With the development of science and technology, the automation of medium and low voltage distribution networks has become a trend in power system development. As the last link in the power transmission and distribution system, the degree of automation of medium and low voltage distribution networks is closely related to the quality and reliability of power supply. Therefore, this paper briefly discusses the necessity of automation of medium and low voltage distribution networks and its implementation schemes. 1. Necessity of Automating Medium and Low Voltage Distribution Networks 1.1 Automating medium and low voltage distribution networks is a requirement for improving people's quality of life and developing the national economy. In modern society, the quality of power supply not only reflects the quality of life, standard of living, and investment environment of a country or region, but is also an important factor affecting economic development, determining the direction and scale of industrial development. In fact, with the advent of the information age, more and more computer devices require uninterrupted power supply, placing higher demands on power supply. Power outages or power rationing can lead to reduced production, while sudden power outages can damage important factory equipment. Only by achieving automation of medium and low voltage distribution networks can the quality of power supply be maximized to meet the needs of people's daily life, work, and production. 1.2 Achieving automation of medium and low voltage distribution networks is a necessity for the development of power enterprises. Automation of medium and low voltage distribution networks can improve the quality and reliability of power supply. It can reduce the number of faults, narrow the scope of accidents, and shorten the time between accidents, providing effective evidence for power restoration, rapid analysis, diagnosis, and reporting of accident causes. Automation of medium and low voltage distribution networks can improve the economic efficiency of the entire power system: reduce the labor intensity of maintenance personnel; reduce the number of operators; enhance the maintenance-free nature of the power system; and improve the safety and health of equipment, extending its service life. Automation of medium and low voltage distribution networks can improve the management level of the entire power grid. This mainly includes: providing more detailed and richer data and information for the automatic, accurate, and timely computer management of the power system; enabling planned power outages and power supply to any location and any user; and facilitating convenient and intuitive monitoring of the power consumption and supply status of each user within the overall system, achieving overall control. 1.3 Medium and low voltage distribution networks are a weak link in my country's distribution network automation. Although the construction of distribution network automation in my country started relatively late, it has undergone nearly 20 years of research and practice, achieving initial results. However, most research and practical results are conducted at the high-voltage distribution network (above 35 kV), while the automation of medium and low-voltage distribution networks (at the substation level) remains largely unexplored, lacking both overall planning and unified technical principles. Furthermore, current vertical monitoring is generally limited to the area before the substation's outgoing lines. For monitoring of the area from substation feeders to end users, which falls under the scope of power management, aside from load control for a few large users, there are no other monitoring methods. 2. Medium and Low-Voltage Distribution Network Automation Schemes 2.1 Comparison of Existing Power System Automation Schemes The automation of medium and low-voltage distribution networks (mainly referring to switching stations, switch rooms, and substations) and substations share certain similarities. Therefore, analyzing the implementation methods of substation automation is crucial for correctly determining the automation scheme for medium and low-voltage distribution networks. A substation automation system consists of five parts: a master station, remote terminal units (RTUs), line sensors, remote control SF6 or vacuum switches, and communication cables. The RTU (Remote Control Unit) is located at the substation site and can automatically collect various switch status quantities (remote signaling) and analog quantities (remote measurement), transmitting them to the main station system of the monitoring center via a dedicated channel. Some RTUs can also execute specific remote control operations according to the intentions and instructions of monitoring personnel and send the operation results back to the main station system of the monitoring center. From the functions achievable by substation RTUs, substation automation includes three aspects: remote signaling, remote measurement, and remote control. In addition, some systems can also realize the total electrical energy summation function based on the results of remote measurement. Correspondingly, substation automation systems can be divided into two categories: one category only implements the functions of remote signaling and remote measurement, i.e., the traditional SCADA system; while the latest SCADA system belongs to the other category, which should be able to realize all three remote functions. These two types of systems correspond to different stages and levels of power system automation. From the perspective of the method of implementing remote measurement by substation RTUs, there are two implementation schemes: a) DC sampling scheme: This type of RTU device first uses a transmitter to convert AC to DC before collecting analog quantities, and then uses an RTUA/D conversion element to represent the DC quantity as a digital quantity. The device primarily uses analog circuits, supplemented by a small amount of digital circuits. Its characteristics include the need for converters, a lack of advanced digital processing units (CPUs, etc.), difficulty in reflecting instantaneous changes in analog quantities, inability to perform harmonic analysis, and