The following describes the tracking control method of the maximum power point control link:
1. Constant pressure tracking method
The principle of constant voltage tracking method: when the temperature is constant, the maximum power point voltage of the solar cell is near both sides of a certain voltage point, as shown in Figure 1.
When ignoring the temperature effect, the maximum power output points a’, b’, c’, d and e’ of the photovoltaic array under different sunshine intensities are always approximately around a certain constant voltage value Um. If the curve L is the load characteristic curve, a, b, c, d and e are the working points when they are directly matched under the corresponding care intensity. Obviously, if direct matching is adopted, the output power of the array is relatively small. To compensate for the power loss caused by impedance mismatch, the constant voltage tracking (CVT) method can be used to pass a certain impedance transformation between the photovoltaic array and the load, so that the system can realize the function of the voltage stabilizer, so that the operating point of the array is always stable near Um. This not only simplifies the entire control system, but also ensures that its output power is close to the maximum output power, as shown in 1. The power difference using constant voltage tracking (CVT) control and direct matching can be seen in Figure 1 as the area between curve L and curve U=Um. Therefore, under certain conditions, the constant voltage tracking (CVT) method can not only obtain higher power output than direct matching, but also can be used to simplify and approximate maximum power point tracking (MPPT) control.
The constant pressure tracking method has the advantages of easy implementation, simple control and good stability. But since this method ignores the influence of ambient temperature, when the temperature changes, the solar cell will deviate from the maximum power point, resulting in a large power loss.
2. Disturbance observation method
The perturbation observation method is currently the most commonly used method in MPPT. The working principle of this method is to increase or decrease the output voltage of the photovoltaic array at regular intervals, and observe the change direction of its output power to determine the next control signal. This control algorithm generally adopts the power feedback method, which samples the output voltage and current of the photovoltaic array through two sensors, and calculates the output power.
Although the algorithm is simple and easy to implement in hardware, the response speed is slow, and it is only suitable for those occasions where the sunlight intensity changes slowly. In steady state, this algorithm will cause the actual operating point of the PV array to oscillate slightly around the maximum power point, thus causing a certain power loss. When the insolation changes rapidly, the tracking algorithm may fail and judge the wrong tracking direction.
The following is a brief description of the classical disturbance observation algorithm as follows: the photovoltaic system controller changes the output of the photovoltaic array with a small step size in each control cycle, that is, increases or decreases the output voltage or current of the photovoltaic array. This process is called “disturbance,” and the output power of the PV arrays before and after the disturbance period is compared. When the given reference voltage increases, if AP>0, it means that the reference voltage is adjusted in the correct direction, and you can continue to “interfere” in the original direction; when the given reference voltage increases, if AP<0, it means that the direction of reference voltage adjustment is wrong, and the direction of “interference” needs to be changed. In this way, the actual operating point of the photovoltaic array can gradually approach the current maximum power point, and finally reach a steady state in a small range near it.
3. Incremental conductometric method
The incremental conductance method is based on the P-U characteristic curve of the photovoltaic cell, and is a tracking method for realizing the maximum power point by adjusting the operating voltage of the system to gradually approach the maximum power point voltage. It can determine the relationship between the operating point voltage and the maximum power point voltage.