An inverter is an electronic device that converts direct current to alternating current. In a photovoltaic system, the inverter can convert the DC output of the photovoltaic array into alternating current with a stable voltage and frequency. Similarly, the output power of the wind turbine may be AC or DC depending on its model. If it is DC, an inverter is also required to convert DC to AC.
Many inverters have two functions: ① convert DC voltage to AC voltage; ② maximize power extraction from PV modules using maximum power point tracking technology. Figure 1a and Figure 1b show the changes in cell voltage, current, and power due to solar radiation intensity and temperature. Changes in these conditions will move the maximum power point. To maintain maximum power transfer from the source to the load, the inverter must adjust the source load to operate at the source’s MPP point to maximize power draw. This function of adjusting the load is called maximum power point tracking (MPPT).
The inverter is designed to be independent of the grid, which means that it can work both independently of the grid and connected to the grid. When in grid-connected working mode, if the grid is energized, the inverter can either obtain power from the grid or output power to the grid.
Power inverters control the voltage and current between the power source (photovoltaic array, wind turbine, or other DC power source) and the electrical load, converting the varying DC output into a high-quality sinusoidal waveform. Both the efficiency of photovoltaic cells and the efficiency of inverters are critical factors for the success of photovoltaic renewable energy systems.
In Figure 2, a solar photovoltaic array provides DC power to an inverter that outputs AC power for a building. In the application shown in Figure 3, the unstable AC output from a wind turbine is first rectified into a relatively stable DC, and then inverted into a stable AC. The public grid and user loads must obtain stable AC voltage waveforms.
In the United States and around the world, the vast majority of electricity is alternating current (AC). The advantage of alternating current is that the voltage can be stepped up by a transformer, thereby reducing the current. Smaller currents help reduce voltage drops and power losses in power transmission lines and wires of all types. The AC voltage output by the inverter can be connected to the public grid through a transformer, or directly to the AC loads of households and commercial users near the system site.
The polarity of the AC waveform shown in Figure 4a changes back and forth between positive and negative, so it can be transformed by a transformer, while the DC waveform in Figure 4b cannot be transformed by a transformer. In order to convert the voltage between the primary and secondary windings of the transformer, the polarity of the input voltage to the primary winding must be periodically changed, which is clearly not the case with the waveforms in Figure 4b.