1. Current research status of high-efficiency grid connected solar micro inverters
We mainly conducted theoretical analysis on the main technical defects in the design of grid connected solar micro inverters, including circuit design and demonstration, implementation and demonstration of innovative algorithms, modification and innovation of control strategies, stability and improvement of inverter performance. We also classified and tested the overall inverter efficiency of the solar inverter. Through experimental data, we verified the proposed innovations, optimizations, and correction of relevant theories The correctness of the analysis and the high efficiency and stability of the designed grid connected solar micro inverter.
Firstly, the research significance and background of grid connected solar micro inverters were elaborated, and the development status and reasons for the development gap of solar inverter technology at home and abroad were analyzed. The main research tasks and innovative points of the paper were determined. Through the study of relevant materials and literature on the development of solar inverters in the early stage, the design stages of solar inverters were divided and the main technologies of each stage were classified. Based on the relevant problems encountered during the experimental process, relevant design plans were formulated. Secondly, a profound analysis of the main technologies in each link and the reasons for the main problems in the design of solar inverters is conducted. Based on the design scheme, the main technologies are implemented or optimized. By comparing the waveforms before and after modification, the correctness of the design scheme is demonstrated, relevant defects are solved, and the overall inverter efficiency of the solar inverter is improved. Finally, this article provides hardware measurement physical images, overall efficiency measurement data, and experimental data graphs during the entire machine design process, and analyzes the correlation between various technologies in the design of grid connected solar micro inverters.
The research focus of this article, namely the main work and innovation completed in this article, is as follows:
(2) Repeatedly absorbing and reusing excess leakage energy to reduce energy loss during the conversion process and improve the inverter efficiency of solar inverters. Due to the leakage inductance energy in the half wave conversion circuit of grid connected solar micro inverters, and the high frequency of solar micro inverters, the energy consumption problem it brings cannot be ignored. A new clamp circuit based on RCD has been implemented in the DC conversion process, which can absorb and feedback leakage inductance energy and suppress parasitic Coss capacitor discharge. Modify the half wave conversion circuit of the original solar inverter, design the current circuit, theoretically derive the circuit parameters, and demonstrate the correctness of the improved circuit.
(3) Implement an MPPT control algorithm based on power variable step size to stabilize the output of the solar inverter at its maximum power point and improve its inverter efficiency. Due to the inability of traditional fixed voltage methods to achieve real-time tracking of power points, traditional disturbance observation methods require additional disturbances, and traditional variable step conductance increment methods can greatly increase the complexity of circuit topology and other adverse factors. Combining the advantages of traditional methods, this paper proposes and implements an MPPT control strategy based on power variable step size. And the correctness of this strategy was demonstrated through mathematical theory deduction and test waveforms.
(4) Suppress the phenomenon of peak voltage, reduce energy loss, and ensure stable and reliable waveform output of solar inverters. Due to the hard switching effect during the conduction and turn off process of high-frequency switching tubes in the design process of solar inverters, the accumulation of leakage inductance energy and the recoil phenomenon caused by the high-frequency switching tubes, the inability of the output power point to stabilize at the maximum power point, and the occurrence of rapid left-right oscillation, unclear soft switching effect, which can easily lead to the superposition of component terminal voltage, and the phenomenon of parasitic capacitor discharge leading to a sudden increase in voltage between the leakage and source electrodes of power switching tubes Severe zero point drop phenomenon and resonance effect of resonant circuits can cause peak voltage, resulting in waveform distortion and reduced safety factor of solar inverters. After completing the absorption and feedback reuse of leakage inductance energy and stabilizing the maximum power point of MPPT, this article further implements a control strategy based on PI method for suppressing peak voltage in soft switching design. Combined with the control strategy of the full wave inverter stage, the entire inverter system is able to suppress peak voltage.
(5) Correct the phenomenon of full wave phase shift and zero point drop, improve the accuracy of the output waveform, and reduce energy loss. Due to the dead time in the design of the dual interleaved system and the inability of traditional H-bridge inverter circuits to perform zero crossing detection, phase shift and zero drop problems occur during the synchronous inverter process. This paper modifies the traditional H-bridge inverter circuit and adds software algorithm regulation to achieve a synchronous inverter strategy based on thyristor H-bridge combined with external interrupt algorithm.
(6) Real time measurement and statistics of input and output efficiency, voltage and current values of AC output, dynamic MPPT tracking status, gradually optimizing the functions of related technologies in each part, and classifying the efficiency of the designed high-efficiency grid connected solar micro inverter based on DSC.
2. Development of high-efficiency grid connected solar micro inverters
In today’s rapidly developing society of science and technology, replacing non renewable resources such as coal with solar energy is a major direction for energy development.
Although the related technologies of solar inverters are becoming more mature, there are still many areas that need to be optimized and improved. For example, for maximum power point tracking technology, environmental conditions such as external temperature, lighting, and shadow area will have a direct impact on the maximum power point output of solar inverters. When the photovoltaic array is subjected to uneven illumination or part of the array is within the shadow area, shadow effects may occur. The PV curve of the photovoltaic inverter may shift from a single peak seeking problem to a multi peak seeking problem. However, since the essence of the MPPT algorithm design in solar inverters is to find the optimal solution for a single peak, when there is a multi peak problem, commonly used MPPT algorithms often encounter the problem of local optimal solutions, Although the output power remains stable at a certain peak at the maximum power point, it does not meet the actual maximum power point output requirements, resulting in an impact on the inverter efficiency of the solar inverter. With the continuous promotion of intelligent algorithms and the continuous improvement of algorithm types, applying multi value optimization intelligent algorithms to the design of solar inverters will also greatly solve the problems caused by the shadow effect of the MPPT algorithm. However, multi value optimization algorithms will inevitably increase the MPPT algorithm, prolong the tracking response time, and even increase the response circuit components and change the topology structure, Moreover, due to the dynamic variation of the IV curve of the photovoltaic array input, there will not be long-term multi peak phenomena. Adopting the MPPT algorithm for single peak optimization is a good control strategy. Therefore, further research and practice are needed for intelligent algorithms for shadow effects and multi value optimization to achieve a win-win situation in terms of cost and benefits.
In addition, in the promotion of solar inverters, how to reduce the cost of solar inverters while ensuring that the working performance and safety factor of solar inverters are not affected is the breakthrough point that will be sought in the future promotion of grid connected solar micro inverters. Otherwise, it is difficult to promote and popularize a solar inverter device that has reached its service life limit before the cost is recovered. If solar energy technology is not promoted and popularized, it will seriously affect the progress of clean energy transformation. The design of grid connected solar inverters is a massive system engineering that involves multiple complex technologies.
Through relevant research, it has been found that the main issues affecting its inverter efficiency are soft switching technology, leakage inductance absorption technology, voltage spike suppression technology, MPPT technology, and full wave inverter technology. So, through continuous improvement and optimization of relevant technologies, the efficiency of solar inverters can be continuously improved, which is also the main research direction in the design of solar inverters in the future.