1. The Role of Communication Systems in Distribution Automation Systems Distribution automation feeder systems (hereinafter referred to as distribution automation) generally consist of a master station (front-end unit, back-end unit, and server, etc.), field devices (switching unit FTU, circuit breaker FTU, pole-mounted switch FTU, communication controller CCU, transformer monitoring unit TTU, and substation CCU, etc.), and a communication section (master station communication host, field device communication slaves, adapters, and communication media, etc.). The main functions of each part are as follows: a. The master station is responsible for coordinating and commanding the entire system. It obtains data and status from field devices through the communication section, compares and analyzes it with existing data, draws correct conclusions, and issues action commands. It also notifies field devices to perform desired actions or preset parameters through the communication section. b. Field devices perform real-time monitoring and data and status acquisition for each relevant device. They receive various commands and data from the master station through the communication section and upload information from various devices to the master station. c. The communication section, through certain communication media and methods, and following certain communication protocols, transmits information between the master station and various field devices in a timely and rapid manner, playing a very important bridging and linking role. Depending on the needs, the entire communication system can be divided into several physical or logical layers, and each different layer or different areas divided according to needs may use different communication methods. Different communication media are converted or coupled in a certain way. In the power distribution automation system, there are many and scattered field devices (measurement and control points), which makes the network design less systematic and the distance cannot be actively controlled. However, the various measurement and control functions of power distribution automation must be completed through timely and reliable communication. Therefore, the performance of communication equipment directly determines the quality of the entire power distribution automation system. If a modern centralized control power distribution automation system does not have a high-quality communication system, then even the best automation functions are meaningless. Therefore, communication occupies a very important position in the power distribution automation system. [b]2 Requirements of the Power Distribution Automation System for the Communication System[/b] a. It can work outdoors in all weather conditions for a long time and adapt to various weather conditions. The digital transmission has good real-time performance and a high transmission rate. b. It has good electromagnetic compatibility, good anti-electromagnetic interference performance, low requirements for the surrounding environment, and the ability to recover normal operation after encountering strong interference; it has the ability to resist lightning strikes. c. Low bit error rate, less than 10⁻⁶ for wireless and less than 10⁻⁹ to 10⁻¹¹ for fiber optic. d. Wide operating power range, low power consumption, long mean time between failures and long service life. e. Good compatibility with various devices (complete interface standards, with a certain degree of adaptability). f. Convenient installation, maintenance, replacement, and adjustment; complete and accurate indication of various working statuses. g. Possesses a certain degree of intelligence; flexible network topology, easy to change; can accommodate various communication media, easy to form large networks; convenient for network management. h. Supports certain network communication protocols, with good compatibility with various devices. i. High performance-price ratio, suitable for national conditions, and not technologically backward. [b]3 Communication Methods[/b] 3.1 Introduction and Comparison of Various Communication Methods 3.1.1 Wireless Transmission Method This method uses electromagnetic waves in the air to transmit information, and there are two transmission methods: analog radio and digital radio. 3.1.1.1 Analog Radio consists of a regular radio and an external modem, such as the Japanese-made M338. Most radios used in power load control are of this type. Its characteristics are low price and slow speed, with a common data transmission rate of ≤1200 bits/s, slow transmit/receive conversion (typically hundreds of ms), and actual power consumption of 25 W. It is suitable for low-speed, non-critical occasions where real-time requirements are not high. 3.1.1.2 Data Transmitter The data transmitter is an improvement on the internal circuit of the analog transmitter, with the addition of a CPU (microprocessor), modem and control part to form a module. It provides a standard 25-pin serial port and operation commands. It has fast transmit/receive conversion, has key relay functions, supports certain network protocols, and has a data transmission rate of 300~19200 bits/s. Compared with the analog transmitter, it shortens the data transmission time and improves anti-interference. The main station uses 25 W of power, and the slave station uses 3~5 W of power. The low power consumption makes the configuration of the equipment power supply and backup power supply very convenient. If the power distribution automation communication system adopts wireless