Abstract: This paper introduces the application of fieldbus technology in intelligent low-voltage power distribution monitoring systems, taking into account the characteristics of power systems. A practical low-voltage power distribution monitoring system was constructed using the PMAC9900E2 integrated power monitor, and the hardware composition, software design, and specific implementation of the monitoring system are explained. Keywords: Fieldbus; Low-voltage power distribution; Integrated power monitor; Intelligent monitoring system 1 Introduction With the rapid development of power systems and the increasing demands for power quality in modern society, the realization of integrated automation in the operation, control, and management of power systems has become imperative. In recent years, the development of intelligent systems in low-voltage power distribution networks has been extremely rapid, and many low-voltage power distribution monitoring network systems based on intelligent electrical appliances have emerged. Many large companies worldwide, such as Schneider Electric of France, Siemens of Germany, and ABB, have launched their own intelligent electrical appliance monitoring, protection, and information network systems, many of which are already fieldbus-based products. Since the 1990s, a major transformation with far-reaching influence has occurred in the field of automation. The birth of fieldbus technology has triggered a revolution in traditional industrial automation instrumentation and control systems. Intelligent automation instruments and open automation systems based on fieldbus technology in industrial processes have attracted widespread attention and high importance, becoming a hot topic in the development of automation technology both domestically and internationally. The PMAC9900E2 integrated power monitoring instrument is a widely used power monitoring instrument. In addition to measuring 36 power parameters, it features 4 status inputs and 4 relay outputs, and can be programmably configured to provide overvoltage, undervoltage, overcurrent, and overfrequency alarms. The PMAC9900E2 adopts a standard mounting structure, high-brightness display, and is aesthetically pleasing and practical. Its communication capabilities are powerful, featuring an opto-isolated RS485 communication port with a communication rate of 300–19200 bps, and supporting open communication protocols. It can be said that using the PMAC9900E2 in low-voltage power distribution monitoring systems is an effective method for applying fieldbus technology to integrated low-voltage power distribution systems, achieving intelligent power distribution systems. It improves the information processing capabilities and operational reliability of the control system, saves on system hardware and wiring costs, and facilitates system configuration, management, and maintenance for users. 2. Hardware Composition of the Monitoring System In this system, the low-voltage power distribution intelligent monitoring system based on fieldbus consists of an industrial control computer, a card control box, a system switch cabinet, and two types of loads. Its block diagram is shown in Figure 1. [align=center] Figure 1 Block Diagram of the Monitoring System[/align] 2.1 Monitoring System Principle The intelligent field electrical measuring instrument is installed in the low-voltage distribution cabinet. The physical layer of its connection with the monitoring host uses an RS485 interface, the medium is a shielded twisted-pair cable, and the data link layer uses the MODBUS communication protocol. Fieldbus technology is used for the main circuit of the system. Through the user-friendly human-machine interface on the monitoring host, it completes the digital acquisition of electrical measurement data and switch status, and performs intelligent switch control and protection functions. The use of pure digital communication greatly improves the system's reliability, stability, and intelligence. 2.2 Energy Management Section (Low-voltage Switchgear) (1) Integrated Power Monitoring Instrument The PMAC9900E2 integrated power monitoring instrument is an automated instrument applicable to power measurement and control. It can measure data in real time with high precision and can be widely used in automation fields such as power system and industrial production monitoring. Its digital acquisition system block diagram is [align=center] Figure 2 Block diagram of digital acquisition system[/align] (2) CI-134 Four-port Serial Card It supports 4 independent RS-422/485 serial ports. In one-to-many communication applications, it can control up to 128 devices. In order to make RS-485 2-wire half-duplex operation simpler, each CI-134 card has automatic data flow control (ADDC) without software operation. Therefore, in Windows applications, no additional coding is required to control the RS-485 half-duplex protocol. (3) Power Supply Switching Devices The main power supply switching devices include air switches, current transformers, AC contactors, thermal relays, push button switches, etc. (4) Digital signal control detects the status of the switch controlled by the contactor through the remote signal card PCLD-782B; remote control of the system load (important load, secondary load) is performed through the remote control card DAT-7685. 3 Monitoring system software design 3.1 Main functions of the software (1) Real-time data acquisition of various parameters of the integrated power monitoring instrument PMAC9900E2; (2) Remote control of various switch quantities of PMAC9900E2; (3) Display of various parameter data through the interface. 