Generator content

First, introduction

(1) Research background

In recent years, scientific research has focused on tackling global warming. The massive emission of carbon dioxide is the root cause of global warming. Scientists are trying to find suitable renewable energy sources to replace fossil fuels to slow this trend. Renewable energy storage systems should have the advantages of high energy density and high security. Due to the gradual improvement of the system and the continuous development of science and technology, the solar photovoltaic industry has developed rapidly [1]. Since the 1980s, developed countries such as the United States, Germany, Japan and Canada have developed long-term development plans, which have greatly promoted the development of the photovoltaic industry. Over the past decade, the average annual production of solar cells worldwide has increased by 33%. In 2004, the world's solar system capacity exceeded 1,200 megawatts, and continued to grow to more than 600 megawatts in 2005. By the end of 2006, global PV capacity exceeded 8 MW. With 5gwp, more than 80% of grid-connected photovoltaic power generation occupies a leading position in the world photovoltaic market. Grid-connected photovoltaic power generation system is the only way to achieve the development of large-scale photovoltaic power generation [2].

China's energy reserves are far below the world average, accounting for only 10% of the world's total reserves [3]. The most attractive and resource-rich solar energy has many unparalleled advantages. In recent years, the state advocates the scientific concept of development, the application of new energy technology, and vigorously develop renewable energy. The photovoltaic industry in Jilin Province is developing very rapidly, mainly concentrated in large public buildings. Solar photovoltaic system installation process is simple, no mechanical moving parts in the working process, large fuel consumption, large emissions. Compared with wind energy and other new energy production technologies, its resources are the richest and cleanest, so photovoltaic power generation technology has good social and economic benefits [4].

(2) The purpose and significance of the research

"China National Green Building Innovation Award Management Measures" clearly defines the significance of green buildings, that is, to provide healthy, clean, comfortable and pollution-free living and working space in People's Daily activities [5], affected by environmental factors, policy factors and application scope of Jilin Province, due to the incomplete simulation and benefit analysis of photovoltaic power generation projects in Jilin Province, it is impossible to predict the photovoltaic development in Jilin Province The benefits of electrical projects. Due to the lack of accurate and accurate data analysis as a foundation and targeted services, many developers are afraid to try this area. Therefore, the application and promotion of photovoltaic power generation technology in our province is very slow. transient system simulation (TRNSYS) program) and a photovoltaic engineering example, the system can clearly analyze the advantages of the photovoltaic power generation industry [6], and make use of the powerful data processing and collection functions of virtual instruments, through the comparison and analysis of the data obtained by the two methods, the maximum power point tracking method is obtained. It improves the scientificity and accuracy of the data, improves the integrity of the photovoltaic power generation simulation, improves the power generation efficiency, and visually analyzes the benefits of the project. In order to solve the problem of slow development of photovoltaic industry in Jilin Province, developers can boldly carry out photovoltaic project construction experiments to speed up the development of photovoltaic industry.

(3) Research status at home and abroad

1. Domestic research status

Most of China's solar photovoltaic power generation systems are energy utilization systems that solve the lives of people in remote areas and some enterprises. The research of solar photovoltaic power generation system is still in its infancy. The photovoltaic systems that are built provide very little electricity to a small number of users and produce very little electricity. Its role in the entire power grid is negligible, and it can be considered that it will not affect the power grid [10]. The research of photovoltaic grid-connected power generation technology is highly valued." During the 11th Five-Year Plan period, China conducted in-depth research on grid-connected technology and large-scale photovoltaic power plants. Encouraging progress has been made. On this basis, China has built 5 kW and 10 kW grid-connected photovoltaic demonstration power stations. China's Ministry of Science and Technology has listed photovoltaic power generation technology as a national key development topic and increased investment in photovoltaic power generation system design. R& controller research and development and the development of light inverters have accelerated the development of the photovoltaic industry, the state has invested a lot of money, and many important provinces have built photovoltaic demonstration power stations, vigorously promote the development of the photovoltaic industry in the province, and have made important contributions to China's photovoltaic industry.

