In photovoltaic off-grid power supply systems, the main role of the inverter is to convert the DC power from the battery into AC power. Inverters are commonly used in off-grid power supply systems, with input connected to photovoltaic controllers and batteries, and output to loads. Off-grid power supply systems are widely used, and inverters come in various forms. Based on output waveforms, they can be divided into modified wave inverters and sine wave inverters; based on electrical isolation methods, they can be categorized into high-frequency sine wave inverters and low-frequency sine wave inverters; designing controllers and inverters separately with distinct wiring is called split-type; integrating the controller and inverter is called all-in-one, or inverter-controller all-in-one.
The output waveform of inverters mainly falls into two categories: sine waves and modified waves. Modified sine wave inverters generate a modified output using PWM pulse width modulation. Due to the approximately 20% harmonic distortion, they cannot power inductive loads like air conditioners but can be used for resistive loads like lamps. Modified sine wave inverters use non-isolated coupling circuits, which are simple and efficient. Pure sine wave inverters employ isolated coupling circuit designs; the circuit is more complex and costly but can connect to any common electrical equipment (including TVs, LCD monitors, etc., especially inductive loads like refrigerators) without interference.
Pure sine wave 3-phase off-grid solar inverters have electrical isolation between the input and output ends. Based on electrical structure, they can be categorized into high-frequency isolation and low-frequency isolation. High-frequency isolation transformers are placed at the DC boost end, following a scheme where DC power is first inverted to high-frequency AC through the high-frequency transformer, stepped up, then rectified to DC, and finally inverted to low-frequency AC. High-frequency inverters use small, lightweight high-frequency magnetic core materials, reducing the inverter's weight and size, and improving efficiency, though the circuit is relatively complex. Low-frequency isolation transformers are placed at the AC output end; their circuits are simpler and more resistant to impact, but they are bulkier and heavier.
Off-grid power supply systems, due to the battery, must include a controller for charging the battery from the modules. If the controller and inverter are separate, forming two distinct devices, it is called split-type. If the controller and inverter are integrated, it is called all-in-one, also known as inverter-controller all-in-one. Split-type systems allow separate selection of the controller and inverter but involve more complex wiring, suitable for systems where module and inverter power differ significantly and for highly powered systems. Inverter-controller all-in-one systems have a simple structure and easy wiring, suitable for systems where module and inverter power differ little.
In terms of cost, modified wave inverters are the most economical, while low-frequency inverters are the most expensive. Regarding load capacity, low-frequency inverters are the strongest. Therefore, when choosing an inverter, consider the application scenario. For simple lighting applications, a modified wave inverter is recommended to save initial costs. For inductive loads with motors like air conditioners, washing machines, or water pumps, a low-frequency inverter is recommended for its strong load capacity. For comprehensive loads, a high-frequency 3-phase off-grid solar inverter is advised, balancing cost and load capacity.
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