Working Principle of Solar Power Low Pressure Sodium Lamp Intelligent Controller

In recent years, renewable energy technology has received wide attention, and solar power generation has become one of the most promising renewable energy sources due to the assistance of advanced power electronics technology. Low-pressure sodium lamps (LPS), as the light source with the highest light efficiency and the smallest light decay, are the best choice for road lighting. When combined with solar photovoltaic systems, the energy-saving effect is more significant. There are various solar power intelligent controller products on the market for different types of loads, but there are few controllers designed for high (low) pressure sodium lamps as loads. In order to distinguish it from other solar power controllers, this article uses low-pressure sodium lamps as loads. The intensity of solar energy is affected by various factors (season, location, climate, etc.) and cannot be maintained constant, resulting in the solar energy comprehensive utilization system being greatly affected by conditions such as sunshine, temperature, and load changes, making the system nonlinear and increasing the difficulty of system control. In order to achieve stable and efficient operation between power supply and load, improve power supply quality, and reasonably control the cycle charge and discharge and charge and discharge depth of the battery in the system, and extend the service life of the battery, it is necessary to design an efficient, reliable, and intelligent solar power charge and discharge controller to complete these tasks, so that the charging and on/off process can be automatically controlled by light control without manual operation, stable and reliable operation, saving electricity bills, and maintenance-free. Therefore, it is of great significance to carry out research on solar power intelligent controller for low-pressure sodium lamps.

1 System Structure

The independent solar photovoltaic system is mainly composed of solar cell array, battery pack, controller and load. Its composition block diagram is shown in Figure 1.

Analysis of the composition of the solar energy comprehensive utilization system shows that the load uses sunlight as energy, the solar cell module/array collects electricity during the day, the lead-acid battery stores electricity, and through the reasonable control of the controller, it provides electricity to the load at night, and the use time and working mode can be flexibly set. A low-pressure sodium lamp solar power controller for a small photovoltaic system has been developed, in which the battery is 24 V, the load is a 36 W low-pressure sodium lamp, and the load output is single-channel.

2 System Hardware Design

The hardware part of the intelligent controller device has three interfaces: one for the solar cell array interface, one for the battery interface and one for the low-pressure sodium lamp load interface. The overall structure design block diagram of the controller is shown in Figure 2. The controller part is in the dotted box.

2.1 P87LPC767 MCU

The system control is implemented by the P87LPC767 microcontroller of PHILIPS Semiconductor Company. When it works at 100 kHz to 4 MHz and the power supply voltage is 3.3 V, its power consumption is only 0.044 to 1.7 mA, which is suitable for battery-powered systems. It provides high-speed and low-speed crystal oscillators and RC oscillation modes, which can be selected by programming, and has a wide operating voltage range; programmable I/O port line output mode selection, Schmitt trigger input, LED drive output can be selected; it contains a watchdog timer and I2C bus; its internal two analog comparators can form an 8-bit A/D converter; it also has power-on reset detection and undervoltage reset detection functions; it ensures that the I/O port drive current reaches 20 mA. P87LPC767 adopts the 80C51 accelerated processor structure, and the instruction execution speed is twice that of the standard 80C51 CPU.

This system is based on the P87LPC767 microcontroller, and the peripheral circuits are mainly composed of voltage acquisition circuit, load output control and detection circuit, LED display circuit, mode selection circuit, and E2PROM chip that can add additional functions to the system. P87LPC767 and its peripheral circuits are shown in Figure 3. Table 1 is a description of the pin functions of the P87LPC767 microcontroller combined with the hardware circuit design.

2.2 Battery charging and discharging circuit

Based on the particularity of photovoltaic power generation, it is necessary to design a charge and discharge control circuit with good performance. In order to extend the service life of the battery, its charge and discharge conditions must be restricted to prevent the battery from overcharging and deep discharge. The charge and discharge circuit is equivalent to a voltage acquisition and battery management module.

When there is sunlight during the day, the microcontroller detects the voltage values ​​of the solar cell array and the battery respectively, and controls the on/off state of the switch MOSFET tube VQ9 (Figure 3) of the battery charging circuit. When the microcontroller detects that the PV + level is higher than the BAT+ level, the switch device VQ9 is turned on, and the solar cell array charges the battery in a direct charging manner; when the battery is charged to an overvoltage, the switch device Q9 is turned off, and the solar cell array charges the battery with a small current (floating charge), which can play an "overcharge protection" role.