2. Foreign research status

Energy is a powerful direct driving force for economic growth and social development. The international economy has shifted from high-speed growth to medium-to-high growth. We see the determination of the international community to develop green resources, recognizing the characteristics of high energy consumption, high pollution, high energy efficiency, low energy efficiency and serious environmental pollution. The high cost treatment of old development roads is difficult to maintain. Countries all over the world are vigorously promoting the application of photovoltaic power generation systems. Grid-connected photovoltaic power generation is the main trend of the development of solar photovoltaic power generation technology in the world. The use of photovoltaic power generation systems mainly starts from the initial investment subsidy and application layer [11]. For example, in 1998 and 1999, in order to promote the development of the photovoltaic industry, Germany has launched 1,000 roof projects and 100,000 roof projects. The South Korean government has adopted policies to support initial investment and subsidies. South Korea has built the world's largest 24-megawatt photovoltaic plant to improve power generation efficiency. The system uses the latest technology to achieve sunlight tracking, the solar panels automatically swing, and the final production efficiency is 15% higher than the fixed installation [12]. Canada has developed a series of policies. According to these policies, photovoltaic systems below 500 kW can provide excess power on grid-connected lines, so as to obtain certain benefits, under the premise of meeting their own use requirements, all systems above 500 kW can be connected to the grid to achieve profitability.

Second, photovoltaic system and photovoltaic industry analysis

1. Classification and power generation principle of photovoltaic power generation system

Solar energy systems can be divided into independent systems and grid-connected systems according to whether they are connected to the grid. The difference, however, is that stand-alone photovoltaic systems are very complex and require batteries as an energy reserve. Grid-connected photovoltaic power generation system can be directly connected to the public grid, fewer projects, simple structure.

Whether it is an independent power generation system or a grid-connected power generation system, its main components are solar panels, controllers and inverters [16]. The solar cell and the inverter work in coordination under the control of the controller. The principle of solar cells is relatively simple, and the common types on the market are usually semiconductors. When sunlight hits the surface, the photon energy is absorbed by the electrons, causing the electrons to change from P-type semiconductors to N-type semiconductors, resulting in the accumulation of electrons and holes. When the conductors are connected to form a loop, the voltage at the semiconductor contact surface will drop and the external current will drop [17]. The power generation principle of solar cells is shown in the figure. Another panel structure is emerging on the market. The organic matter is uniformly suspended on the surface of the film, which has the advantages of light weight and low cost.

2. Solar photovoltaic power generation system composition

Photovoltaic power generation system consists of solar array, charge and discharge controller, inverter, AC distribution cabinet, filter circuit and so on. Solar photovoltaic power generation can be divided into one-stage energy conversion and multistage energy conversion. The widely used energy conversion method is first order energy conversion.

There are some common parameters for photovoltaic systems. The most commonly used are installed capacity and power generation. Installed capacity is equal to the product of panel area. The average sunshine intensity and photoelectric conversion efficiency of the photovoltaic system, where the light intensity is the average light energy intensity per unit area, the unit is w/m2[20]. The power generation capacity of photovoltaic power generation system is equal to the product of installed capacity, effective sunshine time and comprehensive coefficient. The comprehensive coefficient in the formula is affected by many factors, so the Angle of photovoltaic panel, transmission loss and energy conversion loss should be considered when determining the final efficiency coefficient [21]. The installation of solar photovoltaic systems is also one of the hot issues discussed, namely the optimal location of solar panels. Depending on the regional environment, lighting direction and other factors may have different requirements for the installation position.

Third, the application of solar photovoltaic technology

1. Grid-connected inverter technology

According to the different characteristics of DC power supplies, inverters can be divided into current sources and voltage sources. Because of the simple structure of voltage inverter, the control technology is mature and reliable. Voltage inverters are more efficient than current inverters. Therefore, voltage-type inverters are usually used in photovoltaic grid-connected systems. Grid-connected photovoltaic inverters can be divided into insulated and non-insulated types. According to whether there is electrical insulation between the input side and the output side of the grid-connected photovoltaic system, the grid-connected photovoltaic inverter can be divided into power frequency insulation type and high frequency insulation type, which depends on the working frequency of the grid-connected photovoltaic inverter. Operation of insulated transformers.

In practical applications, nonlinear loads exist in both the client and the network. Half wave load and rectifier bridge circuit are common nonlinear load forms. Rectifier bridge circuit is the most common input circuit form in power electronic devices, and also the most common nonlinear load form at present. The biggest harm caused by nonlinear load is the distortion of the system voltage and current waveform, which leads to the degradation of the output waveform quality. Therefore, the current relevant standards in China clearly specify the technical indicators of the output waveform quality of grid-connected inverters. Harmonics are two key factors that affect the output waveform quality of grid-connected inverters. The output of PV grid-connected inverter has the problem of DC injection, that is, there is DC component in the grid-connected current. In recent years, with the wide application of uninsulated grid-connected inverters, the harm caused by this problem has become more and more serious, and has attracted wide attention.

