Research on Modeling and Simulation of Detailed Model of Photovoltaic Power Generation System

With the photovoltaic, fan and other new energy in the proportion of distribution network is growing, the photovoltaic power generation system and the process of accurate simulation is of great significance. Based on Matlab / Simulink, a detailed model of PV array, detailed model of MPPT controller and detailed model of PV inverter control strategy are established according to the actual principle of grid connection, and the process of grid connection of photovoltaic power generation system is simulated. The photovoltaic grid connected waveforms of the mixed-mode simulation system of distribution network of China Electric Power Research Institute are compared with Simulink model simulation waveforms, and the accuracy of the model is verified. The experimental results show that the detailed simulation model of photovoltaic power generation system can accurately reflect the grid connection process of photovoltaic power generation system.


Introduction
In recent years, with the traditional energy shortage and environmental problems become increasingly serious, and the rapid development of clean energy and distributed power generation technology, photovoltaic, fan and other new energy distributed power generation technology in the proportion of distribution network increased dramatically. Among them, the photovoltaic power generation system because of its flexible application, convenient and so on, in the distribution network has a very good application prospects. The modeling and simulation of the grid-connected process of the photovoltaic power generation system has a guiding effect on the actual photovoltaic power generation system. Therefore, it is very important to establish a realistic simulation model of PV power generation system for the development of PV power generation system. Researchers at home and abroad have carried out some research on the modeling and simulation of photovoltaic power generation system. The photovoltaic power generation model is reviewed in [1]. Literature [2] ~ [5] carried out preliminary modeling work on PV array and MPPT. In the literature [6], the characteristics of PV arrays under different conditions were analyzed. The control strategies of the inverter are discussed in [7] ~ [10]. Among them, the control strategy of PQ, V / f and Droop is the focus of these four literatures.

Mathematical Model of Photovoltaic System and Simulink Model
As shown in Figure 1, the solar photovoltaic array of photovoltaic power generation system of light into DC power, the boost circuit MPPT controller, and then through the DC / AC inverter control strategy into alternating current through the filter and isolation transformer into the power grid. The detailed Matlab / Simulink model of each part of the grid-connected model of the photovoltaic power generation system will be established in order to carry out the next step-by-step process simulation study.

Mathematical model of photovoltaic array
Based on the theory of electronics, photovoltaic panels are affected by light to produce photovoltaic effects, solar energy into electrical energy. The equivalent circuit diagram of the PV array is shown in the following figure: In Figure 2, is the current generated by the photovoltaic effect, is the current flowing into the diode, is the photovoltaic array capacitance, is the photovoltaic parallel resistance, is the photovoltaic series resistance, the photovoltaic array equation:

Solar energy
Formula: q is charge, K is Boltzmann constant, T is temperature. Since some of the parameters in (1) are difficult to determine, the mathematical model commonly used in the project is: In the m Formula Conside formula (2) Formula is the of is 1  As can be seen in Figure 4, if you want the photovoltaic inverter in the photovoltaic power generation system to output the maximum power, the voltage needs to take the red dot position in the figure. Since the light intensity S and the photovoltaic array temperature T have a significant effect on the curve, in practice, S and T are in a state of constant change in general. Therefore, need to constantly track the maximum power point to ensure that the photovoltaic power generation system to maximize the output power.

MPPT controller algorithm
The maximum power MPPT controller research tracking algorithm commonly used constant voltage tracking method (Constant Voltage Tracking CVT (Perturbation), And Observation interference observation method referred to as P&O), the incremental conductance method (Incremental Con-ductance method referred to as INC), the incremental conductance method based on variable step size gradient. In this paper, the MPPT controller is designed based on the incremental conductivity method. The overall flow of the incremental conductivity algorithm is shown in the following figure:

Read the initial voltage Ud and the initial Current Id
Read the perturbed voltage Ud+1 and the perturbed current Id+1 Calculate the voltage change value ∆U and the current change value ∆I ∆U=0？ ∆I=0？ -∆I/∆U=I/U？

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Photo
In the M and dragge isolation tr simulation figure 10.   As shown in Fig. 13, the oscilloscope shows the waveform time: 251.001s to 291.001s, which is the PV output voltage current and the grid current change during the PV stabilization process. It can be seen from the figure that the photovoltaic output voltage and the grid current are not changed obviously, and the PV output current has obvious change process. Affected by the inverter, three-phase current amplitude in a period of time to stabilize in a smaller value, after the A-phase current amplitude in the negative half-axis has a significant fluctuation after a slight increase and stability of about 10s, and then instantaneous increase and stability of about 20s, and then gradually increased 10s after the reduction to 10s before the amplitude, and finally gradually Increase and stabilize.

Comparison of simulation results
By comparing the results of the two simulations, we can see that Simulink simulation of the photovoltaic output current stability faster, but the process and digital analog hybrid simulation results are basically the same. In addition, the simulation results of grid connected PV system are consistent with the simulation results of the digital analog hybrid simulation. Therefore, the detailed model of the photovoltaic power generation system described in this paper can accurately realize the simulation of the real process of the photovoltaic power generation system.

Conclusion
In this paper, Simulink modeling is carried out for the detailed model of photovoltaic power generation system, and the process of grid connection of photovoltaic power generation system is simulated. Based on the distribution network digital analog hybrid simulation management system, the digital analog hybrid simulation of the grid -connected process of the photovoltaic power generation system was carried out. By comparing the waveforms of the two simulation results, the feasibility of the detailed model of the photovoltaic power generation system described in this paper is proved.