Photovoltaic inverters are one of the important system balancers (BOS) in photovoltaic array systems and can be used with equipment powered by general AC power. Solar inverters have special functions tailored to photovoltaic arrays, such as maximum power point tracking and islanding protection. The core of the inverter device is the inverter switching circuit, or simply the inverter circuit. This circuit performs the inversion function by turning on and off power electronic switches.
Solar cells generate direct current (DC) under sunlight; however, DC-powered systems have significant limitations. For example, fluorescent lamps, televisions, refrigerators, and electric fans cannot be directly powered by DC, as can most electric machinery. Furthermore, when the power supply system needs to increase or decrease voltage, an AC system only requires a transformer, while the technology for stepping up or down voltage in a DC system is far more complex. Therefore, except for special users such as those in communications and meteorology who directly use DC power, photovoltaic power generation systems supplying electricity for production and daily life all require photovoltaic inverters.
When solar irradiance and ambient temperature change, the input power of photovoltaic (PV) modules exhibits a non-linear change. PV modules are neither constant voltage sources nor constant current sources; their power changes with the output voltage and is independent of the load. Their output current initially remains horizontal as the voltage increases, then decreases as the voltage rises to a certain level, and finally drops to zero when the module's open-circuit voltage is reached.
Generally, the process of converting alternating current (AC) power into direct current (DC) power is called rectification, the circuit that performs the rectification function is called a rectifier circuit, and the device that implements the rectification process is called a rectifier or inverter. Conversely, the process of converting direct current (DC) power into alternating current (AC) power is called inversion, the circuit that performs the inversion function is called an inverter circuit, and the device that implements the inversion process is called an inverter or inverter. As mentioned above, there are various types of inverters, so special attention must be paid when selecting the model and capacity.
Centralized inverter technology connects several parallel photovoltaic (PV) strings to the DC input of a single centralized inverter. Generally, high-power systems use three-phase IGBT power modules, while lower-power systems use field-effect transistors (FETs). A DSP (Digital Signal Processor) converter controller is used to improve the quality of the generated power, making it very close to a sinusoidal current. This technology is typically used in large-scale PV power plants (>10kW). Its main advantages are high system power and low cost. However, because the output voltage and current of different PV strings are often not perfectly matched (especially when PV strings are partially shaded due to clouds, trees, or dirt), centralized inverter technology can lead to reduced inverter efficiency and decreased power consumption.
Square wave inverters output a square wave voltage waveform. The inverter circuitry is simple, inexpensive, and relatively easy to implement. The disadvantage is that the square wave voltage contains a large number of high-order harmonic components, which can cause additional losses in the load and significant interference to communication equipment, requiring an external filter. This type of inverter is commonly found in earlier, small-capacity inverters with a design power of no more than a few hundred watts.
String inverters are based on a modular concept, with each photovoltaic string (1-5kW) connected to a single inverter. They feature peak power point tracking (MPPT) on the DC side and are connected in parallel to the grid on the AC side, making them the most popular inverter type on the international market. Many large-scale photovoltaic power plants use string inverters. Their advantages include being unaffected by differences between modules and shading, while also reducing the possibility of mismatch between the optimal operating point of the photovoltaic modules and the inverter, thus increasing power generation. These technological advantages not only reduce system costs but also increase system reliability.
A stepped-wave inverter outputs a stepped voltage waveform. The advantage of a stepped-wave inverter is that the output waveform is close to a sine wave, a significant improvement over a square wave, and with reduced high-order harmonic content. When the stepped wave has more than 16 steps (f), the output waveform becomes a quasi-sine wave, resulting in high overall efficiency. However, this type of inverter often requires multiple DC power supplies and a large number of power switching transistors, which can cause inconvenience for photovoltaic array grouping and battery grouping.
The above is a detailed analysis of some information about photovoltaic inverters that is worth learning. I hope it can be of some help to you when you are just getting started. If you have any questions, you can also discuss them with me.
