From the above analysis, it can be seen that the open circuit voltage and short circuit current of solar cells are largely affected by the intensity of sunlight and temperature, and the system work will also be erratic, which will inevitably lead to a decrease in system efficiency. For this reason, the solar cell must realize the maximum power point tracking control, so that the battery continuously obtains the maximum power output under the current sunshine.
In the photovoltaic grid-connected system, the photovoltaic cell MPPT device structure adopts the DC/DC circuit, and the realization of the maximum power point tracking function is at the DC/DC level. This stage is used as the load of the photovoltaic cell, and the match with the output characteristics of the photovoltaic cell is changed by changing the duty cycle.
According to the functional classification of DC/DC conversion circuits, there are Buck Converter, Boost Converter, Boost Buck Converter, and Cuk Converter and so on.
Since the input of the Buck circuit works in an intermittent state, if the energy storage capacitor is not added, the photovoltaic cell will work intermittently and cannot be in the best working state. After the energy storage capacitor is added, the Buck circuit power switch will be disconnected. The photovoltaic cell charges the energy storage capacitor, which makes the photovoltaic cell always in the state of generating electricity. At this time, adjusting the duty cycle of the Buck circuit can effectively track the maximum power point. Therefore, the energy storage capacitor is essential to realize the MPPT function by using the Buck circuit. Under heavy load conditions, the energy storage capacitor is always in a state of high current charging and discharging, which is detrimental to its reliable operation. At the same time, because the energy storage capacitor is usually an electrolytic capacitor, this increases the volume of the device and makes the entire system bulky. That is, in order to obtain the normal input working voltage of the DC/AC circuit of the rear stage, the output voltage of the front stage should not be too low, and the voltage of the photovoltaic cell changes greatly with factors such as sunlight. When the output voltage is low, if the Buck circuit is used again Step-down, the inverter stage cannot work normally, so the Buck step-down circuit is not used.
In contrast, the Boost converter can always work in the state of continuous input current. As long as the input inductor is large enough, the ripple current on the inductor can be as small as close to a smooth DC current, so only a small capacitor with a smaller capacity is needed. The sense capacitor does not even add a capacitor, which avoids all the disadvantages caused by adding a capacitor. At the same time, the Boost circuit is also very simple, and because one end of the power switch tube is grounded, the design of the drive circuit is more convenient. And because the DC voltage of the distributed photovoltaic array is generally 170-300v, in order to facilitate the next level of DC/AC to have a better inverter efficiency, a Boost boost circuit should be considered here. The disadvantage of the Boost circuit is that its input terminal voltage is relatively low. Under the same power, the input current is relatively large, which will result in relatively large line loss. But Boost circuit has unique advantages and is still an attractive solution. The circuit is shown in Figure 1.
Figure 1 shows the basic circuit of the Boost converter, assuming that all components in the circuit are ideal components. There is no power loss from the input to the output of the circuit. When the power switch S is turned on, the input voltage charges the inductor L, and the current in L rises; when S is turned off, the inductor L begins to discharge, and the voltage across the inductor is superimposed with the voltage of the input power supply, so that the output is higher than the input. Voltage, the voltage relationship between the input and output of the Boost circuit is:
Uo=Uin/(1-D) ——(1)
In the described MPPT system, since the load of the Boost converter is an electrolytic capacitor, the value of the output voltage UO will be clamped to the voltage across the electrolytic capacitor, because the input voltage is the highest open circuit voltage UOC of the photovoltaic cell, and UOC< If the D value of UO is too small, it can be seen from formula (1) that the voltage generated by the Boost circuit at the output terminal will be less than that of the electrolysis
The voltage across the capacitor makes it impossible to charge the capacitor. Therefore, there is a lower limit of D, Dmin. Only when D>Dmin can the photovoltaic cell have an impact on the charging current of the capacitor. This value can be obtained by the following method. Assuming that the voltage at the input terminal is the open circuit voltage Uoc of the photovoltaic cell, it can be obtained from equation (1):
Uo=Uoc/(1-Dmin) ——(2)
From the above formula:
Umin=1-(Uoc/Uo) ——(3)
When D changes in the interval of Dmin~100%, the voltage at the input and output terminals of the Boost circuit should satisfy equation (1), in U. Under the same condition, changing D will change the voltage at both ends of the photovoltaic cell connected to the input terminal of the Boost converter. Therefore:
Uin=Uo/(1-D) ——(4)
Therefore, the input voltage Um of the Boost circuit can vary from 0 to Uoc. As long as the photovoltaic cell has a suitable open circuit voltage, by changing the D of the Boost converter, the Uin value corresponding to the maximum power point of the photovoltaic cell can be found. At this time, the output power of the photovoltaic cell is the largest. When the capacitor is charged, if the duty cycle is too large and the output filter capacitor is small, a very pulsating charging current will be generated, which will affect the life of the capacitor. However, the output power of the photovoltaic system has an upper limit, so the maximum charging current of the capacitor also has an upper limit. When selecting the output filter capacitor C, you only need to ensure that it can supply the load current at the maximum power during the on-time of the power switch. Can.