A photovoltaic (PV) inverter is a specialized inverter used in the field of solar photovoltaic (PV) power generation. It converts the direct current (DC) generated by solar cells into alternating current (AC) energy that can be directly connected to the grid and loads using power electronic conversion technology. It is an indispensable core component of a PV system. As the interface device between PV cells and the grid, the PV grid-connected inverter converts the electrical energy from the PV cells into AC energy and transmits it to the grid, playing a crucial role in PV grid-connected power generation systems.
Definition of photovoltaic grid-connected inverter
Photovoltaic grid-connected inverters, also known as power conditioners, can be categorized into two types based on their application in photovoltaic power generation systems: stand-alone power supply inverters and grid-connected inverters. Based on waveform modulation methods, they can be classified as square wave inverters, stepped wave inverters, sine wave inverters, and combined three-phase inverters. For inverters used in grid-connected systems, they can be further classified into transformer-type inverters and transformerless inverters, depending on whether they contain a transformer.
1. High efficiency is required. Due to the current high price of solar cells, in order to maximize the utilization of solar cells and improve system efficiency, it is necessary to find ways to improve the efficiency of the inverter.
2. High reliability is required. Currently, photovoltaic power generation systems are mainly used in remote areas, and many power stations are unattended and unmaintained. This requires the inverter to have a reasonable circuit structure, strict component selection, and various protection functions, such as input DC polarity reverse protection, AC output short circuit protection, overheat and overload protection, etc.
3. A wide range of DC input voltage is required. Since the terminal voltage of solar cells varies with the load and solar radiation intensity, although the battery plays an important role in the voltage of solar cells, the voltage of the battery fluctuates with the remaining capacity and internal resistance of the battery. In particular, when the battery ages, its terminal voltage varies greatly. For example, the terminal voltage of a 12V battery can vary between 10V and 16V. This requires the inverter to operate normally within a wide range of DC input voltage and ensure the stability of the AC output voltage.
4. In medium- and large-capacity photovoltaic power generation systems, the inverter output should be a sine wave with low distortion. This is because if a square wave is used in a medium- or large-capacity system, the output will contain more harmonic components, and higher-order harmonics will generate additional losses. Many photovoltaic power generation systems are loaded with communication or instrumentation equipment, which have high requirements for grid quality. When medium- and large-capacity photovoltaic power generation systems are connected to the grid, in order to avoid power pollution from the public grid, the inverter is also required to output a sine wave current.
Working principle of photovoltaic grid-connected inverter
An inverter converts direct current (DC) to alternating current (AC). If the DC voltage is low, it is boosted by an AC transformer to obtain the standard AC voltage and frequency. For large-capacity inverters, since the DC bus voltage is high, the AC output generally does not require a transformer to reach 220V. In medium- and small-capacity inverters, since the DC voltage is low, such as 12V or 24V, a boost circuit must be designed.
Small and medium capacity inverters generally come in three types: push-pull inverter circuits, full-bridge inverter circuits, and high-frequency boost inverter circuits. In a push-pull circuit, the neutral plug of the boost transformer is connected to the positive power supply, and the two power transistors work alternately to output AC power. Because the power transistors are connected to a common ground, the drive and control circuits are simple. Furthermore, because the transformer has a certain leakage inductance, it can limit short-circuit current, thus improving the reliability of the circuit. However, its disadvantages include low transformer utilization and poor ability to drive inductive loads.
The full-bridge inverter circuit overcomes the shortcomings of the push-pull circuit. The power transistors adjust the output pulse width, thus changing the effective value of the output AC voltage. Because this circuit has a freewheeling loop, the output voltage waveform will not be distorted even with inductive loads. The disadvantage of this circuit is that the power transistors of the upper and lower bridge arms are not grounded together, therefore a dedicated drive circuit or an isolated power supply must be used. In addition, to prevent the upper and lower bridge arms from conducting simultaneously, a turn-off circuit must be designed before turn-on, i.e., a dead time must be set, making the circuit structure relatively complex.
