Photovoltaic grid-connected inverter is an essential key component in photovoltaic power generation system. It is mainly used in the special inverter power supply in the field of solar photovoltaic power generation.
The grid-connected inverter converts the AC generated by solar panels into AC that can be directly divided into the power grid through power electronic conversion technology.
Let’s understand the working principle of the photovoltaic grid-connected inverter and its role in photovoltaic power generation system.
1. Working principle of photovoltaic grid-connected inverter
When the public power grid is powered off, the power grid side is equivalent to a short-circuit state. At this time, the grid-connected inverter will be automatically protected due to overload. When the microprocessor detects overload, it will not only block the SPWM signal, but also disconnect the circuit breaker connected to the power grid.
At this time, if the solar cell array has energy output, the inverter will operate in a separate operation state. When operating alone, the control is relatively simple, that is, the negative feedback state of AC voltage.
The microprocessor detects the output voltage of the inverter and compares it with the reference voltage (usually 220V), and then controls the PWM output duty cycle to realize inverter and voltage stabilizing operation.
Of course, the premise of separate operations is that the solar cell array can provide sufficient power at that time. If the load is too large or the sunshine condition is poor, the inverter cannot output enough power, and the terminal voltage of the solar cell array will drop, so as to reduce the output AC voltage and enter the low-voltage protection state. When the power supply of the power grid is restored, it will automatically switch to the feedback state.
2. Function of photovoltaic grid-connected inverter
The inverter not only has the DC-AC conversion function but also has the function of maximizing the performance of solar cells and system fault protection.
To sum up, it has an automatic operation and shutdown function, maximum power tracking control function, anti separate operation function (for grid-connected system), automatic voltage adjustment function (for the grid-connected system), DC detection function (for the grid-connected system), and DC grounding detection function (for the grid-connected system).
（1）Automatic operation and shutdown function
After sunrise in the morning, the solar radiation intensity gradually increases, and the output of the solar cell also increases. When the output power required by the inverter is reached, the inverter will automatically start to operate.
After operation, the inverter will monitor the output of the solar cell module at all times. As long as the output power of the solar cell module is greater than the output power required for the operation of the inverter, the inverter will continue to operate;
The inverter can operate even on cloudy and rainy days until the shutdown at sunset. When the output of the solar cell module becomes smaller and the output of the inverter approaches 0, the inverter will form a standby state.
（2）Maximum power tracking control function
The output of the solar cell module varies with the solar radiation intensity and the temperature of the solar cell module itself (chip temperature). In addition, because the solar cell module has the characteristic that the voltage decreases with the increase of current, there is an optimal operating point to obtain the maximum power.
The solar radiation intensity is changing, and obviously, the optimal operating point is also changing. Relative to these changes, the operating point of the solar cell module is always at the maximum power point, and the system is always from The solar cell module obtains the maximum power output.
This control is the maximum power tracking control. The biggest feature of the inverter used in solar power generation system is that it includes the function of maximum power point tracking (MPPT).
（3）Power grid detection and grid connection function
Before grid-connected power generation, the grid-connected inverter needs to take power from the power grid, detect the parameters such as voltage, frequency, phase sequence, and so on, and then adjust the parameters of its own power generation, synchronize with the grid’s electrical parameters.
（4）Zero (low) voltage ride through function
When the power system accident or disturbance causes the voltage sag of the parallel node voltage of the photovoltaic power station, the photovoltaic power station can ensure continuous operation without disconnection within a certain voltage drop range and time interval.
（5）Detection and control of islanding effect
During normal power generation, the photovoltaic grid-connected power generation system is connected to the large power grid to transmit active power to the power grid. However, when the power grid loses power, the photovoltaic grid-connected power generation system may still work continuously and operate independently of the local load.
This phenomenon is called the island effect. The islanding effect of the inverter will cause great potential safety hazards to personal safety, power grid operation, and the inverter itself. Therefore, the grid access standard of the inverter stipulates that the photovoltaic grid-connected inverter must have the detection and control function of the islanding effect.
