The classification and introduction of photovoltaic systems generally divide photovoltaic systems into independent systems, grid-connected systems and hybrid systems. If the photovoltaic power supply system is divided according to the application form, application scale and load type of the photovoltaic system, the photovoltaic system can be divided into the following six types: small solar power supply system (Small DC); simple DC system (Simple DC); Large-scale solar power supply system (Large DC); AC, DC power supply system (AC / DC); grid connection system (UTIlity Grid Connect); hybrid power supply system (Hybrid); The working principle and characteristics of each system are explained below. 
Type solar power system (Small DC)
The characteristic of this system is that there is only DC load in the system and the load power is relatively small. The whole system is simple in structure and easy to operate. Its main purpose is a general household system, the load is a variety of civilian DC products and related entertainment equipment. For example, this type of photovoltaic system has been widely used in remote areas in the northwest of China. The load is DC energy-saving lamps, tape recorders and televisions. It is used to solve the basic lighting problems of households in areas without electricity.
Simple DC system (Simple DC)
The characteristic of this system is that the system load is a DC load and there is no special requirement for the use time of the load. The load is mainly used during the day, so no battery is used in the system, and no controller is required. The system has a simple structure, and directly uses solar solar cell components to supply power to the load, eliminating the loss caused by the storage and release of energy in the storage battery, and the energy loss in the controller, and improving the utilization efficiency of solar energy. It is often used in photovoltaic water pump systems, some temporary equipment electricity during the day and tourism facilities. Figure 4-4 shows a simple DC photovoltaic pump system. This system has been widely used in the areas of pure tap water for drinking in developing countries, and has produced good social benefits.
4.2.3. Large-scale solar power system (Large DC)
Compared with the above two photovoltaic systems, this photovoltaic system is still suitable for DC power supply systems, but this solar photovoltaic system has a large load power. In order to ensure a stable power supply to the load, its corresponding system scale is also Larger, it needs to be equipped with a larger array of solar cell modules and a larger battery pack. It is often used in communications, telemetry, monitoring equipment power supply, rural centralized power supply stations, navigation beacon lighthouses, street lights and other fields. In China's "light project" implemented in the western region, some rural photovoltaic power stations constructed in some areas without electricity use this form; the communication base stations built by China Mobile and China Unicom in remote areas without power grid also use this type of photovoltaic system. powered by.
AC, DC power supply system (AC / DC)
Different from the above three solar photovoltaic systems, this photovoltaic system can provide power for both DC and AC loads. In the system structure, there are more inverters than the above three systems, which are used to convert DC power to AC power to meet AC load requirements. Usually the load power consumption of such a system is relatively large, so the scale of the system is also large. It is used in some communication base stations with both AC and DC loads and other photovoltaic power plants with AC and DC loads.
Grid-connected system (UTIlity Grid Connected)
The biggest feature of this photovoltaic system is that the direct current generated by the solar cell components is converted into alternating current that meets the requirements of the mains power grid through the grid-connected inverter, and then directly connected to the public grid. The power generated by the photovoltaic square array in the grid-connected system is supplied to the AC load. In addition, excess power is fed back to the grid. On rainy days or nights, when the solar cell module does not produce electrical energy or the electrical energy produced cannot meet the load demand, it is supplied by the power grid. Because the electrical energy is directly input into the grid, the configuration of the storage battery is eliminated, and the process of storage and release of the storage battery is eliminated. The power generated by the photovoltaic square array can be fully utilized to reduce energy loss and reduce the cost of the system. However, a dedicated grid-connected inverter is required in the system to ensure that the output power meets the requirements of the grid power for voltage, frequency and other electrical performance indicators. Because of the inverter efficiency problem, there will still be some energy loss. This kind of system can usually use the city power and solar solar cell module array as the power source of the local AC load in parallel, which reduces the load shortage rate of the entire system. And the grid-connected photovoltaic system can play a peak-shaving role on the public power grid. However, the grid-connected photovoltaic power supply system, as a decentralized power generation system, will have some adverse effects on the grid of the traditional centralized power supply system, such as harmonic pollution and island effect.
Hybrid power supply system (Hybrid)
In addition to the array of solar solar cell modules, this solar photovoltaic system also uses a fuel generator as a backup power source. The purpose of using a hybrid power supply system is to comprehensively utilize the advantages of various power generation technologies and avoid their respective shortcomings. For example, the advantages of the above independent photovoltaic systems are less maintenance, and the disadvantage is that the energy output depends on the weather and is unstable. Compared with a single energy independent system, a hybrid power supply system that comprehensively uses diesel generators and solar cell components provides less energy dependence on the weather. Its advantages are:
 The use of hybrid power supply system can achieve better utilization of renewable energy. Because renewable energy is variable and unstable, the system must be designed according to the period of least energy production. Because the system is designed according to the worst case, the capacity of the system is too large at other times. The excess energy generated during the period of the highest peak of solar irradiation cannot be used and is wasted. As a result, the performance of the entire independent system is reduced. If the worst month situation is very different from other months, it may result in wasted energy equal to or even exceeding the design load demand.
