Abstract: Reliable grounding resistance is of great significance to the safety of power systems. Currently, grounding resistance measurement mainly relies on manual measurement, which requires a significant amount of time and money for each measurement. To address this issue, this paper designs an automatic grounding resistance monitoring system based on automatic testing theory, network communication, and embedded technology. The system's testing terminal can automatically measure grounding resistance, and the measurement results from multiple terminals are automatically uploaded via the network. Monitoring personnel only need to operate the software to complete the measurement process. The design and implementation of this system improves the level of automatic grounding resistance measurement technology, and the system has been put into operation in the power sector of the railway system. Keywords: Grounding resistance; Automatic measurement; Multi-point monitoring 1 Introduction All types of buildings and equipment, especially power system terminals, must have reliable grounding for lightning protection grounding wires to ensure the safe discharge of current into the earth. Numerous investigation reports on lightning damage accidents and practical experience in the safe operation of power grids show that the quality of grounding resistance is related to the safety of the power system. In engineering, to improve the lightning protection grounding wire's resistance to lightning strikes, it is necessary to reduce the grounding resistance of the grounding wire. However, due to factors such as damage, corrosion, and drought, grounding resistance can change significantly. Therefore, real-time monitoring of grounding resistance is crucial for ensuring the safe discharge of static electricity from equipment casings and the high current from lightning rods into the ground during lightning strikes. Currently, common methods for measuring grounding resistance include the megohmmeter method, the 0.618 method, and the isosceles triangle method. The megohmmeter method is based on the principle of measuring pure resistance and is not suitable for measuring the grounding resistance of large-area grounding grids. The 0.618 method and the isosceles triangle method both belong to the voltage-current method. Both compensate for the influence of inter-electrode potential by adjusting the relative positions of the current and voltage test electrodes to obtain the true resistance value. The only difference lies in the geometric position of the test electrodes. This paper will design a real-time monitoring system for lightning protection grounding wires based on the voltage-current method. This system changes the current situation where grounding resistance is measured manually, realizing automatic measurement and simultaneous monitoring of multiple points, thus improving testing efficiency and accuracy. 2. Grounding Resistance Measurement Method of This System The voltage-current method is shown in Figure 1: [align=center] Figure 1 Schematic diagram of voltage-current method Figure 2 Comparison of inter-electrode positions of 0.618 method and isosceles triangle method[/align] Wherein, C2 is the grounding point to be measured, and P and C1 are two auxiliary piles. I is the AC current source, and V is the inter-electrode potential difference. The resistance value can be obtained by measuring the current in the grounding loop C1-C2 and the potential of C2 relative to P. However, the actual measurement environment is complex and variable, with many factors affecting current and voltage measurements, such as the skin effect of the earth, ground current interference in the earth loop, the external electric field of a specific natural environment, the temperature, humidity, soil characteristics, and contact condition of the grounding pile test point. These factors will all affect the resistance measurement. If these effects are not considered, the measurement error will be large or even completely wrong. The voltage-current method measures by means of an electric field, and the magnitude of the potential of the inserted ground pile is the main factor determining the measurement result. In order to shield the influence of the inter-electrode electric field between the auxiliary pile and the test grounding point, attention must be paid to the relative position between the test point and the auxiliary pile. Grounding resistance is mostly measured using the 0.618 method and the isosceles triangle method. These methods are suitable for testing situations where the center of the grounding device under test is clearly defined, and the surrounding terrain is flat with uniform soil resistivity. The relative positions of the three grounding stakes are shown in Figure 2. In the 0.618 method, d1/d2 is near 0.618, and d1 is more than 4 times the diameter of the ground grid where test point C2 is located. In the isosceles triangle method, d1 and d2 are equal and are more than 3 times the diameter of the ground grid where C2 is located. The farther the auxiliary stake is from the test point, the weaker the superposition of the electric field between the electrodes, and the smaller the influence. Both the 0.618 method and the isosceles triangle method are specific implementations based on the voltage-current method. 