Main types and working principles of photovoltaic inverters
Photovoltaic inverters can be mainly divided into four categories: centralized, string, distributed and micro inverters. The centralized inverter system has a large total power and is mainly used in large-scale projects such as ground photovoltaic power stations with good lighting conditions; distributed inverters can be divided into string inverters and micro inverters, which are usually used in small and medium-sized Industrial, commercial and household photovoltaic power generation systems, in which string type is the main distributed inverter product type. Distributed inverters have both centralized and string-type features, and are widely used in projects such as mountain frontrunners. The micro-inverter is to track the maximum power peak value of each photovoltaic module independently, and then merge into the AC grid after inversion. The single-unit capacity of the micro-inverter is generally below 1kW.
The number of centralized access photovoltaic group strings is large, and the single capacity is usually more than 500KW. The centralized inverter is a common kind of photovoltaic inverter in the market. Its working principle is to merge the DC current generated by multiple photovoltaic modules and track the maximum power peak (MPPT), and then the centralized inverter converts the direct alternating current and boosts the voltage, so as to realize the grid power generation. A single MPPT is equipped with 2-12 photovoltaic groups. The power of each MPPT can reach 125-1000kW, and the capacity of each MPPT is usually above 500KW, which has the advantages of high power and large capacity.
Centralized inverters can reduce the number of use, reduce system costs and losses, and facilitate centralized management. Due to the advantages of large capacity of centralized inverters, the use of centralized inverters for photovoltaic power stations of the same scale can greatly reduce the number of inverters used, reduce the overall circuit loss of the system, and facilitate centralized installation and management. At the same time, the centralized inverter itself has a high degree of integration, simple control, relatively mature technology, and low unit cost. The combination of the two factors can greatly reduce the equipment cost of the power station system.
The application of centralized inverters can effectively reduce harmonics and improve the overall power generation quality of the system. When performing the Fourier decomposition of the non-sinusoidal charge, we will get a part of the charge greater than the fundamental frequency, that is, the harmonic, whose frequency is usually an integer multiple of the fundamental frequency. Harmonics will produce harmonic voltage drop on the short-circuit impedance of the power grid, thereby affecting the voltage waveform; it is easy to cause local series-parallel resonance in the system, resulting in equipment damage. The number of centralized inverters used is small, which can reduce the number of series and parallel, and effectively reduce the harmonic content, thereby ensuring the proportion of fundamental waves in the power generation, and improving the overall power generation quality.\
Connecting to multiple sets of DC inputs, the centralized MPPT voltage range is narrow, which affects the overall power generation performance. The number of PV strings connected to a single MPPT of the centralized inverter is large, and it is impossible to precisely control each group of PV strings, so it cannot guarantee that each string is in the best working point, thus reducing the overall system cost. power generation efficiency. The voltage range of the centralized MPPT is generally in the range of 500-850V. Due to the narrow MPPT voltage range, the adjustability of the centralized inverter is poor. Under unsatisfactory lighting conditions such as cloudy rain, the voltage of the system is lower than the minimum voltage of the inverter MPPT, and normal power generation cannot be performed, which affects the power generation time. At the same time, due to its characteristics of accessing multiple sets of DC inputs, photovoltaic systems require good adaptation performance between components. Once one of the components fails, it will affect the overall power generation and power generation efficiency of the system.
The centralized inverter is large in size and needs to be placed in a dedicated computer room, which increases the difficulty of installation. Due to the large capacity of the single unit, the volume and weight of the centralized inverter are large, and a special equipment room needs to be established outdoors for placement. The dedicated computer room occupies a large area, which increases the difficulty of installation while increasing the overall land cost of the system. In addition, due to the airtightness of the equipment room, placing the inverter in the equipment room will lead to poor ventilation inside the equipment room, resulting in thermal problems.
The string inverter adopts a modular design, which can realize decentralized MPPT optimization. The power station system using string photovoltaic inverter usually converts the DC current generated by the modules first through the inverter, and then merges it into the AC grid after confluence, step-up transformation and AC power distribution. Compared with the centralized inverter, the string inverter adopts a modular design and has multiple MPPTs; the number of PV modules connected to each MPPT is less, usually 1-4 groups, which can realize distributed MPPT. Search for the best. Since there are few access terminals, when a single component fails, it will only affect the power generation of the module corresponding to the component, ensuring that the power generation efficiency of the overall photovoltaic system is not affected by a single component, and solving the mismatch problem of centralized photovoltaic power plants .
The string MPPT has a wide voltage range, which can improve the power generation time and power generation of the system. The MPPT voltage range of the string inverter is wide, usually 200V-1000V, and the adjustability is good. In the case of insufficient light or unfavorable weather for power generation, the overall voltage of photovoltaic modules will be low. The wider MPPT voltage range can cover low input voltage, thus ensuring the power generation time of the system and improving the overall power generation.
Paralleling multiple inverters increases wire losses and is prone to resonance problems. Compared with centralized inverters, the individual capacity of string inverters is smaller, usually 100KW or less; for photovoltaic power stations of the same scale, choosing to use string inverters will increase the number of inverters. Multiple string inverters will be connected in parallel, and the wire loss will increase as the number of inverters used increases. At the same time, the parallel connection of multiple inverters will lead to the increase of total harmonics, the difficulty of suppression will be increased, the resonance problem will be more serious, and it will easily cause the failure and burning of electrical equipment.
Distributed inverter is a new type of inverter that combines the advantages of centralized and string type. Distributed inverter is a relatively new type of photovoltaic inverter, which has the characteristics of centralized inverter and string inverter. Distributed inverters can be understood as centralized inverter and decentralized optimization. First, the maximum power peak tracking (MPPT) is carried out separately through multiple string inverters, and then centralized inverters are converted into AC power grids after confluence. Compared with centralized inverters, distributed inverters have the advantages of excellent independent performance, high power generation and overall system stability; compared with string inverters, distributed inverters use decentralized optimization. The latter centralized confluence inverter greatly reduces the equipment cost of the system. Currently, it is mainly used in some leading demonstration base projects in China. Due to the late development of the distributed inverter solution, the project experience is not sufficient, and large-scale application has not yet been formed; at the same time, due to the centralized inverter method, this solution needs to use a dedicated computer room to dissipate heat from the centralized inverter , increasing the usable area occupied by the system.
The micro-inverter can perform MPPT control of individual components, and the power generation efficiency and power generation level are high. Different from other inverters, the micro-inverter is integrated with each photovoltaic module, and can perform maximum power peak tracking (MPPT) control on a single module, thereby greatly improving the overall power generation efficiency and power generation of the system. At the same time, the micro-inverter has the characteristics of small size and light weight, and does not require additional storage space, which greatly enhances the convenience of installation. It is mainly suitable for small and medium-sized power station projects such as households. For a power station of the same scale, the use of micro-inverters will require more equipment, and the overall cost of the system is significantly higher than that of systems using centralized or string inverter solutions.