In photovoltaic systems using uninsulated grid-connected inverters, DC components will be directly injected into the grid, causing serious harm to the grid, such as DC magnetic bias of transformers at all levels of substations. In grid-connected photovoltaic systems using grid-connected inverters, although the isolation transformer can inhibit the output DC component of the grid-connected inverter to a certain extent, the magnetic core saturation, output waveform distortion, increased loss, and shortened service life will also lead to a significant increase in cost, and the DC component will also damage the transformer itself. The research results of Spanish scholars on the output DC component of inverters show that there is still DC component in the grid-connected current of photovoltaic inverters. Therefore, the research on DC injection of PV grid-connected inverters has important practical significance.

2. High and low voltage crossing technology

In recent years, with the rapid development and wide application of photovoltaic grid-connected power generation, the proportion of photovoltaic power generation in the world's energy consumption and power supply is increasing. Once the grid fails, the security of the grid becomes increasingly high. The traditional high penetration rate (proportion in the grid) of photovoltaic power supply is disconnected, which further worsens the operation of the grid, causing serious accidents, and cannot meet the requirements of the normal operation of the power system. The State Grid Corporation of China has also issued the "Technical Regulations for Photovoltaic Power Station grid-connection", which clearly stipulates that when the power grid is abnormal, small photovoltaic power stations are used as the load and the power grid is cut off as soon as possible; For large and medium-sized photovoltaic power stations, it should be considered as a power supply, with a certain low-voltage overcurrent capacity, and provide reactive power support for the system to maintain grid stability. The research of grid-connected inverter overcurrent technology mainly focuses on the low-voltage overcurrent of wind power generation. To -D. When an accident or power system interruption causes the power supply and grid voltage to drop, the grid-connected power supply can ensure continuous operation within a certain voltage drop range and time interval (without connecting the grid). This paper mainly studies the low-voltage overcurrent problem of large-scale photovoltaic power plants. This is because when the grid voltage drops, the grid-connected inverter as the grid-connected power interface reduces the injected power of the grid due to its current capacity limitation. Therefore, the imbalance of input and output power will lead to DC side overvoltage. If the DC voltage remains stable, the output current of the grid-connected inverter is too large, endangering the safety of power electronic equipment.

In addition, according to the characteristics of the photovoltaic array, the output power of the photovoltaic array decreases with the increase of the output voltage until the open circuit voltage is reached and the output power is zero. Therefore, the current research on low-voltage overcurrent of photovoltaic grid-connected power supply is mainly to suppress the overcurrent of photovoltaic grid-connected inverters. The technology of high voltage switch corresponding to low voltage switch has not been paid enough attention. At present, the relevant regulations of our country are mainly off-grid operation. Since wind power is generally located at the end of the grid, the voltage fluctuation of the grid is mainly caused by the drop, and the intersection must be mainly low-voltage. Therefore, once the grid voltage suddenly rises, it is also necessary to study the high-voltage switch of the photovoltaic grid-connected power supply.

3. No interconnection line parallel technology

With the development of the world economy and the increasing number of power equipment, people have put forward higher and higher requirements for the power level and reliability of the power system. Due to the limitation of the power level of the inverter power supply, people have higher and higher requirements for the inverter power supply. The power supply mode of a single inverter can not meet the requirements of high power and ultra-high power. Therefore, the focus is on increasing the power level of the inverter by running multiple inverter modules in parallel. Parallel inverter technology is not only an important means for power system to develop to high power. At the same time, it is also a key technology for the development from centralized to distributed. It appeared in the early 21st century and has been rapidly developed and applied in the following years. The United States, Germany, Japan and other developed countries have conducted in-depth research on this.

In the practical application of parallel inverters, because of the control signal line of inverters, parallel interconnection line has become the main way of parallel inverters, and information sharing between inverter modules has been realized. Compared with parallel connection lines, parallel connection lines without parallel connection lines have the advantages of good redundancy, high reliability, convenient capacity and maintenance, and broad application prospect. Although the parallel without interconnection can not meet the requirements of practical application, it has greater value and significance for the development and application of parallel inverter technology.

For grid-connected inverters, it is generally believed that the grid is an infinite ideal power supply, so it is unnecessary to consider the parallel control problem. In recent years, with the rapid development of grid-connected photovoltaic power generation in the world and the vigorous promotion of our country, for single-phase grid-connected civil photovoltaic inverters running on the low-voltage side of the grid, the grid can not be simply idealized. At this point, the grid-connected PV inverter can be considered to operate in parallel mode without interconnecting lines. Therefore, it is very necessary to study the non-interconnected parallel technology of PV grid-connected inverters from this point of view.

