Are you familiar with photovoltaic inverters? I believe that many friends are not very clear about it, with the development of China's photovoltaic industry, we will certainly have more opportunities to contact photovoltaic in the future, then, together to understand the principle of photovoltaic inverter it .
I. Working principle and characteristics.
Working principle: the core of the inverter device, is the inverter switch circuit, referred to as the inverter circuit. This circuit completes the function of the inverter through the conduction and shutdown of the power electronic switch.
Features.
(1) Requires high efficiency.
Due to the current high price of solar cells, in order to maximize the use of solar cells and improve the efficiency of the system, it is necessary to find ways to improve the efficiency of the inverter.
(2) Requires high reliability.
At present, PV power station systems are mainly used in remote areas, and many power stations are unattended and maintained, which requires the inverter to have a reasonable circuit structure, strict component selection, and requires the inverter to have various protection functions, such as: input DC polarity reversal protection, AC output short circuit protection, overheating, overload protection, etc.
(3) requires a wide range of input voltage adaptation.
As the terminal voltage of the solar cell changes with the load and sunlight intensity. Especially when the battery is aging, its terminal voltage changes in a wide range, such as 12V battery, its terminal voltage may change between 10V ~ 16V, which requires the inverter to ensure normal operation in a wide range of DC input voltage.
Second, the PV inverter classification
There are many ways to classify the inverter, for example: according to the number of phases of the inverter output AC voltage, it can be divided into single-phase inverter and three-phase inverter; according to the different types of semiconductor devices used in the inverter, it can be divided into transistor inverter, thyristor inverter and turn-off thyristor inverter, etc. According to the different inverter line principle, it can also be divided into self-excited oscillating inverter, step wave superposition inverter and pulse width modulation inverter, etc. According to the application in the grid-connected system or off-grid system can be divided into grid-connected inverter and off-grid inverter. In order to facilitate the selection of inverters for photovoltaic users, here is only the classification of inverters according to the different applications.
1、Concentrated inverter
Centralized inverter technology is a number of parallel photovoltaic strings are connected to the DC input of the same centralized inverter, generally using three-phase IGBT power modules for large power, and field-effect transistors for smaller power, while using DSP conversion controller to improve the quality of the output power, so that it is very close to the sine wave current, generally used in large photovoltaic power plant (>10kW) system. The most important feature is the high power and low cost of the system, but since the output voltage and current of different PV strings are often not exactly matched (especially when the PV strings are partially shaded due to clouds, shade, stains, etc.), the use of centralized inverter will lead to a reduction in the efficiency of the inverter process and a decrease in the electric household energy. At the same time, the reliability of the whole PV system is affected by the poor working condition of one PV unit. 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.
2. String inverters
String inverter is based on the modular concept, each PV string (1-5kw) through an inverter with maximum power peak tracking at the DC side and parallel grid connection at the AC side, has become the most popular inverter in the international market now.
Many large PV plants use string inverters. The advantage is that they are not affected by module differences and shading between strings, while reducing the mismatch between the optimal operating point of the PV modules and the inverter, thus increasing power production. These technical advantages not only reduce system costs, but also increase system reliability. At the same time, the concept of "master-slave" between the strings makes the system to link several PV strings together and let one or more of them work when a single string of power cannot make a single inverter work, thus producing more power.
3、Micro-inverter
In a traditional PV system, the DC input of each string inverter will be connected by 10 or so PV panels in series. When 10 pieces of series connected panels, if one can not work well, then the string will be affected. If the inverter uses the same MPPT for multiple inputs, then each input will also be affected, significantly reducing the power generation efficiency. In practical applications, clouds, trees, chimneys, animals, dust, snow and ice and other shading factors can cause the above factors, and the situation is very common. In a PV system with micro-inverters, each panel is connected to a separate micro-inverter, and when one of the panels does not work well, only that one panel will be affected. All the other panels will operate in the best working condition, making the overall system more efficient and generating more power. In practice, if a string inverter fails, several kilowatts of panels will not function, while the impact of a micro-inverter failure is quite small.
4, power optimizer
Solar power system with power optimizer (OptimizEr) can significantly improve the conversion efficiency, and the inverter (Inverter) function into a simple cost reduction. To achieve an intelligent solar power system, the installation of a power optimizer ensures that each solar cell performs at its best and monitors the status of battery depletion at all times. A power optimizer is a device that sits between the power generation system and the inverter, and its main task is to replace the inverter's original optimal power point tracking function. By simplifying the wiring and by using a single solar cell for a power optimizer, the power optimizer can perform an analogous and extremely fast optimal power point tracking scan, so that each solar cell can actually reach the optimal power point tracking.
