As the core equipment in photovoltaic power plants, solar grid connected inverters convert the direct current of solar photovoltaic cell modules into alternating current. Therefore, with the vigorous development of the photovoltaic industry, the research and production of solar inverters have also ushered in spring.
Solar inverters are mainly used for power generation in desert power stations, additional land use power generation, industrial and commercial rooftop power generation, and household power station power generation. In terms of the scale of solar inverters, desert power stations have the largest scale, while solar inverters used in other occasions have smaller scales. For different application scenarios, solar inverters are often divided into centralized solar inverters, string solar inverters, and miniature solar inverters. Centralized solar inverters are mainly aimed at the needs of large photovoltaic power plants, so it is necessary to centrally invert and feed photovoltaic modules into the grid after large-scale series parallel connection. String and micro solar inverters are suitable for smaller applications, so only each string of photovoltaic modules needs to be inverter fed into the grid, or each photovoltaic module needs to be independently fed into the grid after being inverter fed.
At the current level of solar inverter technology, the conversion efficiency has reached 97%~99%, and the maximum power point tracking efficiency has reached 98%~99.9%. The topology of solar inverters is mainly two-level, with IGBT as the main power device and voltage level below 1kV. However, there is still enormous room for technological development in solar inverters.
Developing high-performance controllers and power components is an effective way to improve the efficiency of photovoltaic power generation systems. At the same time, it is necessary to improve the operational efficiency of photovoltaic modules and arrays, reduce the losses of the entire system, and improve the reliability of solar inverters. Reducing power loss and improving equipment reliability are also of great significance for solar inverters themselves.
From the current development status, it is common to use more complex control technologies, modern control algorithms, and friendly operating platforms to improve the efficiency of solar inverters.
In the research and development of solar inverters, theoretical and application research is relatively active abroad, and modern control theories such as fuzzy control are widely used. The maximum power point tracking algorithm, pulse width modulation and other control modes are already very mature technologies abroad. Currently, foreign research mainly focuses on topology combinations for various specific scenarios, as well as improving the efficiency, reliability, power density, and intelligence level of photovoltaic inverters. In terms of low-power inverters, it is required to reduce costs and improve efficiency. In terms of high-power inverters, in addition to considering costs and efficiency, the goal should also be to optimize the topology structure, improve system stability and adaptability. At present, the mainstream power levels of high-power photovoltaic inverters in the industry are concentrated at 100kW, 250kW, 500kW, 630kW, and 1000kW, while the power module levels of photovoltaic inverters in major production enterprises are roughly divided into two types: 50kW power module as the basic power unit and 125kW power module as the basic power unit. With the continuous improvement of the power level of large-scale photovoltaic inverters, photovoltaic inverters with 125kW power modules have become the main research direction at present.
According to the Global Market Research Report on Solar Photovoltaic Inverters, the total global shipment of photovoltaic inverters in the fourth quarter of 2009 was 3.7GW. In the first quarter of 2010, the inverter market slightly declined, but also reached 3.1GW. According to the market analysis of Solar Buzz, nine countries had a photovoltaic installation capacity of 250MW in 2010. At the same time, Italy, Czech Republic, France, Japan, Spain and other countries have a promising photovoltaic future, which indirectly reflects the broad international market for photovoltaic inverters.
In terms of enterprise development, photovoltaic inverters cover various fields such as off grid inverters and grid connected inverters, and can achieve personal power generation strategies as well as power station power generation strategies. According to the report “Global Photovoltaic Inverters and MLPE Patterns” from the United States, Solar Technology AG (SMA) from Germany became the world’s largest producer of photovoltaic inverters in 2015. Its product direction mainly targets the demand for large photovoltaic power stations, while also combining small commercial systems and residential systems. It is the world’s earliest manufacturer of photovoltaic inverters and the most mature and comprehensive manufacturer of technology. Its products are exported to North America, the United Kingdom, Japan, Australia, and China.
In the Chinese photovoltaic inverter market, Huawei, as a rising star, became the largest representative of Chinese photovoltaic inverters in 2015. Its products overturned traditional concepts and gradually replaced central inverters with series inverters, committed to a product portfolio of full series inverters; Secondly, as the manufacturer with the largest inverter sales and shipment volume in the Chinese market in 2014, Sunshine Energy ranked third in 2015. The main sales of domestic photovoltaic enterprises are indeed distributed as shown in the figure.
China is one of the countries with the richest solar photovoltaic lighting resources in the world, and also the largest photovoltaic cell production base in the world. However, the relatively backward level of inverter technology has led to a low utilization rate of solar photovoltaic energy in China, and at the same time, it has led to a situation where the installation volume of photovoltaic inverters in China is extremely mismatched with the production of photovoltaic cells. Therefore, grasping the future development direction of photovoltaic inverters, accelerating the research pace of photovoltaic inverters, and improving the efficiency of grid connected inverters in MW level large-scale photovoltaic power stations are of great significance for China’s economic development, environmental protection, and rational energy allocation.
In order to achieve the goal of improving the conversion efficiency of photovoltaic inverters, the following research has been conducted:
(1) By analyzing the output characteristics of photovoltaic cells, a mathematical model of photovoltaic cells was established, and a simulation model of solar photovoltaic cells was established using MATLAB software. Finally, the rationality of the simulation model was verified.
(2) Based on the output characteristics of the established solar photovoltaic cell simulation model, the principles of three traditional maximum power tracking algorithms, namely constant voltage method, disturbance observation method, and conductivity increment method, were analyzed, as well as the advantages and disadvantages of each algorithm.
(3) On the basis of the existing maximum power point algorithm theory, an improved disturbance observation method with conductance increment as the relaxation factor is proposed, and pure simulation models of the traditional variable step size algorithm and the improved algorithm are established by combining MATLAB Simulink. Finally, the rationality of the algorithm is verified by comparing and analyzing the structures of the two algorithms.
(4) A solar photovoltaic cell model was established using RTDS, and semi physical simulation experiments were conducted on two algorithms. The experiments verified that the improved algorithm was superior to the traditional variable step algorithm, and the practicality of the algorithm was also verified.
(5) Theoretical analysis was conducted on traditional two-level, E-type three-level, and T-type three-level inverters. Based on the structural characteristics of the three inverters, the switching characteristics and power loss characteristics of the three inverters were theoretically analyzed.
(6) Based on the theoretical analysis of power losses in three types of inverters, the power losses and switch tube temperature changes of the three types of inverters were obtained through semi physical simulation. Finally, by comparing the power loss and temperature changes of the three types of inverters, the most ideal inverter structure was selected, achieving the goal of improving inverter conversion efficiency.
(7) Analyzed the working principle and function of key devices in photovoltaic inverter systems. In response to the needs of the experimental platform, the selection and parameter tuning of various key devices were carried out. Finally, through grid connected power generation experiments on the entire photovoltaic inverter system, the rationality of the maximum power point tracking algorithm, inverter structure selection, and key device selection was verified.