The selection of information channels is one of the key aspects of distribution network automation. Due to the large volume and wide range of information involved in distribution network automation, coupled with economic and technological limitations, communication in distribution network automation must be multi-layered, employing different communication methods at different levels to form a flexible and effective automation system. At the terminal, substation or secondary master station, and control center levels, suitable communication media such as fiber optics, wired television, radio, medium and low voltage carrier waves, and microwave can all be found. Users can rationally select the communication method based on the application, transmission rate, real-time performance, reliability, and data volume requirements. Since the mid-1980s, the power sector has extensively installed and used radio load management systems to alleviate the imbalance between power supply and demand. After years of operation, radio load management systems have matured, and large urban power grid load control systems can simultaneously manage thousands of terminal devices. The radio power load management system uses a dedicated 230 MHz frequency band, which can be divided into 15 full-duplex communication frequencies. The load management channel has relatively few real-time tasks and low co-channel interference, making it perfectly suitable for distribution network automation. The advantages of this approach are: firstly, it has rich operational experience and mature technology; secondly, it shares the frequency band with the load management system, saving channel resources; and thirdly, when the system scale is small, it can share the frequency point with the original system and share some communication equipment, saving money. Based on the above ideas, a series of distribution automation projects were successfully implemented using the 230 MHz load management channel. Two application examples are introduced below. [b]1 Medium-voltage feeder automation scheme[/b] From the perspective of power grid operation control functions, medium-voltage feeder automation should complete the following three tasks: firstly, fault protection, that is, to isolate faults and automatically restore power supply to non-faulty sections; secondly, to monitor line operating parameters and the operating status of switching equipment and record faults; and thirdly, to realize remote control of line switching equipment, which is the basis of distribution network dispatch automation. The first task can be the field automation (pole automation) function provided by smart appliances, or the system function of distribution network SCADA, while the latter two tasks must rely on the distribution network SCADA system. The system introduced below uses the "pole automation + distribution network SCADA system" model. This scheme was implemented in Yanzhou, Shandong. The involved lines include four 10 kV outgoing lines, each taking power from two busbar sections of the same substation, forming two loops (as shown in Figure 1). The Yandong line also has a branch line within the loop. During normal operation, tie switches 1400 and 2200 are open; in the event of a fault or line maintenance, the corresponding tie switches can be closed to transfer the load. The automatic distribution switchgear is a Toshiba VSP5 JSAT type vacuum switch, which can automatically isolate line faults. The remote terminal is a Xinlian Electronics PD-0211 feeder monitoring terminal, equipped with a 230 MHz radio, which collects line current and voltage, performs over-limit timing based on remotely adjusted settings, and is also responsible for collecting switch status and executing remote control commands. [img=630,183]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9911/image11/20.gif[/img][align=left] Fig.1 Main circuit of feeder automation system using radio communication. The communication system architecture is shown in Fig.2. The feeder automation master station and communication front-end unit are connected to the dispatch center computer network. The master station radio communicates with the terminal through a dedicated 230 MHz channel. The substation reclosing action signal is reported to the master station via a modem and twisted pair cable. The front-end unit and master station radio are equipment from the power supply bureau's existing load control system and do not need to be purchased separately. [/align][img=225,205]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9911/image11/20-1.gif[/img] Fig.2 Communication structure of the feeder automation system System function settings: (1) Remote control operation of switch closing (unlocking) and opening (locking). (2) Remote signaling switch position change and energy storage status. (3) Real-time remote measurement of line voltage, current (two phases), active/reactive power, power factor, energy, etc. (4) Data storage of hourly data is the same as remote measurement data. Daily data: energy, maximum power, maximum demand, maximum voltage and occurrence time, cumulative time of power and voltage over-limit, number of trips and terminal resets. Monthly data: same as daily data. (5) Equipment management is combined with the management information system to manage feeder equipment data, such as conductor type, length, switch type, commissioning time, number of operations, maintenance records, etc. (6) Reports can automatically generate and print daily and monthly operating parameter reports, operation records, etc. After a fault occurs, the substation reclosing monitoring terminal reports the reclosing action information, the main station immediately inspects the line, judges the fault section according to the switch status and issues an audible and visual alarm. More than a year of operation shows that the system has never experienced a switch failure due to a broken channel or a switch malfunction due to an unreliable channel. [b]2 Distribution Transformer Monitoring and Meter Reading System Scheme[/b] The operating data of the distribution transformer is an important basic data of the distribution network. Real-time monitoring of the load and oil temperature parameters of the power grid can provide a guarantee for the safe operation of equipment and the quality of power supply, and provide a