Design of street lighting power saving control system with remote monitoring

0 Introduction
Lighting is an integral part of urban infrastructure. It plays an important role in urban traffic safety, public security, people's lives and urban appearance, and plays an irreplaceable role. It also marks the strength and maturity of the city. More than 70% of the existing urban street lamps use high-pressure sodium lamps, whose design life is 24,000 hours (5 years). However, due to large voltage fluctuations, the fluctuations in many areas even exceed 15% of the rated voltage. Especially in the second half of the night, due to the reduction of electrical load, the grid voltage is sometimes close to 245 V. The high voltage not only wastes electricity, but also shortens the service life of the bulb. In fact, the actual service life of urban street lamps is less than one year on average.
At present, there are two main ways to save energy at the power supply end. One is to use midnight lights, and the other is to use voltage regulation. The night light is to achieve energy saving by turning off some lighting lamps in the second half of the night. It has the characteristics of simplicity and ease of implementation. The disadvantage is that the road lighting is uneven, and it cannot solve the problem of high voltage in the second half of the night affecting the life of the light source. It is not suitable for indoor places such as shopping malls and schools. The method of reducing the power supply voltage can not only save energy, but also extend the life of the light source, which is a better way to save energy.
For this reason, this article provides a design scheme of a power-saving control system based on the C8051F310 single-chip microcomputer. The system can target the high and fluctuating grid voltage and output the best lighting working voltage in real time online according to the actual needs of the user site, so as to stabilize it within the allowable range, thereby improving the power quality, saving lighting electricity, and extending the service life of the bulb. It is very suitable for indoor and outdoor lighting power-saving control such as street lamps, schools, and shopping malls.

1 Basic design ideas
The street light controller designed in this paper uses an autotransformer to adjust the voltage of the load street light. The primary and secondary coils of the autotransformer are not only magnetically connected, but also electrically connected. Therefore, when the output voltage adjustment range is not large, its capacity is relatively small, so the material consumption is small, the cost is low, and the efficiency is high. Its biggest advantage is that it overcomes the defect of thyristor chopper products generating harmonics and realizes the sinusoidal output of the voltage. Its structure and function are simple and the reliability is relatively high.
Figure 1 shows a basic block diagram of a street light lighting energy saving system. The basic design idea of ​​the street light energy saving controller is to close four contact switches with different transformation ratios on the secondary side of the autotransformer so that the voltage at both ends of the street light can be automatically stabilized at a preset value when the grid voltage changes (mainly when the grid voltage is high at night).

Fluorescent lamp is a low-pressure discharge lamp that uses mercury discharge to generate ultraviolet radiation to excite the phosphor layer and emit light. It is currently a major electric light source for lighting factories, buildings, shopping malls, offices, schools and homes. Table 1 shows the relationship between the illumination, life and voltage of fluorescent lamps (assuming that the life at rated voltage is 1).

As shown in Table 1, if the grid voltage is rated at 220 V, when 90% of the rated voltage is used for power supply, the life of the lamp can be doubled and the illumination is attenuated by 7%. From the perspective of human vision, it is known that the human eye's perception of light intensity changes is calculated according to a logarithmic relationship. When the illumination is attenuated by 7%, the human vision can feel that the light becomes 1.6% darker. At this time, 19% of the electricity is saved, and the best energy efficiency ratio can be achieved. If the grid voltage exceeds 220 V, the energy saving rate is generally above 20%. Therefore, 90% of the rated voltage is the optimal operating and energy-saving voltage for indoor lighting power supply.
1.1 Contact switch setting scheme
From the above analysis, it can be seen that 90% of the rated voltage is the optimal operating and energy-saving voltage for street lamps or indoor lighting, that is, 200 volts. The voltage standard of China's grid is 220 V (-15%~+10%), that is, the fluctuation range of the grid voltage is 187~242 V. Assuming that the grid voltage fluctuates within the above voltage range, and the switch ratios of the autotransformer are set to a0, a1, a2, and a3 respectively, then the four voltage points of the grid fluctuation can be taken to calculate and set the transformation ratio of the autotransformer so that when the grid voltage is at these four points, the transformer output is 200V. These four points are 200V, 210V, 220V, and 230V.

Assume that the bypass ratio is 1, that is, when the grid voltage is 200 V, its ratio a0=1, then the ratios of other switches are: a1=0.95, a2=0.90, a3=0.87. With these four points as the center, we can divide the voltage fluctuation range into 4 segments, as shown in Figure 2. Assume that the grid voltage is X, the output voltage is Y, and the ratio is ax.

As shown in Figure 2, the accuracy of its output voltage value can reach at least 94.3%.
Taking winter as an example, the time required for road lighting is from 17:00 to 6:00 the next day. According to the results of voltage observation, the grid voltage should usually be stable at around 220 V, but from 0:00 in the middle of the night to 6:00 the next day, due to the small grid load, the grid voltage will gradually rise to 230 V. Based on this, the power saving rate can be calculated. The power consumption before voltage adjustment:

Through calculation, it can be seen that its power saving rate can reach 21%.
1.2 Voltage control scheme
Let the input voltage be X and the output voltage be Y; the grid input voltage at time t is X(t), and the contact switch jump value is P. Then, the following formula can be used to determine whether the input voltage X is in the rising stage or the falling stage:

