Research on Photoelectric Complementary LED Street Light Controller

introduction

Photoelectric complementary LED street lighting system is a street lighting system that mainly uses solar cells to generate electricity and uses ordinary 220V AC to supplement electricity. With this system, the photovoltaic battery group and battery capacity can be designed to be smaller. Basically, when there is sunshine during the day, solar energy is used to generate electricity and charge the battery at the same time. When it gets dark, the battery discharges and lights up the load LED. In most parts of my country, there are basically more than two-thirds of sunny weather throughout the year. In this way, the system uses solar energy to illuminate street lights for more than two-thirds of the year, and uses municipal electricity to supplement energy for the rest of the time. This not only reduces the one-time investment of the solar photovoltaic lighting system, but also has a significant energy-saving and emission-reduction effect. It is an effective method for promoting and popularizing solar LED street lighting at this stage.

1 Design of optoelectronic complementary LED lighting system

1.1 LED lighting load

Assume that the height of the photoelectric complementary LED street light pole is 10m, the luminous flux is about 25 lm, and 1W, 3.3V, 350mA LED lights are selected to form two street lights, each with 14 strings and 2 parallels, a total of 28W, and two roads are 56W. Assume that the street light is illuminated for an average of 10 hours a day, the LED street light is fully lit for the first 5 hours, and the brightness is halved for the next 5 hours, that is, the battery consumption is reduced by half.

The actual driving current required is:

350mA×2×2=1.4A

Assuming 10 hours per day, the ampere-hours required for the load are:

1.4A×5h+1.4A×0.5×5h=10.5Ah

The voltage is:

3.3V×14=46.2V

1.2 Battery Pack Capacity Design

1.2.1 Selection of batteries

Solar street light batteries are frequently in charge and discharge cycles, and often overcharge or deep discharge occurs, so the battery performance and cycle life become the most concerned issues. Valve-regulated sealed lead-acid batteries have the advantages of no maintenance, no emission of hydrogen and acid mist into the air, good safety, and low price, so they are widely used. Battery overcharge, over discharge, and battery ambient temperature are all important factors that affect the battery life, so protection measures should be taken in the controller.

1.2.2 Calculation of battery capacity

In the photovoltaic complementary street light system, the LED street lights are powered by solar energy and city electricity. Since sunlight varies greatly with weather, when the sunlight is strong during the day, the solar panels charge the battery; at night, the battery supplies power to the load. On cloudy days, the load electricity is obtained from the battery. When the battery discharge voltage drops to the minimum allowable limit, it automatically switches to city electricity. The capacity of the battery is very important to ensure reliable power supply. If the battery capacity is too large, the cost price will increase. If the capacity is too small, it will not be able to fully utilize solar energy to achieve energy saving.

Battery capacity Bc calculation formula:

Bc = A×QL×NL×T0/CC Ah (1)

In formula (1), A is the safety factor, which is between 1.1 and 1.4. In this formula, A=1.2; QL is the average daily power consumption of the load, which is the working current multiplied by the daily working hours, QL=10.5Ah; NL is the longest continuous rainy days. Due to the use of photovoltaic complementarity, NL=1 day can be taken; T0 is the temperature correction coefficient, which is generally 1.1 above 0℃ and 1.2 below -10℃. In this formula, T0=1.1; CC is the battery discharge depth, which is generally 0.75 for lead-acid batteries and 0.8 for alkaline nickel-cadmium batteries. In this formula, CC=0.75.

Therefore, Bc = A×QL×NL×T0/CC=1.2×10.5×1×1.1/0.75=18.5Ah. In the actual design, we choose 48V, 40Ah maintenance-free valve-regulated sealed lead-acid battery.

1.2.3 Solar cell array design

A certain number of solar cell modules are connected in series to obtain the required working voltage. However, the series connection of solar cells must be appropriate. If the number of series connections is too small, the series voltage will be lower than the floating charge voltage of the battery, and the solar cell array will not be able to charge the battery; if the number of series connections is too large, the output voltage will be much higher than the floating charge voltage, and the charging current will not increase significantly. Therefore, the optimal state can only be achieved when the series voltage of the solar cell modules is equal to the appropriate charging voltage.

The output voltage of the solar cell group is generally 1.2 to 1.5 times the battery voltage. When it is 1.35 times, the battery voltage is 48V×1.35=64.8V. Here we take 65V.