Third, the application of solar photovoltaic technology

1. Economic benefits

Because solar energy resources are free, with the development of science and technology, the installation cost of the entire system has been within a reasonable range, the economic efficiency of solar photovoltaic system is very high. According to the simulation results, the daily and monthly measured power generation of photovoltaic power generation is analyzed, and the variation law of power generation from low solar radiation to high solar radiation is obtained. The average daily power generation is 31KW and the monthly power generation is 930kw. Due to environmental factors in Jilin Province, power generation efficiency can vary significantly from month to month throughout the year. Through the relationship between availability and month, the photovoltaic power generation efficiency of Jilin Province is obtained, and the annual power generation is 8300kw. Based on the simulation results, measured and calibrated power PV output per hour, measured and predicted instantaneous power comparison, measured and predicted energy PV output, the energy conversion efficiency is obtained.

2. Environmental benefits

There is no exhaust emission during the operation of the photovoltaic power generation system, which will not affect the ambient air quality. When there is no waste water, it will not affect the water environment and the soil environment. This project has no obvious noise source and will not affect the surrounding environmentally sensitive targets. After the completion of the project, the vegetation was not damaged and no large-scale civil works were used. Will not cause soil erosion, will not affect the landscape environment. After the completion of the project, the way of electricity will not affect the ecological environment.

In 2014, Jilin Province built its first civilian photovoltaic power station with the support of Power companies of China. The power plant is located in a residential area of Jilin City, Jilin province. Specific parameters for the installed capacity of 3.5kw, annual power generation of 4200 degrees. According to the domestic 0.42 yuan/degree calculation, the annual income may exceed 5,000 yuan, depending on the relevant project personnel. The annual output can reach about 12%, and the investment can be recovered after 8 years of normal work. It has significant social significance and can reduce a large amount of coal and carbon dioxide emissions every year.

In 2003, Konoweiye Technology Co., LTD., a leading enterprise in Beijing photovoltaic industry, first developed and built the 50KW Beijing Daxing Temple Tower photovoltaic system. This sets a precedent for the application of BIPV in China. For the first time, China has realized the perfect combination of photovoltaics and buildings. The completion and perfect operation of the project provides a good reference for the design of large-scale photovoltaic power stations in China, and lays the foundation for the combination of photovoltaic systems and buildings.

Fourth, the current problems and strategies of solar photovoltaic power generation

(1) The current problems of solar photovoltaic power generation

1. Relatively excess production capacity, guide the healthy development of the polysilicon industry, avoid excessive competition in the industry, and promote energy conservation and emission reduction. The state divides the polysilicon industry into excess capacity industries, and resolutely curbs the trend of polysilicon overcapacity and repetitive construction through economic and administrative means. China's regions and cities are facing a new wave of energy development. More than 100 cities have proposed to build new energy bases. In 2008, China's polysilicon production capacity was 20,000 tons, and production was about 4,000 tons. The capacity under construction is about 80,000 tons, which is obviously excess capacity. In just two years, a sunrise industry with high input, high yield and high threshold has rapidly transformed into an industry with excess capacity, which once again shows that in China's big industrial era, we have formed a common industry with insufficient supply, combination of large and small projects and excess capacity, and eliminated backward production capacity.

2. China's overseas PV market and raw materials are developing slowly. At present, the solar photovoltaic industry is at both ends of upstream polysilicon materials and downstream foreign applications, and 98% of domestic solar cells are exported. This amounts to a shortage of large amounts of domestically produced energy. Raw materials are abroad. There are four countries in the world, the United States, Japan, Germany and Russia, that can produce high-quality polysilicon. All four countries are semiconductor producers. China can also produce polysilicon, but the industrial level and technical content are different. In all aspects of environmental protection, energy consumption is far behind the advanced level of foreign countries, it should be said that China has no particularly mature enterprises. The photovoltaic industry is a typical "foreign double-end" industry, which means that China's energy consumption is large and pollution is serious. However, the generation of clean energy in China is also an important reason for people to question the photovoltaic industry.

2. Promote the development of solar photovoltaic industry countermeasures

At the 2019 China PV High-Level Forum, experts agreed that although the global solar PV industry has entered a stage of adjustment, the development trend will not change. Solar energy industry is the most promising industry in the new energy industry. Therefore, the production pace of polysilicon enterprises has a good foundation, which can be adjusted, practice internal skills, stabilize the system, improve quality, reduce consumption, and adapt to market changes.

(1) Many energy enterprises urgently need to promote industrial upgrading in order to quickly adapt to the new situation, accelerate structural optimization, improve management level, and avoid overheating investment. Countries began to restrict high-energy polysilicon projects, but encouraged low-energy polysilicon projects. New polysilicon project scale of more than 3000 tons/year.

(2) Conclusion

Although the prospect of new energy is vast, it is only just seeing a glimmer of light. While vigorously developing new energy, we must also keep a clear head - new energy is far from the "protagonist" time. New energy is the hope of the future, but not the backbone of today. We need decades of efforts to truly build a new energy industry