According to the capacity and control mode of the grid-connected photovoltaic power generation system, the grid environment where the system is located can be divided into the following structures.
(1) The grid-connected photovoltaic power generation system supplies power to the local load (cell), and at the same time, the excess electricity is transmitted to the local grid. Usually, the capacity of this system is limited, and its main function is to supply power to the local load. This kind of structure mostly appears on the rooftop photovoltaic system, which is one of the most popular methods in the current grid-connected photovoltaic power generation system. Its power grid structure (power grid topology 1) is shown in Figure 8-16.
(2) The grid-connected photovoltaic power generation system is directly connected to the distribution network, and all the electric energy is transmitted to the power grid. Each photovoltaic system is equivalent to a distributed independent power generation system. The characteristic of this structure is that the PV system can be installed in any location depending on the grid configuration, and its capacity requirements are not very high. As long as the grid-connected conditions of the PV system are met, power can be transferred to the grid at any time. This distributed generation system can also optimize the power supply structure of the grid, reduce the imbalance of power supply between regions, and reduce the line loss. Figure 8-17 shows the power grid structure (power grid topology 2).
(3) The grid-connected photovoltaic power generation system consists of a large power station, which directly supplies power through the transmission grid. Usually, this system has a large capacity. The advantage of this system is that the combination of grid-connected photovoltaic system is flexible. It can adopt the cascade inverter mode, which means that multiple inverters are connected in parallel and supply power in a centralized manner, or adopt the central inverter mode, which means that some battery banks are connected in series and then connected in parallel to the DC side of the inverter, and finally supply power externally through one inverter. Because the photovoltaic systems are concentrated together, this structure is convenient for system maintenance and is suitable for areas with high sunshine intensity and long sunshine duration. If the variation of sunshine intensity in this area is large, it is not suitable to adopt this system structure, so this system is more restricted by the region. Figure 8-18 shows the power grid structure (power grid topology 3).
(4) Photovoltaic grid-connected power generation system and other renewable energy power stations (such as wind power generation, fuel cell power generation, etc.) form complementary power stations to supply power to the grid. The advantage of this power supply mode is that various energy modes are complementary to each other, which can ensure that the output power of the power station meets the generation requirements. However, due to the variety of power supply, it is bound to increase the initial input cost of the power plant, and it is relatively difficult from the perspective of control and maintenance. Figure 8-19 shows the power grid structure (power grid topology 4).
(5) Photovoltaic grid-connected power generation system and other renewable energy power stations (such as wind power generation, fuel cell power generation, etc.) form an independent power grid to supply power to a certain area, without connecting with the external large power grid. This method is very suitable for the remote areas that are difficult to reach by the large grid of traditional power generation methods, but the capacity of photovoltaic grid-connected power generation system and other renewable energy plants is required to be large enough to maintain the stable operation of the grid. At the same time, as both the photovoltaic grid-connected system and the wind power generation system are intermittent power generation systems, the fluctuation of their output power may affect the stable operation of the system, so it is very difficult for the whole power grid system to rationally configure the capacity and control each power generation system. Figure 8-20 shows the power grid structure (power grid topology 5).
We mainly study the grid-connected PV system below 10kW. This type of system can be directly connected to the 380V distribution network and transmit excess power to the grid on the basis of meeting the local load demand, which belongs to the first grid structure. As a distributed power supply (such as rooftop photovoltaic power generation, etc.), this grid-connected system will be the trend of future photovoltaic application development. When the PV is centralized, its total capacity is sufficient to support local loads, but cannot supply power for the entire network. Since the change of solar irradiation intensity will lead to the change of the energy converted by solar energy, the power output of the grid-connected photovoltaic system may be in a state of fluctuation at any time. When the sunshine intensity is high, the power output of the grid-connected system is also large. When the output capacity of the photovoltaic inverter is greater than the local load demand, the excess energy is transferred to the grid for other loads. When the sunshine intensity is weak or the photovoltaic cell stops working completely at night, the output power of the grid-connected system decreases or even becomes 0, and the energy required by the load is provided by the grid.