I. Working Principle of Photovoltaic Inverters
Inverters are mainly composed of switching elements such as transistors. By repeatedly switching these elements on and off in a regular pattern, a DC input is converted into an AC output. However, the output waveform of an inverter generated simply by an on/off circuit is not practical. Generally, high-frequency pulse width modulation (PWM) is used to narrow the voltage width near the ends of a sine wave and widen it in the center. The switching elements are kept operating in one direction at a certain frequency throughout half a cycle, thus forming a pulse train. This pulse train is then passed through a simple filter to form a sine wave.
II. The role of photovoltaic inverters
Photovoltaic inverters not only have DC-AC conversion functions, but also functions to maximize the performance of solar cells and system fault protection functions. In summary, they have automatic operation and shutdown functions, maximum power point tracking control functions, anti-single-operation functions, automatic voltage regulation functions, DC detection functions, and DC grounding detection functions.
1. Automatic operation and shutdown function
After sunrise, as solar radiation intensifies, the output of the solar cells increases accordingly. Once the output power required for the inverter's operation is reached, the inverter automatically starts operating. Once operational, the inverter continuously monitors the output of the solar panels. As long as the output power of the solar panels exceeds the inverter's required output power, the inverter continues to operate until sunset, even on cloudy or rainy days. When the output of the solar panels decreases and the inverter output approaches zero, the inverter enters standby mode.
2. Maximum Power Point Tracking (MPPT) function
When solar irradiance and ambient temperature change, the input power of photovoltaic (PV) modules exhibits a non-linear change. PV modules are neither constant voltage sources nor constant current sources; their power changes with the output voltage and is independent of the load. Their output current initially remains horizontal as the voltage increases, then decreases as the voltage rises to a certain level, and finally drops to zero when the module's open-circuit voltage is reached.
3. Detection and control functions for the island effect
During normal power generation, a grid-connected photovoltaic (PV) system is connected to the power grid, supplying effective power to it. However, when the grid loses power, the PV system may continue to operate independently of the local load; this phenomenon is known as islanding. Islanding in inverters poses significant safety hazards to personnel, grid operation, and the inverter itself. Therefore, grid connection standards stipulate that PV grid-connected inverters must have islanding detection and control functions.
4. Power grid detection and grid connection function
Before a grid-connected inverter can generate electricity, it needs to draw power from the grid, detect parameters such as voltage, frequency, and phase sequence of the grid power supply, and then adjust its own power generation parameters to be synchronized with the grid parameters before it can generate electricity.
5. Low voltage ride-through function
When a power system accident or disturbance causes a voltage dip at the grid connection point of a photovoltaic power station, the photovoltaic power station can ensure continuous operation without disconnecting from the grid within a certain voltage drop range and time interval.
