Photovoltaic grid-connected inverter is an essential key component in photovoltaic power generation system, which is mainly used in the field of solar photovoltaic power generation. The grid-connected inverter transforms the alternating current generated by solar panels into alternating current that can be directly transferred into the power grid through power electronic conversion technology. Let's understand the working principle of photovoltaic grid-connected inverter and its role in photovoltaic power generation system.
First, the principle of photovoltaic grid-connected inverter
When the public power grid is off, the power 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 overload, in addition to blocking the SPWM signal, it will also disconnect the circuit breaker connected to the power grid. At this time, if the solar cell array has energy output, the inverter will run in a separate state. When running alone, the control is relatively simple, that is, the negative feedback state of 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 ratio to achieve inverter and voltage stable operation.
Of course, running alone assumes that the solar array can provide enough power at the time. If the load is too large or the sunshine condition is poor, the inverter can not output enough power, the terminal voltage of the solar cell array will drop, so that the output AC voltage will be reduced and enter the low-voltage protection state. When the power grid is restored, it will automatically switch to the feedback state.
Two, the role of photovoltaic grid-connected inverter
The inverter not only has the function of direct AC conversion, but also has the function of maximizing the performance of solar cells and system failure protection. To sum up, it has automatic operation and shutdown function, maximum power tracking control function, anti-separate 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 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 automatically starts to run. After entering the operation, the inverter will always monitor the output of the solar cell module, as long as the output power of the solar cell module is greater than the output power required by the inverter, the inverter will continue to run; Until the sunset shutdown, the inverter can operate even on rainy days. When the solar cell module output becomes smaller and the inverter output approaches 0, the inverter will form 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, the voltage of solar cell modules decreases with the increase of current, so there is an optimal working point to obtain the maximum power. The intensity of solar radiation is changing, so is the optimal working point. 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 most important feature of solar power system inverter is to include maximum power point tracking (MPPT) this function.
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 voltage, frequency, phase sequence and other parameters of the grid, and then adjust its own power generation parameters to be synchronized with the grid electrical parameters. After completion, the grid-connected inverter will generate power.
4, zero (low) voltage crossing function
When power system accident or disturbance causes voltage sag of photovoltaic power station connection point, within a certain range of voltage sag and time interval, photovoltaic power station can ensure continuous operation without off-grid.
5. Detection and control of island 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 power grid. However, when the power grid loses power, the photovoltaic grid-connected power generation system may continue to work and operate independently with the local load. This phenomenon is called island effect. When the island effect occurs in the inverter, it will cause great security risks to personal safety, power grid operation and the inverter itself. Therefore, according to the grid entry standard of the inverter, the photovoltaic grid-connected inverter must have the detection and control function of island effect
