The solar photovoltaic power generation system connected to the public grid is called grid-connected photovoltaic power generation system, also called grid-connected photovoltaic power station. The grid-connected type can be divided into a reverse current system and a non-reverse current system. A reverse power flow system is a system where the power company purchases surplus power. A non-reverse power flow system is a system where the power demand in the system is greater than the power provided by solar cells, and there is no need for the power company to buy surplus power.
The grid-connected photovoltaic power generation system converts the direct current output from the solar cell array into alternating current with the same amplitude, frequency and phase as the grid voltage, and realizes the connection with the grid to deliver electrical energy to the grid. Figure 1 is a schematic diagram of a common grid-connected photovoltaic power generation system structure. The system includes a solar cell array, a DC DC converter, a DCAC inverter, an AC load, a transformer, and a battery pack connected in parallel at the output of the DC-DC converter. The battery pack can improve the reliability of the system’s power supply. When the sunshine is strong, the photovoltaic power generation system first meets the power consumption of the AC load, and then sends the excess power to the grid; when the sunlight is insufficient and the solar cell array cannot provide enough power for the load, it can be obtained from the grid or battery pack to supply power for the load.
Of course, if the cost is taken into consideration, it is not necessary to connect the battery, and when the light is insufficient, it can directly request the power supply for the load from the grid.
Compared with the independent system, the grid-connected AC power generation system eliminates the need for energy storage equipment.
There is also a grid-connected extended hybrid power generation system. The hybrid type is between the grid-connected type and the independent type. This kind of system is usually integrated with the controller and the inverter, and the computer chip can be used to control the entire system to achieve the best working condition. Figure 2 shows a hybrid photovoltaic power generation system, which is different from the above two systems in that there is an additional backup generator set, when the photovoltaic array is insufficient to generate electricity or the storage battery is insufficient. It can start the standby generator set, which can directly supply power to the AC load, and can also charge the battery after the rectifier. Hybrid photovoltaic power generation systems are mainly used in power-using occasions that are far away from the grid and need to ensure the continuity of power supply, such as field hospitals and scientific research stations. In a day with insufficient sunlight or rainy weather, the solar battery cannot work, and the power stored in the battery cannot meet the demand, the generator set will replace the solar battery to supply power to the system.
Different levels of photovoltaic grid-connected voltages are different. For example, grid-connected photovoltaic power stations above MWp generally need to be connected to public grids above 10kV, and other designs are roughly the same.
The following describes the grid-connected photovoltaic power generation system with a 30kWp grid-connected solar power generation system.
Figure 3 shows the circuit design of the grid-connected system, which consists of solar modules, inverter devices and AC and DC lightning protection power distribution cabinets. Photovoltaic components convert solar energy into DC power under the photovoltaic effect, and the DC power flows into the grid-connected inverter through the lightning protection power distribution cabinet. The inverter converts it into AC power that meets the power quality requirements of the power grid, and connects to the 380V/150Hz three-phase AC station power system to connect to the grid for power generation. In the daytime, photovoltaic power generation supplies power to the station power load and feeds the excess power into the grid; at night or when the power generation is insufficient in cloudy and rainy days, the public power grid supplies power to the station power load. The photovoltaic grid-connected power generation system is equipped with a local monitoring device with an Ethernet communication interface, and provides the working status and operating data of the system to the integrated automation system of the unattended station through the interface, realizing remote centralized control station monitoring.