 Has high system practicality. In the independent system, the change and instability of the renewable energy will cause the system to fail to meet the load demand, that is, there is a load power shortage. Using a hybrid system will greatly reduce the load power shortage rate.
 Compared with a single-use diesel generator system, it has less maintenance and uses less fuel.
 Higher fuel efficiency. In the case of low load, the fuel utilization rate of the diesel engine is very low, which will cause fuel waste. In the hybrid system, comprehensive control can be performed to make the diesel engine work near the rated power, thereby improving fuel efficiency.
 Load matching is better. After using the hybrid system, because the diesel generator can provide greater power immediately, the hybrid system can be applied to a wider range of load systems, such as the use of larger AC loads, shock loads, etc. It can also better match the load and the power generation of the system, as long as the backup energy is turned on during the peak load period. Sometimes, the size of the load determines the need to use a hybrid system. Large loads require large currents and high voltages. If you just use solar energy, the cost will be high.
But the hybrid system also has its own disadvantages:
 Control is more complicated. Because multiple energy sources are used, the system needs to monitor the working conditions of each energy source, deal with the mutual influence of each sub-energy system, and coordinate the operation of the entire system. Processing chip for system management.
 The initial project is relatively large. The design, installation, and construction of hybrid systems are larger than independent projects.
 Need more maintenance than standalone systems. The use of the oil machine requires a lot of maintenance work, such as replacing the oil filter, fuel filter, spark plug, etc., and adding fuel to the fuel tank.
 Pollution and noise. Photovoltaic systems are clean, energy-free, and non-emissions, but because diesel engines are used in hybrid systems, this inevitably produces noise and pollution.
Many communication power supplies and civil aviation navigation equipment power supplies in remote areas without electricity, because of the high power requirements, all use a hybrid system to achieve the best cost performance. Many rural photovoltaic power stations constructed in Xinjiang and Yunnan in China use hybrid light / firewood systems.
Hybrid power supply system (Hybrid)
With the development of the solar photovoltaic industry, there has been a grid-connected hybrid power supply system that can comprehensively utilize solar photovoltaic arrays, mains power and backup oil machines. This kind of system is usually integrated with the controller and inverter, using a computer chip to comprehensively control the operation of the entire system, comprehensive utilization of various energy sources to achieve the best working state, and can be equipped with a battery. Further improve the system's load power supply guarantee rate, such as AES's SMD inverter system. The system can provide qualified power for local loads and can work as an online UPS (uninterruptible power supply). It can supply power to the power grid and can also obtain power from the power grid. It is a bidirectional inverter / controller. The working mode of the system is to work the city power and photovoltaic power in parallel. For the local load, if the power generated by the solar cell module is enough for the load, it will directly use the power generated by the solar cell module to supply the load demand. If the electrical energy generated by the solar cell assembly exceeds the immediate load demand, the excess electrical energy can also be returned to the grid; if the electrical energy generated by the solar cell assembly is not enough, the mains electricity will be automatically activated to use the mains electricity to supply the local load demand; and, When the local load power consumption is less than 60% of the rated mains capacity of the SMD inverter, the mains will automatically charge the battery to ensure that the battery is in a floating charge state for a long time; if the mains fails, the mains power outage or mains power If the quality of the power supply is unqualified, the system will automatically disconnect the mains power and switch to an independent working mode. The storage battery and inverter provide the AC power required by the load. Once the mains power returns to normal, that is, the voltage and frequency return to within the normal state, the system will disconnect the battery and change to the grid-connected mode to work, and the mains power supply. In some grid-connected hybrid power supply systems, system monitoring, control, and data acquisition functions can also be integrated into the control chip.
Characteristics of solar photovoltaic system Solar photovoltaic power generation system has its own unique characteristics:
â‘ No danger of exhaustion;
â‘¡ Absolutely clean (no pollution, except battery);
â‘¢ Not restricted by the geographical distribution of resources;
â‘£ You can generate electricity at the place where you use electricity;
⑤ High energy quality;
â‘¥ The user is easy to accept emotionally;
⑦ The time it takes to obtain energy is short;
⑧ The power supply system works reliably.
The disadvantages are:
â‘ The irradiated energy distribution density is small;
â‘¡ The energy obtained is related to the meteorological conditions such as four seasons, day and night, and overcast;
â‘¢ The cost is relatively high. Some of the above characteristics determine that the photovoltaic power supply system has its unique advantages and related constraints in the application.
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