In the past, most of my country's power distribution networks adopted radial power supply. This power supply method can no longer adapt to the development of the social economy and meet the power supply quality requirements of users, because once a line fault occurs at a certain point, it will cause the entire line to go out of power, and the maintenance time due to the inability to quickly determine the fault point is too long, which greatly reduces the reliability of power supply [1]. For this reason, the power supply network now widely adopts ring network connection, that is, two lines are connected by a tie switch in the middle. During normal operation, the tie switch is in the open state and the system operates in an open loop; when a fault occurs in a certain section, the load can be transferred through network reconstruction to ensure the normal power supply of non-faulty sections, thereby greatly improving the reliability of power distribution network. At present, my country has invested huge sums of money to transform urban and rural power grids to improve the reliability of the entire power system. Under this situation, choosing a power distribution method that conforms to the actual situation of my country's power industry and has both high reliability and good economy is an urgent task before us. [b]1 Location, isolation and power restoration mode of feeder fault[/b] Distribution network automation mainly includes substation automation and feeder automation. Power outages caused by feeders occur frequently in power distribution networks. After a fault occurs, how to restore power supply as quickly as possible is an important aspect of feeder automation. In fact, the most fundamental task of power distribution automation is to locate, isolate, and restore power supply in the shortest possible time. Their development can be divided into three stages [2]: (1) Using fault indicators installed on the power distribution line, power maintenance personnel locate the fault section and manually isolate the fault section using pole-mounted switchgear to restore power supply to the normal section. This method results in a long outage time and slow power restoration. (2) Using intelligent switchgear (such as reclosers, sectionalizers, etc.), the fault is automatically isolated and power is restored on-site through their mutual cooperation. This method has a high level of automation, can achieve control functions without communication, and has a low cost. The disadvantage is that the switchgear needs to increase the number of closing and opening actions to complete the fault isolation and power restoration. (3) Integrating switchgear and feeder terminal units (FTUs) into intelligent devices with data acquisition, transmission, and control functions, and communicating with the computer control center in real time, and being centrally controlled by the control center in a remote control manner. This method utilizes advanced computer and communication technologies to complete fault location, isolation, and power restoration in one go, avoiding multiple impacts of short-circuit current on lines and equipment. Its main drawbacks are: reliance on communication, complex structure, and numerous factors affecting the reliability of the power distribution system. The purpose of power distribution network feeder automation is to improve power supply reliability; therefore, while the system's functionality is important, its operational reliability and economy are the most pressing concerns for the power sector [2]. Therefore, relatively speaking, the second of the three models best suits the actual situation of my country's power industry. Its main features are: (1) It can use the recloser itself to cut off the fault current, realize the local isolation of the fault, and reduce the power outage range; (2) It does not require communication means, and can use the recloser to reclose multiple times and the mutual coordination of the protection action time to realize the automatic location, isolation and power restoration of the fault; (3) It can directly obtain power from the power grid and does not require an external uninterruptible power supply; (4) It has strong anti-interference ability against overvoltage, lightning, high frequency signals and strong magnetic fields, and high reliability; (5) With the addition of communication equipment, it can be easily upgraded to the above third mode, so that the distribution network automation can be carried out step by step. [b]2 Comparison of several feeder automation modes mainly composed of reclosers and sectionalizers[/b] In the ring network distribution mode mainly composed of reclosers and sectionalizers, it can be divided into three modes: circuit breaker + voltage type sectionalizer, recloser + sectionalizer (using sectionalizer as a connection), and completely using reclosers. Each of these modes has its own advantages and disadvantages, which are analyzed in detail below [3]. (1) Distribution mode of “circuit breaker + sectionalizer” and “recloser + sectionalizer (with sectionalizer as connection)”. Features: No communication equipment is required. The line is segmented by the sectionalizer. The voltage signal is detected by the sectionalizer. According to the voltage application time