In today’s rapidly developing society of science and technology, environmental issues are increasingly valued by people. With the recognition and attention to the phenomenon of haze, the threat of environmental problems to human daily health has become a focus of public opinion. From the perspective of environmental protection and sustainable development strategies, collecting and converting resource rich solar energy to replace the development mode that mainly relies on non renewable resources such as coal as the main energy supply will be a major direction for energy development.
The National Development and Reform Commission issued the Medium – and Long Term Plan for Renewable Energy in 2007 and the Eleventh Five Year Plan for Renewable Energy in 2008, respectively. According to the plan, China will achieve a total installed capacity of 1600MW by 2020, with grid connected power generation accounting for 75%.
Due to the late start of China’s photovoltaic industry and the recent increase in public awareness of environmental protection, although the total installed capacity of China’s solar inverter industry has rapidly increased from 250WM in 2010 to 27GW in 2016, the total installed capacity is still less than 1% of the global installed capacity, which is significantly different from developed countries abroad. In addition, the core technologies involved in the development of solar inverters, as well as the cost and benefits of photovoltaic power generation, are also one of the factors that restrict the development of the photovoltaic industry.
In recent years, with the rapid development of the photovoltaic industry, all technical indicators involved have been rapidly developed and improved, and photovoltaic inverter technology has gradually matured. However, product performance is uneven. According to relevant statistics in 2016, currently, the top 50 photovoltaic inverter manufacturers in China produce solar inverters. According to their product performance rankings, the top ten solar inverter equipment has an inverter efficiency ranging from 92% to 97.88%. The lower ranked solar inverter equipment has an inverter efficiency of only about 83.5%, or even lower. However, even among the top ranked solar inverter devices, only a few can be classified as first-class inverter devices. It is easy to see that although photovoltaic inverter technology has become popular, factors such as execution efficiency, operational effectiveness, energy utilization efficiency, stability, performance advantages, and cost of each technology are the key to the huge differences in the solar inverter industry. According to relevant statistics in 2016, the highest inverter efficiency of foreign solar inverters is 98.04%. So, whether from the perspective of photovoltaic materials, hardware technology, or algorithm implementation, there is still a lot of room for improvement and research value in the actual conversion and utilization efficiency of solar inverters in terms of converting light energy to DC power, converting DC to half wave output, and converting half wave to AC full wave output.
According to the current usage of grid connected solar micro inverters, they can be divided into three types: centralized, distributed, and self supplied. Among them, centralized power plants are mainly used for the construction of large-scale ground power stations, which are more common in centralized power plants in the western region. Based on the vast geographical advantages, they fully absorb solar energy resources and concentrate on grid connected power generation work. However, due to the numerous photovoltaic arrays placed in a centralized manner, the distribution of light is not uniform at the same time, resulting in significant errors in selecting the maximum power point for solar inverters, which cannot fully utilize all energy. Therefore, centralized solar inverters are a way of sacrificing spatial resources for energy. In order to fully utilize the useful energy converted from photovoltaic arrays and save limited space, distributed solar inverters have gradually developed. Distributed solar inverters adopt a one-to-one approach, where one solar inverter device corresponds to a set of photovoltaic arrays, forming a unit. Multiple units work together to perform grid connected power generation. So distributed solar inverters are a way of exchanging energy utilization and regional space by investing more solar inverter equipment. Distributed is mainly built on the roof of housing, with the advantages of more flexible utilization, wider coverage area, and richer collection of resources. In 2015, the cumulative installed capacity of photovoltaic power generation in China was 37.95GW, of which 31.7GW was installed through centralized distribution, accounting for 83.5% of the total installed capacity of ground power stations in China. 6.25GW was installed through distributed photovoltaic power generation, accounting for only 16.5% of the total installed capacity of ground power stations in China. This is a significant contrast to developed countries abroad, mainly due to the cost and energy utilization efficiency of solar inverters.
Calculated at 50 cents per kilowatt hour, a one kilowatt solar inverter theoretically generates about 1 kilowatt hour of electricity. The first market price is generally 10 yuan per watt, so a one kilowatt solar inverter is about 10000 yuan, requiring continuous operation for 20000 hours, and each hour must meet the requirement of generating at least 1 kilowatt hour to just make up for the cost of the solar inverter. If a solar inverter works at full load for 6 hours every day, its service life is about 10 years. In addition, it is impossible for the solar inverter to achieve 100% inverter efficiency, which means that it is difficult to recover the cost of the solar inverter before its service life expires.
According to the Research Report on the Operating Situation and Investment Strategy of China’s Photovoltaic Market from 2017 to 2022, the capacity utilization rate of the top ten Chinese enterprises in 2016 was over 95%, a year-on-year increase of 35%, and the total installed capacity exceeded 27GW, a year-on-year increase of 172%, ranking first in the world. Due to various factors such as the late start and slow promotion of China’s photovoltaic industry, there is still a significant gap in overall installed capacity compared to developed countries abroad. At the 10th International Solar Industry and Photovoltaic Engineering Exhibition held in Shanghai, China on May 24, 2016, the authoritative classification of China’s efficiency was released. Among them, solar grid connected inverters (including transformers) with a weighted efficiency of over 97% in China were classified as Level 1, 96% -97% were classified as Level 2, and 94% -96% were classified as Level 3. The inverter efficiency of foreign solar inverters reached 97.67% -98.04%, The inverter efficiency of China’s cutting-edge solar inverters reaches 92%~97.88%, while the inverter efficiency of poorly performing solar inverters is 83.5%.
