Overview of Maximum Power Point Tracking (MPPT) control and constant voltage MPPT algorithm
In order to realize the maximum power tracking control, it is necessary to understand the output characteristics of the photovoltaic panel. Figure 1 shows the output characteristic curve of a certain type of photovoltaic panel under different lighting conditions. It can be seen from Figure 1 that the open circuit voltage of the photovoltaic panel remains basically unchanged. When the radiation intensity increases, the short-circuit current and output power of the photovoltaic panel increase. In the case of different irradiance, the corresponding output voltage when the photovoltaic panel outputs the maximum power is approximately distributed near a specific voltage, that is to say, under different irradiance conditions, the bus voltage at the maximum power point remains approximately unchanged.
Figure 2 shows the output characteristic curve of photovoltaic panels under different temperature conditions. It can be seen from the figure that as the temperature increases, the short-circuit current of the photovoltaic panel remains basically constant, while the open-circuit voltage and maximum output power decrease.
It can be seen that the output of photovoltaic panels has a great relationship with the intensity of sunlight and ambient temperature where the panels are located. In order to make full use of photovoltaic panels, it is necessary to achieve the output power of photovoltaic panels at the maximum power point through MPPT.
The realization of the MPPT algorithm should first be based on the parameter characteristics of the photovoltaic panel itself, such as open circuit voltage and short circuit current; then, the operating point of the photovoltaic panel should be calculated in real time to find the output voltage corresponding to the maximum power point on the output characteristic curve; finally, adjust the input voltage reference of the converter, and use closed-loop adjustment to make the output voltage of the photovoltaic panel equal to the output voltage corresponding to the maximum power point. The MPPT process actually uses a regulator to change the input impedance of the converter itself to match the output impedance of the photovoltaic panel. At present, the more common MPPT algorithms mainly include the constant voltage method, the conductance increment method and the disturbance observation method.
Constant voltage method
It can be seen from Figure 1 that when the light intensity is constant and the temperature does not change much, the maximum output power point of the photovoltaic panel is approximately maintained at a certain constant voltage. Related research pointed out that there is an approximate linear relationship between the maximum power point voltage UmaxP of photovoltaic panels and the open circuit voltage Uopen of photovoltaic panels. The ratio is generally between 0.6 and 0.9. The specific size can be obtained by measuring the output characteristics of photovoltaic panels. The simple method of controlling the output voltage of the photovoltaic panel to a constant value is the constant voltage method.
The constant voltage method is an open-loop MPPT algorithm, which is simple and easy to implement, but since the influence of temperature on the output voltage of the photovoltaic cell is ignored, the photovoltaic power generation system cannot automatically track to the other maximum power point of the photovoltaic cell when the environment where the photovoltaic cell is located changes significantly, so the control accuracy is poor. However, this method can be applied to the startup process of the photovoltaic grid-connected system to make the output power of the photovoltaic panel quickly approach the maximum power point, and then cooperate with other MPPT algorithms to complete the maximum power output of the photovoltaic panel, which can improve the computational efficiency of the algorithm.