1 Introduction For many years, due to the prominent contradiction between power supply and demand in China, the focus of power investment has been on the transmission and transformation system. When the contradiction between power supply and demand is slightly alleviated, the weakness of the distribution system, which has been neglected, becomes apparent, becoming a "bottleneck" between the power supply system and the ever-increasing user demand. With the continuous improvement of the power sector's requirements for power supply reliability and economic efficiency, coupled with the introduction of national industrial policies to promote economic development and increase investment in infrastructure, distribution automation has been put on the agenda [1]. The realization of distribution automation will make power supply more reliable, provide more comprehensive services to users, save on personnel input, and make the operation of the entire system more efficient. Feeder automation is an important part of power system distribution automation. For a long time, since China's 10 kV lines are mainly overhead lines, the realization of feeder automation of 10 kV overhead lines is the primary task in the work of urban grid transformation. Faced with a large number of 10 kV distribution lines, how to realize the basic functions of automation in an economical and efficient manner is the main task at present. The continuous improvement of computer and power electronics technologies has made feeder automation, an automation technology that integrates computer, automatic control, electronic technology, communication technology, and new power distribution equipment, easier to implement and more effective. This article introduces a voltage-type feeder automation system scheme. This scheme is implemented through three stages: pole-mounted distribution automation, telemetry and remote control automation, and computer-controlled distribution automation. These three stages are from the application of outdoor primary equipment (stage 1), signal acquisition and transmission (stage 2), to the realization of computer management (stage 3). This scheme has been implemented in Japan for nearly 30 years. The neutral point ungrounded operation mode of my country's 10 kV power grid is similar to that of Japan's power grid, so this scheme is more suitable for application to my country's overhead distribution network system. [b]2 Voltage-type Feeder Automation System[/b] The voltage-type feeder automation system scheme is shown in Figure 1. 2.1 Pole-mounted Distribution Automation Stage The pole-mounted distribution automation facility consists of a pole-mounted vacuum automatic distribution switch (PVS), a remote terminal unit (RTU) with fault diagnosis function, and a power transformer (SPS) installed on the same pole. The pole-mounted vacuum automatic switch (PVS) uses a vacuum interrupter for arc extinguishing and SF6 gas for external insulation. The switch has an isolation break connected in series with the vacuum interrupter, giving it superior arc extinguishing and voltage withstand performance [2]. Since both the arc extinguishing and insulation media are oil-free, the risk of fire or explosion is avoided. The high-voltage section, low-voltage control circuit, and operating mechanism of the switch are all sealed within the switch box. The main circuit leads are protected by insulated cable outlets, and the live parts are not exposed, providing a 15-year maintenance-free feature. The switch has a compact structure and features suspended mounting hardware and cable leads, making pole-mounted installation extremely convenient. It has manual/automatic operation modes, making switch operation more flexible. The controller can be divided into two types: a fault detection controller (FDR) and a remote terminal unit (RTU). Using a fault detection controller in conjunction with the pole-mounted switch can achieve isolation of faulty sections and restoration of power supply to non-faulty sections, but communication with the station's computer requires a separate RTU unit. In addition to the functions mentioned above, the remote terminal unit also has the ability to communicate with the computer in the station. By cooperating with a communication system suitable for the local geographical terrain, it can realize telemetry, remote control, remote signaling and remote adjustment of power distribution automation. [img=312,293]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9907/image7/142.gif[/img] Fig.1 Configuration of feeder automation system CB: Circuit breaker; PVS: Vacuum switch; SPS: Power transformer; RTU: Remote 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 power transformer collects the voltage on the distribution line from both sides of the line as a switching power supply and a detection signal for fault detection and judgment. The entire system uses voltage delay to calculate the time it takes for a circuit breaker to close and open, thus identifying the faulty section. Based on this, it enables the independent and automatic isolation of faulty sections and automatic fault location using primary equipment. The following describes the process of isolating faulty sections and restoring power to non-faulty sections when this system is used in a ring network. Figure 2 shows a schematic diagram of the ring network operation. In the diagram, CB1 and CB2 are circuit breakers in two substations, LS1, LS2, LS3, LS5, and LS6 are sectionalizing switches on the lines, and LS4 is the loop point switch (normally open) for both lines. The delay time for the sectionalizing switch is 7 seconds, and the delay time for the loop point switch is 45 seconds. Assume that when the fault occurs in section c, circuit breaker CB1 in the substation trips, and LS1, LS2, and LS3 disconnect due to voltage loss. Subsequently, CB1 recloses once, and LS1 and LS2 close in a predetermined delay sequence. When closing to the fault section c, if the fault was transient, power is restored to all lines since the line has already returned to normal. If the fault is permanent, LS2 will close sequentially to the short circuit, causing CB1 to trip again. At this time, LS2 senses the short circuit fault, and LS3 senses the abnormal low voltage at the front end and stores the fault latch. Afterward, CB1 is energized, restoring sections a and b to normal. Simultaneously, due