If the input voltage of a certain road at time t is greater than the input voltage at time (t-1), the input voltage is in the rising stage. Set the voltage jump value to P, that is, when the voltage rises to P+2. The contact switch jumps. On the contrary, if the input voltage of a certain channel at time t is less than the input voltage at time (t-1), the input voltage is in the falling stage. If the voltage jump value is set to P, that is, when the voltage drops to (P-2), the contact switch jumps.
Among them, a hysteresis of 4 V can be set so that the voltage has two different thresholds in the rising and falling stages. The hysteresis is set to ensure that the contact switch will not jump frequently when the input voltage fluctuates frequently.
1.3 Sampling scheme
In view of the fact that the amplitude of the grid voltage of the system does not change very quickly, the sampling speed and sampling accuracy requirements of the street light controller do not need to be very high. The accuracy of the 10-bit AD converter of the microcontroller C8051F310 can meet the system requirements. After the signal is converted by AD, the microprocessor calculates the effective value of the corresponding voltage and further judges it. The design has a simple structure, low cost, and is sufficient to meet the requirements of sampling speed and sampling accuracy. The
conversion accuracy of the A/D converter is determined by its reference voltage and output field length. The so-called conversion accuracy refers to the ability of an A/D converter to monitor the minimum voltage change. In fact, the conversion accuracy is the minimum step voltage of the A/D converter. The voltage value can be obtained by dividing the reference voltage of the MD converter by the number of conversion values ​​of the converter.
Since C8051F330 uses 3.3 V as the reference voltage, its conversion accuracy is 3.3/1024, that is, 3.22 mV, which can fully meet the accuracy requirements of this system.
1.4 Remote communication control scheme
Remote communication refers to the communication between multiple street light controllers and PCs, which is convenient for users to monitor the status of street light controllers distributed in various places. This system communicates through the UART port of C8051F310 and the serial port of the PC to transmit the detected various status information to the host computer (such as the voltage of the power grid, the current of each phase A, B, and C, and the various states of the street light controller, etc.), and the user can select the street light controller to be monitored according to the software of the host computer, and then issue various control commands according to its status, such as bypass or select the energy-saving gear.
Since one PC is used for multiple street light controllers, the communication protocol can be specified by listening mode, that is, a communication address is assigned to each controller. Before the communication starts, the host sends the communication address first, and each controller receives this address and then compares it with its own address. The street light controller with matching address starts to communicate with the host, and the others remain in listening mode. In this way, users can easily monitor any controller.
Considering the transmission distance and anti-interference ability of the bus, this design uses RS-485 interface on the control board and uses differential mode to transmit signals, so it has strong anti-common mode interference ability.

2 Hardware Design
The hardware part of this power-saving control system mainly includes a microprocessor, a voltage detection circuit, a current detection circuit, a contact switch drive circuit, a clock and a communication interface circuit. When the grid voltage is collected by the rectifier and filter circuit to the microcontroller, the microcontroller will perform AD conversion through its own AD conversion circuit, and then determine the position of the transformer secondary switch that needs to be closed at this time through calculation, and at the same time issue a command to drive the corresponding relay to close the corresponding contact switch, thereby realizing its basic functions. The KEY circuit contains some external manual logic controls. Figure 3 shows the hardware circuit block diagram of this system.

2.1 Voltage detection circuit
The block diagram of the voltage detection circuit in this system is shown in Figure 4. The circuit includes various signal conditioning circuits. The grid voltage passes through a 220 V/7.5 V transformer, and then adjusts its amplitude through a proportional amplifier composed of an op amp. It is then converted into DC through a full-wave rectifier circuit, and then enters the microcontroller for AD sampling after passing through an RC filter circuit.

The structure of the current detection circuit is basically the same as that of the voltage detection circuit, except that the current is converted into a voltage signal after passing through a current transformer, and the signal conditioning process is basically the same.

2.2 Contact switch drive circuit
Figure 5 shows a schematic diagram of the contact switch drive circuit. In Figure 5, the output of the microcontroller can be connected to the transistor through the drive circuit to control its on and off, thereby controlling the on and off of the relay coil, and then controlling the on and off of the autotransformer contact switch through the relay.

2.3 Communication interface circuit
The communication between this power saver and PC is serial communication, using the UART port of the microcontroller, where RX0 is the microcontroller receiving signal and Txo is the microcontroller sending signal. The UART port sends signals to or receives signals from the bus through a dedicated RS485 chip. The circuit diagram is shown in Figure 6.

The optocoupler in the figure plays an isolation role to prevent the microcontroller from being interfered by other signals. Transient diodes are generally used for lightning protection.

3 Software Design
Figure 7 shows the software structure flow chart of this system. When the system is turned on, the microcontroller is first initialized, and then the internal timer is started to sample the current and voltage. If the current is overcurrent, the controller will bypass the entire system to avoid burning components. After that, the microcontroller analyzes the collected grid voltage to determine what gear the autotransformer should be in. Combined with the command sent by the PC to the controller, it comprehensively analyzes the actions that should be taken.

The communication program should ensure smooth communication between the computer and the single-chip microcomputer and enable multi-machine communication. After the computer is turned on, each device connected to the bus will be assigned a communication address. The computer can select which device to communicate with through the control interface, thereby monitoring the information of each device and sending control commands remotely. At this time, each device on the bus receives the information sent by the computer, and only the device whose communication address matches the address contained in the information can make relevant actions and obtain the right to send its own status information to the computer.

4 Conclusion
The experiment shows that the power-saving device is simple and convenient to implement, and has good reliability. Through the square lighting experiment in Chongqing Locomotive Depot, its power saving rate can reach more than 20%, which is basically in line with the results of theoretical calculations, can meet the commercial requirements of power-saving lighting, and can achieve long-term safe and reliable operation.