Pulse width modulation controller
The principle of the pulse width modulation (PWM) controller circuit is shown in Figure 1. The controller switches the input of the photovoltaic module in pulse mode. When the storage battery gradually tends to be full, with the gradual increase of its terminal voltage, the frequency and time of the PWM circuit output pulse will change, which will extend the on-time of the switch and shorten the interval, and the charging current will gradually approach zero. When the battery voltage drops from the full point to the point, the charging current will gradually increase. Compared with the previous two controller circuits, although the pulse width modulation charging control method does not have a fixed overcharge voltage disconnection point and recovery point, when the battery terminal voltage reaches the vicinity of the overcharge control point, its charging current approaches zero. This charging process can form a relatively complete state of charge, and the instantaneous change of the average charging current is more in line with the current charging status of the battery, which can increase the charging efficiency of the photovoltaic system and extend the total cycle life of the battery. In addition, the pulse width modulation controller can also realize the maximum power tracking function of the photovoltaic system, so it can be used as a high-power controller in a large-scale photovoltaic power generation system. The disadvantage of the pulse width modulation controller is that the controller’s own work has a power loss of 4% to 8%.
Multi-channel controllers are generally used in high-power photovoltaic power generation systems of several kilowatts or more, and divide the entire photovoltaic power supply into multiple branches to connect to the controller. When the battery is full, the controller disconnects each branch of the photovoltaic array one by one; when the battery voltage drops to a certain value, the controller turns on the photovoltaic array one by one to adjust the charging voltage and current of the battery pack. This control method is an incremental control method, which can approximate the effect of a pulse width modulation controller. The more channels, the smaller the increase, and the closer to linear adjustment. But the more the number of channels, the higher the cost. Therefore, when determining the number of photovoltaic arrays, the control effect and the cost of the controller must be considered comprehensively.
The circuit principle of the multi-channel controller is shown in Figure 2. When the battery is fully charged, the control circuit will control the mechanical or electronic switch to sequentially disconnect the branches Z1~Zn of the photovoltaic square array from K1~Kn. After the first circuit Z1 is disconnected, if the battery voltage has fallen below the set value, the control circuit waits; until the battery voltage rises to the set value again, disconnect the second circuit Z2 and wait again; if the battery voltage no longer rises to the set value, the other branches remain connected to the charging state. When the battery voltage is lower than the recovery point voltage, the photovoltaic square array branches that are disconnected are connected in sequence, until they are all connected before dark. In the figure, D1~Dn are the anti-reverse charge diodes of each branch, A1 and A2 are the charging ammeter and the discharging ammeter respectively, and V is the battery voltmeter.
The intelligent controller uses a microprocessor such as CPU or MCU to collect the operating parameters of the photovoltaic power generation system in real time at high speed, and according to a certain control law, the single-circuit or multi-circuit battery module is cut off and connected intelligently by the program in the single-chip microcomputer. The medium and high-power intelligent controllers can also control and transmit data through the computer through the RS232/485 interface of the single-chip microcomputer, and carry out long-distance communication and control.
In addition to the protection functions of overcharge, overdischarge, short circuit, overload, and anti-reverse connection, the intelligent controller can also use the battery discharge rate to control the discharge with high accuracy. The intelligent controller also has a high-precision temperature compensation function. The circuit principle of the intelligent controller is shown in Figure 3.
Maximum power point tracking controller
The principle of the maximum power point tracking controller is to multiply the voltage and current of the battery module or photovoltaic array to obtain the power after detection, and judge whether the output power of the battery module or the rectangular array at this time reaches the maximum. If it is not running at the maximum power point, adjust the pulse width, modulate the output duty cycle, change the charging current, perform real-time sampling again, and make a judgment on whether to change the duty cycle. Through this optimization tracking process, it can be ensured that the battery modules or photovoltaic arrays are always running at the maximum power point. The maximum power point tracking controller can keep the battery module or photovoltaic array at the maximum power point state at all times, so as to make full use of the output energy of the battery module or photovoltaic array. At the same time, the PWM modulation method is adopted to make the charging current into a pulse current to reduce the polarization of the battery and improve the charging efficiency.