There are many principles for the classification of inverters. For example, according to the number of phases of the inverter output AC voltage, it can be divided into single-phase inverters and three-phase inverters; according to the different types of semiconductor devices used in the inverter, it can be divided into transistor inverters, thyristor inverters and turn-off thyristor inverters; acording to the principle of the inverter circuit, it can also be divided into self-excited oscillation inverter, stepped wave superposition inverter and pulse width modulation inverter. In order to facilitate the selection of inverters for solar photovoltaic power plants, here only the different output AC voltage waveforms of the inverters are classified, and the characteristics of the inverters with different output waveforms are briefly explained.
1. Square wave inverter
The AC voltage waveform output by the square wave inverter is a square quilt, as shown in Figure 1(a). The inverter circuits used by such inverters are not exactly the same, but the common feature is that the circuits are relatively simple and the number of power switch tubes used is small. The design power is generally between tens of watts to hundreds of watts. The advantages of square wave inverters are: cheap and simple maintenance. The disadvantage is: because the square wave voltage contains a large number of high-order harmonics, additional losses will be generated in the electrical appliances with the transformer as the load, and it will also interfere with the radio and some communication equipment; in addition, some of these inverters have insufficient voltage regulation range, and some have insufficient protection functions, and the noise is relatively large.
2. Step wave inverter
The AC voltage waveform output by this type of inverter is a staircase wave, as shown in Figure 1(b). There are also many different circuits for the inverter to realize the step wave output, and the number of steps of the output waveform is also different. The advantage of the ladder wave inverter is that the output waveform is significantly improved compared with the square wave, and the content of the high-order wave is reduced. When the ladder reaches more than 17, the output waveform can achieve a quasi-sine wave; when the transformerless output is used, the overall efficiency is very high. The disadvantage is that there are many power switch tubes used in the ladder wave superposition line, and some of the line forms also require multiple sets of DC power input, which brings trouble to the grouping and wiring of the solar cell array and the balanced charging of the battery pack; in addition, there is still some high level of interference from the stair wave voltage to radios and some communication equipment.
3. Sine wave inverter
The AC voltage waveform output by this type of inverter is a sine wave. The advantages of a sine wave inverter are: good output waveform, low distortion, no interference to communication equipment, and low noise; in addition, it has complete protection functions and strong adaptability to inductive and capacitive loads. The disadvantage is: the circuit is relatively complicated, the maintenance technology is required to be high, and the price is relatively expensive. Early sine wave inverters mostly used discrete electronic components or small-scale integrated circuits to form analog waveform generating circuits, directly use the analog 50Hz sine wave to cut the triangular wave of several kilohertz to tens of kilohertz to generate a high frequency pulse waveform of sine wave pulse width modulation (SPWM), and the 220V/50Hz single-phase sine wave AC voltage output is obtained through the power conversion circuit, step-up transformer and LC sine filter.
However, the circuit structure of this analog sine wave inverter is complex, the number of electronic components is large, and the working reliability of the whole machine is low. With the development of large-scale integrated microelectronics technology, dedicated SPWM waveform generation chips (such as HEF4752, SA838, etc.) and intelligent CPU chips (such as MCS51, PIC16H INTEL80196, etc.) gradually replace small-scale discrete component circuits, which greatly improves the technical performance and working reliability of the sine wave inverter, and has become the preferred solution for the current medium and large sine wave inverters.
The classification characteristics of the above three types of inverters are only for reference by developers and users of solar photovoltaic power generation systems when identifying and selecting inverters. In fact, inverters with the same waveform are still very different in terms of circuit principles, devices used and control methods.