limit, the fault is automatically isolated by multiple reclosing through the circuit breaker or recloser. The investment is low and it is easy to coordinate. Disadvantages: Fault isolation requires multiple reclosing, which increases the number of impacts on the system; when isolating a fault, it will affect the non-faulty section, causing the non-faulty section to lose power; the longer the feeder and the more segments, the longer the step-by-step delay time, which makes the time required to restore power supply longer. (2) Distribution mode of “completely using reclosers”. Features: No communication equipment is required. The recloser itself cuts off the fault current. Through multiple reclosing and the mutual coordination of the protection action time limit, the feeder fault is automatically isolated on the spot, avoiding the situation where the entire line loses power due to a fault in a certain section. At the same time, it reduces the number of outgoing switch operations. Disadvantages: High investment; the more segments, the more difficult the protection coordination; the longer the instantaneous overcurrent protection delay of the substation outgoing line switches; and the greater the impact on the system when a fault occurs at the outgoing line. To address the advantages and disadvantages of the above three power distribution methods, we designed a new and more practical power distribution mode: the outgoing lines of the two substations in the ring network power supply are modified ordinary reclosers, and the intermediate tie switch is a tie-type segmented recloser (combining the functions of a tie switch, segmenter, and recloser). The line is segmented by the modified segmenter. Although this method still consists of reclosers and segmenters, by improving these reclosers and segmenters and using the tie-type segmented recloser as a tie switch, this coordination method can possess the advantages of the above three modes while avoiding most of the disadvantages. The system wiring is shown in Figure 1. [img=527,109]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2000-7/34-1.jpg[/img] The following describes the working process of this mode using transient and permanent faults in section b of the line. Assume a transient fault occurs in section b. After VW1 trips, it recloses after a delay of T1. After QO1～QO3 loses voltage, it trips again after a delay of T2. Setting T1 < T2, when VW1 recloses, QO1～QO3 have not yet completed their tripping action and are still in the closed state. In this way, VW1 can clear the transient fault within T1 (0.5s), avoiding the step-by-step delay of the sectionalizer and greatly reducing the power outage time when a transient fault occurs. Assume a permanent fault occurs in section b. After VW1 recloses once, QO1 closes and locks out. VW1 then recloses again, and power is supplied from substation 1 to section a. During this process, QO2 detects a short-lived small voltage. QO2 performs closing and locking out simultaneously with QO1, thus locking out both ends of the faulty section b, achieving fault isolation. After the fault occurs, VW3 closes after a delay of XL after detecting a single-sided loss of voltage. QO3 also closes after a delay of X after VW3 closes, and power is supplied from substation 2 to sections c and d. If a fault occurs in section c or d during this process, or if QO2 fails to complete closing and locking out (the probability of this happening is extremely small), VW3 will detect the fault after closing and trip again. After the first reclosing, fault isolation and power restoration are achieved. Therefore, in either case, this distribution mode can avoid power outages on the line section from VW3 to substation 2. In other words, isolating the faulty section will not affect non-faulty lines, preventing unnecessary power outages on non-faulty line sections. After a fault occurs, while the reclosers and sectionalizers on the line operate, the fault locators installed inside the substation, based on the coordinated action times of each switching device, can quickly determine the precise location of the faulty section for maintenance. As the above analysis shows, although this power distribution method cannot complete the location, isolation, and restoration of power supply to the fault in one go, it can quickly isolate transient faults; in the event of a permanent fault, it can simultaneously lock out both ends of the faulty section. Compared to the traditional "recloser + sectionalizer" power distribution method, this method shortens the power outage time and reduces the number of short-circuit current impacts on the line. Because reclosers are only installed at the substation outgoing terminals and in the middle of the line, protection coordination is easy to achieve; although the line has many sections, the instantaneous overcurrent protection delay of the substation outgoing circuit breakers does not need to be too long, so the impact on the power distribution system when a short circuit occurs at the substation outgoing terminals is relatively small. At the same time, since sectional reclosers are used as tie switches, power outages in non-faulty sections are avoided when isolating faults. In addition, although this power distribution method is not as fast and powerful as the third power distribution mode, it has its own advantages. It does not require communication equipment and relies entirely on the intelligent switching equipment in the line to complete the location, isolation and power restoration of the fault on the spot, which simplifies the structure of the power distribution system and greatly reduces the factors affecting reliability. Moreover, these intelligent switching equipment have communication interfaces, and if necessary, communication functions can be easily added to make the feeder automation of the power distribution network reach a higher level. 