Photovoltaic power generation system has the following characteristics: no rotating parts, no noise; No air pollution, no waste water discharge; There is no combustion process and no fuel is needed; Simple maintenance and low maintenance cost; Good operation reliability and stability; As a key component, solar cells have a long service life, and the life of crystalline silicon solar cells can reach more than 25 years; It is easy to expand the power generation scale as needed.
As shown in the schematic diagram of DC load photovoltaic system. It includes several main components in the photovoltaic system: photovoltaic module array: it is composed of solar cell modules (also known as photovoltaic modules) connected in series and in parallel according to the system requirements, which converts solar energy into electric energy under the irradiation of sunlight and is the core component of the solar photovoltaic system. Generally speaking, qualified solar charge and discharge controllers have the following charge and discharge protection methods:
A direct charging protection point voltage: direct charging is also called fast charging, which belongs to fast charging. Generally, when the battery voltage is low, the battery is charged with a large current and a relatively high voltage. However, there is a control point, also called a protection point, which is the value in the above table. When the terminal voltage of the battery is higher than these protection values during charging, direct charging should be stopped. The direct charging protection point voltage is also the "overcharge protection point" voltage. The terminal voltage of the battery should not be higher than this protection point when charging, otherwise it will cause overcharge and be harmful to the battery.
B. Equalize the control point voltage: After the battery is directly charged, the charge-discharge controller is generally allowed to stand by for a period of time to let its voltage naturally drop, and when it drops to the "recovery voltage" value, it will enter an equilibrium state. Why design equilibrium? That is, after the direct charging is completed, some batteries may be "reversed" (the terminal voltage is relatively low). In order to pull these single molecules back and make the terminal voltage of all batteries uniform, it is necessary to charge them with high voltage and medium current for a short time, which is called uniform charging, that is, "balanced charging". Generally, the charging time should not be too long, usually a few minutes to ten minutes, but it is harmful to set the time too long. For a small system equipped with one battery and two batteries, average charging is of little significance. Therefore, street lamp controllers generally have no unified charging, only two stages.
C floating charge control point voltage: generally, after the battery is fully charged, let the battery stand for a period of time, so that its terminal voltage naturally drops. When it drops to the "maintenance voltage" point, it enters the floating state. Adopt PWM (pulse width modulation) mode, similar to "trickle charging" (small current charging). When the battery voltage is low, charge it a little, and when it is low, charge it a little, one by one. In fact, PWM mode is mainly to stabilize the terminal voltage of the battery and reduce the charging current of the battery by adjusting the pulse width. This is a very scientific charge management system. Specifically, in the later stage of charging, the remaining capacity (SOC) of the battery is >: when it is 80%, the charging current must be reduced to prevent excessive gas (oxygen, hydrogen and acid gas) from being released due to overcharge.
D over-discharge protection termination voltage: this is easy to understand. The battery discharge cannot be lower than this value, which is the national standard. Although battery manufacturers also have their own protection parameters (enterprise standards or line standards), they will eventually move closer to the national standard. It should be noted that, for the sake of safety, 0.3V is artificially added to the over-discharge protection point voltage of 12V battery as the temperature compensation or zero drift correction of the control circuit, so that the over-discharge protection point voltage of1.10v is 22.20V for the 24V system, and many manufacturers of charge and discharge controllers adopt 22.2V(24V.. be filled/suffused/brimming with
As an interface circuit between photovoltaic cells and lead-acid cells, electric controllers are generally expected to work at the maximum power point to achieve higher efficiency. However, while realizing MPPT, battery charging control should also be considered. Commonly used main circuit topologies mainly include Buck converter, Boost converter and Cuk converter. Generally, the output voltage of photovoltaic cells fluctuates greatly, but Buck converter or Boost converter can only carry out buck or boost conversion. As a result, photovoltaic cells can not work at the maximum power point in a large range, which leads to the decline of system efficiency. At the same time, the input current ripple of Buck converter is large. If the energy storage capacitor is not added at the input end, the system will work in an intermittent state, which will lead to the intermittent output current of photovoltaic cells, and it will not be in the best working state. However, the output current ripple of Boost converter is large, so charging the battery with this current is not conducive to the service life of the battery; Cuk converter has both step-up and step-down functions. Applying Cuk converter to the charging controller of photovoltaic system can realize the maximum power point tracking in a wide range, which is beneficial to improve the system efficiency. Therefore, the main circuit of the charging controller often chooses the Cuk converter, and its system topology is shown in the main circuit diagram of the Cuk charging controller.
