The process of converting DC electric energy into AC electric energy is called inverter, the circuit that completes the inverter function is called inverter circuit, and the device that realizes the inverter process is called inverter or inverter equipment. The inverter used in the photovoltaic power generation system is a conversion device that converts the DC power generated by the photovoltaic module into AC power. It makes the voltage and frequency of the alternating current after conversion consistent with the voltage and frequency of the alternating current in the power system to meet the needs of powering various alternating current loads and grid-connected power generation. Figure 1 is the outline drawing of a common inverter.
The basic requirements of photovoltaic power generation systems for inverters: ①Reasonable circuit structure, strict selection of components, and various protection functions; ②Wide DC input voltage range; ③ Fewer power conversion intermediate links to save costs and improve efficiency; ④ High conversion efficiency; ⑤ High reliability, unattended and maintenance; ⑥The output voltage and current meet the power quality requirements, with low harmonic content and high power factor; ⑦It has a certain overload capacity.
Classification of inverters
There are many types of inverters, which can be classified in different ways.
According to the number of phases of AC output by the inverter, it can be divided into single-phase inverter, three-phase inverter and multi-phase inverter.
According to the different working frequency of the inverter conversion circuit, it can be divided into power frequency inverter, intermediate frequency inverter and high frequency inverter.
According to the different waveforms of inverter output voltage, it can be divided into square wave inverter, step wave inverter and sine wave inverter.
According to the principle of the inverter circuit, it can be divided into self-excited oscillation inverter, stepped wave superposition inverter, pulse width modulation inverter and resonance inverter.
According to the difference of the inverter main circuit structure, it can be divided into single-ended inverter structure, half-bridge inverter structure, full-bridge inverter structure, push-pull inverter structure, multi-level inverter structure, forward inverter structure and flyback inverter structure, etc. Among them, low-power inverters mostly adopt single-ended inverter structure, forward inverter structure and flyback inverter structure, medium-power inverters mostly adopt half-bridge inverter structure, full-bridge inverter structure, etc, and high-voltage high-power inverters mostly adopt push-pull inverter structure and multi-level inverter structure.
According to the different output power of the inverter, it can be divided into small power inverter (<5KW), medium power inverter (5～50KW), and high power inverter (>50KW).
According to the inverter isolation (conversion) method, it can be divided into the method with power frequency isolation transformer, the method with high frequency isolation transformer, and the method without isolation transformer.
According to the destination of the inverter’s output energy, it can be divided into active inverters and passive inverters. For solar photovoltaic power generation systems, active inverters are required in grid-connected photovoltaic power generation systems, while passive inverters are required in off-grid independent photovoltaic power generation systems.
In solar photovoltaic power generation systems, inverters can also be divided into off-grid inverters (inverters used in stand-alone photovoltaic systems) and grid-connected inverters.
Grid-connected inverters can be divided into centralized inverters, string inverters, micro (module) inverters, two-way energy storage inverter, etc.