Photovoltaic grid-connected inverter

Inverters are used in photovoltaic power plants to convert direct current into alternating current. The grid-connected system puts forward higher requirements for the inverter part.

①The inverter output is a sine wave, and the high-order harmonics and DC components are small enough to not cause harmonic pollution to the power grid.

② The inverter can operate efficiently under the condition that the load and sunshine vary greatly, that is, the inverter is required to have the maximum power tracking (MPPT) function, which can automatically adjust regardless of the changes in sunshine and temperature to achieve the maximum power output .

③It has advanced anti-islanding operation protection function, that is, the system is automatically cut off from the power grid when the power grid loses power, so as to prevent the damage caused by the independent power supply to the operation, maintenance and maintenance personnel.

④It has the function of automatic grid connection and decoupling. When the sun rises in the morning and the sunshine reaches the power generation output power requirement, it is automatically put into the power grid for power generation operation, and when the output power is insufficient at sunset, it is automatically disconnected from the power grid.

⑤It has the function of automatic adjustment of output voltage. When the grid-connected upstream is upstream, the voltage and upstream power can be adjusted at any time with the change of the voltage at the grid-connected point.

⑥It has complete grid-connected protection function. When an abnormality occurs on the system side or the reverser side, the power generation system is quickly cut off, that is, it has over-voltage and under-voltage protection, over-frequency and under-frequency protection, etc., to meet the requirements of unattended remote monitoring. The circuit structure of the grid-connected inverter is shown in Figure 1. Through the three-phase full-bridge inverter, the DC voltage of the photovoltaic array is converted into a high-frequency three-phase AC voltage, filtered by a filter into a sine wave voltage, and isolated and boosted by a three-phase transformer, and then integrated into the grid to generate electricity.

Photovoltaic grid-connected inverter
Figure 1 – Grid-connected inverter circuit structure

Photovoltaic grid-connected inverter can use DSP control chip, use current-controlled PWM active inverter technology, and the wide DC input voltage range is 220~450V; the grid-connected inverter in the system continuously detects whether the photovoltaic array has enough energy to connect to the grid to generate electricity. When the grid-connected power generation condition is reached, that is, when the array voltage is greater than 240V and maintained for 1min, the inverter power supply is transferred from the standby mode to the grid-connected power generation mode, and the DC power of the photovoltaic array is converted into AC power and connected to the grid. At the same time, in this mode, the inverter power supply has been using the MPPT method to maximize the output energy of the photovoltaic array, which effectively improves the utilization rate of solar energy by the system. When the solar radiation is very weak, that is, the array voltage is less than 200V or at night, the photovoltaic array does not have enough energy to generate electricity, and the inverter automatically disconnects from the grid.

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