired units;on the other hand,they have been going to seek to replace conventional power sources with new energy to maximize energy sustainable development.With the rapid development of solar power generation,it has become an effective way to replace conventional power supply.The photovoltaic power generation system is connected to power grid by the power electronic converter,and there are significant differences with conventional power supplies such as hydropower and thermal power.Therefore,with the growth of photovoltaic access capacity,its impact on the stability of the system has become a hot topic of scholars nowadays.The simulation and modeling and the characteristic analysis of PV grid-connected system is basis for the study on impact on the stability of large-scale PV grid connected to system.In this thesis,based on the PSD-BPA,electromechanical transient model of PV is established,and makes a contrastive analysis of the photovoltaic power generation unit and the electrical difference of conventional power.Under the electromechanical transient simulation scale,the transient power characteristics of the photovoltaic grid system are analyzed by simulating the process of photovoltaic small systems disturbed.Aiming to the influence of large-scale PV grid-connected systems on the oscillation damping characteristics,this paper is based on photovoltaic grid-connected systems,and the process of drop and recovery of voltage of grid-connection point is simulated,when scale PV grid-connected system is under different control modes,the influence of the U_s-B_pv trajectories on the oscillation damping is analyzed.And according to the equal area principle,the effect mechanism of photovoltaic system on the oscillation damping is revealed.In this paper,the two-zone four-machine system disturbed time domain simulation is taken as an example.The effectiveness of influence on the system damping evaluated according to the U_s-B_pv trajectories characteristic of photovoltaic grid connected system is verified.On this basis,the damping optimization measure of the photovoltaic grid connected system with SVG additional damping control is proposed.Based on the Heffron-Phillips model,the influence of the photovoltaic grid-connected on the two links of its electrical oscillation and mechanical oscillation is analyzed.The effect of the dynamic response of the photovoltaic system on the damping of the unit is briefly expounded.It is pointed out that PV grid-connected at the sending-end power grid,which has the effect of strengthening or weakening system damping characteristics,and it needs to be analyzed in combination with specific scenarios.Making the simulation and analysis of Central Tibet power grid,by increasing or decreasing the power of hydropower,the different levels of photovoltaic penetration are adjusted,and the changes of system damping under various conditions are considered.The simulation shows that,with the increasing of photovoltaic penetration,the system damping is monotonically increasing,and the different control mode of photovoltaic inverter has no obvious effect on the system damping.The countermeasures of DC power modulation or SVC additional damping control are put forward and its effectiveness is verified.Photovoltaic power generation system has problems such as lack of inertia,frequency modulation,and lack of frequency and voltage withstand capacity.It will have a significant impact on the frequency stability characteristics of power grid with photovoltaic high Penetration.Based on the analysis of the features of photovoltaic power generation and its control system,this paper discriminates the three differences in the electrical aspect of photovoltaic power generation system,and analyzes the influence of conventional power source substituted by PV on frequency stability characteristics.For the Tibet power grid with PV high penetration and aiming at the impact of unbalanced power impact,the frequency stability characteristics and its peak value variation characteristics are analyzed under different PV generation ratio,the measures to improve the frequency stability of power grid with photovoltaic high penetration are put forward such as perfecting emergency control and utilizing hvdc power regulation.