3 System Composition The system designed in this paper consists of two parts: hardware and monitoring software. The hardware part is called the lower-level machine, which is mainly responsible for the automatic measurement of grounding resistance, calculation of measurement data, display of resistance value, and data transmission with the upper-level machine (computer). The lower-level computer provides three measurement methods: timed and fixed-point measurement, manual button measurement, and upper-level computer command measurement, to meet the needs of automatic measurement, local manual measurement, and remote monitoring measurement. The upper-level computer installs monitoring software to monitor all lower-level computers, obtains measurement data, sends measurement commands, and sends time synchronization commands through communication with the lower-level computers. The system composition is shown in Figure 3: [align=center] Figure 3 Block Diagram of Automatic Grounding Resistance Monitoring System Figure 4 Lower-level Computer Structure Diagram[/align] As shown in Figure 3, the measurement data from n lower-level computers can be transmitted to the computer monitoring center via communication lines. The computer center can also control each measurement terminal by sending measurement commands. This enables remote multi-point testing. The monitoring software installed on the upper-level computer is written in VC++ and has a user-friendly interface, allowing administrators to obtain the current grounding resistance value through simple operations. The software's operating mode can also be set to automatically measure the resistance of designated machines or multiple lower-level computers at fixed times and locations every day, sending the data to the upper-level computer for automatic storage. When the measurement data exceeds the normal range, both the site and the monitoring center will automatically alarm. This saves staff the time and effort of observing computer measurement data. The communication line can be changed depending on the specific measurement scenario, such as RJ45 wired LAN, CDMA wireless WAN, serial port, etc. This system uses a wired connection via RS485 bus to connect the measurement terminal equipment and the computer during testing. 4 System Implementation 4.1 Test Terminal Hardware Structure Figure 4 shows the structure of the lower-level machine, i.e., the terminal measurement equipment. The analog voltage and current signals obtained from the test circuit pass through a filter circuit to the amplification module, then the A/D conversion module obtains digital signals which are fed to the subsequent digital signal processing unit to calculate the grounding resistance value. Finally, the result is transmitted to the display module. If data transmission to the upper-level computer is required, data will also be sent to the communication module. Among the various modules, the digital signal processing module is the core module of the entire lower-level machine, serving as the carrier of the lower-level machine's measurement and control software. The operation of this module is controlled by the program, thereby realizing digital filtering, resistance calculation, control of all peripheral hardware circuits of the measurement terminal, and connection with other functional modules through interface circuits. This module can be powered by various processor chips, such as ARM, DSP, and C8051F series embedded microcontrollers. The controllable power supply module can provide multi-frequency test power supplies, selecting the least interference-prone frequency based on the actual measurement environment. The clock chip provides the lower-level machine's time reference and maintains the clock even after power loss. Downloading the C program to the flash memory of the digital signal processing module enables functional control of the entire lower-level machine. After power-on, the program begins running. If a test command arrives, it will control the test current source to turn on via interrupt, and the lower-level machine will automatically measure the grounding resistance value according to Figure 4. When the upper-level machine is operating in multi-point monitoring mode, it issues test commands and receives measurement data through data communication with the lower-level machine. 4.2 Test Terminal Software Flowchart [align=center] Figure 5 Lower-level Computer Program Flowchart[/align] [align=center] Figure 6 Upper-level Computer Active Acquisition and Communication Flowchart[/align] Figure 5 describes the main flow of the lower-level computer program in detail. As shown in the figure, the lower-level computer has two testing modes: fixed-point testing and waiting for test commands. The test commands are broadly defined, including both actual test commands sent by the upper-level computer and manual button testing on-site. The program displays the calculated results after each resistance measurement and automatically compares whether the resistance is within the normal range; if it exceeds the range, it automatically alarms. The program can also select whether to send test data according to the staff's requirements and can execute time synchronization commands to align the test results of multiple test terminals with the same time standard. This allows the monitoring center to obtain a table of test results—test times—which is very useful for comparing the grounding resistance values ​​of different terminals at the same time. The change curve of the grounding resistance value over time at the same site can be achieved by recording measurement data multiple times; all of this can be achieved by setting the working mode of the upper-level computer software. 