[b]1 Introduction[/b] Overhead power distribution systems using voltage-controlled feeder automation as a basis can be implemented in three stages. Figure 1 shows the basic structure of the overall scheme. [img=367,232]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9912/image12/28.gif[/img] CB—Circuit breaker; PVS—Vacuum switch; SPS—Power transformer; RTU—Remote control terminal unit; FSI—Fault indicator; TCR—Remote control receiver unit; TCM—Remote control master station unit; CPU—Central processing unit; CD—Control console; CRT—Display; G-CRT—Graphic display; LP/PRN/HC—Printing equipment Fig.1 Structure of automated voltage controlled type distribution feeders In the first stage, the vacuum automatic distribution switch PVS, remote control terminal unit RTU, and power transformer SPS are erected on the same pole. Utilizing the intelligent detection capabilities of the RTU itself, it can cooperate with switchgear to isolate faulty sections and restore power to non-faulty sections. Furthermore, by using fault section indicator equipment within the station and calculating the opening and closing times of circuit breakers, it can identify the faulty section and notify operators for maintenance. The characteristic of this stage is that it requires no communication system; the intelligent functions of the pole-mounted equipment alone can independently complete the basic functions of overhead power distribution automation. The completion of this stage reduces outage areas, shortens outage time, improves power supply reliability, and realizes the basic functions of feeder automation. To further advance power distribution automation, aiming to improve power supply reliability, enhance power quality, achieve excellent power information management, provide comprehensive services to users, and reduce operating costs and the workload of operators, after completing the basic functions of the first stage, the next step is to realize computer-managed power distribution automation linked by telemetry and remote control automation. This represents a higher level of development in power distribution automation. In this stage, the telemetry and remote control automation stage, serving as the intermediate link between the pole-mounted equipment and the comprehensive computer management within the station, plays a crucial bridging role. This article focuses on the hierarchical expansion of distribution automation systems, starting with voltage-type feeder automation. It discusses the basic approach to developing from pole-mounted distribution automation to telemetry and remote control automation, and compares the communication methods of the basic telemetry and remote control automation modes. [b]2 Basic Approach to Telemetry and Remote Control Automation[/b] As shown in Figure 1, the basic voltage-type feeder automation scheme is characterized by the RTU being the fundamental device connecting the pole-mounted distribution automation stage and the telemetry and remote control automation stage. In the first stage, the RTU utilizes its intelligent fault detection capabilities to identify and isolate faulty sections. However, this stage of RTU cannot be considered a true RTU, as it only functions as a fault detector. Only when it fully utilizes the communication function between the pole-mounted equipment and the station's remote control master console (TCM) can it be considered a true RTU. The telemetry and remote control automation stage is essentially an important component of the computer-managed distribution automation stage. The main purpose of discussing this as a separate stage is to strengthen the hierarchical nature of the distribution automation implementation process, namely, from the application of outdoor primary equipment (stage one) to signal acquisition and transmission (stage two) to computer management (stage three). Distribution automation is a large-scale systems engineering project, and the initial planning of the power sector will play a decisive role in the economic and rational implementation of this project. Telemetry and remote control automation are important links in distribution automation and must be considered in the initial planning. Therefore, the choice of communication method is a key issue in realizing telemetry and remote control automation. In the implementation of a distribution automation system based on voltage-type feeder automation, if one wants to avoid the current complexities of communication method selection and expand after the communication scheme is determined, then one can choose to leverage the intelligent fault detection function of the RTU in this equipment, while reserving interfaces to add independent communication RTUs when telemetry and remote control are needed, thus realizing telemetry and remote control automation; if one wants to avoid the complicated installation on poles and complete the deployment of distribution automation pole-mounted equipment in one step, then after selecting the communication method, one can directly adopt an integrated RTU with both fault query and remote communication functions to achieve pole-mounted equipment selection in one step. Regardless of the method, the system can complete telemetry and remote control automation, and develop towards computer-aided power distribution automation. [b]3 Characteristics of different communication methods[/b] 3.1 Power line carrier communication Power line carrier communication uses power lines as channels. Communication between almost all points can be formed without laying dedicated channels from the power source point to the substation point to the distribution point and all users. Therefore, there is no need to move communication channels when installing new or mobile equipment. Moreover, this communication network is consistent with the material flow of the power production process, making it easy for information flow and data processing to meet the requirements of hierarchical and layered management [1]. Power line carrier communication has been used more in high-voltage transmission networks in the past. However, for distribution networks, due to their very complex wiring and component parameters, there are overhead lines and cable networks, many branches of different lengths and cross-sections connected to the main line, and multiple connections on the branches. In addition, there are distribution transformers of different capacities distributed in various places, resulting in greater signal attenuation. Moreover, the noise composition of medium-voltage distribution networks is different from that of high-voltage transmission lines. Although the random noise caused by corona discharge and leakage discharge on the insulation surface caused by high