The main tasks are as follows:
(1) By consulting relevant literature and materials, understand the development status of solar inverters outside of China in recent years. Analyze the main gaps and the reasons for hindering promotion. Summarize the relevant issues in the design process of grid connected solar micro inverters, the various technologies involved, as well as the implementation methods and control strategies of various technologies. Familiar with the programming style, operating interface, and debugging process of the DSC controller.
(2) Identify the reasons for the low efficiency of solar inverters, summarize and classify them, identify all the shortcomings of the solar inverter designed in this article, divide the design of solar inverters into technical categories, prescribe targeted solutions, correct errors in the previous design, add important missing links in the original design, and combine them reasonably based on the correlation between various technologies to improve the overall performance of solar inverters.
(3) Analyze the theoretical basis, implementation methods, and shortcomings of the main technologies involved in the design of the complete solar inverter, analyze the existing problems, and propose corresponding solutions based on the defects encountered in the development of the entire machine and various problems existing in the original machine.
(4) Innovative design of half wave conversion circuit and selection and demonstration of leakage inductance energy absorption circuit parameters in the design of grid connected solar micro inverters, implementation, innovation and optimization of MPPT algorithm, derivation and modification of MPPT theoretical formula, derivation and modification of soft switch theoretical formula, implementation of soft switch control strategy, correction of zero drop and phase shift problems in full wave inverter, The improvement of the overall inverter efficiency and the optimization of the overall performance of the solar inverter are the focus of this study and design. On the premise of completing significant modifications and optimizations, and improving the overall performance and efficiency of the solar inverter, the research focus is analyzed one by one, and a practical solution for the involved problems is provided. The rationality of this innovative point is demonstrated by combining theoretical formula derivation and hardware physical waveform acquisition.
(5) Improve and build hardware measurement physical devices, measure and record the input and output efficiency, AC voltage and current values, maximum power point tracking status of solar inverters in real time, provide experimental renderings of grid connected solar micro inverters, and classify the efficiency of the designed high-efficiency grid connected solar micro inverters based on DSC according to the latest literature and data.
(6) Summarize and extend the more stringent functional requirements that need to be implemented for some of the main technologies, as well as the future research directions of related technologies.
The innovative points are as follows:
(1) The design and implementation of a circuit based on RCD leakage sensing energy absorption and suppression of peak voltage: According to the principle of clamp circuit, the resistance and capacitance parameters of circuit components are derived through mathematical theory, and a reasonable current absorption circuit is set to enable the leakage sensing energy in each cycle to be fed back to the output end of the solar inverter through the circuit, suppressing the back impact of leakage sensing energy on the power switch tube, And the phenomenon of energy loss and jitter at the half wave connection caused by the presence of leakage energy, and the suppression of peak voltage caused by the discharge of parasitic capacitor Coss during the conduction and turn off processes of power switching tubes.
(2) The design and algorithm implementation of MPPT control strategy based on power variable step size: Adopting the advantages of traditional MPPT algorithm, through mathematical formula derivation and demonstration, it realizes the tracking and stability of the maximum power point only by collecting voltage without the need for voltage disturbance. Simplify the circuit topology structure, solve the problem of unstable output and power point oscillation caused by voltage disturbances, and solve the problem of maximum power point not being able to track in real time due to fixed voltage. The stable output waveform and maximum power output greatly improve the overall inverter efficiency and dynamic MPPT tracking efficiency of the solar inverter.
(3) The design and implementation of a soft switching control strategy based on PI method to suppress peak voltage: Combining the mathematical formula derivation of the MPPT control strategy with variable power step size, extending the soft switching mathematical derivation formula, suppressing the peak voltage problem caused by transformer parasitic capacitance and power switch parasitic capacitance resonance, highlighting the soft switching phenomenon, and reducing the energy loss of power switch during conduction and turn off processes, Improve the stability and inverter efficiency of the entire solar inverter system.
(4) The design and implementation of a full wave inverter control strategy based on a controllable silicon H-bridge combined with an external interrupt algorithm: Based on the characteristics of the zero cutoff of the controllable silicon, two thyristors are used to replace the two switching tubes on the traditional H-bridge, and combined with an external interrupt algorithm, zero crossing point detection is achieved during the full wave inverter stage, and the problem of zero drop at the zero point of the inverter full wave caused by dead time is solved.
(5) Construct an efficiency testing circuit for the entire solar inverter, measure the overall inverter efficiency and dynamic MPPT efficiency of the solar inverter, classify the solar inverter designed in this paper based on the inverter efficiency level index, and apply the relevant theories, mathematical formula derivation, algorithm design, control strategy, etc. involved in grid connected solar inverters to actual hardware systems, Test the input and output waveforms of the designed high-efficiency grid connected solar micro inverter hardware circuit, gradually improving the inverter efficiency of the solar inverter designed in this article, and ensuring stable output of the solar inverter at the maximum power point. The full load inverter efficiency of the entire machine reaches 96.2%~97.6%, and the dynamic MPPT efficiency reaches 99.62%, completing the development of a 230V solar inverter for the entire machine.