Photovoltaic power generation system composition
Solar photovoltaic power generation system-generally consists of photovoltaic cell array, combiner box, DC power distribution cabinet, battery pack, battery charge and discharge controller, inverter, AC power distribution cabinet, power generation monitoring system, solar tracking system, environmental monitoring system, It is composed of lightning protection system and other equipment.

Inverter is a converter device that converts DC power into 50Hz AC power, and it is one of the core equipment of photovoltaic power generation system. The direct current generated by the photovoltaic panel generally needs to be converted into alternating current through an inverter, which is supplied to the alternating current load or integrated into the alternating current grid.

According to different power output goals, inverters can be divided into off-grid inverters, on grid inverters, and parallel-off-grid inverters. The AC output of the off-grid inverter is not connected to the grid. The solar cell modules store the generated electrical energy in the battery through the DC/DC converter, and then the off-grid inverter converts the DC power in the battery into amplitude frequency The stable alternating current is used for the load. As shown in Figure 1.

Figure 1 Off-grid photovoltaic power generation system

on grid inverters directly invert the direct current from solar panels into high-voltage power to feed into the grid, and battery storage is not a necessary intermediate link. as shown in picture 2.

Figure 2 on grid photovoltaic power generation system

On grid and off-grid inverters are inverters that can operate both on-grid and off-grid. They are mainly suitable for micro-grid systems, especially household photovoltaic micro-grid systems. When the external power grid fails, the internal and external power grids will be disconnected, and the off-grid inverter will immediately switch from on grid operation to off-grid operation to ensure the stable power supply of the internal grid. When the external power grid is restored, the internal and external power grids are reconnected, and the off-grid inverter switches to on grid operation at the same time.

Classification of photovoltaic on grid inverters
There are many implementation schemes for on grid inverters, which are mainly divided into voltage type and current type. Voltage-type on grid inverter schemes are more common. This is mainly because the energy storage components in voltage-type inverters are capacitors. Compared with the inductance of energy storage components in current-type inverters, the energy storage efficiency and the volume of energy storage devices are It has obvious advantages in terms of price, price, etc. The drive control of the fully-controlled power device is relatively simple and the control performance is relatively good. Photovoltaic on grid inverters can be classified according to topology,isolation mode,output phase number,power level,power flow direction,and photovoltaic string mode.

According to the classification of topological structure, the topological structures currently used include: full-bridge inverter topology, half-bridge inverter topology, multi-level inverter topology, push-pull inverter topology, forward inverter topology, flyback inverter topology, etc. Among them, high-voltage and high-power photovoltaic on grid inverters can use multi-level inverter topology, medium-power photovoltaic on grid inverters mostly use full-bridge and half-bridge inverter topologies,and low-power photovoltaic on grid inverters use forward,Flyback inverter topology.

According to the classification of isolation methods, it includes two types: isolated and non-isolated. Among them, isolated on grid inverters are divided into power-frequency transformer isolation and high-frequency transformer isolation. At the beginning of the development of photovoltaic on grid inverters, industrial However, due to the obvious shortcomings in volume, weight, and cost, the on grid inverter of high-frequency transformer isolation has developed rapidly in recent years. Non-isolated on grid channel converters have gradually gained recognition for their high efficiency, simple control and other advantages. Currently, they have begun to be promoted and applied in Europe,but they need to solve key issues such as reliability and common mode current.

According to the number of output phases, it can be divided into two types: single-phase and three-phase on grid inverters. Generally, single-phase mode is generally used for small and medium power occasions, and three-phase on grid inverters are mostly used for high power occasions. According to the classification of power level, it can be divided into small power on grid inverters with power less than 1kVA, medium-power on grid inverters with power level 1~50kVA and high-power on grid inverters with power above 50kVA. Since the development of photovoltaic on grid inverters, the most mature development is the medium-power on grid inverter. At present, commercial and commercial mass production has been achieved, and the technology is becoming mature. The future development of photovoltaic on grid inverters will be parallel in the two directions of low-power micro-inverters, that is, photovoltaic module integrated inverters and high-power on grid inverters. Micro-inverters have unique advantages in photovoltaic building integrated power generation systems, urban residential power generation systems, and small and medium-sized photovoltaic power stations. High-power photovoltaic on grid inverters have obvious advantages in large-scale photovoltaic power stations, such as desert photovoltaic power stations.

According to the classification of power flow, it is divided into two types: unidirectional power flow on grid inverter and bidirectional power flow on grid inverter. One-way power flow on grid inverters are only used for on grid power generation. Two-way power flow on grid inverters can be used for on grid power generation and can also be used as rectifiers to improve grid voltage quality and load power factor. The two-way power flow on grid converter began to gain attention in 2009, and it is one of the future development directions. The future photovoltaic on grid inverters will integrate on grid power generation, reactive power compensation, active filtering and other functions. They can realize on grid power generation when there is sunlight during the day, and realize reactive power compensation and active filtering when power is used at night.

Photovoltaic on grid inverters can be divided into string inverters, centralized inverters and micro inverters according to the combination of photovoltaic panels. This is the most commonly used classification method in the application field.