When the utility grid is powered off, the grid side is equivalent to a short-circuit state, and the grid-connected inverter will be automatically protected due to overload. When the microprocessor detects an overload, in addition to blocking the SPWM signal, it will also disconnect the circuit breaker connected to the grid. At this time, if the solar cell array has energy output, the inverter will operate in a separate operation state. The control is relatively simple when running alone, that is, the negative feedback state of the AC voltage. The microprocessor detects the inverter output voltage and compares it with the reference voltage (usually 220V), and then controls the PWM output duty cycle to achieve inverter and stability. pressure operation.

01 The working principle of photovoltaic grid-connected inverter

Of course, the premise of operating alone is that the solar array can provide enough power at the time. If the load is too large or the sunshine conditions are poor, the inverter cannot output enough power, and the terminal voltage of the solar cell array will drop, thereby reducing the output AC voltage and entering a low-voltage protection state. When the power grid is restored, it will automatically switch to the feedback state.

02 The role of photovoltaic grid-connected inverters

The inverter not only has the function of DC-AC conversion, but also has the function of maximizing the performance of the solar cell and the function of system fault protection. To sum up, there are automatic operation and shutdown functions, maximum power tracking control function, anti-independent operation function (for grid-connected system), automatic voltage adjustment function (for grid-connected system), DC detection function (for grid-connected system), DC grounding detection Function (for grid-connected systems).

1. Automatic operation and stop 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 starts to run automatically. After entering into operation, the inverter will monitor the output of the solar cell module all the time. As long as the output power of the solar cell module is greater than the output power required for the inverter to work, the inverter will continue to run; it will stop at sunset, even if it is cloudy and rainy. The inverter can also operate. When the output of the solar cell module becomes smaller and the output of the inverter is close to 0, the inverter will form a standby state.

2. Maximum power tracking control function

The output of a solar cell module varies with the intensity of solar radiation and the temperature of the solar cell module itself (chip temperature). In addition, since the solar cell module has the characteristic that the voltage decreases with the increase of the current, there is an optimum operating point where the maximum power can be obtained. The intensity of solar radiation is changing, and obviously the optimal working 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 always obtains the maximum power output from the solar cell module. This control is the maximum power tracking control. The biggest feature of inverters for solar power systems is that they include the function of maximum power point tracking (MPPT).

3. Power grid detection and grid connection function

Before the grid-connected inverter is connected to the grid for power generation, it needs to take power from the grid, detect the parameters such as voltage, frequency, phase sequence, etc. of the grid power transmission, and then adjust the parameters of its own power generation to be synchronized with the grid electrical parameters. It will be connected to the grid to generate electricity.

4. Zero (low) voltage ride through function

When an accident or disturbance in the power system causes a voltage sag at the grid-connected point of the photovoltaic power station, within a certain voltage drop range and time interval, the photovoltaic power station can ensure continuous operation without being disconnected from the grid.

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 and transmits active power to the grid. However, when the grid loses power, the photovoltaic grid-connected power generation system may continue to work and operate independently from the local load. This phenomenon is called the islanding effect. When the islanding effect of the inverter occurs, it will cause great safety hazards to personal safety, power grid operation, and the inverter itself. Therefore, the grid connection standard of the inverter stipulates that the photovoltaic grid-connected inverter must have the detection and control function of the islanding effect.