The intermittent variable step length search method is a new method that organically combines the constant voltage method and the variable step length disturbance observation method. The method is specifically divided into the following three stages, as shown in Figure 1. In the figure, ① represents the first stage-the start-up phase, ② represents the second stage-the timing T1 variable step search phase, ③ represents the third stage-the timing T2 fixed duty cycle stage.
The first stage is the variable step-length start-up stage. At first, the step size is large, and the step is quickly located near the maximum power point, and then the step size is reduced until it oscillates near the maximum power point.
The second stage is to track and search the maximum power point with variable step length within a short time period of T1, and judge the change of the external environment at this stage according to the size of the current step size and the accumulated value of step length in T1. If the current step length is small and the step length accumulated value is close to zero, it is determined that the change in the external environment is small. If the current step size is large or the accumulated value of the step size deviates from zero, re-timing T1, search for the maximum power point of this stage, and turn to the third stage until the condition of small changes in the external environment is met.
The third stage adopts constant duty cycle control. From the second stage, it can be seen that the current external environment changes very little, and the power change is basically negligible. Therefore, the average duty cycle in the previous T1 is used as the optimal duty cycle to control the power conversion. After the long time T2.T2 of this stage is over, return to the second stage and repeat.
When the external environment changes greatly, the system always uses T1 timing variable step size tracking search, which is basically the same as the conventional variable step size search method. When the change of the external environment is small, the optimal constant duty ratio control with a longer timing time is maintained. For the constant duty cycle stage, when the external environment such as light changes, if the constant duty cycle is still used, a certain amount of power will be lost. Therefore, at this stage, it is necessary to judge the degree of changes in the external environment in real time. The judgment criterion is no longer based on step. The length and the cumulative value of the step length are judged according to the differential size of the output power Ppv of the photovoltaic array to the output voltage Vpv. If it is determined that the external environment has changed greatly, return to the second stage to re-timing variable step search, and quickly locate near the maximum power point; otherwise, it will not return to the second stage until the timing T2 ends. The process is shown in Figure 2.
It can be seen that the third stage uses the optimal duty cycle obtained in the second stage as a constant duty cycle, which effectively avoids the power fluctuations and the resulting power fluctuations caused by the continuous disturbance caused by the conventional search method. Track power loss. The magnitude of the change in the external environment is judged based on the differentiation of the output power Ppv of the photovoltaic array to the output voltage Vpv. In each T2 constant duty cycle control, the value of dPpv/dVpv is always calculated in real time. When it is continuously greater than the average absolute value of dPpv/dVpv in the previous stage, it is judged that the external environment changes greatly, otherwise the change is small. The step length adjustment is calculated by formula (1), △d is the minimum setting step length. M is an integer greater than zero, a large value of M indicates a large adjustment step length, otherwise the adjustment step length is small. The fixed duty cycle of the third stage is determined by formula (2). When the external environment is stable, the average duty cycle of the T1 timing search in the previous stage is taken as the optimal duty cycle of this stage, where h represents the number of searches performed within T1. The benchmark for changes in the external environment is represented by equation (3).
dk=dk-1±M△d——(1)
The simulation was carried out using PSIM software. Simulation parameters: Voc=38V, Isc=3.2A under standard conditions, the search time T1 of the timing variable step size is 0.1s, and the time T2 of the timing constant duty cycle is 0.2s.
Figure 3 is the result of the output voltage and current of the photovoltaic array when the light intensity is constant at 1000W/㎡. The results show that after a short start-up process, the system quickly enters the two phases of timed variable step search and constant duty cycle, and these two phases change alternately. It can be seen that the current external environment remains basically unchanged.
Figure 4 shows the output voltage and current results of the photovoltaic array using the intermittent variable step size search method when the illumination changes. The response is relatively fast during the start-up phase and the sudden change of illumination, and when the system is in the constant duty cycle phase, even if the illumination changes, for example , In the 1.2s and 2.1s of the fixed duty cycle stage, the light is suddenly weak and strong, and it can also be judged that the external environment has changed, and then the fixed duty cycle stage ends and enters the second stage (T1 variable step search stage) , And quickly reach the new maximum power point.
Under basically the same conditions as the above simulation parameters, the control system with TMS320LF2407A digital signal processor as the core is used to verify the disturbance observation method and the intermittent variable step size search method respectively. T1=0.4s, T2=2s .
Figure 5 shows the experimental results of the perturbation observation method and the intermittent variable step size search method under constant light intensity. Figure 5(a) shows that the system has been oscillating near the maximum power point under the disturbance observation method. Figure 5(b) shows that the 0.4s timing variable step search and the 2s timing constant duty ratio control alternately. It can be seen that the 2s timing constant duty ratio control works stably at the maximum power point.
In the experiment, the light-shielding and non-shielding methods are used to simulate the sudden change of light. Figure 6(a) shows the output voltage and current waveforms of the photovoltaic array of the disturbance observation method under the light sudden change. Figure 6(b) shows the output voltage and current waveforms of the photovoltaic array using the intermittent variable step method under the condition of sudden light changes. The results show that the perturbation observation method responds slowly when the light intensity changes, while the intermittent variable step search response is relatively fast. At the same time, when the system is in the constant duty cycle stage, even if the illumination changes, it can immediately leave the constant duty cycle stage and restart. Enter the search phase, and then quickly track the new maximum power point.
In summary, the intermittent variable step size search method adopts timing and constant duty ratio control when the illumination changes are small, which realizes the stable operation of the system at the maximum power point and avoids the tracking power loss caused by continuous oscillation to a certain extent. Make the system have good static performance. In the start-up phase and when the illumination changes drastically, the timed variable step perturbation observation method is used to speed up the search, so as to ensure that the system also has excellent dynamic performance.