Under the condition of continuous load current, the steady-state process of uk converter circuit is as follows:
1, conduction period of switch tube Vr
During this period, the switching tube Vr is turned on, and the voltage on the capacitor C2 makes the diode D2 reverse biased and turned off. At this time, the input current iL2 makes L 1 store energy. The discharge current iL2 of C2 makes L2 store energy and supply power to the load, as shown in the equivalent circuit diagram (A) of Cuk converter in continuous operation mode.
2. During the period when the switch tube Vr is turned off, the switch tube Vr is turned off, the diode D2 is turned on in a positive bias, and the energy release current il 1 of the power supply and Ll is charged to C2, while the energy release current iL2 of L2 is used to maintain the load, as shown in the equivalent circuit diagram (b) of Cuk converter in continuous operation mode. Therefore, C2 is charged during the Vr off period and discharged to the load during the Vr on period, and C2 plays the role of energy transfer. The circuit principle of solar lawn is relatively simple. Its controller is realized by a boost circuit.
Component selection: BT 1 uses 3.8V/80mA solar panels, and monocrystalline silicon is the best, followed by polycrystalline silicon; Two 1.2V/600mA nickel-cadmium batteries are selected for BT2. If it is necessary to improve the luminosity or extend the time, the power of solar panels and batteries can be increased accordingly. The β of VQ2, VQ3 and VQ5 is about 200, and VQ4 needs transistors with large β values. VD 1 Try to choose low-voltage transistors, such as germanium tubes or Schottky diodes. LED can be white, blue and green with ultra-high brightness astigmatism or spotlight. When red, yellow, orange and other low drop LEDs are selected, the circuit needs to be reset. R3 and R5 suggest choosing 1% precision resistor; R4 adopts photoresistor, with bright resistance10kω ~ 20kω and dark resistance 1mω. Other resistors can be common carbon film (1/4)W and (1/8)W resistors. L 1 adopts (1/4)W color inductor, and the DC impedance is smaller.
Circuit working principle: when there is sunlight in the daytime, BT 1 converts light energy into electric energy, and VD 1 charges BT2. Due to illumination, the resistance of the photosensitive resistor is low, and VQ4 b is extremely low, so it is turned off. When there is no light at night, the resistance of the photosensitive resistor is very high, VQ4 is turned on, and VQ2 b is also turned on at a very low level. The DC boost circuit consisting of VQ3, VQ5, C2, R6 and L 1 works, and the LED emits light.
The core of DC boost circuit is a complementary tube oscillation circuit, and its working process is as follows: When VQ2 is on, the power supply charges C2 through L 1, R6 and VQ4. Because the voltage across C2 cannot change suddenly, VQ3 b is extremely high and VQ3 is not conductive. With the charging of C2, its voltage drop is higher and higher, and the potential of VQ3 b is lower and lower. When the turn-on voltage of VQ3 is reached, VQ3 turns on and VQ5 turns on one after another. C2 discharges through VQ5 ce junction, power supply and VQ3 eb junction (because VQ2 is on, we assume that its ec junction is short-circuited, and VQ3 e electrode is directly connected to the positive electrode of power supply).
After discharging, VQ3 turns off, VQ5 turns off, and the power supply charges C2. Then VQ3 turns on, VQ5 turns on, and C2 discharges. Repeatedly, the circuit oscillates. During the oscillation, when VQ5 is on, the power supply is grounded through L 1 and VQ5 ce, and the current is stored through L 1. When VQ5 is off, L 1 generates induced electromotive force. This can increase the battery voltage to directly drive the LED to improve the efficiency, but with the increase of the battery voltage, the price of the corresponding solar cell is also greatly increased. As long as the circuit components are set properly, the efficiency is acceptable. When the battery is not fully charged during the day (such as rainy days), BT2 may be overdischarged and damage the battery. Therefore, R5 is added to form over-discharge protection: when the battery voltage drops to 2V, the base potential of VQ4 is not enough to make it conductive due to the partial voltage of R5, thus protecting the battery. Increasing R5 will affect the conduction depth of VQ4.