a relatively complex and difficult implementation of the total electrical energy summation function. b) AC Sampling Scheme: This type of RTU device directly uses A/D conversion elements to collect and calculate AC electrical quantities, eliminating the need for conversion equipment such as transmitters. However, it requires a fast digital processing unit to analyze and synthesize the collected data. It can not only reflect instantaneous changes in electrical quantities but also perform harmonic analysis, calculate frequencies, and simply implement the total electrical energy summation function. These devices often use microcomputers (such as 8x86) in conjunction with multiple microcontrollers (such as 8051, 8098, etc.) and a large number of A/D conversion circuits to acquire switch and analog quantities. Currently, with the full development and application of digital technology, the AC sampling scheme is a reasonable choice for distribution network automation. It primarily uses digital circuits, supplemented by a small amount of analog circuits, offering powerful functionality, easy expansion, significantly improved reliability compared to the DC sampling scheme, and low overall cost. 2.2 Application Characteristics of Medium and Low Voltage Distribution Network Automation A medium and low voltage distribution network automation system consists of five parts: a master station, remote terminal units (RTUs), line sensors, remote control SF6 or vacuum switches, and communication cables. The application of medium and low voltage distribution network automation has its own distinct characteristics: a) Traditional substation RTUs emphasize remote signaling and telemetry, but the number of automated objects (switch rooms, substations, and distribution rooms) in medium and low voltage distribution networks is large, and switching operations are frequent, making remote signaling and remote control functions more important. b) The automated objects in medium and low voltage distribution networks are distributed across various environments, including urban and rural areas, and are operated by different levels of power management personnel (including rural electricians). Therefore, it requires features such as flexible installation, ease of operation, no professional maintenance required, and resistance to harsh environments. c) RTUs applied to medium and low voltage distribution networks should have a modular structure, and the simpler and more reliable the hardware, the better. Ideally, a single set of simple hardware should be sufficient to meet the requirements of different occasions and scales with only simple configuration. Therefore, it is necessary to develop new RTUs with structures different from the traditional ones to suit the characteristics and needs of medium and low voltage distribution network automation. 2.3 PLC Implementation of RTUs for Medium and Low Voltage Distribution Network Automation Programmable Logic Controller (PLC) technology has matured considerably after decades of development. It offers a complete range of models and numerous functions, and is widely used in various fields of industrial control. Using PLCs to implement the RTU functions for medium and low voltage distribution network automation can well meet the unique requirements of RTUs. In the domestic market, there are PLC products from many well-known manufacturers. These products range from simple to complex, each forming its own series to meet the specific requirements of different applications. Most mid-to-low-end PLC products include discrete point inputs and outputs (the number of points can be increased or decreased depending on the application), analog sampling inputs, clock, and communication functions. Utilizing the existing functions of these PLCs, the RTU functions for medium and low voltage distribution network automation can be easily implemented. Remote signaling is achieved using the PLC's discrete input points, remote control is achieved using the PLC's discrete output points, telemetry is achieved using the PLC's analog sampling inputs, and communication with the host computer is achieved using the PLC's communication function. All these functions can be accomplished without additional hardware; only simple programming of the PLC based on the actual conditions of the switch room is required. Furthermore, the PLC's analog output function can even be used to remotely adjust the distribution network. For example, adjusting the turns ratio of the voltage regulating transformer, and adjusting the voltage, current, and phase angle of the static var compensator. This PLC-based RTU implementation scheme for medium and low voltage distribution network automation can fully meet the special requirements of medium and low voltage distribution network automation. It has the following characteristics and advantages: simple hardware structure, completely maintenance-free; scalable to various sizes, simply by connecting the PLC's expansion modules together to increase remote control points, remote signaling points, and telemetry points; resistant to harsh environments; high reliability; programming to implement various functions, eliminating the need for hardware debugging; and low cost. The specific design of the PLC scheme includes the following steps: a) Obtaining the number of operation points. a) Understand the basic situation of the power distribution network and the specific requirements for automation. Determine the equipment that the system needs for remote control, remote signaling, remote measurement, and even remote adjustment. Count the specific number of points in each power distribution room that require these four types of signals. b) Determine the communication scheme. Based on the scale and distribution of the power distribution network, determine the overall design scheme, mainly the design and selection of the communication scheme. c) PLC selection. Based on the number of operation points in each location and the determined communication scheme, select an appropriate PLC model to implement the RTU function. Since the RTU needs to receive instructions from the monitoring center and upload information from the power distribution network and switchgear, communication functionality