4.3 Communication Mechanism between Monitoring Center and Test Terminal To achieve data and command communication between the monitoring center and the test terminal, a communication protocol must be established that both parties adhere to, and a mutually agreed-upon communication mechanism must be generated based on this protocol. Figure 6 shows the communication flowchart of the host computer actively acquiring data in this system. After calibration, the host computer sends an acquisition command and begins listening to the communication interface for measurement data. If correct data is successfully received, it begins sending acquisition commands to the next slave computer, continuing until measurement data from all slave computers with the set acquisition channels is successfully received. If the host computer does not receive data after sending an acquisition command, it enters the error handling unit. There may be various reasons for not receiving data, such as a communication link failure. Receiving data but it being incorrect may also be due to various influencing factors, such as baud rate offset. A reliable system not only needs good inherent stability but also fault tolerance to handle unexpected problems. The error handling unit here is designed to handle sudden communication failures. The error handling unit can be used to handle all errors during the communication process. Due to the high stability of this system, a simple error retransmission mechanism can be adopted here. For more complex measurement and communication environments, the error handling mechanism can be modified. Furthermore, the quality of the communication protocol directly affects the quality of communication. This system also considers another measurement scenario: the testing personnel press the lower-level machine's test button on-site. In this case, after measuring the grounding resistance value, the lower-level machine automatically sends the measurement data to the upper-level machine. The upper-level machine determines the correctness of the data according to the protocol; if correct, it receives the data; if incorrect, it handles the error accordingly. The communication process flow diagram in this scenario is similar to that in Figure 6, the difference being that the lower-level machine actively completes the measurement and automatically sends the data, without waiting for the upper-level machine to send time synchronization and acquisition commands. Multi-point monitoring is achieved through polling. The upper-level machine sends acquisition commands to each lower-level machine sequentially according to the communication flowchart in Figure 6. After each measurement is completed, the lower-level machine sequentially sends the data to the upper-level machine, allowing the upper-level machine to receive the measurement results from all lower-level machines. 5. System Testing The automatic grounding resistance monitoring system can be applied to various power systems that require grounding resistance monitoring. Table 1 shows the measurement data of three lower-level machines at a certain power system site over a continuous time period. The true values ​​of the resistances to be measured are 0.65, 0.67, and 0.7 ohms, respectively, measured using the isosceles triangle method. Table 1: Grounding Resistance Field Measurement Values. As can be seen from Table 1, the measurement results are very close to the true values, and the fluctuations are relatively stable within +0.02 to -0.02 ohms. Table 2 shows the data obtained during the metrology bureau's verification, providing a comparison table of the measured values ​​and true values ​​of one lower-level machine under various resistance values. This table only shows the measurement results of this system for some resistance values. As can be seen from Table 2, the measurement results of the lower-level machine are still relatively accurate under different resistance values. The communication link of this system has also undergone rigorous testing and can meet the requirements of multi-point monitoring. Table 2: Resistance Measurement Data. Table 6: Conclusion Grounding resistance measurement is an essential detection task in power systems. Manual measurement requires a lot of manpower and costs. This paper designs an automatic grounding resistance monitoring system that organically combines grounding resistance measurement technology, automation technology, and communication technology. It achieves automatic grounding resistance measurement and simultaneous multi-point monitoring through three methods: timed measurement, on-site test button measurement, and remote software-controlled measurement. The system not only displays measurement data on-site but also automatically uploads and saves it to a computer for easy data accumulation and comparison. Its automatic alarm function reduces the time-consuming and labor-intensive task of frequent grounding resistance monitoring, thus lessening the workload. The system's accompanying operating software has a user-friendly interface, facilitating operation and replacing complex manual testing processes. Furthermore, the system boasts advantages such as high stability, low power consumption, and small terminal measurement equipment size. Extensive experimental and testing results demonstrate that the designed automatic grounding resistance monitoring system meets the design requirements. The innovation of this paper lies in the application of modern electronic technology to achieve automatic and remote monitoring of power systems, a major development direction for power system measurement technology. The automatic grounding resistance monitoring system designed in this paper is a concrete example of introducing automation control technology into the field of grounding resistance measurement to achieve remote automatic monitoring. Economic Benefits: 1 million RMB References : 1. Luo Zhijia, Di Zheng, Application of Ethernet-based Distributed Data Acquisition and Monitoring System, Microcomputer Information, 2006, Issue 01, Page 29. 2. Shen Lianfeng, Song Tiecheng, et al., Embedded Systems and Their Development and Application, Beijing: Electronic Industry Press, 2005. 3. Zhang Lun, Hardware Design and Debugging of 32-bit Embedded Systems, Beijing: Machinery Industry Press, 2005. 4. National Standard GBJ65-83, Grounding Design Specification for Industrial and Civil Power Installations. 5. Tong Shibai, Hua Chengying, Fundamentals of Analog Electronics, Beijing: Higher Education Press, 2001. 6. 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