PV inverters can be divided into four main categories: centralized, string, distributed and micro inverters.
PV inverters can be divided into four main categories: centralized, string, distributed and micro inverters. The total system power of centralized inverters is large and thus centralized inverters are mainly used in large-scale projects such as ground-mounted photovoltaic power plants 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. Distributed inverters have the characteristics of both centralized and string inverters, and are used on a certain scale in projects. Micro inverters are used to track the maximum power peak of each PV module individually, and then are inverted and connected to the AC grid. The capacity of micro-inverter is usually less than 1kW.
The centralized access to a large number of PV strings, and the capacity of a single unit is usually over 500KW. The centralized inverter is one of the most common types on the market. Its working principle is to converge and maximize power peak tracking (MPPT) of DC current generated by multiple PV modules, and then the centralized inverter works for direct AC-DC power conversion and voltage boosting to realize grid-connected power generation. The centralized inverter generally adopts a single MPPT, and a single MPPT is equipped with 2-12 groups of PV strings, and the power of each MPPT can reach 125-1000KW, and the single capacity is usually over 500KW, which has the advantage of high power and large capacity.
Centralized inverters can reduce the number of use, reduce system costs and losses, and facilitate centralized management. Since centralized inverters have the advantage of large capacity, the use of centralized PV power plants of the same scale can greatly reduce the number of inverters used, which can 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 significantly reduce the equipment cost of the power plant system.
The application of centralized inverters can effectively reduce harmonics and improve the overall power 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 produce harmonic voltage drops on the short-circuit impedance of the grid, thus affecting the voltage waveform; they can easily cause series-parallel resonance in the system, resulting in damage to the equipment. Centralized inverters used in smaller quantities can reduce the number of series-parallel connections and effectively reduce the harmonic content, thus ensuring the proportion of fundamental waves in power generation and improving the overall power quality.
The centralized MPPT has a narrow voltage range when multiple DC inputs are connected, which affects the power generation performance. The centralized inverter overall can’t guarantee that each string is at the best working point because of the large number of strings connected to the single MPPT, thus reducing the overall power generation efficiency of the system. The voltage range of centralized MPPT is generally in the range of 500-850V, and the adjustability of the centralized inverter is poor due to the narrow MPPT voltage range. In the case of poor lighting conditions such as rain, the system voltage is lower than the minimum voltage of the inverter MPPT, which prevents normal power generation and affects the power generation time. At the same time, due to the characteristics of accessing multiple DC inputs, the PV system requires good adaptability among the modules, and if one of the modules fails, it will affect the overall power generation and efficiency of the system.
The centralized inverter is large and requires a dedicated room, making it more difficult to install. Due to the large capacity of a single unit, the centralized inverter is larger and heavier, requiring a dedicated room to be built outdoors for placement. The large area of the dedicated room increases the overall land cost of the system and also makes the installation more difficult. In addition, due to the airtightness of the room, the inverters are placed in the room, which leads to poor ventilation and heat dissipation problems.
String inverters have a modular design that allows for decentralized MPPT seeking. A string inverter system usually converts the DC current generated by the modules to AC through the inverter, and then connects to the AC grid after sinking, boosting, and AC distribution. Compared with centralized inverters, string inverters have a modular design with multiple MPPTs; the number of inputs to each MPPT is small, usually 1-4 groups, which enables decentralized MPPT seeking. Because of the small number of access points, when a single module fails, it only affects the power generation of the corresponding module, which ensures that the power generation efficiency of the overall PV system is not affected by individual modules and solves the mismatch problem of centralized PV power plants.
String MPPT has a wide voltage range, which can improve the power generation time and capacity of the system. The MPPT voltage range of the string inverter is wide, usually 200V-1000V, which has good adjustability. In case of insufficient light or unfavorable weather, the overall voltage of PV modules will be low, and the wide MPPT voltage range can cover low input voltage, thus ensuring the power generation time of the system and improving the overall power generation.
The parallel connection of multiple inverters increases the wire loss and is prone to resonance problems. Compared with centralized inverters, string inverters have a smaller capacity, usually 100KW or less, and the number of inverters will be increased when string inverters are used for the same size PV plant. Multiple string inverters will be connected in parallel, and the wire loss will increase as the number of inverters increases. At the same time, the parallel connection of multiple inverters will lead to the increase of total harmonics, which will be more difficult to suppress and cause more serious resonance problems, and will easily cause the failure and burnout of electrical equipment.
The central distributed inverter is a new type of inverter that combines the advantages of both centralized and string inverters. It can be understood as a centralized inverter and decentralized optimization search, firstly, the maximum power peak tracking (MPPT) is performed separately by multiple string inverters, and then it is inverted into AC power and connected to the grid after convergence. Compared with the centralized inverter, the central distributed inverter has the advantages of excellent independent performance, high power generation and overall system stability; compared with the string inverter, the central distributed inverter adopts the centralized inverter after decentralized optimization, which greatly reduces the equipment cost of the system. Since the development of a central distributed solution is late and the project experience is not sufficient, it has not yet been applied on a large scale; meanwhile, due to the centralized inverter method, this solution needs a special room to dissipate the heat of the centralized inverter, which increases the use area of the system.
The micro inverter can control MPPT on individual modules, resulting in higher power generation efficiency and capacity levels. Unlike other inverters, micro inverters are integrated with each PV module, allowing MPPT control of individual modules, which significantly improves the overall system efficiency and power production. At the same time, microinverters are small in size and light in weight, and do not require additional land for installation, which greatly enhances the ease of installation. It is mainly suitable for small and medium-sized power plant projects such as household use. To build a power plant of a certain scale, more micro inverters are needed to put into use, and the overall cost of the system is significantly higher than that of a centralized or string inverter system.
SOURCE: SWS RESEARCH