In the construction and operation of photovoltaic power plants, the inverter plays a very important role in the electrical equipment, which, like the technology of photovoltaic power systems, constantly improves the operational efficiency and conversion power, and works together with the photovoltaic modules and other power generation equipment to form the most efficient photovoltaic system.
The process of converting AC energy into DC energy is usually referred to as rectification, the circuit that performs the rectification function is referred to as a rectifier circuit, and the device that implements the rectification process is referred to as a rectifier device or rectifier. In contrast, the process of converting DC energy into AC energy is called inverting, the circuit that performs the inverting function is called an inverter circuit, and the device that implements the inverting process is called an inverter device or inverter. Inverters are also known as power conditioners and can be divided into two types according to their use in photovoltaic power systems: stand-alone power supply and grid-connected power supply. According to the waveform modulation method, they can be divided into square wave inverters, step wave inverters, sine wave inverters and combined three-phase inverters. Inverters for grid-connected systems can be divided into transformer inverters and transformerless inverters according to the presence or absence of a transformer. There are various types of inverters, so special attention needs to be paid when selecting the type and capacity of the machine. Especially in solar power systems, the efficiency of the PV inverter is an important factor in determining the capacity of the solar cells and the size of the battery.
The inverter is a power conditioning device consisting of semiconductor devices, mainly used to convert DC power into AC power. It generally consists of a step-up circuit and an inverter bridge circuit. The step-up circuit steps up the DC voltage from the solar cell to the DC voltage required for the inverter output control; the inverter bridge circuit converts the stepped-up DC voltage equivalently to an AC voltage of the usual frequency. The inverter consists mainly of switching elements, such as transistors, which, by repeatedly switching them on and off in a regular manner, turn the DC input into an AC output. Of course, it is not practical to produce an inverter output waveform purely from the on and off circuit in this way. Generally a high frequency pulse width modulation (SPWM) is required to narrow the width of the voltage near the ends of the sine wave and widen the width of the voltage in the centre of the sine wave, and to keep the switching elements moving in one direction at a certain frequency for half a cycle, thus forming a pulse train (a sine wave). The pulse wave is then allowed to pass through a simple filter to form a sine wave.