0 Introduction Most of the 10 kV distribution networks in urban and rural areas of China adopt radial power supply systems, which are dendritic in shape with equipment distributed along the lines, resulting in significant dispersion. Once a fault occurs, it is often difficult to locate the fault point by manpower searching within a radius of several kilometers or even tens of kilometers, leading to long power outages, large affected areas, and serious impacts on users. In response to the existing situation of the distribution network, we have successfully developed a "Power Grid Fault Microcomputer Control Alarm System." After a fault occurs, the system can quickly transmit fault information to the dispatch center via automatic alarm units installed in the switching stations and determine the fault area, thereby greatly improving the reliability and efficiency of the distribution network. [b]1 System Principle[/b] This system mainly consists of two parts: one part is the automatic alarm unit located in the switching stations as the control level; the other part is the microcomputer monitoring center located in the dispatch center as the management level. Both parts are equipped with modems and are connected via the public telephone network. When a fault current occurs, the signal relay in the automatic alarm unit of the switching station activates. The main controller collects, organizes, and processes the information, immediately activates the modem to dial, connects to the dispatch center's microcomputer system via the public telephone network, and then transmits the fault information to the dispatch center. After this data is organized, recorded, and analyzed by the microcomputer monitoring center, an alarm can be triggered for the dispatch center operator. Since multiple switching stations along a feeder are equipped with such alarm units, the operator can identify which section has experienced a ground fault through the display. As shown in Figure 1, if a fault occurs in the cable downstream of switching station 7, the alarm units at stations 1, 2, and 7 will all sound, thus confirming that the ground fault occurred between switching stations 7 and 8. [img=254,174]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9912/image/39-1.gif[/img] Fig.1 The tree style of power supply network. Under normal circumstances, in order to check the working status of the alarm unit, self-test communication can be performed periodically. The time interval of this self-test can be adjusted by the dispatch center. The automatic alarm unit placed in the unmanned switch station is designed with a modular combined structure, which is convenient for capacity expansion, function expansion and maintenance. Various forms of hardware and software anti-interference measures are adopted, and it has strong self-diagnostic function. Once a problem is detected, it will automatically notify the monitoring computer of the dispatch center immediately. In view of the current development status of distribution network management automation, some data of the system is processed in the form of a general database to provide data sharing, so that the information of the most basic level of the distribution network can be easily sent to other computer networks such as the "Distribution Management System (DMS)". [b]2 System Configuration and Electrical Block Diagram[/b] The automatic alarm unit consists of four parts, as shown in Figure 2. [img=319,143]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxtzdh9912/image/39-2.gif[/img] Fig.2 Structure of auto-alarm unit a. Zero-sequence current detector: When a short-circuit fault occurs in the cable line, the short-circuit current is detected by the zero-sequence current transformer and connected to the automatic alarm unit. It is then sent to the overcurrent detector through the secondary current transformer (TA) in the automatic alarm unit. The overcurrent detector determines whether the zero-sequence current exceeds the set value. If it does, a signal is sent out. b. Main controller: When the signal from the overcurrent detector is received, a delay is started. If the duration of the overcurrent signal exceeds the set delay, the main controller sends an alarm to the dispatch center through the Modem. c. Modem: Features automatic dialing and enables communication via the public telephone network under the control of the main controller. d. Backup Battery: Automatically engages the backup battery when the 220V AC power supply fails, allowing the alarm unit to continue operating. Normally, the 220V AC power supply provides float charging to the battery through a voltage regulator. [b]3 System Software[/b] 3.1 8031 ​​Microcontroller Software This software features self-testing, testing, and fault alarm functions. Because several switching stations may simultaneously dial to the microcomputer monitoring center, to prevent missed alarms, relevant information is retained in the 8031 ​​microcontroller for a period of time. It also records faults occurring several times consecutively (including the local time at the time of the fault) and sends them out together once communication is established. During dialing, if the dispatcher is busy, the call will automatically hang up, pause for 5 seconds, and then dial again until the call is connected and the relevant information is sent (this process can last up to 10 hours). Depending on user needs, the self-test interval can range from 12 hours to several days. The dispatching terminal microcomputer monitoring center can monitor the entire communication process from a PC. 3.2 Dispatch Terminal Microcomputer Monitoring Center System Software The dispatching terminal microcomputer monitoring center system software is built on the Windows 95 Chinese platform and is written in Visual Basic for Windows programming language. The Windows 95 Chinese operating system is universal and standardized, and can be applied to various hardware platforms. It ensures the portability and compatibility of the application software, allowing users to reap long-term benefits from their investment. Windows 95 provides powerful multi-tasking parallel processing capabilities, supports networks, has an excellent multi-window graphical interface, and is familiar and mastered by most operators. The entire monitoring software is written in Visual Basic. Visual Basic is a modular visual programming language developed on Windows, providing a large number of tools for developing applications in Windows, and is particularly suitable for interface creation in the Windows environment. a. Communication Function: Real-time reception of alarm, self-test, retransmission, and other information. b. Database Management Function: Processes the information collected by the system (including distribution network feeder conditions, faults, system self-tests, etc.) into a standard database format, facilitating sharing with MIS at all levels via the network. c. Query, display, and print functions: Real-time display and printing of alarm and self-test information for each switch station and feeder; historical data can be queried, displayed, and printed. d. Online editing or modification of switch station attributes: For example, switch station number, substation to which the switch station belongs, and switch stations connected before and after the 10 detectors. e. Online editing or modification functions: Including modification of the dispatch center telephone number, setting of detector delay time intervals, and setting of self-test time. f. Online editing or modification functions for other basic parameters: Including input and change of passwords, input and change of display time for various abnormal situations, etc. g. Simulation operation function. [b]4 Field Operation Status[/b] The device has been operating in the field for nearly 2 years. During this period, tests were conducted on the alarm unit with action and non-action values. The operation showed that the automatic alarm unit is accurate, sensitive, and highly reliable; communication is rapid, reception is accurate, and judgment is error-free. The action value is zero-sequence current of 47 A, and the non-action value is zero-sequence current of 42 A. During this period, a grounding fault occurred in the section of the distribution network equipped with the automatic alarm unit. The automatic alarm unit proved to be able to report fault information to the dispatch center in a timely and accurate manner, without any malfunctions or failures. This system connects to the public telephone network via a modem. Due to the modem's poor lightning protection, it was struck by lightning in some cases. This problem has since been resolved through improvements. A relay, controlled by the main control board, was added between the modem and the public telephone network. The relay only connects the modem to the public telephone network when an alarm signal is received. This effectively solves the lightning protection problem because the modem is not permanently connected to the public telephone network.