voltage is relatively low in medium and low voltage networks, the frequent operation of a large number of electrical appliances in medium voltage networks increases the pulse noise component, which will cause signal interference. Therefore, the following problems exist when the power line carrier method is used in the distribution network: (1) When the carrier signal is transmitted in the distribution network, the distribution signal is attenuated and reflected due to the branches. Especially at each node, different degrees of signal reflection and refraction will inevitably occur. The signal strength at any point is always the vector sum of the superposition of related signals, which varies with the wiring method, operation mode and climate, which brings difficulties to the reception. (2) When the structure of the distribution line changes (such as the number of branches, the length of the line or the number of switches), the phenomenon of not receiving the signal will occur, and the adjustment is more difficult. (3) There are many disconnections in the distribution network. For the case of switch disconnection not caused by fault, the signal transmitted by the power line carrier needs to be judged as whether it is caused by fault or normal switch disconnection. (4) If similar frequencies are used on the power line, interference will occur, so multiple frequencies need to be prepared. (5) The transmission rate of power line carrier is generally between 50 and 200 b/s. When using RTU polling communication, if there are too many units, the signal transmission is slow and the cycle is long, which is not practical. This is also not conducive to the improvement of function and future expansion. The Tokyo Electric Power System in Japan implemented distribution automation earlier and adopted the power line carrier method. At present, it is facing the problem that the communication network limits the improvement of the overall system function and is considering adopting the fiber optic communication method. Therefore, the above issues need to be carefully considered when using the power line carrier communication method. 3.2 Audio wired communication method Audio wired communication method is a widely used communication method, mainly because of its good reliability, economy and scalability. This method requires the laying of communication lines, but there are no special requirements for the laying of communication lines and the connection of each communication end. The characteristics of this method are: (1) The transmission rate is above 1200 b/s. It is suitable for systems with a large amount of information to be transmitted. (2) It can realize multi-channel parallel processing. Since the signal transmission path is relatively fixed, the same address can be used when different paths are independent. In this way, it is very simple to send signals from the master station to the RTU in the implementation of the programming software. (3) When the substation needs to be restored after a fault, a common command can be sent to all RTUs to control them at the same time, and the switch status can be confirmed in a short time. (4) It is convenient to add paths, which is beneficial to improve the function and future expansion. Basic parameters of a typical communication line transmission method: Communication cable: ESI twisted pair cable, 2 or 4 lines per channel; Modulation method: FSK; Transmission rate: 1200 b/s; Carrier frequency: Uplink 1.3 kHz (according to user requirements); Downlink 2.1 kHz; Output level: Maximum 0 dBm; Receive level: 0 to -30 dBm; Signal-to-noise ratio (S/N): 25 dB. 3.3 Fiber Optic Communication Characteristics of Fiber Optic Communication: (1) High transmission speed and the ability to transmit voice, data and images. Considering a communication rate of 9600 b/s, each fiber optic communication ring can connect up to 100 communication nodes [2], which can fully meet the needs of the expansion of the communication network of the distribution network automation system; (2) Low transmission loss, approximately 0.2 to 1.0 dB/km, which can realize long-distance transmission. Using single-mode fiber, the transmission distance is greater than 20 km; using multimode fiber, the transmission distance is generally greater than 6.5 km; (3) High reliability and strong anti-interference ability, not affected by electromagnetic waves or other strong electromagnetic fields; (4) Using ring network communication, each is a hot backup. Once the communication ring fails, the optical terminal equipment can automatically select a route and heal itself, which improves reliability; (5) Flexible configuration and convenient expansion. If a new point needs to be added, a loop can be opened in the nearest fiber optic communication ring network for direct connection. However, the cost of laying optical fibers will increase significantly when optical fiber communication is applied to power distribution networks; the price of optical terminal equipment varies depending on the type of fiber, and each segment point requires optical-to-electrical conversion equipment, resulting in high equipment costs; maintenance requires specialized technicians. Furthermore, the application of optical fiber communication in power distribution network automation is still in its early stages, and technical problems will continue to arise in practical applications. Therefore, if it is currently limited to information transmission within the power distribution network, optical fiber communication is very uneconomical. However, if used in conjunction with other systems, optical fiber communication is undoubtedly a good communication method. 3.4 Wireless Communication Methods Due to the development of science and technology, wireless communication technology has advanced rapidly, and its applications have become more widespread. Comparing the three methods of wireless communication: microwave communication equipment and the entire project are expensive and not suitable for multi-point communication in the distribution network; spread spectrum communication has the advantages of strong anti-interference ability, low bit error rate and low transmission power, and is suitable for long-distance communication such as substations and dispatch centers. However, when used in urban distribution networks, due to the poor communication environment and its poor diffraction ability, the signal reception will be affected by wave transmission, resulting in poor performance [3]; ordinary radio communication is a more practical method, but it also faces the following problems in application: (1) electromagnetic noise interference reduces communication reliability and increases bit error rate; (2) it is subject to the restrictions