[b]0 Introduction[/b] In recent years, in order to meet market demand, the national power industry policy has been adjusted accordingly, with key investments placed on the construction and renovation of urban and rural power grids, providing conditions for the application and development of distribution network automation systems. Establishing a reliable communication network with a reasonable performance-price ratio is an important part of distribution network automation system engineering. Among the various communication modes available today, optical fiber communication [1] is increasingly valued for its many advantages, including high speed, high reliability, anti-interference, low power consumption, stable operation, convenient installation and operation, and reasonable price. The Shanghai Pudong Jinteng Industrial Park Distribution Network Automation Pilot Project [2] successfully adopted single-loop optical fiber communication, accumulating experience for the promotion and application of optical fiber communication in power distribution systems. However, in a single-loop optical fiber communication network, the reliability of the optical fiber loop will be affected as the number of loop nodes increases. Dual-loop optical fiber communication is an important way to improve the reliability of optical fiber loop communication. Developing a self-healing optical fiber dual-loop transceiver is a prerequisite for promoting the application of dual-loop optical fiber communication. [b]1 Key Technologies of Dual-Loop Fiber Optic Communication Networks[/b] Distribution ring network power supply systems have many nodes and dispersed coverage, but the distance between nodes is relatively short, generally ranging from hundreds to thousands of meters, which is conducive to using a point-to-multipoint fiber optic ring communication method. In distribution network automation systems, the master station connects to each substation (node) sequentially through optical cables and fiber optic transceivers, forming a fiber optic ring. The number of nodes allowed to be linked in the fiber optic ring depends on the data transmission rate R, the percentage Δ of the allowable code distortion (lead and trailing edge delay difference) for correct data identification or detection, and the average code distortion δ caused by each node. The number of nodes N can be approximately estimated by Δ/(Rδ). If calculated based on R=9600 bit/s, Δ=10%, and δ=0.1 μs, the number of nodes N can generally reach nearly 100, which is sufficient to meet the actual needs of engineering projects. Under normal circumstances, the simple single-loop fiber optic method has high reliability. However, once a fault occurs at a certain point (optical cable breakage, node power failure, or other problems), the entire fiber optic system cannot form a ring, affecting the reliability of the communication system. This naturally leads us to consider a dual-loop approach. However, using two fiber optic loops to construct a simple dual-loop fiber optic system presents two problems: a. If the routes are the same (i.e., using different fiber cores of the same cable), the problem caused by cable breakage cannot be completely resolved. Using different routes (i.e., using two separate cables) increases costs. b. The relationship between the output signals of the dual loops. In a fiber optic loop, each substation transmits and receives signals via two fiber optic cables. Due to different paths, each signal experiences a time delay after passing through a relay node, causing the signals from the two fibers to arrive at the same substation at different times, i.e., the two signals are out of phase. Simply superimposing these two signals will cause bit errors, leading to reception confusion and the terminal being unable to identify the signal. Based on the above analysis, it is concluded that simply superimposing the signals from two fiber optic loops (single loop) cannot solve the practical problem. Only a dual-loop fiber optic system with automatic switching (also known as self-healing) can ensure the safe and reliable operation of the power distribution network automation system. Therefore, a dual-loop fiber optic transceiver with self-healing functionality is the key technology for realizing a dual-loop fiber optic communication network. [b]2 Basic Principles and Functional Characteristics of Fiber Optic Dual-Loop Transceivers[/b] 2.1 Basic Principles and Functions of Fiber Optic Dual-Loop Transceivers A fiber optic dual-loop transceiver consists of a fiber optic transceiver and an automatic switching circuit. Its main functions are: a. Transmitting, receiving, and forwarding optical signals; b. Ensuring that only one digital signal is output from each of the two signals in the substation's fiber optic loop, thereby guaranteeing the reliability of the received signal; c. When one signal is interrupted due to a fault, the automatic switch