communication, the high-speed data transmitter should be the first choice. The reasons are detailed as follows: a. Fast transmit/receive switching is one of the advantages of data radios. Their typical transmit/receive switching time is 3-5 ms, while that of analog radios is 200-1200 ms. The high data transmission rate and fast switching of data radios facilitate improved overall performance of power distribution automation systems, shortening the time required for critical functions such as fault location, handling, and line transfer. This is particularly evident when building large power distribution automation network systems. b. Configurable repeater function: Data radios can be configured with 1-8 repeater levels. This function not only makes network design more flexible but also improves the reliability and intelligence of the communication system. c. Low actual power consumption: The overall power consumption is low, resulting in higher reliability and a longer operating time of the internal backup power supply after a power outage. The advantages of wireless communication include easy installation and convenient network adjustment; communication can still be maintained using the internal backup power supply even after all transmission lines at both ends of the equipment are disconnected or de-energized; and low cost. The disadvantages of wireless communication include: susceptibility to interference from electromagnetic waves in the air; limitations imposed by the antenna height (antennas are typically installed a few meters above the pole or on the roof of a substation switching station); and the need to apply for frequency points and pay application and usage fees. The overall performance of wireless communication depends primarily on the functional specifications and product quality of the communication radio, and secondarily on a reasonable network design and compatible communication protocols. 3.1.2 Carrier Communication: The method of transmitting information via power lines using a modem is called carrier communication (here referring to 10 kV and 220/380 V power line carrier communication). The advantage of carrier communication is that it utilizes the power lines of the power company, eliminating the need for dedicated wiring and saving on conventional wiring costs. The disadvantages of carrier communication include low communication speed, short communication distance, and poor reliability due to changes in line impedance caused by load connections and disconnections, making it difficult to meet the requirements of the main communication network for distribution automation. With the adoption of error correction and other technologies, it can be used in other areas with lower real-time requirements (such as meter reading in residential areas). The performance of carrier communication mainly depends on the performance specifications of the modem. 3.1.3 Wired Cable Method This method uses twisted-pair cables or dedicated cables to transmit information via electrical signals. Fieldbus technologies such as RS-485, CAN, and Lonworks can be used. Its characteristics include low cost, lower construction and maintenance requirements compared to optical fiber, lower reliability than optical fiber, and lower data transmission rate. It is suitable for applications where real-time requirements are not high. RS-485 is a commonly used, inexpensive bus (interface) technology, available in half-duplex 2-wire and full-duplex 4-wire versions. In practical use, communication can be achieved simply by laying wires according to network design requirements, connecting various devices, and following certain communication protocols. The data transmission rate depends on the communication distance (up to 1200 bits/s in a network of approximately 1 km; the transmission rate can be significantly increased for short-distance applications). RS-485 bus technology is widely used in power distribution automation for transformer monitoring, data acquisition in substations and switching stations' local area networks, and short-distance communication. Proven in use, RS-485 bus technology is cost-effective and suitable for transformer monitoring in power distribution automation systems, making it usable in China. However, it is not suitable for the main communication network of power distribution automation systems. The disadvantages of RS-485 bus include limited network length, making it unsuitable for long-distance, large-scale networks; modifications to established networks are subject to certain constraints, and inappropriate line impedance can render the modified network inoperable. The overall performance of RS-485 depends primarily on reasonable network design, the specifications of the RS-485 driver chip, and a suitable communication protocol. Since communication lines are installed outdoors along power lines, resistance to wind, sun, and lightning strikes must be considered. CAN and Lonworks buses are excellent fieldbus technologies, but they are not as widespread as RS-485 in China. This may be due to economic reasons, and also because most current devices do not provide standard interfaces of this type, requiring redevelopment or the addition of interface cards. CAN and Lonworks buses can form high-quality local area networks, with much more convenient network topologies and higher transmission rates than RS-485, but they are more expensive. 3.1.4 Fiber Optic Communication utilizes the propagation of light waves in optical fibers to transmit useful information. 