1. System structure of centralized photovoltaic power station
For large-scale centralized photovoltaic power plants, according to the order difference of the inverter access system position, the inverter equipment selection of photovoltaic power generation system is divided into two kinds of inverter modes: centralized and string type (see Figure 2).
The system structure of the centralized inverter is to first convert solar energy into DC power from photovoltaic modules. Multiple modules form a photovoltaic string in series/parallel and are collected to the DC combiner box. The combiner box is integrated and the centralized photovoltaic inverter is input. The current adapted to the terminal voltage is connected to the latter and output AC power of the specified voltage level after being inverted. After being boosted by the transformer, it is merged into the public grid through the output line of the specified voltage level (see Figure 1).
Unlike the “combine first and then reverse” of the centralized inverter, the system structure of the string inverter is “reverse first and then reverse”. The multiple components of the photovoltaic array are connected to the string photovoltaic inverter through series 1 parallel connection to form photovoltaic strings, which are inverted into AC power of rated output voltage level and then connected to the photovoltaic AC combiner box, and then upgraded through the box-type transformer. After voltage, it is merged into the public power grid of the specified voltage level (see Figure 2).
2. Regional types and characteristics of centralized photovoltaic power plants
Large-scale centralized photovoltaic power plants can be roughly divided into deserts, hills, agricultural light (including fish ponds, vegetable sheds and other various agricultural facilities) complementation, and floating on the water according to the different features of the photovoltaic array.
Desert ground photovoltaic power plants refer to photovoltaic power plants developed using vast and flat unused ground resources. It also includes some power plants built on flat lands such as Gobi and grasslands. The main feature of the desert flat ground power station is that the environment is flat, the photovoltaic module array area is relatively regular, the orientation is relatively consistent, and there is little or no shelter. Centralized inverters are generally used in the selection of system inverters, and the installed capacity is relatively large. Compared with other types, the operation and maintenance of desert flatland power stations have advantages in convenience and economy. (See Figure 3)
Hilly photovoltaic power station refers to a photovoltaic power station developed using relatively low-altitude hills, mountains and other land resources (see Figure 4). This type of power station generally covers a large area, but the array area is arranged more scattered, the component orientation is not consistent, and the shelter is common in the morning and evening. There are often shrubs and vegetation in the photovoltaic array area, and there are many auxiliary projects such as road inspections and fences in the photovoltaic area. In the selection of inverter mode, if the location of the power station is relatively regular, but there are still problems of inconsistent component orientation or obstruction sooner or later, a centralized inverter with maximum power point tracking mode is generally used to ensure both orientation and obstruction conditions. The power generation efficiency greatly improves the convenience of construction and the friendliness of grid connection. If the terrain of the power station is very complex, string inverters are generally used to achieve full adaptation to smaller unit components and improve flexibility in design and construction.
Fishery/agricultural photovoltaic power station refers to the installation of photovoltaic modules on the water surface of existing ponds, agricultural and forestry facilities, and breeding sheds, and at the same time, the functional compound photovoltaic project of fishery/agricultural nursery planting or aquaculture under the modules (see figure 5). The Yuguang/Agriculture and light complementary power station has no pollution and zero emissions, and does not occupy additional land. It can realize the three-dimensional value-added utilization of land and fish pond resources, and achieve a win-win situation for photovoltaic development and agricultural production. Fishery/agricultural power stations generally have a slightly smaller installed capacity, and the project location is often close to the production and living areas of villages and towns. The surrounding environment has many interference factors on the production and power generation of the power station, and the operation and maintenance management environment is relatively complicated.
Surface floating power stations are usually those formed by using idle water area, using floating bodies such as pontoons and pontoons, and installing photovoltaic modules (see Figure 6). The surface floating power station has the function of not occupying land resources, reducing water evaporation, and blocking the sunlight and inhibiting the growth of algae. According to theoretical calculations, due to the cooling effect of the water body on the photovoltaic modules, the surface temperature of the modules can be restrained from rising. If the temperature of the solar panels is reduced by 1°C, the power generation can be 10% to 15% higher than that of the ground in the same area.