3.2 Software platform (1) System platform This system uses the Windows 98 operating system. This operating system is adopted because it is convenient to directly operate the underlying system and can easily access the thousands of processes that constitute the Microsoft Windows operating system. (2) Application software adopts Visual Basic 6.0 Chinese version. VB is used to develop the user graphical interface and various controls, and ADO (ActiveX Data Objects) is used to access and process data from a certain data source. (3) Database software The database software adopts Microsoft Access 97. The system implements functions such as database table creation and indexing, and implements a security mechanism for the database to prevent data loss, generate backup and restore data, etc. Moreover, all Microsoft products are used to ensure that the software has satisfactory compatibility. 4 Implementation of Low-Voltage Power Distribution Intelligent Monitoring System 4.1 Serial Communication Protocol of PMAC9900E2-MODBUS The role of the communication protocol is to enable information and data to be effectively transmitted between the industrial control computer and PMAC9900E2. It includes: (1) All RS-485 loop communication should follow the master/slave mode, in which information and data are transmitted between a single master station and up to 32 slave monitoring devices; (2) The master station will initialize and control all information transmitted on the RS-485 communication loop; (3) Communication should never start from a slave station; (4) All communication on the RS-485 loop occurs in a package. The bytes of this package constitute standard asynchronous serial data and are transmitted in the form of 7 data bits, 1 stop bit, and no parity bit; (5) In any case, the slave station can only respond to one request from the master station. 4.2 Implementation of data acquisition In this system, the PMAC series instrument communication library (ver 1.2.1.7) is used to implement data acquisition. It can be achieved by calling pmac.dll. 4.3 Interface design The interface design mainly uses various controls of Visual Basic 6.0. Using these controls, buttons, checkboxes, labels, combo list boxes and text boxes can be added to the program, and files on the hard disk and databases can be accessed in the program. In the interface design, the main display, information input and output are implemented using these internal controls. 4.4 Overall software flowchart The digital monitoring system consists of an entry quick display interface, a login interface, a monitoring management interface, an information management interface, etc., and its flowchart is shown in Figure 3. [align=center] Figure 3 Flowchart of Digital Monitoring System Software[/align] (1) System Entry Quick Display Interface The entry display screen that appears when the system program starts mainly displays the system name, copyright information and a simple bitmap. (2) Login Interface Users log in according to different permissions. The system gives different permissions to different users according to the preset values ​​to avoid misoperation. (3) Monitoring Management Interface Users can select the port, address and transmission rate to enter the system monitoring state. They can monitor system data in real time and adjust parameters to realize remote communication, remote control and remote measurement. (4) Information Management Interface It can realize comprehensive and secure management of various aspects of system data (such as real-time data, historical data and system parameters) to facilitate system maintenance. 5 Conclusion The low-voltage power distribution intelligent monitoring system based on fieldbus technology is composed of PMAC9900E2 integrated power monitoring instrument, which improves the information processing capability and operational reliability of the control system, saves the hardware and wiring costs of the system, and improves the security and reliability of the system. However, digital monitoring systems using the PMAC9900E2 also have a problem that cannot be solved yet: the limitation on the number of control lines. This greatly hinders the promotion of digital acquisition systems. Therefore, in low-voltage power distribution monitoring systems requiring numerous control lines, a comprehensive monitoring system combining digital monitoring and analog control must be adopted. With the further integration of power control with microelectronics, computer technology, and network communication technology, the shortcomings of the PMAC9900E2 in remote control will be overcome, enabling low-voltage power distribution systems using intelligent instruments to exhibit superior performance in remote signaling, telemetry, remote control, and remote adjustment. The application of intelligent instruments in the system will transform the control system into a two-level system: a field level and a workstation level. It will be easily networked upwards to form a plant-level or company-level network. Through this network, it will connect to the global Internet, forming a truly networked control system. It is foreseeable that networked control equipment replacing traditional control equipment is an irresistible historical trend. With the continuous improvement of fieldbus technology, low-voltage power distribution systems employing field intelligent instruments similar to the PMAC9900E2 will become the mainstream, bringing about an unprecedented revolution in power distribution monitoring systems. References: 1. Bai Yan, Wu Hong, Yang Guotian, eds. Distributed Control Systems and Fieldbus Control Systems—Fundamentals, Evaluation, Design and Application. Beijing: China Electric Power Press, 2001. 2. Wu Qiufeng, ed. Automation Systems Computer Networks. Beijing: Machinery Industry Press, 2001.