This chapter focuses on the topological structure of the DC microgrid system integrating photovoltaic and energy storage, and conducts mathematical analysis of the equivalent circuit models of the photovoltaic panel power generation module, the battery module, and the inverter required for grid connection. It also builds and simulates each module using MATLAB/Simulink.
- Photovoltaic Unit Module Modeling and Simulation
- Photovoltaic Cell Simulation Model
- The photovoltaic power generation control system is mainly composed of structures such as photovoltaic panels (PV), current conversion devices (DC/DC), and inverter controllers (DC/AC). The steps of photovoltaic power generation include converting solar energy into electrical energy through the photovoltaic array, delivering the energy to the DC bus through the Boost converter, stabilizing the output by connecting energy storage elements such as batteries or supercapacitors, and finally converting AC and DC through the inverter and integrating into the power grid.
- The internal structure of the photovoltaic panel is formed by doping different impurity elements in crystalline silicon to form two large semiconductors of different types, forming a PN junction. When the single photovoltaic cell is irradiated from the back, electrons move towards the N region, and holes move towards the P region, generating a potential difference. When a load is connected, current is generated, converting light energy into electrical energy.
- Based on the equivalent circuit diagram of the photovoltaic cell, the I-V and P-V characteristic curves of the photovoltaic panel under different Light intensity and temperatures are analyzed by controlling variables in MATLAB/Simulink. The results show that the output power of the photovoltaic panel is positively correlated with the Light intensity and is more significantly affected by the Light intensity than the temperature.
- Maximum Power Point Tracking
- The output characteristics of the photovoltaic cell change with external environmental factors, and there is a maximum power point under each set of conditions. The goal of maximum power point tracking (MPPT) technology is to ensure that the photovoltaic cell operates at the maximum power point.
- The basic idea of MPPT is to adjust the duty cycle D by continuously comparing the internal impedance of the photovoltaic cell with the equivalent impedance on the load side, so that the two are equal, and the photovoltaic cell can output the maximum power.
- The disturbance observation method is used to track and control the photovoltaic output power to improve the energy utilization efficiency. The control process is based on the rules that when the voltage and power change in the same direction, the voltage should be increased; when they change in different directions, the voltage should be decreased.
- Photovoltaic Power Generation System Simulation Analysis
- The photovoltaic power generation system is improved by modeling the photovoltaic cell as a controlled power source. The simulation results show that the relationship between the output power of the photovoltaic array and the Light intensity is proportional, and the system outputs a stable rated power when the Light intensity is at the rated value. The DC side voltage fluctuation range is small, and the designed MPPT control is effective under different Light intensity.
- Photovoltaic Cell Simulation Model
- Grid-Connected Inverter Module Modeling and Simulation
- Structure of the Grid-Connected Inverter
- The inverter has the functions of direct AC/DC conversion, maximizing the performance of solar cells, and system fault protection. According to the project requirements, 8 sets of 110kW photovoltaic grid-connected inverters are used.
- The filter of the inverter is diverse, and the LC filter is selected in this project. The filter’s order determines the drop speed of the transition region.
- Control Strategy of the Grid-Connected Inverter
- The traditional control methods of single-phase photovoltaic inverters include single-loop control, double-loop control, and repetitive control. Single-loop control is simple and easy to implement but has poor anti-interference ability. Double-loop control forms an internal current loop and an external voltage loop, improving the stability and response speed of the control system but is susceptible to nonlinear signal disturbances. Repetitive control has good suppression ability for fundamental and harmonic disturbances and can improve the quality of the grid-connected current.
- In this project, double-loop control is used to control the grid-connected inverter, forming a voltage outer loop and a current inner loop to enhance the system stability and response speed.
- Mathematical Model of the Grid-Connected Inverter
- Based on the basic LC filter structure, the relationship between the inverter outlet voltage, output voltage, inductance current, and output current is obtained.
- The parameters of the LC filter are designed according to requirements such as the inductance voltage drop, current ripple, and power factor.
- Modeling and Simulation of the Grid-Connected Inverter
- A three-phase LC full-bridge inverter model is built using MATLAB/Simulink. The simulation results show that the DC side voltage can quickly stabilize, and the output current lags the grid voltage by 90°, enabling the grid-connected inverter to operate stably.
- Structure of the Grid-Connected Inverter
- Battery System Modeling and Simulation
- Working Principle of the Battery Unit
- The battery plays a crucial role in the operation of the optical storage integrated microgrid, and stabilizing the bus voltage within a certain range is important. By controlling the battery module, the stability of the microgrid system can be improved.
- Lead-acid batteries are widely used in photovoltaic microgrid systems due to their low cost, large capacity, and mature technology. The main parameters of the battery include terminal voltage, battery capacity, depth of discharge, and state of charge.
- The equivalent circuit of the battery is represented by a controlled voltage source, and the open-circuit voltage and output voltage are expressed by equations considering the influence of the internal resistance.
- Battery System Simulation Analysis
- Based on the Battery equivalent circuit model, a simulation model is built in MATLAB/Simulink. Using a battery as the power unit, the simulation results show that the battery can discharge normally, and the voltage and SOC change smoothly. When the input power increases, the battery charges, and the voltage rises. The established energy storage unit model and the implemented energy storage control strategy are feasible.
- Working Principle of the Battery Unit
In summary, this chapter analyzes and studies the selected energy storage battery type, inverter architecture, and mathematical models of each system in the microgrid. It realizes MPPT control of the photovoltaic system, charge and discharge control of the battery, and inverter control. The corresponding modules are built in the MATLAB/Simulink software, and the simulation of the optical storage integrated system without grid connection is carried out. The simulation results verify the effectiveness of the system and the feasibility of the control strategy.
The following is a summary of the key information in a table format:
| Module | Key Points |
|---|---|
| Photovoltaic Unit Module | Photovoltaic Cell Simulation Model, Maximum Power Point Tracking, Photovoltaic Power Generation System Simulation Analysis |
| Grid-Connected Inverter Module | Structure, Control Strategy, Mathematical Model, Modeling and Simulation |
| Battery System | Working Principle, Simulation Analysis |