Because of the limitation of the low level of science and technology and the high cost of photovoltaic power generation, the application effect of photovoltaic power generation system is unsatisfactory for a long time, which affects its promotion and development to a large extent. Therefore, in order to promote the improvement and development of photovoltaic power generation and the improvement of photovoltaic power generation technology, many people have improved the controller of independent photovoltaic power generation system, which has a very important value for the development of photovoltaic technology. This paper first introduces the types of photovoltaic power generation system, and then expounds the working principle and application of the stand-alone photovoltaic power generation system charge controller. In the twenty-first century, the global resource shortage is becoming more and more serious, which has become a global problem. Therefore, every country in the world began to put forward the development strategy of protecting the environment and saving resources, and vigorously advocated the use and development of renewable energy. Among so many renewable resources, solar energy is the most representative and most valuable for development and utilization. Many countries in the world have also studied the use and development of solar energy, and have achieved good results. Among them, the most effective and successful research is the design of photovoltaic power generation system, which has been widely applied in many industries. However, due to the limitation of low technology level and the high cost of photovoltaic power generation, the application effect of photovoltaic power generation system is unsatisfactory for a long time, which affects its promotion and development to a large extent. Therefore, in order to promote the improvement and development of photovoltaic power generation and the improvement of photovoltaic power generation technology, many people have improved the controller of independent photovoltaic power generation system, which has a very important value for the development of photovoltaic technology. Photovoltaic power generation system is a system that converts the unstable power generated by solar photovoltaic cells into stable AC power through a controller. According to the application situation, photovoltaic power generation system can be divided into three categories: independent, grid-connected and hybrid photovoltaic power generation systems. Independent photovoltaic power generation systems usually include photovoltaic arrays, lead-acid batteries, DC/AC converters, off-grid inverters and loads. Because it is not connected in parallel with the grid, it is called an independent or off-grid system. Batteries are needed to store the excess power generated by the photovoltaic array. When the light is insufficient or there is no light at all, the batteries can discharge, and the load can be supplemented to supply power. Independent photovoltaic systems are usually used in nomadic or mountainous areas because it is difficult to connect the power grid in these areas.
In addition, stand-alone photovoltaic power generation system is also widely used in satellite radio and television, mobile communication base stations and other areas. The structure is shown in Figure 1. Grid-connected photovoltaic power generation system refers to the parallel connection of power grid and power generation system, which usually includes DC, AC converter, photovoltaic array, load and grid-connected inverters. Because in parallel with the grid, there is no need for the power generation system to use batteries to store excess energy, but to use grid-connected inverters to connect the energy and the grid, and the power generation system is the auxiliary of the grid.
In this system, the core is grid-connected inverters. In order to ensure that the grid does not interfere with the grid, grid-connected inverters must maintain the same frequency and phase of AC power with the grid. Compared with stand-alone and grid-connected photovoltaic power generation systems, hybrid photovoltaic power generation system has more than one set of generators. If the sun does not shine enough or seriously exceeds the load, the system can not supply the required electricity. At this time, starting the generating units can build a power supply guarantee for the system. Generally speaking, photovoltaic charging system includes not only solar panels, inverters, batteries, but also controllers and charging controllers. The photovoltaic array converts the solar energy into DC power, and then connects it to the charging controller.
After tracking with the maximum power, the controller can charge the battery or connect it to the DC input of the inverter, and it can also provide external DC load. In the inverters, when DC is converted to AC, the output is for AC load, as shown in Figure 2. Maximum power tracking of photovoltaic arrays. Because of the changing external environment, in order to maximize the utilization of the output power of the photovoltaic array, and then achieve the maximum power output of the charging system, it is necessary for the charging controller to track the maximum power point of the photovoltaic array.
Achieve the purpose of fast intelligent charging. Because of the lack of stability of the input power in photovoltaic power generation system, in order to meet the demand of load, if the input power of photovoltaic array is insufficient, it needs to be discharged by batteries. If the input power of photovoltaic array is enough, the batteries will be charged. Therefore, when the battery is in the state of continuous charging and discharging, the system can combine its capacity and discharging state, in different charging state, and then realize the role of fast intelligent charging of the battery.
