Application characteristics of string inverters and multi-string inverters

1. String inverter

The string inverter is based on the concept of modularity, which inputs each photovoltaic string in the photovoltaic array into a designated inverter, multiple PV strings and inverters are modularly combined, and all inverters are connected to the grid in parallel at the AC output, as shown in Figure 1(b). The capacity of this type of inverter is generally 1-10KW, and the main features of the string inverter are as follows.

What are the main application characteristics of inverters
Figure 1 – Schematic diagram of the access methods of various grid-connected inverters

①The inverter of each string has its own MPPT function and island protection circuit, which is not affected by the performance difference and local shading of the battery components between the strings, and can handle different orientations and different types of battery components, improve the overall efficiency of the power generation system, and is very suitable for application in distributed photovoltaic power generation systems.

②The string inverter system has a certain redundant operation function. Even if a PV string or a certain inverter fails, it will only reduce the system capacity, which can effectively reduce the power loss caused by the power failure of the entire system caused by partial failure, and improve the stability of the system.

③ The string inverter system can be scattered and connected to the grid nearby, which reduces the use of DC cables, thereby reducing the cost of system cables and the power loss of cables.

④The string inverter is small in size, light in weight, easy to carry and install, does not require professional tools and equipment, nor does it require a special power distribution room. DC line connection does not require DC combiner boxes and DC distribution cabinets, etc.

⑤ The string inverters are scattered in the solar photovoltaic system, which puts forward relatively high requirements for information communication technology. However, with the continuous development of communication technology and the continuous emergence of new communication technologies and methods, this problem has been basically solved.

2. Multi-string inverter

Multi-string inverter is to obtain the advantages of string inverter and centralized inverter at the same time. On the basis of the string inverter, a multi-string input mode is formed, and the system enables several groups of strings associated with it to work together without affecting each other, thereby producing more electric energy. This form of multi-string inverter uses DC-DC converters to connect multiple strings to a common inverter system, but can still complete the individual MPPT functions of the strings, thus providing a complete solution that is more economical than ordinary string inverter systems

The multi-string inverter system not only reduces the number of inverters, but also enables strings with different ratings (such as different rated power, different sizes, different manufacturers and different number of modules), strings with different orientations, strings with different tilt angles and different shades to be connected to a common inverter. At the same time, each group of strings works at their respective maximum power points, so as to minimize the loss of power generation caused by the difference between groups of strings, and the whole system works in the best efficiency state. The capacity of multi-string inverters is generally 10-80KW.

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