3.1.4.1 Characteristics and Applications of Fiber Optic Communication Fiber optic communication has unparalleled advantages over other methods: a. High-speed communication, strong adaptability, and compatibility with various standard equipment. b. High reliability, with opto-isolation between input and output, strong anti-interference and lightning strike capabilities, making it particularly suitable for use in power systems. c. Wide bandwidth, long transmission distance, and a bit error rate of less than 10⁻⁹, superior to other communication methods. d. Self-healing loops can be used, offering higher reliability than single-loop methods. e. Flexible configuration and convenient expansion. Adding a new node can be achieved by opening a nearby fiber optic loop and directly connecting. f. Small size, light weight, low power consumption (not exceeding 5W), and modular structure of optical terminal equipment. g. High protection level of optical cables, unaffected by electromagnetic signals, and can be installed on the same poles or in trenches as power cables, both above and below ground. The disadvantages of fiber optic communication are that equipment and construction costs are higher than high-speed data transmission radios, and network adjustments are less convenient than wireless methods. Before breakthroughs in fiber optic and optical device performance, the overall performance of fiber optic communication mainly depends on the optical terminal equipment, communication protocols, and fiber itself. Fiber optic communication, as an ideal communication method, has been widely used in various departments of the telecommunications industry, between power bureau main stations and some substations, and between various units within the bureau. However, the biggest obstacle to its application in distribution automation systems is the high cost of equipment and installation. With social progress and technological development, the large-scale use of fiber optic communication in power supply departments has become possible. On the one hand, distribution automation requires communication systems to provide high-speed, high-capacity, and highly reliable data communication, making fiber optic communication the natural first choice; on the other hand, with the large-scale production of optical cables in China, the successful development of optical terminal equipment suitable for distribution automation, and the continuous decline in the price of fiber optics and optical terminal equipment, power supply departments can now afford it. 3.1.4.2 Networking methods of fiber optic communication a. Single-ring method: Single-fiber point-to-point or direct link method, as shown in Figure 1. In the diagram, D1 represents the host or communication controller (or a secondary master station), and D2 to Dn represent the various field devices. Data flows unidirectionally (or in the opposite direction shown in the diagram) within the loop. If a break occurs at any point in the loop (e.g., point A) or a field optical terminal device malfunctions, the entire loop communication will be blocked. In fact, as long as the communication line remains open and the optical terminal device does not malfunction, communication will not be affected. [img=300,140]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hebdljs-2/24-1.jpg[/img] b. The self-healing ring method adds one more optical fiber to the single-fiber ring, forming a ring structure with a dual-core optical cable, as shown in Figure 2. Of the two optical fibers, one (fiber a) is the primary fiber, and the other (fiber b) is the backup fiber. Data flows in opposite directions within the two fiber rings. When a break occurs at a point or a fault occurs in an optical terminal device, such as at point A, where optical terminal devices D4 and D3 are in a state of not receiving (optical) signals, the backup fiber will automatically be activated to form a new loop, returning to the main station along their respective channels (see Figure 3), which is the so-called self-healing ring function. The cost increase of adopting the self-healing ring method is not significant, but the reliability of the network is improved. [img=300,319]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hebdljs-2/24-2.jpg[/img] With the development of technology, more optical fiber devices, optical terminal devices, and new technologies will be introduced into power distribution automation communication, making network design more convenient and reasonable, and more suitable for power system applications. 3.1.5 Satellite communication utilizes a USAT Ultra-Small Aperture Terminal (0.5–1.2 m) for two-way satellite communication, operating in the Ku or C band. This is a master-station controlled satellite communication system. Compared to terrestrial wireless communication, its main advantages are higher reliability, higher speed, and greater network flexibility, making it suitable for various terrains, especially mountainous areas. Examples of its use in master control stations, substations, centralized control stations, and dispatch centers exist in China. However, this method has high equipment costs and requires annual satellite rental fees. 3.1.6 Spread Spectrum Wireless Spread spectrum communication is rarely used in power distribution automation and requires further trial and testing. 