According to the admittance increment algorithm, the S function in MATLAB environment is written. Its input is the PV output voltage and current, and its output is the reference voltage of the BOOST circuit duty cycle. Considering that in most cases, the photovoltaic cells after the BOOST circuit to battery charging or connected to the inverter dc side, in a relatively small system of sampling time, BOOST circuit output voltage change is small, can be considered as a constant, so the load on the maximum power tracking experiment with a constant voltage source in series to simulate a resistance. Figure 8-11 shows the maximum power tracking (MPPT) simulation model. Some simulation parameters are as follows: IL is 10A, A is 0.02, switching frequency is 40kHz, sampling period of solar energy battery output voltage and current is 2ms, scanning time of MPPT algorithm implementation module (S function) is 0.2ms, duty cycle reference voltage increment is 0.01V.
Figure 8-12 shows the PV output power curve. It can be seen that the PV cell output basically stays around the maximum power point (2.1kW) after stabilization, with a downward fluctuation of about 50W. Figure 8-13 shows the corresponding BOOST circuit duty cycle reference voltage waveform, and the peak value of the triangle wave is 50. When the system output is stable, the duty cycle reference voltage is about 34, that is, the duty cycle is about 70% when the maximum power is output. In fact, even if the output voltage of the BOOST circuit changes to some extent (such as the voltage rise on the DC side), the photovoltaic cell can still maintain the maximum power output. However, considering the limit of the duty cycle of the BOOST circuit, the output voltage cannot rise without limit, otherwise it will lead to excessive duty cycle and imbalance. Therefore, it is necessary to estimate the output voltage range of the BOOST circuit in advance to select the appropriate PV cell array when designing the PV cell MPPT system. Figure 8 to 14 shows the photovoltaic battery output power curve, along with the change of output voltage and output current facts above curve is composed of several curve overlapping, in front of the photovoltaic cells output into the steady state, output voltage and output current are over the maximum power point, but eventually returned to near the maximum power point (the results will be launched after the curve). The BOOST circuit duty cycle reference voltage in Figure 8-13 has an overshoot, which is also one of the reasons for the output power fluctuation
Figure 8-15 (a) shows the variation curve of photovoltaic cell output power and the variation of U-I curve when the sunshine intensity increases. The short-circuit current rises from 6A to 7.4A from 0.1s, and the output power of the photovoltaic cell also increases. As can be seen from FIG. 8-15 (b), when the output characteristics of photovoltaic cells transition from U-I curve 1 to U-I curve 2, the output voltage does not change much, but the current changes greatly. This characteristic conforms to the theoretical analysis above, that is, the output voltage changes little when the maximum power point of photovoltaic cells changes. At the same time, it also shows that the maximum power tracking runs normally in the whole process, which makes the dynamic performance of the output curve change better.
A smaller capacitor in parallel at the PV output is able to maintain the voltage across the PV during the BOOST circuit switch, substantially reducing PV output voltage and current fluctuations. Through comparison, it is found that this measure has obvious effect and can reduce the value of BOOST inductance. The capacitor current is positive or negative, indicating the role of the capacitor, that is, to provide inductor current when the switch is closed, so as to reduce the fluctuation of photovoltaic current; When the switch is off, the current on the inductor decreases while the current on the capacitor increases, which can also maintain the photovoltaic output current basically unchanged. This capacitor can not change the current fluctuation of the inductor, but it can improve the waveform of the output current of the photovoltaic cell. According to the simulation results, when the capacitor is not added, the photovoltaic cell at the beginning of the simulation shows a constant voltage source characteristic, and the output voltage fluctuates greatly. When capacitance is added, the photovoltaic cell shows a constant current source characteristic at the beginning of simulation, and the output voltage gradually rises and finally reaches the maximum power point and becomes stable. When the voltage fluctuation is stable, it significantly reduces.
When designing the BOOST circuit, the problem to be considered is that the output current of the photovoltaic cell may be relatively large when the BOOST circuit is in the closed state. In this case, from the perspective of energy, the output voltage is bound to drop, which will lead to the deviation of the output power from the maximum power point and cause the fluctuation of the output power. The solution is to parallel multiple photovoltaic arrays to increase the current capacity. Second, reduce duty cycle; The third is to increase the inductance value and switching frequency of BOOST circuit. The first method to solve the flow of multiple arrays and the problem of local MPP, the second method is involved in regulating the limit, such as the duty ratio is reduced to a certain degree will lead to intermittent current and output voltage is reduced, and the third way is more flexible in application, is a good way to solve the problem of photovoltaic battery output power fluctuation, need to face the new problems is to control the cost and electromagnetic compatibility. During the simulation, it is found that with the increase of switching frequency, the BOOST output voltage increases slightly, the PV output voltage ripple is small and its DC value is slightly larger, and the output current ripple is also small. This is mainly because the inductor enters the current continuous working state at this time, and the equivalent input impedance of the BOOST circuit is independent of the switching frequency. The larger the inductance, the smaller the power output fluctuation, but the time to rise to the maximum power becomes longer. The average value of PV output voltage is large and the ripple is small, and the current ripple is also significantly reduced, which indicates that the inductor has a great influence on PV output.
In addition, to increase the output voltage of the BOOST circuit, you need to increase the duty cycle. In the simulation case, the BOOST ratio M of the boost circuit rises monotonically with the duty cycle D, and when D is greater than 0.8, M rises faster. However, in practical circuits, there is often a limit, when D continues to increase, M decreases, because there is a parasitic resistance in the inductor L, which limits the power transfer to the next stage.
When designing the initial duty cycle reference voltage of MPPT, it should be estimated in combination with the photovoltaic output characteristics. In operation, there is no need to worry that the duty cycle will change greatly, because according to the PV output characteristics, when the light intensity changes, the voltage at the maximum power point does not change much. However, when the ambient temperature changes strongly, the duty cycle will change greatly, but the ambient temperature changes slowly compared with the sunshine intensity. In practice, the duty cycle should be controlled between 30% and 80%. In order to avoid the duty cycle adjustment range being too small or beyond the above range, the BOOST ratio M of the boost circuit should be set appropriately, generally speaking, 2~3 is more appropriate. In order to achieve high voltage output, multiple PV modules can be used in series to increase the PV output open-circuit voltage to keep the boost ratio M unchanged. It is also found from the simulation that the load resistance at the maximum power point is equal to the ratio of the open circuit voltage and the short circuit current, which conforms to the impedance matching principle. The guiding significance of this result is that when designing the capacity of photovoltaic power generation system, the open-circuit voltage and short-circuit current of photovoltaic cells can be obtained by measuring method first, so as to estimate the load impedance of photovoltaic cells at the maximum power point. Based on this impedance value, the appropriate BOOST circuit parameters can be set.