When there is insufficient sunlight at night or on cloudy days, the relay is turned on and the battery is discharged to ensure that the load does not stop power. The relay RELAY1 designed in this system is a battery discharge switch, and the control signal of RELAY1 on/off is output by the I/O port of the single-chip computer. The load of this system is a low-pressure sodium lamp, which is used for road lighting. Therefore, it should have a light control function, that is, when there is sunlight, RELAY1 is turned off; when there is insufficient sunlight at night or on cloudy days, RELAY1 is turned on, the battery is discharged, and the street lamp is illuminated. From the perspective of protecting the battery, when it is necessary to supply power to the load but the battery voltage is less than the "over-discharge voltage", RELAY1 is also turned off to perform "over-discharge protection" to avoid emptying the battery and damaging the battery; when the solar cell array is powered again and only when the battery voltage rises to the floating charge voltage again, RELAY1 is turned on again when it is necessary to supply power to the load, and the load circuit is connected.

2.3 DC/AC low-frequency electronic ballast system circuit for low-pressure sodium lamp

The electronic ballast is connected between the power supply and one or more gas discharge lamps, and limits the working current of the gas discharge lamp to the specified value, which is used for output control and detection of the load. Low-pressure sodium lamps belong to gas discharge lamps. Since gas discharge lamps have negative resistance working characteristics, they should be equipped with control devices such as ballasts to make them work normally. These related control devices should complete the following control functions: 1) limit and stabilize the working current of the gas discharge lamp; 2) ensure that the lamp voltage, lamp current and lamp power are stable within the allowable variation range of the battery terminal voltage, so that the lamp can work normally. 3) provide the ignition voltage required for the gas discharge lamp. 4) provide the required lamp electrode preheating function before the gas discharge lamp load is ignited.

At present, there have been many studies on solar power controllers, low-pressure sodium lamps and their supporting electronic ballasts. However, in actual engineering applications, it is found that since the above three devices or products are independently developed, they have poor compatibility in work. Some low-pressure sodium lamps are unstable when started, and some even cause damage, which greatly limits the application of low-pressure sodium lamps. Therefore, it is recommended to incorporate the DC/AC low-frequency electronic ballast system circuit module of the low-pressure sodium lamp into this controller system, and design it into a solar low-pressure sodium lamp lighting system intelligent controller and ballast all-in-one machine.

2.4 LED display circuit

This controller uses a two-color LED as the system status indicator. The two-color LED display is very intuitive and replaces the previous multiple indicator lights. The microcontroller controls the output level of pin 2 (P1.7) and pin 3 (P1.6) by comparing the value of pin 17 (AD1, i.e. BAT+ voltage) with the set value to determine the color and state of the system status indicator. The status of the status indicator is shown in Table 2.

2.5 Controller working mode selection circuit

This controller has 8 preset working modes for users to choose from (see Table 3). Users only need to turn the DIP switch J1. The microcontroller will automatically detect the controller mode selected by the individual user and implement the functions in different modes according to the program flow.

3 System Software Design

The software program of this design includes: main program, timer interrupt program, A/D conversion subroutine, external interrupt subroutine, charge and discharge management subroutine, load management subroutine, LED display subroutine, etc. Figure 4 is the block diagram of the software structure design of this system. Taking "debugging mode" as an example, the software design program flow of this system is shown in Figure 5.

4 Experimental Results

According to the above design ideas, a prototype was trial-produced. The experimental platform consisting of two 12 V 7 AH valve-controlled sealed lead-acid batteries, a 36 W low-pressure sodium lamp and an 80 Wp photovoltaic array system replaced by a DC power supply simulation was used to conduct experimental research on various performance indicators of the prototype. Figure 6 is the waveform of the low-pressure sodium lamp oscilloscope in the stable stage. The results show that the system operates stably and reliably.

5 Conclusion

After experimentation and debugging, this system has achieved the expected functions, effectively and reasonably completed the management of system status and real-time control of energy flow. The microprocessor is used to realize the charge and discharge control of the solar controller, and its various performance indicators are significantly better than those of conventional controllers. It can also set parameters and perform temperature compensation for different batteries, greatly expanding its use functions.