In electrical technology, the process of converting AC power into DC power is called rectification. The circuit that completes the rectification function is called a rectifier circuit, and the device that realizes the rectification process is called a rectifier or rectifier. Correspondingly, the process of converting DC power into AC power is called inverter, the circuit that completes the inverter function is called inverter circuit, and the device that realizes the inverter process is called inverter equipment or inverter.
According to the use of inverter in photovoltaic power generation system, it can be divided into independent power supply and grid-connected use. According to the waveform modulation method, it can be divided into square wave inverter, step wave inverter, sine wave inverter and combined three-phase inverter. The inverters used in grid-connected photovoltaic power generation systems can be divided into transformer-type inverters and transformer-less inverters according to the presence or absence of transformers.
According to the different applicable occasions of the inverter to classify:
(1) Centralized inverter
Centralized inverter technology is that several parallel photovoltaic strings are connected to the DC input of the same centralized inverter. Generally, three-phase ICBT power modules are used for higher power, and field effect transistors are used for lower power. Digital signal processing technology (Digital Signal Process, DSP) converts the controller to improve the quality of the generated electric energy, making it very close to the sine wave current, and is generally used in the system of large photovoltaic power stations (installed capacity> 10kW). The biggest feature is the high power and low cost of the system. However, because the output voltage and current of different photovoltaic strings are often not completely matched (especially when the photovoltaic strings are partially shaded due to cloudy, shade and stains, etc.), a centralized inverse is adopted. The method of change will cause the efficiency of the inverter process to decrease. At the same time, the power generation reliability of the entire photovoltaic system is affected by the poor working state of a photovoltaic module string. The latest research direction is the use of space vector modulation control and the development of new topologically connected inverters to obtain high efficiency under partial load conditions. The centralized inverter is shown in Figure 1.
(2) String inverter
The string inverter is realized based on the modular concept and has become the most popular inverter method in the international market. Each photovoltaic string (1~5kWp) passes through an inverter and has a very High-power point tracking function, parallel and grid-connected at the AC end, as shown in Figure 2.
Many large-scale photovoltaic power plants use string-type converters. The advantage is that it reduces the mismatch between the optimal working point of the photovoltaic module and the inverter when it is not affected by the module difference and shadow between the strings, thereby increasing the power generation. The technological trend not only reduces the cost of the system, but also increases the reliability of the system. At the same time, under the concept of “group one follower”, the system can connect several groups of photovoltaic groups together when the power of a single square array cannot make a single inverter work, and one or several of them work to produce more Of electrical energy.
(3) Micro inverter
In the traditional PV system, there will be about 10 photovoltaic panels connected to the DC input end of each string inverter. If one of the 10 panels connected in series does not work well, this string will be affected. If the inverter uses the same MPPT for multiple inputs, each input will also be affected, greatly reducing the power generation efficiency. In practical applications, various blocking factors such as clouds, trees, chimneys, animals, dust, ice and snow will cause the above-mentioned situations, which are very common. In the PV system using micro-inverters, each panel is connected to a micro-inverter. When one of the panels does not work well, only this one will be affected, and the other photovoltaic panels will be affected. Running in the best working state makes the overall system more efficient and generates 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. The latest concept is that several inverters form a “team” to replace the “master-slave” concept, which makes the reliability of the system a step further. The micro inverter is shown in Figure 3.
(4) Power optimizer
The installation of a power optimizer (Opimizer) in the solar power generation system can greatly improve the conversion efficiency. In order to realize a smart solar power generation system, the installation of a power optimizer can allow each solar cell to exert its best performance and monitor the battery consumption status at any time. The power optimizer is a device between the power generation system and the inverter. The main task is to replace the original best power point tracking function of the inverter. The power optimizer simplifies the circuit, so that the single solar cell corresponds to a power optimizer, and performs extremely fast maximum power point tracking and scanning, so that each solar cell can indeed reach the maximum power point. In addition, it can also The communication chip is implanted to monitor the battery status anytime and anywhere, and immediate feedback of problems allows relevant personnel to repair as soon as possible.
