Solar energy is a clean and efficient new energy source, and its utilization is related to the promotion of sustainable energy development strategies. Its development has attracted widespread attention from all sectors of society. Grid connected power generation is an inevitable direction for large-scale solar power generation, and solar inverters are the core components of photovoltaic grid connected power generation systems. The utilization and development of solar energy promote the development of photovoltaic grid connected systems, and promote the grid connected power supply of solar power generation.
1.Solar grid connected inverter
1.1 Principles of solar inverters
Solar inverters are an important part of photovoltaic grid connected systems, achieving the conversion of direct current output from photovoltaic cells to three-phase alternating current, and meeting the requirement of alternating current energy being in the same frequency and phase as the connected grid voltage. The regulation of AC output from solar inverters is related to its control. The grid connection of photovoltaic systems needs to ensure that the phase of the grid connected current is synchronized with the voltage of the grid, and the regulation of active and reactive output of the system is achieved by controlling the grid connected current.
1.2 Classification and topological structure of solar grid connected inverters
There are various inverters used in photovoltaic grid connected systems, and their topological structure is mainly a full bridge structure. The solar grid connected inverter can be divided into low-frequency link grid connected inverter and high-frequency link grid connected inverter according to the design and selection of isolation transformers.
(1) Low frequency link grid connected inverter
Due to limitations in technological development, the early solar inverters used in China were low-frequency link grid connected inverters. It has multiple advantages in use, simple structure, high work efficiency, and easy to control operation. However, its high cost, low integration, and high noise limit its use. The circuit structure is shown in Figure 1.
(2) High frequency link grid connected inverter technology
The research and development of high-frequency grid connected inverters has promoted the development of solar inverters. The use of high-frequency transformers has achieved reliable isolation of the system, and the ratio adjustment of voltage gain has achieved optimal tracking of maximum power, while meeting the range of changes in system DC current. In fact, the highly integrated inverter circuit has been realized, and the circuit structure is shown in Figure 2.
2. Controller control strategy and analysis
The solar inverter control center used for grid connection of photovoltaic power generation systems includes monitoring and control of current and power. The output current is mainly controlled by adopting various optimization control strategies to track and control the given current. The article adopts PWM current optimization control method. The power tracking control of solar inverters is achieved by tracking the maximum power point.
2.1 Maximum Power Tracking MPPT
The current photovoltaic power generation system widely adopts maximum power point tracking as the system power point control method, and practice has proven that the system operates stably and the power tracking effect is good. Photovoltaic cells are a nonlinear component, therefore the output characteristics of photovoltaic arrays are also nonlinear, and the battery output is easily affected by environmental factors such as ambient temperature and light intensity. Theoretical analysis shows that photovoltaic cells can work reliably under different output voltages, but they only appear at a certain output voltage. The working point at which photovoltaic cells are at their maximum output power is the maximum power point of the photovoltaic power supply system. It is necessary to track the maximum power point (MPPT) during the actual operation of batteries, which not only helps to maximize the efficiency of photovoltaic cells, but also improves the overall operational efficiency of photovoltaic grid connected systems. MPPT control essentially detects the output power of a photovoltaic system in real-time through certain optimization control algorithms, in order to achieve real-time tracking of the maximum power operating point and achieve the maximum power output effect.
2.2 Grid connection control
At present, the control methods for AC output of solar inverters can be divided into voltage control method and current control method. Current control can simultaneously ensure the response speed and stability requirements of the system, and can be classified into:
(1) Current hysteresis tracking method
The most commonly used PWM solar inverter with current tracking controller is the current hysteresis tracking control method. The current hysteresis tracking comparison method is used for real-time control, with fast response and no need for chopping. The output voltage does not contain specific frequency harmonics.
(2) Timing controlled current hysteresis tracking method
The current hysteresis tracking method with timing control can effectively prevent the switching frequency of power electronic switching devices from being too high, but its disadvantage is that the compensation current following error is not fixed.
(3) Triangular wave comparison method for real-time current
The triangular wave comparison method of real-time current has a large error in following the current, contains harmonics in the output voltage, and the amplifier gain is limited. The current response is slower compared to the current hysteresis tracking control.
At present, the current hysteresis tracking method is widely used to control DC/AC solar inverters. This control hardware circuit is simple, but it belongs to real-time control mode, with fast current response and no need to use carrier waves. Therefore, the output voltage waveform does not contain specific frequency harmonic components, and the stability is also good.
3. Detection and control of islanding effect
Distributed power systems generally suffer from islanding effects, as do photovoltaic power generation projects. The islanding effect refers to the sudden failure of the power supply network during normal power supply or the need for power outage maintenance. The system automatically trips, and each distributed grid connected power supply fails to monitor the power outage status and take measures in a timely manner, resulting in the power supply breaking away from the mains network and operating independently, ultimately forming a state where it supplies power to the load in the surrounding small area. This is the state where distributed power supply breaks away from the control of the power company and forms self-sufficient islanding power supply. From the perspective of electricity safety and power quality, islanding effect is not allowed. When islanding occurs, it is necessary to quickly and accurately cut off the solar grid connected inverter, which leads to research on detecting and controlling islanding effect.
The anti islanding control strategy based on active power disturbance achieves periodic control of the output current of the solar inverter, and detects power imbalance in the event of a power outage in the power grid system. The detection system exceeds the overvoltage/undervoltage protection threshold, thereby detecting the islanding effect of the photovoltaic system.
4. Simulation experiment
A simulation model of the voltage source solar grid connected system inverter and its control was established using the SimPower Systems functional module of simlink in MATLAB 2010, and a photovoltaic grid connected experimental device with a rated power of 2kW was completed. The input voltage range of the system is 100-300V, the output power is about 1.5kW, and the AC frequency is 50Hz. The current of low-power photovoltaic grid connection is the same frequency as the voltage connected to the grid, and the operating power factor of the system is 0.95.
5. Conclusion
We have studied low-power solar grid connected inverters. The principle and classification of solar grid connected inverters were introduced, and the system was designed using maximum power tracking control and anti islanding effect control during operation. The operation of a low-power solar grid connected inverter was simulated using MATLAB, and it was shown that the research system has good dynamic response and grid connection efficiency. The experiment proves that the designed system has good operational performance and stable and reliable operation.