3.2 The choice of communication method should be based on local conditions and a performance-price ratio, paying attention to the following points: a. Whether the adopted technology is mature and feasible, and whether it has a certain degree of advancement and forward-looking nature, as modern society and technology are developing rapidly, with new technologies emerging constantly. b. The scale of investment and the number of functions the system is intended to achieve. c. The number and distribution density of monitoring and control points, and the actual effective range of communication. d. When using wired cables or optical cables, is the installation (aerial, ground, or along buildings) convenient? When using wireless transmission, is the applied channel clean and has minimal interference (considering the impact of different seasons and different times within a day)? What is the future usage of wireless frequencies in this area? e. Topography, terrain, and electromagnetic interference in the air and around the equipment. f. Good compatibility between the test network and the future large network to ensure full utilization of investment; the sharing or compatibility of the distribution automation communication network with existing communication equipment and future communication equipment. Several methods can be organically combined, taking advantage of their strengths and compensating for their weaknesses, and comprehensively considered in the overall system design. 3.2.1 Using wireless as the primary method: If the urban area is small, the distribution area of ​​each measuring point is small, and the investment is small, then wireless as the primary communication method can be selected; for transformer monitoring with low real-time requirements and large data volume, wired methods can be used; for meter reading in residential areas, low-voltage carrier waves can be used. When the urban area is large and there are many monitoring points, and the wireless communication distance is too long or obstructed by buildings, one or more repeater stations (secondary stations) can be added as needed. If the location is mountainous, and the terrain significantly impacts communication, and construction is difficult, wireless communication with repeater stations may be a better choice. 3.2.2 Using fiber optics as the primary method: Using fiber optic communication as the main network, supplemented by wireless communication or wired local area networks in areas far from the central main station or with few monitoring points, may be a more practical and ideal approach. The main network uses a highly reliable self-healing ring, while the sub-networks use a layered main-supplement combination of single-ring networks, which helps improve system reliability. Using different communication methods to accommodate different communication media is economical and reasonable, and conducive to widespread adoption. See Figure 4 for a schematic diagram of the communication network hierarchy. [img=300,180]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/hebdljs-2/25-1.jpg[/img] [b]4 Case Studies of Operational Projects[/b] Representative projects that have been successfully put into operation include distribution automation projects in Zigong, Beiliu, Kunming, Shijiazhuang, and Tangshan. The first four all use wireless methods. Among them, Zigong City is located in a mountainous area, making it difficult to lay wired cables over a large area. The urban area is relatively small, so wireless is a more suitable choice. The main station is located in the main building of the Ziliujing branch, with the antenna on the roof. Considering the large elevation changes in the urban area, a relay station is set up on the microwave antenna tower of the highest substation in the city, Tudipo, which can cover the entire urban area and relay signals that cannot be directly connected to the main station in the entire urban area. This solves the problem of signal obstruction by high mountains and tall buildings. The transformer monitoring adopts a local area network RS-485 wired cable method. The successful operation of the Zigong power distribution automation project proves that this communication method is relatively reasonable and effective. The Tangshan power distribution automation project adopts a combination of fiber optic communication and wired communication. The first layer is the main loop, which adopts a highly reliable fiber optic self-healing loop method. All important field equipment, such as pole-mounted sectionalizing switches, pole-mounted tie switches, line outlet circuit breakers, switching stations, and communication controllers, are located in this layer to meet the system's reliability and real-time requirements. The second layer is equipped with transformer monitoring instruments, community meter reading equipment, etc. This layer contains non-critical data (compared to switch status and data) with low reliability and real-time requirements. It adopts a single-fiber non-self-healing loop and wired cable method, which helps to reduce costs without affecting the overall performance of the system. [b]5 Conclusion[/b] In power distribution automation communication systems, the self-healing loop method of fiber optic communication, supplemented by wired, wireless and other methods, is likely to be the mainstream communication networking technology in power distribution automation in the next few years. The successful development of new technologies will provide more communication options, such as satellite communication. When it becomes possible to use ultra-small aperture ground receiving equipment, there are enough satellites in the sky, and the cost of satellite leasing is acceptable to the power supply department, this communication method is likely to be a better one.