The concept of inverter
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.
Modern inverter technology is a science and technology that studies the theory and application of inverter circuits. It is a practical technology built upon the foundations of industrial electronics, semiconductor device technology, modern control technology, modern power electronics, semiconductor converter technology, and pulse width modulation (PWM) technology. It mainly comprises three parts: semiconductor power integrated devices and their applications, inverter circuits, and inverter control technology.
Inverter Classification
There are many types of inverters, which can be classified in different ways.
1. Based on the frequency of the AC power output, inverters can be divided into power frequency inverters, medium frequency inverters, and high frequency inverters. Power frequency inverters have a frequency of 50-60Hz; medium frequency inverters typically have a frequency of 400Hz to tens of kHz; and high frequency inverters typically have a frequency of tens of kHz to MHz.
2. Based on the number of phases output by the inverter, it can be divided into single-phase inverters, three-phase inverters, and multi-phase inverters.
3. Based on the destination of the output electrical energy, inverters can be divided into active inverters and passive inverters. Inverters that transmit the output electrical energy to the industrial power grid are called active inverters; inverters that transmit the output electrical energy to a certain electrical load are called passive inverters.
4. According to the form of the main circuit of the inverter, it can be divided into single-ended inverter, push-pull inverter, half-bridge inverter and full-bridge inverter.
5. Based on the type of main switching device, inverters can be classified into thyristor inverters, transistor inverters, field-effect inverters, and insulated-gate bipolar transistor (IGBT) inverters. They can also be categorized into two main types: "semi-controlled" inverters and "fully controlled" inverters. The former lacks self-turn-off capability; the device loses its control function after being turned on, hence the term "semi-controlled." Ordinary thyristors belong to this category. The latter, however, possesses self-turn-off capability, meaning that the on and off states of the device can be controlled by the control electrode, hence the term "fully controlled." Electric field-effect transistors and insulated-gate bipolar transistors (IGBTs) belong to this category.
6. Based on the DC power supply, inverters can be divided into voltage source inverters (VSI) and current source inverters (CSI). In the former, the DC voltage is nearly constant, and the output voltage is an alternating square wave; in the latter, the DC current is nearly constant, and the output current is an alternating square wave.
7. Based on the waveform of the inverter's output voltage or current, inverters can be divided into sinusoidal output inverters and non-sinusoidal output inverters.
8. According to the inverter control method, inverters can be divided into frequency modulation (PFM) inverters and pulse width modulation (PWM) inverters.
9. Based on the operating mode of the inverter switching circuit, inverters can be divided into resonant inverters, fixed-frequency hard-switching inverters, and fixed-frequency soft-switching inverters.
10. According to the inverter's commutation method, it can be divided into load-commutated inverters and self-commutated inverters.
Photovoltaic inverters are the core power regulation devices in photovoltaic (PV) systems. Solar photovoltaic modules generate direct current (DC) electricity, which must be converted to AC by a photovoltaic inverter to connect the PV power generation to the grid and load. The inverter capacity is determined by the nature of the load and is generally smaller than the PV system capacity. In a PV system, the inverter is the most expensive component, accounting for about 10% of the total cost, making it the second most expensive component, but also possessing high added value.
Photovoltaic systems are divided into grid-connected and off-grid systems, and correspondingly, photovoltaic inverters are also classified as grid-connected and off-grid. Generally, the inverter capacity of a grid-connected system is the same as the photovoltaic system capacity, while the inverter capacity of an off-grid system is determined by the load capacity and nature, and is generally smaller than the photovoltaic system capacity. According to historical statistics on global photovoltaic system installations, grid-connected photovoltaic systems account for the absolute majority of total installed photovoltaic capacity, exceeding 90% under government grid-connected power generation subsidies. Currently, grid-connected inverters are the focus of development for various manufacturers, and their technical specifications represent the level of photovoltaic inverter technology.
Centralized grid-connected photovoltaic inverters have an early start and a complete product series, making them the mainstream products in the current photovoltaic inverter market and occupying a dominant position. However, with the increasing maturity of technology and the continuous development of commercial applications, the problems existing in centralized grid-connected photovoltaic power generation systems have gradually attracted attention:
(1) Reliability: In a centralized photovoltaic grid-connected power generation system, the inverter is the key and weakest link in the entire system. The failure of a single inverter can lead to the collapse of the entire system.
(2) MPPT tracking efficiency: Although most manufacturers of centralized photovoltaic inverters claim that the tracking efficiency can reach 99.9%, in fact, since their MPPT tracking is aimed at the entire photovoltaic array and cannot take into account each photovoltaic module, the power loss caused by module matching, partial shading and other factors is considerable;
(3) System scalability: The characteristics of centralized systems determine that their system scalability is poor.
Distributed microinverter technology was developed to address the numerous problems associated with centralized grid-connected photovoltaic inverters. Although its development started later and its product range is still incomplete, it remains in its early stages. However, in terms of current product performance, microinverters are in no way inferior to, and may even outperform, traditional centralized inverters. By directly connecting to individual photovoltaic modules, microinverters can achieve true MPPT (Multi-Level Productivity) for each module, maximizing its power generation efficiency. They offer flexible installation and expansion, are plug-and-play, and can be modularly produced. Microinverter manufacturers can collaborate with photovoltaic module manufacturers to customize inverters for specific photovoltaic modules, thereby accelerating their commercialization and marketization.