to the power outage in section d, switch LS4 at the ring network point begins a delay after sensing the power outage at one end. After a period of time, LS4 closes, supplying power to section d from CB2. According to the above time settings, the section with the longest power outage time due to the fault is section d, lasting 45 seconds. The system achieves isolation of the faulty section and restoration and allocation of power to non-faulty sections. [img=289,83]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9907/image7/143.gif[/img] Fig.2 Demonstration of ring network application 2.2 Telemetry and remote control automation stage The facilities in the telemetry and remote control automation stage consist of the following devices: a control console that issues on/off commands, a CRT that displays information, a remote control master console (TCM) that communicates with the master console, and a remote terminal unit (RTU) that communicates with the master console. The second stage, as an extension of the first stage, can specifically realize the transmission of line information and the control and monitoring of primary equipment. As the intermediate link between primary equipment and power distribution automation system, this stage can realize the following functions: (1) When a fault or power outage occurs, the switch on the remote control stick is turned on or off; the system is switched when overloaded. (2) Monitor the voltage and current of transformers and feeders in the substation, as well as the operation of relays in the station; monitor the working status of switches and RTUs on the poles. (3) Display the status and system information of circuit breakers, switches, and other equipment on the screen through a computer. (4) Record operations, print status change records, copy CRT screen content, and record maintenance work data. The key issue in this stage is to select a suitable communication method. Currently, communication methods can be divided into two types: one is peripheral communication, which is mainly a data and voice channel; the other is computer software communication. Peripheral communication is divided into two main categories: wired and wireless. Wired communication includes fiber optic communication, audio cable communication, and power line carrier communication; wireless communication includes microwave communication, spread spectrum communication, and radio communication. Computer software communication mainly realizes the exchange of data between databases, various remote control devices, and various computer software according to certain protocols. Each communication method has its own advantages and disadvantages, and the effects, economy, and practicality of different communication methods when applied to feeder automation vary greatly. After selecting a suitable communication method, the second stage can be easily extended from the first stage. 2.3 Computer-controlled distribution automation stage [3] Computer-controlled distribution automation is an extension of the second stage. Information from each substation is sent to the computer system of the main station through the Telecommunication Control Unit (TCM) of the main station for comprehensive computer management. This stage is an automation stage with computers as the main body. After realizing computer-controlled distribution automation, the following functions can be realized: (1) Distribution network monitoring function: The system collects information of the distribution network and monitors the status of the distribution network. (2) Distribution network control function: The system remotely controls the switches and RTUs according to the operator's instructions through the TCM. (3) Distribution network single-line diagram display function: Each feeder of the distribution network is displayed in a single-line diagram format. (4) Recording function: The status of the distribution network, operation records, system faults, etc., processed by monitoring, control and data maintenance can be printed out through the recording function. (5) Data maintenance function: The operator can use the data maintenance function to update the database in the computer. (6) Simulation function: The system can simulate the equipment action and network status during an accident and provide training functions. (7) Graphical display function of the distribution network: Displays the distribution network diagram corresponding to the street map. (8) Substation monitoring function: remotely monitors the status and measurement data of equipment within the substation. (9) Distribution network dispatching function: calculates and dispatches the operation process of the distribution network, that is, by using data such as the location of the fault area and the electrical status of the distribution network, it provides power to the fault-free area to restore the power supply to the power outage area. The difference between this stage and the second stage is that the second stage only realizes the management within the substation, while the third stage realizes the management of the feeder automation with full computer control, making the management system a higher level. In addition, the automation of distribution system management and dispatch can be realized on a larger scale by completing the mutual backup connection between the master stations. [b]3 Characteristics of voltage-type systems[/b] At present, the application of distribution network automation systems in China is roughly divided into two types: one is voltage-type systems and the other is current-type systems. The two systems have their own advantages and disadvantages. Here, we focus on analyzing the basic starting point of applying voltage-type systems to distribution networks. (1) From the perspective of the operation mode of the 10 kV distribution network, since most of the 10 kV systems in my country are currently neutral-point ungrounded systems, which are similar to the grid structure of Japan, voltage-type equipment is more suitable. In addition, this mode has nearly 30 years of operating experience in Japan. Therefore, it is more appropriate to apply it to the distribution automation system of 10 kV overhead lines. (2) From the perspective of the reliability of power system operation, the advantages of voltage-type systems are more prominent. Because in the second and