Classification of photovoltaic inverters

- Nov 01, 2018-

There are many methods for classifying inverters. For example, according to the number of phases of the output AC voltage of the inverter, it can be divided into a single-phase inverter and a three-phase inverter; depending on the type of semiconductor device used by the inverter, Divided into transistor inverter, thyristor inverter and turn-off thyristor inverter. According to the principle of the inverter circuit, it can also be divided into a self-excited oscillation type inverter, a staircase wave superposition type inverter, and a pulse width modulation type inverter. According to the application in the grid-connected system or off-grid system, it can be divided into grid-connected inverter and off-grid inverter. In order to facilitate the selection of inverters for optoelectronic users, only the inverters are classified here.

1. Centralized Inverter

The centralized inverter technology is that several parallel PV strings are connected to the DC input terminal of the same centralized inverter. Generally, the three-phase IGBT power module is used for high power, and the field effect transistor with low power is used, and DSP is used at the same time. The conversion controller is used to improve the quality of the electrical energy produced, making it very close to the sinusoidal current, typically used in systems with large photovoltaic power plants (>10 kW). The biggest feature is that the system has high power and low cost. However, because the output voltage and current of different PV strings are not completely matched (especially when the PV strings are partially blocked due to cloudy, shade, stains, etc.), the concentrated inverse is adopted. The changing method will lead to a decrease in the efficiency of the inverter process and a decrease in the power of the electric household. At the same time, the reliability of power generation of the entire photovoltaic system is affected by the poor working condition of a certain photovoltaic unit. The latest research direction is the use of space vector modulation control and the development of new inverter topology connections to achieve high efficiency under partial load conditions.

2. String Inverter

The string inverter is based on the modular concept. Each PV 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. The most popular inverter on the market.

Many large photovoltaic power plants use string inverters. The advantage is that it is not affected by the difference of modules between the strings and the shadowing, and at the same time, the optimal working point of the photovoltaic component is not matched with the inverter, thereby increasing the power generation. These technical advantages not only reduce system cost, but also increase system reliability. At the same time, the concept of "master-slave" is introduced between the strings, so that in the case that a single string of electric energy cannot make a single inverter work, several sets of photovoltaic strings are linked together, and one or several of them are worked. , thus producing more electricity.

The latest concept is that several inverters form a "team" to replace the "master-slave" concept, making the reliability of the system a step further. At present, transformerless string inverters have taken the lead.

3. micro inverter

In the traditional PV system, the DC input terminal of each string inverter will be connected in series by about 10 photovoltaic panels. If one of the 10 series connected panels does not work well, the string will be affected. If the inverter's multiple inputs use the same MPPT, then each input will also be affected, greatly reducing power generation efficiency. In practical applications, various factors such as clouds, trees, chimneys, animals, dust, ice and snow will cause these 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. Other photovoltaic panels will operate at optimum operating conditions, making the overall system more efficient and generating more power. In practical applications, if a string inverter fails, it will cause several kilowatts of the panel to fail to function, and the micro-inverter fault has a relatively small impact.

4. power optimizer

Adding a power optimizer (OptimizEr) to the solar power system can greatly improve the conversion efficiency and simplify the function of the inverter (Inverter). In order to realize a smart solar power system, the device power optimizer can ensure the best performance of each solar cell and monitor the battery consumption state 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 optimal power point tracking function of the inverter. The power optimizer performs an extremely fast optimal power point tracking scan analogously by simplifying the line and a single solar cell corresponding to a power optimizer, so that each solar cell can achieve the best power point tracking. In addition, the battery status can be monitored anytime and anywhere by placing a communication chip, and the immediate return problem allows the relevant personnel to repair as quickly as possible.

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