is a major consideration in PLC selection. Because the operation types and number of operation points in each switchgear and switchgear often vary greatly, whether the PLC has a modular structure and configuration capabilities, and whether it can flexibly and economically form a variable-scale system with input points, output points, measurement points (A/D), and adjustment points (D/A), is another major consideration in PLC model selection. Currently, many manufacturers' products can meet the requirements for communication and modularity. For example, SIEMENS' S7-214 and higher series, Mitsubishi's A1S series, and Panasonic's higher-level PLC series, etc. Depending on the specific situation, in a distribution network automation project, the entire distribution network system can use PLCs from the same manufacturer, or different manufacturers' PLCs can be selected according to the specific conditions of the distribution room, so as to take advantage of the advantages and features of each manufacturer's PLCs. 3. PLC Implementation of RTU Function The implementation of RTU function in PLC includes two aspects: hardware implementation and software implementation. 3.1 Hardware Implementation In terms of hardware, the main issues are how to provide power to the PLC, how to achieve long-distance communication, and how to specifically implement remote control, remote signaling, remote measurement, and remote adjustment. Since PLCs have matching dedicated power supply modules, when designing the RTU, the main consideration should be the power supply problem of the PLC after the power grid is cut off, which is usually solved by configuring rechargeable batteries. Generally, the communication module of PLC only has short-distance communication capability. Although some companies provide matching networking modules for PLCs, the communication distance is limited to within a few kilometers. The characteristics of a power distribution network are numerous points and a wide area; therefore, it is necessary to utilize other methods to extend the communication distance of the PLC. Many methods exist, including telephone modem solutions, dedicated line modem solutions, wireless solutions, paging station service solutions, and fiber optic solutions. In the same power distribution automation project, a single method or a combination of methods can be used depending on the specific circumstances. Regarding the four remote operations of the RTU, since the PLC's voltage level and power capacity cannot perfectly match the operating equipment, and electrical isolation is required, auxiliary modules and devices for potential conversion, power amplification, and electrical isolation must be added. For remote control, when the PLC receives a switching command, the path from the output point to the internal power supply is connected or disconnected. If the output current of the output point is directly used to operate the switching equipment, the power is insufficient. Therefore, the PLC's output point can be used as the excitation power supply for a small-power relay to control the opening and closing of the relay's normally open or normally closed contacts. This relay then controls the operating power supply of the power distribution switch in the power distribution network, causing the switch to operate and the line or power distribution equipment to be switched on or off. For remote signaling, the leads at both ends of the auxiliary contacts of the switch under test are connected to the input point and ground of the PLC. When the power distribution switch operates, the auxiliary contacts open and close accordingly, and the corresponding input point of the PLC is disconnected or short-circuited with ground, thus obtaining a high or low level inside the PLC. For telemetry, the signal from the current transformer must fall within the measurement range of the PLC's A/D conversion module before it can be connected to the input terminal of the corresponding module. In addition, when selecting the PLC's A/D module, the sampling period must also be considered. If the period is too long, accurate values ​​cannot be obtained. PLCs can realize remote adjustment functions, but since their application in power grids is rare, they will not be discussed in detail here. 3.2 Software Implementation In terms of PLC software, since PLCs execute programs in two ways, namely cyclic scanning and interrupt, in order to complete all RTU functions, the PLC software should include: a main program executed by cyclic scanning; a communication program (receiving and sending messages); a received message analysis program; an uploaded message generation program; an input point level interrupt scanning program; and an operation execution program (remote control, remote signaling, telemetry, etc.). In the programming of the above-mentioned modules, the following issues should be considered: a) Is the speed of the PLC's main CPU fast enough? How to program with short execution time? b) Communication between the PLC and the monitoring center requires a complex communication protocol. Can the PLC's program capacity accommodate all the programs? How to program concisely and efficiently? c) The PLC obtains the input status of the input points by cyclically scanning the memory image of the input points. When the power distribution switch operates, the corresponding auxiliary contacts often exhibit brief jitter. Although the mechanical frequency of the jitter is high, it is very low relative to the frequency of the PLC's program scanning execution. Therefore, this jitter will be reflected in the PLC's memory image as multiple unrelated switching actions. How to eliminate this illusion of switching actions in the program? Practice has proven that by using appropriate programming techniques, all of the above problems can be satisfactorily solved. 4 Conclusion Realizing the automation of China's medium and low voltage power distribution network is necessary to improve power supply quality, power consumption reliability, and the overall level of power companies. Using a PLC to implement the RTU function of a medium- and low-voltage power distribution network is a promising solution due to its simplicity, reliability, and ease of use.