Photovoltaic on grid inverters can be divided into string inverters, centralized inverters and micro inverters according to the different combinations of photovoltaic panels.
String inverter
String inverters are becoming the most popular inverters in the international market. It is a design based on a modular concept. Multiple photovoltaic panels are connected in series according to the rated input voltage requirements of the inverter. They are connected in parallel to the grid through an inverter, and the inverter performs maximum power peak tracking at the DC end. There are also string inverters that allow multiple strings to be connected and perform multi-channel MPPT tracking control. Their power capacity is about 1~50kW. They are usually used in photovoltaic buildings BIPV (Building Integrated Photovoltaic), BAPV (Building Attached In photovoltaic systems such as Photovoltaic or Roof Plant, it is also called residential or commercial photovoltaic inverter.
String inverters are particularly suitable for use in distributed power generation systems. Taking the common 250W polysilicon solar panel as an example, a single string can range from 3 panels to 23 panels. The combination is very flexible. The roof of the building can also have a more optimized scheme. The advantage of the string photovoltaic inverter is that each string has an independent MPPT, which does not affect each other, which avoids the imbalance between strings or the influence of shadows on the system. Another advantage of the string inverter is that the DC input range is relatively wide, which can extend the power generation time and increase the power generation. In the past two years, there have been more and more cases of using string inverters to design megawatt-level photovoltaic power stations, and string inverters have certain advantages in the application of large power stations.
According to the integration into the power grid, string inverters can be divided into string single-phase inverters and string three-phase inverters. Generally, string inverters are directly integrated into industrial or civil power grids. Single-phase inverters are mainly used in civil and commercial roofs where single-phase electricity enters the home, and the power of a single-phase inverter is generally 1~5kW. Three-phase inverters are mainly used in commercial and industrial roofs, and the power of a single unit is generally 6~30kW.
Centralized inverter
Centralized inverters are generally used in large-scale power generation systems such as large workshops, desert power stations, and ground power stations with uniform sunlight. The total system power is large, generally above 100kW~1MW. Multiple PV strings are connected in parallel to the combiner box, and then connected to the DC input end of a centralized inverter for on grid power generation. It has the following advantages:
①The inverter has high integration, high power density and low cost:
②The inverter has a small number of components and high reliability:
③The number of inverters is small, which is easy to manage;
④ Less harmonic content, less DC component, and high power quality;
⑤Various protection functions of the inverter are complete, and the safety of the power station is high;
⑥With power factor adjustment function and low voltage ride through function, the power grid has good adjustability.
However, due to the large number of strings connected in parallel, the characteristic matching of photovoltaic modules is different or the influence of partial shadowing results in inconsistent maximum power point tracking (MPPT) characteristics of each string, so the efficiency of the photovoltaic system is low. It needs a dedicated computer room and air-conditioning facilities, its own power consumption is large, and the maintenance workload is large. At the same time, the poor working status of a photovoltaic unit group will affect the power generation reliability of the entire photovoltaic system. A single inverter has a large power, and once a failure occurs, there is no redundancy measure, which has a greater impact on the grid. For considerations such as reliability and efficiency optimization,-“Some large photovoltaic power plants are also using string inverters. The advantage is that they are not affected by module differences and shadows between strings, and at the same time, they reduce the maximum amount of photovoltaic modules. The mismatch between the good point and the inverter increases the power generation. The technical advantage not only reduces the cost of the system, but also increases the reliability of the system. At the same time, the concept of “master-slave” is introduced between the strings, In this way, when a single string of electric energy cannot make a single inverter work, the system connects several sets of photovoltaic strings together, and allows one or several of them to work, so as to produce more electric energy.
Micro on grid inverter
A micro on grid inverter is a converter device that directly boosts, inverts, and merges the DC power of a single photovoltaic panel into the grid. Generally, the power capacity is less than 1000W, so it is called a micro inverter. It has the ability to track the maximum power point at the component level, and can be integrated on the photovoltaic panel assembly as an adapter between a single photovoltaic panel and the grid, which makes the photovoltaic power generation system plug-and-play, even without professional technicians. Operation and Maintenance. In large-scale on grid applications, it is necessary to coordinate and control each module through the communication function, monitor the status of each module, and detect the faulty module. Micro-inverters have the above-mentioned advantages, but their unit power cost is relatively high, and they are not suitable for the use of large-scale photovoltaic electric fields.
The three kinds of on grid inverters have their own advantages and disadvantages, and they all have their own suitable applications. They need to be selected according to the actual application, and they can also be combined to optimize efficiency.