In order to understand the function of inverters and maximum power point trackers in the context of photovoltaic systems, it is necessary to first understand the characteristics of photovoltaic cells. A photovoltaic cell is a constant current and variable voltage device, while a rotary generator in a power plant is a constant voltage converter device. For example, the mains voltage connected to a home is usually stable at 120V, but the current varies depending on the load. The opposite is true for constant current generators, where the current remains constant, but the voltage varies with the load. The characteristics of photovoltaic cells can be described by a current-voltage curve, which is called an I-V curve or a volt-ampere curve. The I-V curve describes the possible operating points for all voltages and currents under a specific set of conditions.
Figure 1a shows two I-V curves. In the first curve, the solar radiation intensity is 1000W/m², and in the second curve, the solar radiation intensity is 800W/m². Both curves are at a temperature of 25°C. The maximum power points of both curves are marked with MPP, representing the current and voltage values (P=I×E) that can deliver the maximum power to the load under this set of operating conditions. Note that the MPP of both curves lies at the inflection point of their I-V curves. At a given solar radiation intensity and temperature, both curves show that the current remains essentially constant until passing the turning point, and drops to zero shortly after. The effect of temperature on the I-V curve of a silicon photovoltaic cell is shown in Figure 1b. where the first curve is at a temperature of 25°C and the second curve is at a temperature of 85°C, note that when the temperature decreases (cooler), the voltage rises, and when the temperature increases (warmer), the Voltage drops. But the current remains essentially constant as the temperature changes.
As the battery voltage continues to rise after passing the MPP point, its current drops sharply and power drops with it. The inverter must react to this change by outputting power to the load at a specified current and voltage. When the specified conditions cannot be met, the connection to the load should be disconnected. Photovoltaic panels to be transported are shown in Figure 2.