activates, automatically connecting the two fiber optic loops on both sides of the fault point to form a new fiber optic path, ensuring normal communication within the loop. The structure of the fiber optic dual-loop transceiver is shown in Figure 1. [img=108,148]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9905/image5/46-1.gif[/img] OFF/REP Repeater (Forwarder) Switch: TA, TB, RA, RB Optical Cable Connector DP Digital Communication Port; HD/FD Half-Duplex (485)/Full-Duplex (422) Switch TE, RE Digital Signal Indicator; TOA, TOB, ROA, ROB Optical Signal Indicator Fig.1 The two-way optical fiber transceiver link outline 2.2 Features of the Optical Fiber Dual-Loop Transceiver The optical fiber dual-loop transceiver retains the advantages of the single-channel optical fiber transceiver, with small size, simple structure, and convenient use. Since only baseband transmission is used and no modulation/demodulation method is used, the power consumption is very low. The fiber optic dual-loop transceiver uses a 12V power supply. When transmitting or receiving signals, the operating current is 100 mA; when not transmitting or receiving signals, the static operating current is less than 30 mA, resulting in low temperature rise and stable operation, making it particularly suitable for harsh outdoor environments. The transceiver often uses fast CMOS circuits, ensuring high speed and reliable operation. [b]3 Application Examples[/b] In actual power distribution network control, the configuration of the dual-loop fiber optic communication system is as follows: the master station uses FD (422) with forwarding mode OFF (no forwarding); each substation uses HD (485) with forwarding mode REP (forwarding, forming a loop). The two fiber optic loops use two fiber cores from the same optical cable. The two fiber optic loops are hot-standby for each other. The optical signals in the two optical fibers are distinct (different phases) but interconnected (facilitating switching). During normal operation, the optical signals in the two optical fibers travel their own paths in opposite directions. Each optical signal is forwarded separately in the fiber optic transceiver of each substation. The dual-loop fiber optic communication system is shown in Figure 2. [img=302,195]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9905/image5/46-2.gif[/img] Fig.2 Scheme of two-way optical fiber loops system 3.1 Normal operation The query command signal TD from the master station is sent from the optical transceiver and transmitted in different directions by the two optical fiber loops A and B; the signals RD1 and RD2 received by the master station come from the two optical fiber loops respectively, where RD1 is the standard 422 output signal of the A loop sent by the dedicated interface, while RD2 is the spare output signal of the B loop. Although RD1 and RD2 arrive at different times or in different phases, they should both contain the response information of each substation and the query command sent by the master station. RD1 and RD2 occupy two serial ports of the master station respectively, receiving information from the A loop and the B loop respectively. Since each substation uses HD mode, both query and response signals are input/output via the RD/TD terminals. 3.2 Fault Analysis a. Optical cable fault between substations II and III (see Figure 3(a)): Master station RD1 can only receive response signals from substations I and II; RD2 can only receive response signals from substation III. In this case, the master station computer can analyze and determine that there is a fault in the optical cable between substations II and III. b. Fault in substation II (see Figure 3(b)): Master station RD1 can only receive response signals from substation I; RD2 only receives response signals from substation III. In this case, the master station can determine that substation II has a fault. Simultaneously, other potential problems in the fiber optic loop can be identified based on the differences between RD1 and RD2. [img=278,285]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9905/image5/46-3.gif[/img] Fig.3 Fault examples of two-way optical fiber loops system [b]4 Conclusion[/b] The use of optical fiber dual-loop technology in distribution network automation systems has greatly improved the safety and reliability of distribution network communication. The successful development of domestically produced optical fiber dual-loop transceivers with self-healing function will greatly promote the application of optical fiber communication in distribution network automation systems. References [1] Lin Gongping, Wang Kaibin. Application of optical fiber communication in distribution network. Automation of Electric Power Systems, 1998, 22(8) [2] Ye Shixun, Zhang Bulin, Ye Xi, et al. Discussion on the distribution network automation system and its technology in Jinteng Industrial Park. Automation of Electric Power Systems, 1998, 22(8)