In recent years, new architectures of “micro inverter” and “micro converter” have emerged, that is, each solar cell module is equipped with a micro inverter, and the output power of each module is optimized to maximize the overall output power. Even if some panels are affected by shadows, dust coverage, etc., the inverter power optimizer can still track the best local MPP (Maximum Power Point). At the same time, the micro-inverter converts the input voltage/current into different output voltage/current to maximize the energy transmission in the system.
Micro-inverter technology proposes to directly integrate the inverter with a single photovoltaic module, and equip each photovoltaic module with an inverter module with AC-DC conversion function and maximum power point tracking function, which directly converts the electrical energy generated by the photovoltaic module It can be converted into AC power for use by AC loads or transmitted to the grid.
If one of the panels does not work well, only this one will be affected. All other photovoltaic panels will run in the best working condition, making the overall system more efficient and generating more power. In practical applications, if the string inverter fails, it will cause the panels of several kilowatts to fail to function, and the impact of the failure of the micro-inverter is quite small. Since each photovoltaic module corresponds to a micro-inverter, by adjusting the voltage and current of each row of photovoltaic panels, all can be balanced, and the problem of system mismatch is not easy to occur. In addition, each module has a monitoring function, which reduces the maintenance cost of the system and makes the operation more stable and reliable.
Due to the flexible configuration of the micro-inverter, solar cells can be installed in the household market according to the user’s financial resources, so it is very suitable for small household photovoltaic power generation systems. The photovoltaic power generation system composed of micro-inverters has low voltage, is safer, simpler and faster to install, has low maintenance and installation costs, and is less dependent on installation service providers, so that the solar power system can be used by some users who have a little understanding of electrical technology. Install by yourself.
Since the output voltage of a single module is low, in order to make the DC side voltage higher than the grid side peak voltage, the micro-inverter should have a step-up link. At present, micro-inverters mostly use high-frequency transformers, which have high power density, high efficiency, and can achieve electrical isolation between solar cells and the grid side.
(1) Classification of micro-inverters
At present, the topological types of micro-inverters are mostly single-stage and multi-stage. The traditional power decoupling scheme using electrolytic capacitors has low reliability, and the micro-inverter adopting the improved power decoupling scheme has higher reliability, which is the trend of micro-inverter research.
The single-stage micro-inverter directly converts the DC power output from the solar cell to the grid-side AC power through the high-frequency transformer, without other conversion links, and is simple in structure, but the control is more complicated. At present, the research on single-stage micro-inverters is mostly focused on the structure of flyback circuits. This type of inverter uses fewer components, low cost, and high reliability, and is suitable for low-power applications.
The two-stage inverter first raises the output voltage of the photovoltaic cell to a voltage value greater than the grid-side peak value through the DC/DC boost link, and performs maximum power tracking, and then converts to grid-connected alternating current through the subsequent inverter.
(2) Comparative analysis of micro-inverter topology
The micro-inverter based on the improved power decoupling scheme has higher reliability, and it is also the focus of current micro-inverter research. However, this type of micro-inverter still has the disadvantages of complicated circuit structure and generally low efficiency.
1) In the single-stage micro-inverter, after introducing an additional power decoupling circuit, although the secondary power disturbance can be effectively suppressed, the micro-inverter has a longer working life, but at the same time the equipment is inevitably increased The volume and cost are reduced, the overall efficiency of the equipment is reduced, and the control and circuit topology have become complicated. Seeking a more efficient and concise decoupling scheme is one of the problems to be solved by single-stage microinverters.
2) The circuit structure of the multi-level micro inverter is complicated, the energy conversion times are many, and the overall efficiency is reduced. This type of circuit mostly reduces the capacitance value by increasing the voltage of the decoupling capacitor terminal through the boost link. Because the micro-inverter based on the high-gain boost DC boost circuit loses the isolation of the transformer, the common-mode leakage current problem needs to be considered. The control and topology of the non-isolated grid-connected device are more complicated. The multi-level micro-inverter requires more components, which increases the cost of the equipment to a certain extent.
3) Three-phase micro-inverters are usually two-stage, and still need a step-up link. The overall circuit requires more components, and the cost is higher than that of a single-phase inverter circuit. Although the three-phase topology without a boost link is more efficient, the current application object is limited to specific high-power output photovoltaic modules, which is not universal. If the boost link is introduced, this type of topology is similar to the multi-stage topology, and the circuit requires more components.
Since micro-inverters mostly use small-capacity inverter designs, their efficiency is relatively low and their cost is relatively high. Analysis of the current micro-inverter structure shows that due to the simple structure of the single-stage micro-inverter, the number of switches required is small, the cost is lower than that of the multi-stage inverter, and the efficiency is relatively high. If it can be further improved Power decoupling circuit, while introducing soft switching technology, so that both power decoupling circuit and inverter circuit work in soft switching state, not only can reduce the loss of the main circuit, improve the overall efficiency, but also reduce the heating of the device, and further improve the system The reliability, high efficiency and low cost single-stage micro inverter will be more attractive. In addition, there are many factors that affect the reliability of micro-inverters. The current research on improving the working life of micro-inverters mainly focuses on how to replace electrolytic capacitors in circuits. In practice, the extreme working environment of micro-inverters , Packaging, manufacturing process, etc. will affect the reliability of the equipment.