third stages of automation development, fault isolation and location are still completed independently by primary equipment, while the communication system is mainly used to complete the load dispatching after power flow calculation and the daily dispatching, maintenance and operation functions, which improves the reliability of the entire system in handling accidents. From the perspective of fault judgment methods for pole-mounted equipment, voltage-type equipment only needs to judge whether there is power supply in the distribution line, while current-type equipment using RTU requires the cooperation of switch CT to judge the location and direction of fault current. The fault judgment criteria of RTU itself need to be set according to the load changes of the segmented section, which is quite troublesome for the distribution network with frequently changing loads; (3) From the perspective of maintainability of pole-mounted equipment, voltage-type equipment takes power from the line and does not need to provide additional power; while current-type equipment RTU can also use power transformer as the power supply during normal operation, but when the line is faulty, it must rely on battery power to ensure normal communication. The battery needs to be maintained and inspected regularly, which greatly reduces the maintenance-free nature of the primary equipment of the system; (4) From the perspective of line power restoration method, although the voltage-type system has more switching operations due to the step-by-step connection method, it is not as good as the current-type equipment in shortening the power outage time. However, in actual application, due to the reclosing of the circuit breaker and the step-by-step connection of the segmented switches, the circuit breaker can be effectively prevented from maloperating due to inrush current. 4 Selection of System Communication Method For feeder automation systems, the selection of communication channels is a key issue. When applied to power grids, although power line carrier, twisted-pair communication, radio and optical fiber can all achieve communication, the effects, economy and practicality of different methods are different. (1) Using power line carrier for communication has the advantage of not needing to run wires and being convenient and easy to implement when the system changes. However, the switching of electrical equipment will interfere with the signal transmitted on the power line, causing significant fluctuations in the transmission and attenuation characteristics of the network, resulting in serious signal interference. In addition, the power grid has many branches, and signal attenuation or signal blind spots are prone to occur at the segmentation points, resulting in low transmission reliability. The transmission speed is slow, and the cost of various coupling equipment is high. Therefore, power line carrier is not an ideal communication method for feeder automation systems. Although there are literatures that have proposed a major breakthrough in PLC technology (DPL technology) [4], it has not yet entered the stage of substantial use. (2) Using twisted-pair communication. This communication method requires additional communication lines, but it has high transmission reliability and moderate transmission speed. When expansion is needed, it is not limited by other factors. After comprehensively considering economy, reliability and practicality, it can be used as a feasible feeder automation communication method; (3) Wireless communication is used, which does not require wiring and is more convenient to use. However, this method has the following problems: First, it is necessary to comply with the regulations of the Radio Wave Law and apply for frequencies; Second, the development of urban telecommunications has caused a lot of radio wave interference and electromagnetic noise, which has resulted in a high bit error rate and decreased reliability; The RTU radio has a large transmission power and a high cost. In comparison, this method is more suitable for small and medium-sized cities with small areas and little air interference; (4) Fiber optic communication is a good communication method, but the amount of engineering work for laying optical cables and the cost of equipment such as optical transceivers are expensive. If it is only used for information transmission in the distribution network, the cost of the entire system is too high and the performance-price ratio is too low, which is uneconomical. Power departments in various places can choose a communication method suitable for the local geography and topography according to the requirements of local distribution network automation planning, and choose a communication method after comprehensively considering economy, reliability and practicality. 5 Conclusions (1) The voltage-type feeder automation system can be gradually implemented in three stages: pole-mounted distribution automation, telemetry and remote control automation, and computer-aided distribution automation. (2) This scheme is more suitable for the domestic 10 kV distribution network neutral point ungrounded system. The primary equipment is maintenance-free, easy to operate and set, and the primary equipment is used to isolate faults, while the secondary equipment is mainly used to complete monitoring, dispatching and computer management, ensuring high system reliability. (3) The choice of communication method is the key to the implementation of this scheme. According to the actual situation in different regions of China, different communication methods can be selected for feeder automation. Twisted-pair communication is a good communication method in terms of reliability, economy and practicality. References 1 Gao Yan. Seize the opportunity, implement the task, and accelerate the pace of urban power grid construction and transformation. Power Supply and Utilization, 1998(8) 2 Ohshima I, Fujisawa A et al. Switching performance of vacuum switch and insulation coordination in overhead power distribution system. Int. Symposium on Adoption of New Techniques for Power Distribution Systems. Dec 1987 3 Kato S, Naito T et al. Computer-based distribution automation. IEEE 1985 Power Industry Computer Application Conference 4 Song Yonghua, Xiao Ying et al. Major breakthrough in power line carrier technology. Power System Technology, 1999, 23(2)