scientific basis for load forecasting, system capacity expansion and new user connection. Considering the dispersion of transformers (substations), a hybrid communication mode of wireless and wired is adopted as shown in Figure 3. In the diagram, the main station and the front-end unit are connected to the computer network of the load management system. The front-end unit and the 230 MHz radio share the load control system equipment. The wireless terminal (PD-0210 type) can communicate directly with the main station radio. For transformers with relatively centralized installation and conditions for laying communication cables, a wired terminal (YPDJ-I type) is used with the communication controller. The communication controller acts as an intermediate station, completing data paging storage and wireless-to-wired channel conversion. [img=301,181]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9911/image11/20-2.gif[/img] Fig.3 Supervising system for MV/LV distribution transformer Public transformer areas (stations) may be equipped with a data collection unit for a low-voltage carrier meter reading system. The above-mentioned wired and wireless terminals can forward the data from the data collection unit through the 485 interface. The distribution transformer monitoring system has the following functions: (1) Real-time operating parameters of the transformer include three-phase voltage, current, active/reactive power, active power, power factor, maximum demand, oil temperature, and three-phase imbalance rate. (2) Hourly data includes three-phase voltage, current, active/reactive power, and active power. (3) Extreme value records include the extreme values ​​of the following quantities and their occurrence times: three-phase voltage, current, active/reactive power, and oil temperature. (4) Limit overrun records include the cumulative time of voltage exceeding the upper and lower limits, the time of voltage exceeding the safe voltage, and the time of current exceeding the set value by 70%, 80%, 90%, and 100%. (5) Power outage records include the time of transformer power outage and power restoration (save 5 times) and phase loss power supply records. (6) Automatic time synchronization and remote adjustment of set values. (7) Forwarding of meter reading data. The monitoring system with the above functions (1) to (6) has been in operation for more than two years. The operation shows that it is entirely feasible for the system to use the 230 MHz wireless channel. The system with (7) functions is about to be put into operation. [b]3 Issues to be noted when applying radio to power distribution automation[/b] (1) Utilization of channel resources 230 MHz radio is a dedicated frequency band for power systems. If the radio equipment is shared with the load management system, no additional frequency point needs to be applied for. If the system is large in scale and has high real-time requirements, new frequency points should be considered. (2) Formulation of specifications When using 230 MHz radio to implement power distribution automation, it is necessary to ensure that the new automation project is compatible with the power load management system. The system specifications must be considered in a comprehensive manner. Since the power load control specification follows the IEC870.5.1:1990 standard [4] and adopts the FT1.2 frame format, before the national standard for power distribution automation specifications is officially issued, the improved load control specification can be considered for use in power distribution automation. (3) Reliability of channels When wireless channels are used for real-time control, strict reliability must be guaranteed, especially when the geographical and electromagnetic environment is complex, the communication network must be robust. When formulating a plan, a detailed field strength test should be conducted, and secondary stations and relay stations should be reasonably arranged to strengthen the communication network when necessary. (4) Real-time response speed In feeder automation, there are many real-time query and control tasks. The delay time of the wireless system mainly includes the data delay of the radio, the RTS/CTS delay, the transmitter power-on delay, the signal transmission delay, and the data processing delay between the master station and the RTU. Among them, the delay caused by the radio is relatively large, which may reach hundreds of ms. The reclosing delay of the outgoing line switch in a general substation is 0.3 to 0.5 s. If there are 3 segmentation points on an outgoing line, the fault handling time of each segmentation point is only 0.1 to 0.2 s. After deducting the feedback verification and the action time of the segmenter, the actual usable communication time of each segmentation point is even shorter. Modern high-quality radios can meet the above application requirements, with data delay time below 10 ms and transmitter startup delay of approximately 2 ms. They can complete full-line inspection within a 300 ms reclosing interval, identify and isolate faulty sections, and ensure successful reclosing on the first attempt. [b]4 Conclusion[/b] The 230 MHz radio channel has been successfully applied to distribution network automation technology, realizing medium-voltage feeder automation and distribution transformer monitoring. Operational experience shows that radio, as a mature communication technology, can be used for distribution network automation. The 230 MHz load management channel can meet the real-time and reliability requirements of distribution network automation systems, and has advantages such as mature technology, saving frequency resources, and saving channel equipment investment. [b]5 References[/b] 1 Urban Power Supply Professional Committee of China Electrotechnical Society. Guidelines for Planning and Design of Distribution System Automation (Trial, Draft for Review). 1998 2 Gu Jinwen, Zhang Bulin. Distribution Automation. Automation of Electric Power Systems, 1999, 23(5) 3 Cheng Zhongyuan, Xu Sujun, Huang Jianguo. Discussion on FI/SR in medium-voltage distribution network automation scheme. Nanjing: The Second Intelligent Electrical Appliance Symposium, 1998, 9 4 Safety and Equipment Department of the Ministry of Electric Power. DL535-93. Data transmission protocol for power load control system (revised edition), 1996, 8