In a single-stage three-phase or single-phase photovoltaic grid-connected power generation system, the stable operating point of the photovoltaic array is in the right area including the maximum power point, and the left area of the maximum power point is the unstable operating area. In the unstable area, the trajectory of the output voltage of the photovoltaic array can be changed by controlling the grid-connected power, so that it can enter the stable working area. Therefore, using the dynamic model of the single-stage grid-connected power generation system and according to the motion characteristics of the output voltage of the photovoltaic array, the maximum power point tracking method can be reasonably designed to realize the real-time tracking control of the maximum power point of the photovoltaic array.
Power stepping method to achieve the maximum power point tracking algorithm idea: when the single-stage photovoltaic power generation system is working, the output power of the grid-connected inverter can be increased by a certain step when the photovoltaic array voltage is stable, if the photovoltaic array voltage drops gradually Decrease and stabilize again, it proves that the output voltage of the photovoltaic array is on the right side of the maximum output power point voltage. At this time, the grid-connected output power of the inverter can be increased; if the voltage drop rate of the array gradually increases, the voltage value cannot be stabilized. , Prove that the photovoltaic array is working on the left side of the maximum power point, and the grid-connected power value should be reduced to restore the working voltage of the photovoltaic array to the right side of the maximum power point voltage. After stability, continue to increase the grid-connected output power by a certain step to make the photovoltaic array work The voltage tends to the maximum power point voltage. The grid-connected reference current amplitude signal Iref is used to replace the output power of the photovoltaic array. Figure 1 is a software flow chart of the power stepping method to achieve maximum power point tracking.
In Figure 1, the photovoltaic array voltage Vpv is first sampled, and the difference ev between the current given voltage signal Vref and Vpv is calculated. If the eν value is equal to the previous calculated value ev-1 for many consecutive times (for example, the counter j1≥5), the output voltage of the photovoltaic array is considered to be stable. At this time, the value of Vpv is assigned to the given voltage signal Vref, and then based on the previous value. The ratio of the disturbance increment dIref of the constant output current to ev determines the magnitude of the new given grid-connected current amplitude increment dIref, and recalculates the grid-connected current amplitude given Iref. When the ev value is greater than the ev-1 value, it means that the voltage of the photovoltaic array is decreasing. If the voltage of the photovoltaic array is continuously decreasing and cannot be stabilized (for example, the counter j2> 10), the proportional controller (kp is the proportional coefficient of the proportional controller) is used to decrease The small grid-connected current amplitude can curb the downward trend of the photovoltaic array voltage and make the photovoltaic array voltage move to a stable point in a stable trajectory. When the output voltage of the photovoltaic array is stable, the output power value of the grid-connected inverter can be increased again, and the observation and processing of the array voltage can be repeated. When the ev value is less than the ev-1 value, it means that when the output power of the grid-connected inverter is constant, the output voltage of the array is rising. According to the previous analysis, the array voltage is running on a stable track. It will converge to a stable point. After it works stably, the output power of the grid-connected inverter will be gradually increased to finally realize the MPPT function.
Figure 2 is the equivalent simulation model of the single-stage photovoltaic grid-connected power generation system, and Figure 3 is the simulation result of using the power stepping method in the simulation model shown in Figure 2. The output characteristic parameters of the photovoltaic array model used are shown in Figure 3(a). In the initial situation, a photovoltaic array model with a short-circuit current of 3A is used. When the system tracks the model and runs to the 8th s, it linearly increases the short-circuit current and increases to 4A at the 10th s. At this time, the increase of the short-circuit current is stopped. , The system continues to track the working point of the maximum power output of the photovoltaic array to the 14th s. At this time, the short-circuit current is linearly reduced, and at the 16th s, the short-circuit current is reduced to 3A. At this time, it stops reducing the short-circuit current.
Figure 3 (b), (c) and (d) respectively show the tracking of the output voltage, current and power of the photovoltaic array to the reference maximum power point voltage, current and power. It can be seen from Figure 3(b) that as the power step increases, the photovoltaic output voltage gradually decreases. When it is less than the maximum power point voltage, it cannot be stabilized. The algorithm will reduce the grid-connected output power value and pull the array output voltage. Return to the right side of the maximum power point voltage, so as to realize the disturbance tracking of the maximum power point voltage; because the voltage fluctuation is small, the output power of the photovoltaic array can track the maximum power point value of the array relatively accurately, and the tracking state does not follow the output power of the array It is not affected by sampling and quantization errors, as shown in Figure 3 (c) and (d).
Because the algorithm does not change the grid-connected reference power value during the process of increasing the maximum power point voltage value caused by external factors, dynamic tracking cannot be achieved in this process. As shown in Figure 3, the reference maximum power point voltage increases During the process, the actual working voltage of the photovoltaic array deviated seriously from the maximum power point voltage, but in this case, the output voltage of the photovoltaic array will only shift to the right of the maximum power point voltage, that is, greater than the maximum power point voltage, so it will not cause the bus voltage to collapse The phenomenon. Refer to the process of voltage drop at the maximum power point, the algorithm will adjust the grid-connected output power to stabilize the output voltage of the array. When the external conditions are stable, it will continue to track to achieve steady-state accurate tracking, so the sudden weakening of the light will cause the maximum power When the point voltage drops, the output voltage of the photovoltaic array will not collapse.
Although this method does not have dynamic tracking performance, it can quickly and accurately achieve maximum power point tracking under steady-state conditions. It only needs to sample and observe the output voltage of the array. Its implementation cost is low, the algorithm is simple and reliable, and it can effectively avoid busbars. Voltage collapse phenomenon. In addition, the algorithm is not affected by the intensity of light, and can achieve effective steady-state tracking in the full power range, so this method is suitable for occasions with weak light and relatively stable light.