Due to its simple structure and easy implementation, the perturbation observation method is currently one of the commonly used methods to implement MPPT. The principle is to increase or decrease the output voltage or current of the photovoltaic array at a certain interval of time, and observe the direction of power change thereafter to determine the next control signal.
Take the classic disturbance observation method as an example to illustrate its working principle as follows: First, change the output voltage of the photovoltaic array, and calculate the power generation based on the actual output voltage and current of the photovoltaic array. Then, compare with the generated power calculated last time. If the current generated power is less than the last power value, it means that the power output is reduced by this control, and the output voltage of the photovoltaic array should be controlled to change in the opposite direction to the original. If the current generated power is greater than the last power value, the original increase or decrease direction of the voltage is maintained, which ensures that the power output of the photovoltaic array always changes in the increasing direction. Such repeated disturbances, observations and comparisons make the photovoltaic array reach its maximum power point and achieve maximum power output. The flow of the disturbance observation method is shown in Figure 1.
In Figure 1, Vpv(k), Ipv(k), Ppv(k) are the measured output voltage and output current value of the photovoltaic array and the calculated output power value, respectively.
Figure 2 is a simulation curve of photovoltaic cell output power and BOOST converter duty cycle under constant light intensity conditions. It can be seen from the figure that the maximum power tracked by the disturbance observation method always fluctuates around the maximum power point.
It can be seen that the tracking method is simple and clear, and easy to implement by software. However, even if it tracks to the maximum power point, the system still continues to oscillate near the maximum power point, which will result in a loss of power. In addition, the selection of the tracking step, that is, the amount of voltage change △V, and the initial value of the tracking have a great influence on the response speed and accuracy of the system. When the △V setting is too large, the tracking speed is fast, but the output power oscillation is large near the maximum power point; if the △V setting is too small, the tracking speed is slow, but the output can be better close to the maximum power point, near the maximum power point The output power oscillation is small. In a steady state, this algorithm will cause the actual operating point of the photovoltaic array to oscillate slightly near the maximum power point, which will cause a certain amount of power loss. Although it can be improved by using a larger initial step size and reducing the step size when approaching the power point, the output power oscillation cannot be avoided. Another disadvantage of this method is that when the external environment changes suddenly, the wrong tracking direction is judged, and even the DC bus voltage collapses.
The perturbation observation method is one of the true maximum power point tracking methods. If you need to know other methods, you can read this article-What Is The True Maximum Power Point Tracking Method?