Photovoltaic power generation systems are connected to the grid or loads through inverters with various technical structures. Due to the diversity of application occasions, it will inevitably lead to the diversity of photovoltaic module installations. In order to maximize the conversion efficiency of solar energy and take into account the economic benefits, the diversification of inverters is required to achieve the best way of solar energy conversion.
The centralized inverter is a more traditional solution, that is, the photovoltaic strings of the power station are combined through the DC combiner box, and after the energy is transmitted to the inverter, the electric energy is converted from DC to AC in a centralized manner. The centralized inverter adopts the “centralized inverter, centralized MPPT tracking” scheme, and the power is relatively large, with a power range of 100~1000kW. It is mainly used in large-scale grid-connected photovoltaic power stations with a system installed capacity of megawatts or more. For example, Large-scale power generation systems such as desert power stations and ground power stations. The number of centralized inverters is small and the number of components is small, so it is easy to manage and maintain; the centralized inverter has high power density and low cost; but the centralized inverter is large in size, so it needs a dedicated computer room and is installed vertically. The power generation loss caused by the failure of the centralized inverter is relatively large, and the professional personnel of the manufacturer can only be repaired on site, and the maintenance time is longer. Since the centralized inverter power device adopts high-current IGBT, its own power consumption and the power consumption of ventilation and heat dissipation of the equipment room are large, so it is necessary to set up a special air duct on the outer wall of the inverter for forced cooling. The number of MPPT modules of the centralized inverter is very small, and the maximum power point tracking is performed on the entire photovoltaic array. Uneven sunlight and module differences will lead to low power generation efficiency of the centralized inverter. Therefore, the centralized inverter is mostly suitable for uniform sunlight. area. The protection level of the centralized inverter is generally IP20. The centralized inverter has complete protection functions and high power station safety; it has power factor adjustment function and low voltage ride-through function, and has good grid adjustment performance, which is suitable for centralized grid connection.
The string inverter directly converts the DC power generated by the photovoltaic modules into AC power, which is boosted and connected to the grid after being aggregated by the AC combiner box. The idea of string inverter is different from the traditional centralized inverter, that is, the inverter of small energy. The string inverter adopts the “decentralized inverter, decentralized MPPT tracking” scheme. The power of the inverter is small, and the power range is generally 3~50kW. It is suitable for small and medium-sized rooftop photovoltaic power generation systems and small ground power stations. String inverters are usually used in distributed photovoltaic power generation systems with small footprint and uneven illumination to more flexibly meet the power generation needs of different photo areas. Since the single-unit capacity of string inverters is relatively small, more inverters need to be equipped per megawatt, the corresponding cost is higher, and the total failure rate increases, which makes system monitoring difficult; Small size, small footprint, no need for a dedicated computer room, can be installed directly outdoors, and flexible installation. The power generation loss caused by the failure of the string inverter is small, and the on-site operation and maintenance personnel can directly replace the failed inverter. The power switch tube of the string inverter uses a small current MOSFET device, which has low loss and can be naturally dissipated. The string inverter has a multi-channel MPPT tracking circuit, which can make each photovoltaic string work at the maximum power point with high power generation efficiency. String inverters have a high degree of protection, mostly IP65. The power device and signal circuit of the string inverter are on the same board, which makes the design and manufacture of the inverter more difficult, and the reliability of the inverter is slightly worse. The number of string inverters is large, there is a risk of resonance, and the total harmonics are high.
The distributed inverter is a newly proposed inverter form in the past two years. It integrates the MPPT and DC/DC boost functions into the photovoltaic intelligent controller, and then concentrates the boosted DC power through the distributed inverter. Convert to alternating current. The distributed inverter adopts the scheme of “centralized inverter, decentralized MPPT tracking”. The distributed inverter is a product that combines the advantages of the centralized inverter and the string inverter, and realizes “the low cost of the centralized inverter and the high power generation of the string inverter. “. Because of the “centralized inverter”, the distributed inverter occupies a large area and requires a dedicated computer room, but the number is small, so the management and maintenance are convenient. The input voltage range of the distributed inverter is wider, and the working voltage range will be expanded from 450~820V (DC) of the traditional centralized inverter to 300~820V (DC), which can maximize the use of light energy, thereby Efficient use of energy in the morning and evening as well as in low light. The distributed inverter solution increases the input DC voltage from the DC voltage of about 600V (DC) of the traditional centralized inverter to a stable DC voltage of about 800V (DC), and the current drops by 25% under the same power, so the cross section of the input DC cable can be reduce. Also for AC output, the AC output voltage of the distributed inverter solution is also increased from the 315V (AC) voltage of the traditional centralized inverter to about 500V (AC), and the current drops by 37% under the same power, so the output AC cable The cross-section of the cable can be reduced, and the purchase cost of the cable can be reduced. The distributed inverter decentralizes the MPPT function to the DC combiner box, and the power generation efficiency is high. Compared with centralized inverters, distributed inverters in large-scale ground power stations with good consistency will increase the power generation of the system by 2% to 3%, and generate power in complex operating environments such as mountains and rooftops. The amount will be increased by 3% to 5%. The DC combiner box of the distributed inverter solution has more functions, and accordingly requires higher reliability of the communication cable. If you encounter a place with a long distance or strong interference, the communication cable needs to be replaced by an optical cable, and the related engineering costs need to be increased. There are few operating performance and manufacturers of distributed inverters. When large-scale application of distributed inverters, whether the performance of low voltage ride-through/zero voltage ride-through can meet the requirements of the grid has not been tested for a long time.
Micro-inverters have low power and are usually connected to several photovoltaic modules in parallel, so they are mainly used in small-scale photovoltaic power generation systems, such as household photovoltaic power generation systems, photovoltaic carports, etc. For household distributed photovoltaics, the safety and power generation of micro inverters, as well as the economical type of string type, these two types of inverters will be the mainstream products of household distributed photovoltaics.
Each photovoltaic inverter has unique advantages and disadvantages, and there is no absolute use limit. It should be selected reasonably according to the construction conditions and construction goals of each project, such as the largest power generation and the lowest cost per kilowatt-hour.