In the grid-connected photovoltaic power generation system, the grid-connected inverters are roughly divided into centralized inverters, string inverters (including bidirectional energy storage inverters) according to the different access methods of photovoltaic modules or square arrays. ) and micro (component) inverters of category 3. Figure 1 is a schematic diagram of the access methods of various grid-connected inverters.
The characteristic of the centralized inverter is just like its name, which is to centrally connect the square matrix composed of multi-channel battery module strings into a large inverter. Generally, several battery components are connected in series to form a string, and then all the strings are combined through the DC combiner box, and one or several channels are collectively output and then input to the centralized inverter, as shown in Figure 1(a). When the primary confluence cannot meet the requirements of the inverter’s input characteristics and the number of input channels, the secondary confluence must be carried out through the DC power distribution cabinet. The capacity of this type of grid-connected inverter is generally 300-2500KW.
The main features of the centralized inverter are as follows.
① Since the photovoltaic array is input to the grid-connected inverter after one or two confluence, it is impossible for the inverter’s maximum power tracking (MPPT) system to monitor the working status and operation of each photovoltaic string. That is to say, it is impossible to make each group of strings reach their respective MPPT modes at the same time. When the photovoltaic array is in poor working condition due to uneven illumination, partial shading, etc., it will affect the inverter efficiency of all the strings and the entire system.
② The centralized inverter system has no spare capacity, and the reliability of the entire system is completely limited by the inverter itself. If it fails, the entire system will be paralyzed, and the system can only be repaired on site, and the repair time will be long.
③ The centralized inverter is usually a high-power inverter, and its related safety technology costs a lot.
④ The centralized inverter is generally large in size and heavy in weight. Special tools, professional machinery and hoisting equipment are required for installation, and the inverter also needs to be installed in a special power distribution room.
⑤ The DC side of the centralized inverter needs more DC cables, and the cable cost and cable power loss are relatively large.
⑥ The solar power system using the centralized inverter can be connected to the grid centrally, which is convenient for management. Ideally, centralized inverters also provide high efficiency at relatively low input costs.