Inverters Suitable for Different Occasions
There are many ways to classify inverters. For example, according to the number of phases of the AC voltage output by the inverter, it can be divided into single-phase inverters and three-phase inverters; according to the different types of semiconductor devices used in the inverter, Divided into transistor inverter, thyristor inverter, and turn-off thyristor inverter, etc. According to the principle of the inverter circuit, it can be divided into self-excited oscillation inverter, stepped wave superposition inverter, and pulse width modulation inverter. According to the application in the grid-connected system or off-grid system, it can be divided into the grid-connected inverter and off-grid inverter. To facilitate optoelectronic users to choose inverters, here are only classified according to the different applicable occasions of the inverter.
（1） centralized inverter
Centralized inverter technology is that several parallel photovoltaic strings are connected to the DC input end of the same centralized inverter. Generally, three-phase IGBT power modules are used for high power, and field-effect transistors are used for low power, and DSP is used at the same time. The conversion controller is used to improve the quality of the generated electric energy, making it very close to the sine wave current, which is generally used in the system of large photovoltaic power stations (>10kW). The biggest feature is the high power and low cost of the system, but 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, stains, etc.), a centralized inverse is adopted. The method of change will lead to a decrease in the efficiency of the inverter process and a decrease in the energy of the electricity users. At the same time, the power generation reliability of the entire photovoltaic system is affected by the poor working status of a certain photovoltaic unit group. The latest research direction is the use of space vector modulation control and the development of new inverter topology connections to obtain high efficiency under partial load conditions.
The string inverter is based on the modular concept. Each photovoltaic string (1-5kw) passes through an inverter, has maximum power peak tracking at the DC end, and is connected in parallel at the AC end. It has become the international most popular inverter on the market.
Many large photovoltaic power plants use string inverters. The advantage is that it is not affected by module differences and shadows between strings, and at the same time reduces the mismatch between the optimal working point of the photovoltaic module and the inverter, thereby increasing the power generation. These technical advantages not only reduce the cost of the system but also increase the reliability of the system. At the same time, the concept of “master-slave” is introduced between the strings, which makes the system connect several PV strings and let one or several of them work when a single string of electric energy cannot make a single inverter work. , To produce more electricity.
In the traditional PV system, the DC input terminal of each string inverter will be connected in series by about 10 photovoltaic panels. When 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 sheltering factors such as clouds, trees, chimneys, animals, dust, ice, and snow will cause the above factors, and the situation is very common. In the PV system of the micro-inverter, each panel is connected to a micro-inverter. When one of the panels does not work well, only this one will be affected. All other photovoltaic panels will run in the best working condition, making the overall system more efficient and generating more power. In actual 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 microinverter is quite small.