If there is no sunlight on that day, the battery discharge capacity to the load at night is:

Bcb = A×QL×NL = 1.2×10.5×1 = 12.6Ah

In Zhengzhou, the battery is charged with 5 hours of sunlight, and the current is:

I = 12.6Ah/5h = 2.52A

So the solar cell array power is:

P = UI = 65V × 2.52A = 163.8W

Actually, 4 36V 48W solar panels can be used, with a total of 192W, divided into two groups, each with 2 panels connected in series, and the voltage is 72V.

2. Introduction to controller and working principle

2.1 Optoelectronic Complementary LED Street Light Controller System Structure

The structure diagram of the optoelectronic complementary LED street light control system is shown in Figure 1. The key component in this system is the controller, and the functions of the controller are mainly:

(1) Detect the voltage and current of the solar panel during the day, and track the maximum output power point of the solar panel through the MPPT algorithm, so that the solar panel can charge the battery with the maximum output power, and control the way the solar cell charges the battery; (2) Control the automatic conversion of photoelectric complementarity, control the battery discharge at night, and drive the LED load lighting; when the sunlight is insufficient or it is rainy, the battery discharge voltage reaches the minimum voltage, and it can automatically switch to the mains power supply to light up the LED street lamp; (3) Implement over-discharge protection, over-charge protection, short-circuit protection, reverse connection protection and polarity protection for the battery; (4) Control the switch of the LED lamp, and control the turning on and off time of the LED lamp by monitoring the external environment.

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2.2 Charging circuit and output control

2.2.1 Charging Circuit

The charging circuit is used to adjust the charging current and voltage so that the solar panel can charge the battery stably. Since the solar radiation energy converted by the solar panel is different at different times of the day, the current and voltage output by the solar cell are different, which requires the necessary charging circuit to control. This circuit is a voltage-type pulse width modulation (PWM) control circuit implemented by TL494, and the circuit diagram is shown in Figure 2.

When the single-chip computer connected to R12 gives a high level to pin 4, the cut-off time of TL494 increases to 100%, and TL494 does not work. In this way, the level of the input at pin 4 can be used to determine whether to charge the battery. Pin 12 of TL494 is connected to the power supply, and the 5V reference voltage output by pin 14 is used for the single-chip computer. At the same time, the voltage division of R5 and R6 is used as the reference voltage signal for constant voltage charging at the in-phase end (pin 2) of the error amplifier 1 in TL494. The positive voltage of the battery is divided by R2 and R3 as the given voltage signal for constant voltage charging input at the inverting end (pin 1) of the error amplifier 1. The deviation between the two is used as a constant voltage regulator. The resistor and capacitor components are introduced between pins 2 and 3 to correct and improve the frequency response of the error amplifier. When the system is working, it detects the output voltage of the solar panel and the voltage of the battery in real time, and controls whether the solar cell charges the battery according to the different conditions of each voltage value, and controls whether the LED street light is lit according to the set street light time control or light control mode, and the reasonable switching of the power supply mode between the battery and the mains when the light is on. TL494 mainly completes the detection of the battery and solar panel and the charge and discharge control under the control of the single-chip program.

The lighting time of the street lamp can be set according to the direct dial switch on H1~H4, and each gear corresponds to 1 hour, 2 hours, 4 hours, and 8 hours. In this way, it can be adjusted within 1 to 15 hours through different combinations. The control flow chart of the system software is shown in Figure 3.

During operation, the MCU will always detect the voltage of the solar cell and the battery. When the output voltage of the solar cell is higher than the battery by more than 2V and the battery is not fully charged, the MCU's 11-pin outputs a low level, and the chip TL494 starts to work and charges the battery through the MOS tube Q1. When it is fully charged, it switches to the floating charge state to compensate for the self-discharge of the battery. The charging of the battery starts with a large current constant current charging state, and the charging current is Imax. When the battery voltage reaches 52.8V, the charger is in a constant voltage charging state, and the charging current continues to decrease. When the current drops to 250mA and the battery voltage rises to about 56.4V, the battery has reached 100% of the rated capacity, and the circuit enters the floating charge stage. The floating charge voltage provided to the battery offsets the self-discharge of the battery. When the battery voltage reaches 57.6±0.2V, the battery reaches the overcharge voltage point, the MCU's 11-pin outputs a high level, the chip TL494 ends its work, and the battery charging ends.

3 Conclusion

Through the design and actual test observation of the optoelectronic complementary LED street light system, the results basically meet the design requirements, but it must be put into actual long-term operation and continuously improved in design to achieve effective use of solar energy, the most reasonable matching of battery capacity, the lowest cost, and the best performance-price ratio.