Analysis Of Nonlinear Phenomenon In Grid-Connected PV System

With the increasing demand of clean-energy under the sustainable development strategy, the scale of PV (photovoltaic) is continuing to expand. The grid-connected voltage and capacity is getting higher and higher. The influence of nonlinear behavior of the grid-connected PV system on the stability is more prominent, which can restrict the large-scale development. Therefore, the deep study of the nonlinear behavior of the grid-connected PV system is extremely urgent. It has the important practical significant.This paper aims at the structure characteristic of grid-connected PV system, and modeling the system based on the correlation theory of nonlinear dynamics. It presents a through study of the mechanism of the nonlinear behavior on the stability of the system. The control structure is proposed and analyzing the stability of the system based on the average model. The time domain model of the analyzed mathematical model is established using MATLAB/SIMULINK. The nonlinear behavior of each component and the interrelationship between cascaded systems are deeply analyzed. The effective way to study the nonlinear behavior is obtained and the measures to improve the stability are proposed. The detailed researches are as follows:(1) The technical situation of current grid-connected PV system is analyzed. Based on the theory of power electronics converter and nonlinear dynamics behavior, the bottleneck hampered the developed the grid-connected PV system is analyzed. The research results of the nonlinear behavior of the DC power electronics converter direct the three-AC system. It can improve the operation condition effectively, and design more reasonable grid-connected PV system.(2) The operating principle of three DC-AC converter main circuit is studied. The switch model of it is established. The state space average is obtained to design the current and voltage loop parameters. The frequency domain analysis is also obtained, which can more intuitive to see the influence of parameters on the stability. The stability margin, and then the stability scope of the system parameters are obtained, which can provide the theoretical basis to design the system.(3) The time domain model of the analyzed mathematical model is established using MATLAB/SIMULINK. The results show that the DC-capacitance voltage can maintain the panel's voltage close to a desired reference^ which is obtained by the MPPT. The grid-current is in phase with the utility voltage. To improve the utilizing efficiency, the PV system can transported unity power to the grid and the waveform of grid-current is good in steady-state.The system access to another stability operation state after a brief period of adjustment when the solar irradiation changes. The results show that the system with strong anti-interference ability.The mathematical modeling, and the structural and parameters of the control system is validity and rationality through compared the simulation result with the theoretical analysis. Compared the time domain model with the average model, it can see that the influence of the variation of parameters on the stability of the system can be easily predicted under the average model and the operation characteristic can be held macroscopically. The physical process on the operation can be detailed observed under time domain model. It is convenient to do meticulous research on the system. Analysis result will slightly different under different methods. In practical, the analysis methods must be selected on particular case.(4) The nonlinear behavior of grid-connected PV system mostly come from power electronics converter. DC-AC inverter is switched regularly in the control system. The topology structural of the main circuit is time-varying. Therefore, grid-connected PV system is a strong nonlinear circuit with particular nonlinear phenomena of power electronics converter. The results show that bifurcation phenomena are occurred as the parameters of the control system are changed. It will make the operation of the system properties become worse, even cause the system to lose stability.So, bifurcation phenomena should be fully considered in the design process to prevent bifurcation behavior. And making it has enough stability margins.(5) The system can be divided to three parts:DC component, DC-AC converter and load component. The stable operation of the system is affected by the cascaded three parts.The double closed loop control strategy of DC-AC converter is proposed. The time constant of inner current loop is small, and it can quickly track the system. The time constant of outer voltage loop is large, which can be considered the output current is equal to the reference value. Because of this, the interrelationship between the DC side and load side is greatly reduced. The result shows that the DC-voltage is rarely changed with the load.The paper analyzes the influence of nonlinear behavior of the grid-connected PV system on the stability combined with the actual system. And the control strategy is proposed. The consistency of average model and the time domain model show that the correctness and effectiveness of the results.The bifurcation phenomena of grid-connected PV system are complicated, which must select appropriate methods and strategies. Seize the principal contradiction, ignore the secondary factors. That can make scientific research along the correct and effective direction. The study can be used as a reference of the high-performance grid-connected PV system.