Inverters can be divided into square wave inverters, step wave inverters and sine wave inverters according to the different output voltage waveforms. The output waveforms are shown in Figure 1. In the solar photovoltaic power generation system, square wave and step wave inverters are generally used in low power applications. The advantages and disadvantages of these three different output waveform inverters are introduced below.
① Square wave inverter. The waveform output by the square wave inverter is a square wave, also called a rectangular wave. Although the circuits used by square wave inverters are not the same, the common advantages are that the circuit is simple (the number of power switch tubes used is the least), the price is cheap, and the maintenance is convenient. The design power is generally between hundreds of watts and several kilowatts. The disadvantage is that the voltage regulation range is narrow, the noise is large, the square wave voltage contains a large number of high-order harmonics, and additional losses will be generated when electrical appliances such as motors are used with inductive loads, so the efficiency is low and the electromagnetic interference is large.
② Step wave inverter. Step wave inverter is also called modified wave inverter. Compared with the square wave, the staircase wave is significantly improved, the waveform is similar to the sine wave, and the high-order harmonic content in the waveform is less, so it can meet the needs of most electrical equipment including inductive loads. When the transformerless output is adopted, the efficiency of the whole machine is high. Due to the moderate price of step-wave inverters, it is widely used in household power supplies in remote areas where the requirements for power quality are not very high. The disadvantage of step-wave inverter is that the circuit is more complicated. In order to correct the square wave into a staircase wave, multiple different complex circuits are required to generate multiple waveforms for superimposition and correction. These circuits also use a lot of power switch tubes, which have serious electromagnetic interference and more than 20% harmonic distortion. There will be problems when driving precision equipment, and it will also cause high-frequency interference to communication equipment. Therefore, step-wave inverters cannot be used in these occasions, let alone grid-connected power generation occasions.
③ Sine wave inverter. The waveform of the sine wave inverter output is the same as that of the AC mains. The advantages of this inverter are good output waveform, low distortion, low interference, low noise, strong adaptability to load, complete protection functions, good overall performance, high efficiency, can meet all AC load applications, and suitable for various electrical occasions. The disadvantage is that the circuit is complicated, maintenance is difficult, and the price is relatively expensive. In the application of photovoltaic grid-connected power generation, in order to avoid power pollution to the public grid, a sine wave inverter must be used.