The development of photovoltaic grid-connected inverter technology and the power generation mode of the power generation system

The on grid inverter, as the interface device between the photovoltaic cell and the grid, converts the electrical energy of the photovoltaic cell into AC power and transmits it to the grid, and plays a vital role in the photovoltaic power generation system. Modern inverter technology provides strong technical and theoretical support for the development of photovoltaic on grid power generation. The improvement of on grid inverter performance is essential to improve the efficiency and reliability of the system, extend the life and reduce the cost.

The development of inverter technology has always been closely integrated with the development of power devices and their control technology. It has gone through five stages since its inception. The first stage: In the 1950s and 1960s, the birth of the thyristor SCR created conditions for the development of sine wave inverters: The second stage: In the 1970s, the turn-off thyristor GTO and bipolar transistor BJT came out. The development and application of inverter technology: The third stage: In the 1980s, the birth of power devices such as power FETs, insulated gate transistors, and MOS controlled thyristors laid the foundation for the development of inverters in the direction of large capacity: Four phases: In the 1990s, the development of microelectronics technology enabled new control technologies such as vector control technology, multi-level conversion technology, repetitive control, fuzzy control and other technologies to be better applied in the field of inverters and greatly promoted The development of inverter technology: The fifth stage: At the beginning of the 21st century, with the continuous improvement of power electronics technology, microelectronics technology and modern control theory, inverter technology is moving towards high frequency, high efficiency, and high power density. , High reliability, and intelligent development.

The photovoltaic on grid power generation system can be divided into:
① Centralized on grid photovoltaic system, the power generated by the system directly enters the grid, but this method obviously cannot take advantage of the wide distribution of solar energy and the wide area.

②Distributed on grid photovoltaic system, that is, a household photovoltaic on grid system, which can be combined with buildings to form a rooftop photovoltaic system. The design can reduce the construction cost and the cost of photovoltaic power generation systems. In a distributed on grid photovoltaic system, the electricity that is not used during the day can be sold to the local public power grid through the inverter, and when electricity is needed at night, it can be purchased from the power grid. A typical household photovoltaic on grid power generation system is shown in Figure 1.

Figure 1 Typical household photovoltaic grid-connected power generation system

Distributed Generation (DG), also known as distributed power generation or distributed energy supply, has no unified definition of distributed generation. Generally, it refers to a power generation/energy supply method in which relatively small power generation devices (generally less than 50MW) are distributed at the user’s load site or near the user. In addition to the characteristics of decentralization and miniaturization, modern distributed power generation systems also have the characteristics of implementing combined heat (cold) power supply, environmental friendliness, fuel diversification, networking, intelligent control, and information management.

Different experts have different descriptions of distributed power generation, but two points are consistent, namely, small-scale and on-site layout. According to this “definition”, it is obvious that my country’s “small generating units”, “small thermal power”, and “small thermal power” can also belong to the category of distributed power generation, but they are not at the same level as modern distributed power generation technology. Due to the technical and economic performance and environment If the performance is not good, it will be gradually eliminated. It can be seen that if the distributed photovoltaic on grid power generation system can be widely applied to users’ homes, it will not only make full use of the wide distribution of solar energy resources, but also improve the quality of the power grid, strengthen the power grid’s peak regulation capacity, disaster resistance and extension Ability and other purposes. At present, the research on distributed photovoltaic on grid power generation systems is on the one hand the research of solar cells, which reduces the cost of each watt of electricity generated by the battery to a practical stage; on the other hand, it is the research on the inverter system for on grid power generation. , Such as improving the efficiency and stability of the system, the control of the maximum power point of solar cells, and the effect of the system on the peak regulation of the power grid. Finally, a distributed power station system is formed.

The coordinated application of distributed power generation and centralized power supply system has the following advantages:
① The power stations in the distributed power generation system are independent of each other. Because users can control by themselves, there will be no large-scale power outages, so the safety and reliability are relatively high;
②Distributed power generation can make up for the lack of security and stability of large power grids, and continue to supply power when accidents occur, which has become an indispensable and important supplement to centralized power supply;
③ Real-time monitoring of the quality and performance of power in districts and cities is very suitable for supplying electricity to residents in rural areas, pastoral areas, mountainous areas, developing small and medium-sized cities, or commercial districts, which can greatly reduce the pressure on environmental protection:
④The transmission and distribution losses of distributed power generation are very low, or even none, and there is no need to build a distribution station, which can reduce or avoid additional transmission and distribution costs, and at the same time, the civil and installation costs are low:
⑤ It can meet the needs of special occasions, such as mobile decentralized power generation vehicles (in a hot standby state) used for important gatherings or celebrations;
⑥The peak shaving performance is good, and the operation is simple. Since there are few systems involved in the operation, the start and stop are fast, and it is easy to realize fully automatic.

Solar photovoltaic power generation technology is a distributed power generation technology based on renewable energy theory. It uses the photoelectric effect of semiconductor materials to directly convert solar energy into electrical energy. Photovoltaic power generation has the advantages of no fuel consumption, no geographical restrictions, flexible scale, no pollution, safety and reliability, and simple maintenance. However, the cost of this kind of distributed power generation technology is very high, so the solar power generation technology at this stage still needs to be technically improved in order to reduce the cost and is suitable for a wide range of applications by users.

In addition to solar cells, distributed photovoltaic on grid power generation system devices mainly have the following research focuses and directions:
①The tracking problem of solar cell maximum power point, theoretical method, control realization;
②DC/DC device research, circuit topology, control scheme;
③The research of DC/AC inverter device, which includes the selection of inverter circuit topology, the research of related control methods and control methods, is the core of the whole system research:
④ Detection method and prevention strategy of islanding effect;
⑤ Development of two-way electrical energy measurement device.