Classification Introduction of Grid-connected Photovoltaic System

Grid-connected photovoltaic system is a beneficial supplement to traditional power generation. In view of its importance to environmental protection and economic development, all developed countries have made every effort to promote solar power generation, and small and medium-scale solar power generation has formed an industry. There are two ways of solar power generation: photovoltaic power generation and solar thermal power generation. Among them, photovoltaic power generation has outstanding advantages such as simple maintenance and variable power, and is widely used as a medium and small grid-connected power supply.

1. Introduction of grid-connected photovoltaic system

Photovoltaic power generation system can be divided into off-grid photovoltaic power generation system and grid-connected photovoltaic power generation system. The investment of grid-connected photovoltaic power generation system is 25% lower than that of off-grid photovoltaic power generation system. Connecting the grid connected photovoltaic system to the large power grid in the form of a microgrid for grid-connected operation, and supporting each other with the large power grid, is an important technical way to increase the scale of photovoltaic power generation. The grid-connected operation of photovoltaic power generation systems is also the main direction of future technological development. Through Grid integration can expand the range and flexibility of solar energy use.

Grid-connected photovoltaic power generation means that the direct current generated by solar modules is converted into alternating current that meets the requirements of the mains grid through a grid-connected inverter and then directly connected to the public grid. It can be divided into grid-connected power generation systems with batteries and without batteries. The grid-connected power generation system with batteries is schedulable and can be integrated into or withdrawn from the power grid as needed. It also has the function of backup power supply. When the power grid fails for some reason, it can provide emergency power supply. Photovoltaic grid-connected power generation systems with batteries are often installed in residential buildings. Grid-connected power generation systems without batteries do not have the functions of dispatchability and backup power, and are generally installed on larger systems.

Grid-connected photovoltaic power generation has a centralized large-scale grid-connected photovoltaic power station, which is generally a national-level power station. The main feature is that the generated energy is directly transmitted to the grid, and the grid is uniformly deployed to supply power to users. However, this kind of power station has a large investment, a long construction period, and a large area, and has not yet developed much. Distributed small-scale grid-connected photovoltaics, especially building-integrated photovoltaic power generation, are the mainstream of photovoltaic power generation grid-connected due to the advantages of small investment, fast construction, small footprint, and strong policy support.

2. Product description of grid-connected photovoltaic system

It is an important feature of grid-connected photovoltaic power generation system to operate in the micro-grid and connect to the interconnected special/ultra-high voltage large grid through the medium and low voltage distribution network.

3. Basic necessary conditions for grid-connected photovoltaic systems

The basic necessary condition for grid connected photovoltaic system to be connected to the grid is that the frequency and phase of the sine wave current output by the inverter are the same as the frequency and phase of the grid voltage.

4. System classification of grid-connected photovoltaic systems

4.1 Grid-connected photovoltaic power generation system with countercurrent

There is a countercurrent grid-connected photovoltaic power generation system: when the electricity generated by the solar photovoltaic system is sufficient, the remaining electricity can be fed into the public grid to supply power to the grid (sell electricity); when the electricity provided by the solar photovoltaic system is insufficient, the electricity can be used to supply power to the load (buy electricity). Since the direction of power supply to the grid is opposite to that of the grid, it is called a grid connected photovoltaic system with reverse flow.

4.2 Grid-connected photovoltaic power generation system without countercurrent

No countercurrent grid-connected photovoltaic power generation system: Even if the solar photovoltaic power generation system has sufficient power generation, it will not supply power to the public grid, but when the solar photovoltaic system does not provide enough power, the public grid will supply power to the load.

4.3 Switchable grid-connected photovoltaic power generation system

The so-called switching grid-connected photovoltaic power generation system actually has the function of automatic two-way switching. First, when the grid connected photovoltaic system has insufficient power generation due to cloudy, rainy days, or its own failure, the switcher can automatically switch to the power supply side of the grid, and the grid supplies power to the load; second, when the grid suddenly loses power for some reason, The photovoltaic system can automatically switch to separate the power grid from the photovoltaic system and become an independent photovoltaic power generation system. Some switching photovoltaic power generation systems can also disconnect the power supply for general loads and connect the power supply for emergency loads when needed. Generally, switching grid-connected power generation systems are equipped with energy storage devices.

4.4 Grid-connected photovoltaic power generation system with energy storage device

Grid-connected photovoltaic power generation system with energy storage device: it is to configure energy storage device according to needs in the above-mentioned types of photovoltaic power generation system. Photovoltaic systems with energy storage devices are highly proactive. When power outages, power cuts and faults occur in the grid, they can operate independently and supply power to the load normally. Therefore, the grid-connected photovoltaic power generation system with energy storage device can be used as a power supply system for important or emergency loads such as emergency communication power supply, medical equipment, gas stations, shelter indication and lighting.

5. Forms of grid-connected photovoltaic systems

There are two forms of grid-connected photovoltaic power generation systems: centralized grid-connected and decentralized grid-connected.

5.1 Centralized grid connection: The characteristic is that the power generated can be directly transmitted to the large power grid, which is uniformly deployed by the large power grid to supply power to users, and the power exchange with the large power grid is one-way. It is suitable for grid connection of large-scale photovoltaic power plants, which are usually far away from the load point. Desert photovoltaic power plants are connected to the grid in this way.

5.2 Distributed grid connection: also known as distributed photovoltaic power generation grid connection, the characteristic is that the generated electric energy is directly distributed to the electric load, and the excess or insufficient power is regulated by connecting to the large power grid, and the power exchange with the large power grid Possibly both ways. It is suitable for small-scale photovoltaic power generation systems. Usually, urban photovoltaic power generation systems adopt this method, especially photovoltaic systems combined with buildings.

6. The development process of grid-connected photovoltaic system

According to the official website of the National Energy Administration on January 20, 2022, according to statistics, China's new photovoltaic power generation grid-connected installed capacity will be about 53 million kilowatts in 2021, ranking first in the world for 9 consecutive years. By the end of 2021, the grid-connected installed capacity of photovoltaic power generation will reach 306 million kilowatts, exceeding the 300 million kilowatt mark, ranking first in the world for seven consecutive years.

7. Three characteristics of grid-connected photovoltaic system

On January 24, 2022, according to the relevant person in charge of the National Energy Administration, in the first year of the "14th Five-Year Plan", my country's photovoltaic power generation construction achieved new breakthroughs, showing three new features:

First, distributed photovoltaics reached 107.5 million kilowatts, exceeding 100 million kilowatts, accounting for about one-third of all photovoltaic power grid-connected installed capacity.

Second, in the newly-added photovoltaic power generation grid-connected installed capacity, distributed photovoltaic power generation increased by about 29 million kilowatts, accounting for about 55% of all newly-added photovoltaic power generation installed capacity, breaking through 50% for the first time in history, and the development of centralized and distributed photovoltaic power generation The trend is obvious.

The third is that among the new distributed photovoltaics, household photovoltaics will exceed 20 million kilowatts in 2021 after exceeding 10 million kilowatts for the first time in 2020, reaching about 21.5 million kilowatts. Household photovoltaic has become an important force for my country to achieve carbon peak and carbon neutrality goals and implement the rural revitalization strategy as scheduled.