3 Measures to improve reliability and reduce line outage time For power distribution automation, the degree of automation and the strength of functions are important, but the reliability of the entire system should be the first priority. In addition, economic efficiency should also be considered [2]. In order to ensure the reliability of the “recloser + sectionalizer” mode with the sectional recloser as the tie switch introduced above, the following measures were taken: (1) The switch body of the recloser is a vacuum circuit breaker, which adopts the patented technology of vacuum arc extinguishing outdoor composite insulation. It has the advantages of being oil-free, gas-free, maintenance-free, long-lasting, fire-free, and explosion-free. The mechanism adopts a spring operating mechanism for rapid energy storage of motors, eliminating the need for high-voltage closing coils. (2) A high-performance PLC (Programmable Logic Controller) is selected as the control center for the recloser and sectionalizer. This simplifies the peripheral circuitry and greatly improves the overall reliability and anti-interference capability. (3) Power is obtained directly from the line without any external power supply. American switching power supply modules are selected, which have strong anti-interference capability, a wide operating range, and can output a stable rated voltage within the input range of 30% to 120%. In addition, there are redundant design and derating measures, which can also improve the overall reliability. In order to reduce the power outage time in this power distribution mode, the following measures were taken: (1) Quickly clearing transient faults to reduce power outage time. In the power system, 62% to 85% of line faults are transient faults. If transient faults are treated as permanent faults, it will cause a long-term (tens of seconds or more) power outage. Therefore, an optional first-time fast reclosing function has been added to the recloser, and a delayed tripping function after complete loss of voltage has been added to the sectionalizer. These two work together to clear transient faults within 0.5 to 1 second, greatly reducing the power outage time during transient faults. (2) Both ends of the fault section are simultaneously locked. When a fault occurs in the traditional sectionalizer, only one end of the faulty line can be locked at a time. The improved sectionalizer can isolate both ends of the faulty section at the same time when a permanent fault occurs in the line, avoiding power outages in non-faulty sections, shortening the time to restore normal power supply, and reducing the number of reclosing times of the recloser or circuit breaker, thus reducing the impact on the system accordingly. (3) Inrush current avoidance function. The most important loads in the power distribution system are transformers and high-voltage motors. Therefore, when the recloser is closed for the first time or reclosed, a starting current much higher than the rated current will appear, which may cause the recloser to malfunction. The improved recloser has added inrush current avoidance measures in both software and hardware. It can automatically distinguish between the inrush current generated by closing and the fault current, and effectively solve the inrush current problem. [b]4 Conclusion[/b] This paper introduces three development stages of distribution network feeder automation. After comparison, it is believed that the distribution system mainly composed of "recloser + sectionalizer" is more in line with the current situation of my country's power industry. Several methods of distribution network feeder automation mainly composed of "recloser + sectionalizer" are analyzed, and a new and practical distribution method is proposed. It can reduce the power outage time and the number of short-circuit current impacts on the line during faults, and it is easy to achieve the coordination of protection time. This distribution mode has been piloted in Huangyan Power Supply Bureau, Zhejiang Province. So far, the operation effect is satisfactory and has met the design requirements. [b]References:[/b] ［1］ Sun Jisheng. Research and application of 10kV ring network power supply technology［J］. China Electric Power, 1999, 32 (2). ［2］ Secretariat of the Distribution Automation Subcommittee of the Automation Committee of the Chinese Society for Electrical Engineering. Issues of concern in the academic discussion of the Distribution Automation Subcommittee［J］. Power System Technology, 1999, 23 (1). [3] Lin Gongping. Distribution network feeder automation technology and its application [J]. Automation of Electric Power Systems, 1998, 21(4).