It can protect the storage battery. In order to avoid excessive charging and discharging of batteries, the purpose of repairing batteries is realized by floating charging, and then the protection of batteries is realized, so that the service life of batteries can be prolonged. Communication function. The photovoltaic charging controller can communicate with the main controller of the photovoltaic power generation system. It can not only charge the current and voltage, but also realize the capacity and charging state of the battery, and adjust the output of the charging controller to ensure its working state. It has the function of indicating and alarming. According to the different working status of the charging controller, the photovoltaic charging controller uses the LED lamp to indicate and alarm. When it charges at the maximum power, the single lamp will be on; when it charges, the double lamp will be on; if it is charged floating, the three lights will be on. If the charging controller is in a fault state, it will beep to alarm, then the charging controller will stop running, combined with the different faults, the LED lights will appear different display, such as in the overcurrent state, there will be single light flashing; if in the overvoltage state, there will be double light flashing. The charging technology of the independent photovoltaic charging controller is affected by environmental factors. In the photovoltaic power generation system, the energy supplied by the photovoltaic array is very limited. In order to make full use of these limited energy, it is difficult to meet the demand of the system by charging the battery in the past way. Therefore, the intelligent charging technology which is consistent with the independent photovoltaic power generation system is tracking. The maximum power utilization of technology, so that solar energy can be converted to the maximum extent, and the converted electricity into the system. This paper introduces the common intelligent charging methods in photovoltaic power generation system.
Constant current charging. When the battery is charged, the accepted current is constant, and the output current is maintained by controlling the output voltage, mainly by two kinds of constant current charging methods: sectional charging and single charging. From the point of view of sectional mode, different stages should choose different currents, similar to constant voltage charging. It is important to select a reasonable junction point for conversion, mainly by the temperature rise and terminal voltage of the battery. For all batteries in series, constant current charging has the same amount of charge, and it will not cause the problem of insufficient charge as constant voltage charging.
It is also suitable for many batteries in series, and the more batteries in series, the better. In a single way, during the whole process of charging, the current of charging does not change. Both kinds of constant current charging have the advantages of low success rate and simple control, but their shortcomings are also obvious. According to the Mass curve, the current that the battery can withstand will decrease with the charging process.
Therefore, in order to avoid the occurrence of gas in the battery after charging, a relatively small current is usually used for single constant current charging. Constant voltage charging. The so-called constant-voltage charging refers to that when the battery is charged, the charging voltage of the battery will remain constant. When the battery is charged, the current will be larger. However, due to the urgent need of the charging process, the terminal voltage of the battery will rise, and at the same time, the current will gradually decrease. Generally speaking, in order to avoid excessive battery charging within a certain range, the charging voltage will be set, thermostatic element and then the charging current will be limited. Compared with constant current charging, the electrolytic water produced by constant voltage charging is relatively less.
When charging is about to be completed, because the charging current is not high, the battery overcharge can be avoided. The advantage of constant voltage charging is that it is easy to complete and control. However, when starting charging, because the terminal voltage of the battery is not high, the current is large and the gas evolution is serious, the active substance inside the battery will fall off, which will lead to the temperature rise of the battery, and it is easy to make the battery electrode plate bend, which directly causes the battery to be discarded. At the end of charging, because the current is too small and the charging time is longer, it will cause the battery to reduce the capacity of lead-acid battery because of insufficient charging for a long time.
Therefore, constant voltage charging is often used in batteries with relatively small Rongxiang and relatively low voltage. Two-stage charging. The charging mode is formed by utilizing the advantages of constant current or constant voltage charging and integrating them. At the beginning of charging, the battery can be charged quickly by using the limited constant current. When the terminal voltage of the battery has reached a certain value, the constant voltage is selected for charging. When the charging current slowly starts to decrease as the charging proceeds, it waits until the battery is fully charged. This kind of charging mode will not cause the problem of excessive current in the initial stage, nor will it occur the situation of too high voltage in the later stage of charging, which avoids a lot of gas evolution in the battery, reduces the temperature rise of the battery, and plays a role in protecting the safety of the battery.