1. Voltage feedback method
The voltage feedback method is the simplest maximum power tracking method, which can include a fixed reference voltage method and a variable reference voltage method, as shown in Figure 1 and Figure 2 respectively. Through the results of the pre-test, we can know the voltage of the maximum power point of the solar panel under a certain sunshine intensity. This method is to adjust the terminal voltage of the solar panel to match the pre-tested voltage to achieve The effect of maximum power point tracking. However, the biggest disadvantage of this control method is that when the atmospheric conditions change drastically, the system cannot automatically track the other maximum power point of the solar panel, thus causing energy loss.
2. Power feedback method
The power feedback method is similar to the voltage feedback method. As shown in Figure 3, since the voltage feedback method cannot automatically track the maximum power point under the rapidly changing atmospheric conditions, the power feedback method adds a logical judgment of the output power to the voltage change rate so as to be able to The maximum power point tracking is achieved due to changes in the atmosphere. It can be seen from Figure 3 that when dP/dU=0, it is the maximum power point of the solar panel. With the control process, it can dynamically track the maximum power point of the photovoltaic cell under different sunlight intensities and temperatures. Regarding the feedback method, although this method is more complicated and requires more calculation processes, its effect of reducing energy loss and improving overall efficiency is very significant.
3. Disturbance observation method
The disturbance observation method is also called the hill climbing method, as shown in Figure 4. Because of its simple structure and fewer parameters to be measured, it is widely used in the maximum power tracking of photovoltaic panels. Periodically increase or decrease the size of the load to change the terminal voltage and output power of the photovoltaic panel, and observe and compare the output voltage and output power before and after the load change to determine the next increase or decrease action, assuming the output power If it is larger than before the change, the load will continue to change in the same direction; on the contrary, if the output power is smaller than before the change, it means that we need to change the direction of the load change in the next cycle. Such repeated disturbances, observations and comparisons make the photovoltaic cell array reach the maximum power point. This is the basic principle of the disturbance observation method.
However, this method relies on constantly changing the output voltage and output power of the photovoltaic cell array to track the maximum power point. When the maximum power point (Pmax) is reached, the disturbance will not stop, but will oscillate around Pmax, thus causing Energy loss, and reduce the efficiency of the solar cell array. Especially when the atmospheric environment changes slowly, the energy loss is more serious. This is the biggest shortcoming of the disturbance observation method. Although we can reduce the amplitude of each disturbance to reduce the amplitude of oscillation at Pmax to reduce energy loss, this approach will change the speed of tracking to another maximum power point when the temperature or sunshine intensity changes significantly. Slow, a lot of energy will be wasted at this time, so when we use the disturbance observation method, the magnitude of the disturbance needs to be made by the user.
4. Incremental conductance method
The incremental conductance method, as shown in Figure 5, is also a more widely used method to find the maximum power point. Its basic idea is the same as the power feedback method. Its starting point is the logical judgment formula dP/dU=0. The power (P) can be represented by voltage (U) and current (I), and dP/dU=0 is rewritten as:
dP/dU=d(IU)/dU=I+U(dI/dU)=0
After finishing the above formula, you can get
dI/dU=﹣(I/U)
In the above formula, dI represents the current difference measured before and after the increment; in the same way, dU represents the voltage difference measured before and after the increment. Therefore, by measuring the increment (dI/dU) and the instantaneous conductance value (I/U) of the photovoltaic cell, the next change can be determined. When the increment value and the conductance value meet the requirements of Figure 6, it means that the maximum power has been reached. Point, that is, do not perform the next disturbance. This is the basic working principle of the incremental conductance method.
Although the conductance increment method is still to change the output voltage of the photovoltaic cell to reach the maximum power point (Pmax), but by modifying the logic judgment formula to reduce the oscillation phenomenon near the Pmax point, making it more adaptable to the rapidly changing atmospheric environment . In theory, the theoretical derivation of this method is perfect, but when the detection circuit cannot achieve a very precise measurement, its error is inevitable, so the probability of the above formula is very small, which means this method There are still large errors in actual applications. It can be seen that the disturbance observation method and the conductance increment method can be said to be the same, the difference lies only in the choice between logical judgment and measurement parameters.