of the radio wave law and requires application to the Radio Management Committee for a dedicated channel; (3) if the transmission distance exceeds a certain range, a relay station needs to be set up; (4) when the transmitted signal encounters interference from proprietary frequencies, it will affect the reliability; (5) due to the development of wireless communication technology, it is easy for individuals to interfere with specific frequencies. Once this happens, it will bring danger to the entire distribution network. [b]4 Comprehensive Evaluation of Different Communication Methods[/b] 4.1 Reliability Analysis From the characteristics of the above four communication methods, power line carrier and wireless communication are easily affected by external noise and human factors due to the openness of the signal transmission path; communication line and fiber optic communication have closed paths, so their reliability is very high. 4.2 Economy Due to the large scale and wide coverage of the power distribution network, the economy of the communication method must also be given priority. (1) Basic Costs Power line carrier has no basic costs, wired communication requires the construction of communication lines, so there are construction costs; the cost of laying optical cables for fiber optic communication is a large investment; radio communication may require the establishment of relay stations depending on the regional conditions, so this cost is also considerable. In comparison, power line carrier has the lowest basic costs. (2) Equipment Costs: Power line carrier communication requires the installation of coupling filters, coupling capacitors, line traps, etc., which are expensive, and the reliability of the equipment greatly affects the smoothness of the channel, hence the high cost. Wired communication only requires a modem, which can be placed inside the controller RTU, making it cheaper. Fiber optic communication requires optical terminal equipment at each sub-port, resulting in higher costs. For radio communication, the modem in the controller RTU is affected by the wireless communication transmission power, resulting in moderate costs. Therefore, wired communication has the lowest equipment cost. Regarding maintenance costs, except for fiber optic communication, other methods are basically maintenance-free. It can be seen that the overall cost of both wired and radio communication is relatively low, making them better communication methods from an economic perspective. 4.3 Transmission Capacity Power line carrier transmission rates range from 50 to 200 b/s; wired communication transmission rates range from 2000 to 1.5 Mb/s, more than 10 times that of power line carrier; fiber optic communication transmission rates can reach 4 Gb/s, capable of transmitting large amounts of information; ordinary radio transmission rates are around 2000 b/s, but the on-time is relatively long. Therefore, fiber optic communication has the highest transmission capacity, followed by wired communication. In summary, from an application performance perspective, fiber optic communication is a good communication method. However, considering reliability, economy, practicality, and the characteristics of the power distribution network, wired communication transmission, under conditions of severe signal interference and limited investment, seems to be the better choice at present. [b]5 Communication Protocols[/b] Communication protocols (mainly for computer software communication) are the rules that various remote control devices, databases, etc., should follow when exchanging data with computer systems. Currently, CDT and POLLING protocols are widely used in my country's power system dispatch automation. The cyclic CDT protocol is unsuitable for communication between pole-mounted equipment and substations in distribution networks due to its low channel utilization. The polling protocol, facing numerous communication points in the distribution network, requires the master station to use a point-to-multipoint approach, employing a question-and-answer method for repeated visits, which is time-consuming and affects the timely transmission of important information (such as fault information). A better approach is for RTUs to proactively report changes to the control master station, while the master station visits each RTU at regular intervals [4]. Currently, IEC 870.5 (developed by the International Electrotechnical Commission) and DNP 3.0 (widely used in North America) are two communication protocols suitable for distribution network automation systems. They have advantages such as lower communication channel speed requirements, high efficiency, support for RTU proactive reporting, and support for multiple master station configurations, and can be used as references. DNP 3.0 is similar to FT2 in IEC 870.5; the two are basically compatible and can be modified according to actual needs. 6 Conclusions (1) Adopting the development approach of voltage-type distribution automation system, the pole-mounted distribution automation stage can independently realize the basic functions of distribution automation without relying on communication methods, while telemetry and remote control automation can be easily extended from the pole-mounted automation stage. (2) The power line carrier method applied to the 10 kV overhead system has certain shortcomings in terms of reliability and economy; the basic investment of radio communication is small, but the reliability is reduced due to current urban development and radio wave interference; fiber optic communication is an excellent communication method under the condition of sufficient investment and compatibility with other systems; wired communication transmission is a better communication method in terms of reliability, economy and practicality under the condition of severe external interference and limited investment. (3) It is important to adopt standard protocols for communication. IEC 870.5 and DNP3.0 are two communication protocols that are more suitable for distribution network automation systems and can be used as a reference. [b]7 References[/b] 1 Chen Chonghao. Medium-voltage distribution line carrier technology and application. Power Supply and Utilization, 1998(4): 22-24 2 Lin Gongping, Wang Kaibin. Application of optical fiber communication technology in distribution network automation system. The First Academic Exchange Conference on Distribution Network Automation in Yantai, 1998 3 Xu Layuan. Current status and development direction of distribution network automation in my country. The First Academic Exchange Conference on Distribution Network Automation in Yantai, 1998 4 Xu Bingyin. Feeder automation technology. Power System Technology, 1998(4)