Inverters, also known as power regulators and power regulators, are an essential part of photovoltaic systems. The most important function of a photovoltaic inverter is to convert the direct current generated by the solar panel into the alternating current used by household appliances. All the power generated by the solar panel must be processed by the inverter before being output.
The core of the inverter device is an inverter switching circuit, referred to as an inverter circuit for short. This circuit completes the inverter function by turning on and off the power electronic switch. The switching of power electronic switching devices requires certain driving pulses. These pulses may be adjusted by changing a voltage signal. The circuit that generates and regulates the pulse is often called a control circuit or control loop. The basic structure of the inverter device includes a protection circuit, an output circuit, an input circuit, an output circuit, and the like in addition to the inverter circuit and the control circuit described above.
The inverter not only has the function of direct AC conversion, but also has the function of maximizing the solar cell function and the system fault maintenance function. In summary, there are active operation and shutdown functions, maximum power tracking control function, anti-standalone operation function (for grid connection system), active voltage adjustment function (for grid connection system), DC detection function (for grid connection system), and DC ground detection Function (for grid-connected system).
Application range classification (1) Ordinary inverter DC 12V or 24V input, AC 220V, 50Hz output, power from 75W to 5000W, some models have AC and DC conversion, that is, UPS function.
(2) Inverter / charging machine In this type of inverter, users can use various forms of power to power the AC load: when AC power is available, use the AC power to power the load through the inverter, or charge the battery; when there is no AC power, use the battery to power the AC load . It can be used in combination with various power sources: batteries, generators, solar panels and wind turbines.
(3) Special inverter for post and telecommunications Provide high-quality 48V inverters for post and telecommunications, communication, with good product quality, high reliability, modular (1KW module) inverter, with N + 1 redundancy function, scalable (power from 2KW to 20KW ).
(4) Special inverters for aviation and military This type of inverter has 28Vdc input and can provide the following AC output: 26Vac, 115Vac, 230Vac, its output frequency can be: 50Hz, 60Hz and 400Hz, the output power ranges from 30VA to 3500VA. There are also DC-DC converters and inverters for aviation.
Output waveform classification (1) Square wave inverter The AC voltage waveform output by the square wave inverter is a square wave. The inverter circuits used in this type of inverter are not exactly the same, but the common feature is that the circuit is relatively simple and the number of power switch tubes used is small. Design power is generally between 100 watts and kilowatts. The advantages of square wave inverter are: simple circuit, cheap price and easy maintenance. The disadvantage is that because the square wave voltage contains a large number of higher harmonics, additional losses will occur in the load appliances with core inductors or transformers, which will interfere with the radio and some communication equipment. In addition, this type of inverter has the disadvantages that the voltage regulation range is not wide enough, the protection function is not perfect, and the noise is relatively large.
(2) Step-wave inverter The AC voltage waveform output by this type of inverter is a step wave. There are also many different lines for the inverter to achieve a step wave output, and the number of steps in the output waveform varies greatly. The advantages of the step wave inverter are that the output waveform is significantly improved than the square wave, and the higher harmonic content is reduced. When the step reaches more than 17, the output waveform can realize a quasi-sine wave. When no transformer output is used, the efficiency of the whole machine is very high. The disadvantage is that there are many power switch tubes used in the step wave superposition line, and some of the line forms also require multiple sets of DC power input. This brings troubles to the grouping and wiring of solar cell squares and the balanced charging of storage batteries. In addition, the step wave voltage still has some high-frequency interference on the radio and some communication equipment.
(3) Sine wave inverter The AC voltage waveform output by the sine wave inverter is a sine wave. The sine wave inverter has the advantages of good output waveform, low distortion, low interference to the radio and equipment, and low noise. In addition, it has complete protection functions and high efficiency. The disadvantages are: the circuit is relatively complicated, the maintenance technology is high, and the price is expensive.
The classification of the above three types of inverters is helpful for designers and users of photovoltaic systems and wind power generation systems to identify and select inverters. In fact, inverters with the same waveform still have great differences in terms of line principle, use of devices and control methods.
Other classification methods 1. According to the output AC power frequency classification, it can be divided into industrial frequency inverter, intermediate frequency inverter and high frequency inverter. Power frequency inverters are inverters with a frequency of 50 to 60 Hz; intermediate frequency inverters generally have a frequency of 400 Hz to more than ten kHz; high frequency inverters generally have a frequency of more than ten kHz to MHz.
2. According to the number of phases output by the inverter, it can be divided into single-phase inverter, three-phase inverter and multi-phase inverter.
3. According to the whereabouts of the inverter’s output power, it can be divided into active inverters and passive inverters. An inverter that sends the power output from the inverter to the industrial grid is called an active inverter; an inverter that sends the power output from the inverter to a certain power load is called a passive inverter Device.
4. According to the form of the inverter’s main circuit, it can be divided into single-ended inverters, push-pull inverters, half-bridge inverters and full-bridge inverters.
5. According to the type of the inverter’s main switching device, it can be divided into thyristor inverters, transistor inverters, field effect inverters, and insulated gate bipolar transistor (IGBT) inverters. It can be summarized into two categories: “semi-controlled” inverter and “full-controlled” inverter. The former does not have self-shutdown capability, and the components lose control after they are turned on, so it is called a “semi-controlled” ordinary thyristor. Both on and off can be controlled by the control electrode, so it is called “full control type”. Power field effect transistor and insulated gate double weight transistor (IGBT), etc. belong to this category.
6. According to DC power supply, it can be divided into voltage source inverter (VSI) and current source inverter (CSI). In the former, the DC voltage is nearly constant and the output voltage is an alternating square wave; in the latter, the DC current is nearly constant and the input current is an alternating square wave.
7. According to the inverter control mode, it can be divided into frequency-modulated (PFM) inverter and pulse-width-modulated (PWM) inverter.
8. According to the working mode of the inverter switching circuit, it can be divided into resonant inverters, fixed-frequency hard-switching inverters and fixed-frequency soft-switching inverters.
9. According to the inverter commutation method, it can be divided into load commutated inverter and self-converted inverter.