With the rapid development of power system automation technology, power grids in various regions are building and implementing unmanned substations. Substation automation systems are the mainstream development trend for newly built unmanned substations, and the technology is becoming increasingly mature. In particular, the hierarchical distributed substation automation system is widely welcomed for its excellent performance and has played a significant role in the safe and economical operation of the power grid. This paper takes the Changsha Mawangdui substation as an example and adopts a hierarchical distributed substation automation system composed of Sepam digital microcomputer protection units from Schneider Electric of France. The structural characteristics of the system are analyzed, and the current situation and development trend of substation automation construction are discussed from the perspectives of application design, on-site acceptance, and operation management. [b]1 Characteristics of the Sepam System[/b] 1.1 System Composition The Sepam system consists of Sepam digital microcomputer protection units and ISIS monitoring system units. The core of the ISIS monitoring system is the ES1000 monitoring system software, which mainly consists of a central computer system, a communication network, and Sepam protection units. The Sepam system's digital microcomputer protection units mainly consist of three types: Sepam2000, Sepam1000, and Sepam100. Each type offers multiple models to suit different voltage levels, protection methods, and protection types. Sepam2000 is primarily used for main equipment protection, Sepam1000 for busbar protection (busbar protection is not used in this example for the 110kV terminal station) and current and voltage transformer measurement and monitoring, and Sepam100 for analog display and switching between local and remote control. 1.2 System Features The Sepam system is powerful, capable of performing protection, control, monitoring, PLC (programmable logic controller), remote sensing, remote control, alarm, and event logging functions at various voltage levels. Its protection functions are advanced, reliable, and comprehensive. To locate fault causes and assess fault severity, it provides the current values ​​for each phase and ground fault at the time of tripping. Based on the characteristics of switch maintenance and pre-test cycles, it records the number of switch actions to promptly identify which switch requires maintenance. The large, high-definition LCD screen can display and retrieve various electrical parameters and non-electrical information. Fault display shows the faulty phase by reading and storing the trip current of each phase. It features a unique EMS (Electromagnetic Compatibility) board, ensuring safe and reliable operation even in environments with strong interference (such as HV substations), fully complying with IEC standards. This allows for the use of advanced digital technology within the substation without any protective measures. Each terminal is independent when energized, simplifying maintenance and making setting and testing very convenient. The powerful control logic function can meet the needs of various application modes through simple parameter settings. In particular, the built-in PLC provides greater flexibility in configuration. The system features intelligent electronic devices with continuous dynamic self-testing capabilities. The number of input/output contacts can be increased by adding expansion boards, and data can be read with phase information, greatly facilitating operation and maintenance personnel. Because protecting current transformers (CTs) typically requires a high saturation ratio, normal measurement accuracy cannot meet the requirements for accurate control, measurement, and alarm. Conversely, measuring a CT with a low saturation ratio is insufficient to meet the requirements for high overcurrent protection. Therefore, Sepam utilizes a coreless CSP coil current transformer (without magnetic core) based on the special Rogo Wski coil principle, which provides a wide dynamic range and outstanding linearity. The ES1000 software is powerful, with a clear graphical interface and a user-friendly interface. It monitors system operating status and measured values, and features real-time monitoring and automatic tracking and recording of past operating trend curves. During operation, it displays alarms, prints displays, logs operating reports, remotely controls various components, automatically reconfigures the network after a fault, manages energy based on user payment, and allocates fees to users in different locations. [b]2 Application Design Analysis[/b] 2.1 System Design Concept The Sepam system is a complete substation automation system. Except for the emergency manual operation tripping and closing methods retained in each control and protection unit (in this example, Sepam100 is used for simulation diagram display and to realize local and remote switching), all other control, monitoring, measurement and alarm functions can be completed through the computer monitoring system ISIS. The substation does not need to set up a separate remote terminal unit (RTU). The monitoring system fully meets the functions of "four remotes" and the needs of unattended operation. From the perspective of system design, the system has the following characteristics: (1) The distributed design system adopts a modular, distributed open structure to ensure the reliability of each control and protection function and the upgradeability of the system. Each control and protection function is distributed on the switch cabinet or the control and protection cabinet as close as possible to the switch. The main and backup protection in the main transformer are separated, and the backup automatic transfer is concentrated in the control room. All control, protection, measurement, alarm and other signals are processed into data signals in the local unit and transmitted to the monitoring computer in the main control room through the RS-485 communication interface and a dedicated communication cable. Each local unit is independent of each other and will not affect each other. The protection function does not depend on the monitoring computer at all. The reliability and availability of the entire system are enhanced. The entire system connects each feeder, station service transformer, main transformer, DC power supply system, capacitor comprehensive compensation device, fire protection and monitoring computer to form a complete computer network system. As shown in Figure 1. [img=349,157]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/dwjs9905/image5/t14-1.gif[/img] Fig.1 Network system diagram of Mawangdui 110kV substation (2) Simple and reliable Due to the adoption of integrated local control and protection units that can be directly installed on switch cabinets and control and protection cabinets near the switches, the secondary wiring is simplified. There is only a communication shielded cable connection between the switch room or switch yard and the main control room, which reduces the number of control cables to a minimum and has a strong anti-electromagnetic interference capability. (3) Scalability The system design adopts a network system structure, which fully considers the needs of future substation scale and function expansion. When the scale of the substation and the system expand, it is easy to achieve this by adding local control and protection units. Moreover, the system can be gradually expanded and improved through network expansion. (4) Good system compatibility: The system consists of standardized software and hardware. The database is integrated using a language similar to industrial BASIC. The database functions are powerful and flexible. It is equipped with a standard RS-485 serial communication interface, a dedicated smart communication card applicom, a protocol converter, a modem, a communication connection box CCA609 for Sepam digital microcomputer protection devices, and a communication protection key dongle. Users can flexibly configure the system according to the needs of the station to form a monitoring network. The system software can also easily adapt to the rapid development of computer technology. (5) Economic efficiency: Due to the simplification of secondary wiring and secondary equipment configuration to the greatest extent, the area of ​​the main control room and protection room is greatly reduced, control cables and cable layers are eliminated, and the system's maintainability is improved. Therefore, the overall cost of the substation can be significantly reduced. 2.2 System Functions The user interface between the system and the user is a graphical window display. Standard methods, such as mouse control of all function keys, allow operators to intuitively perform various operations. The system's program menu is a tree structure, allowing users to easily access various control screens. Each menu function key has a clear text description and the desired screen. All system raw data is acquired in real-time from local control and protection units and transmitted to the monitoring computer for processing and display via a serial communication interface. To prevent misoperation, access control is implemented. By assigning different login passwords to operators, the system can classify users into different levels, each with different permissions. Every function of the system application can be adapted according to user requirements and system design to facilitate modification and meet actual needs. [b]3 Analysis of On-site Commissioning and Operation[/b] 3.1 Initial Commissioning and Acceptance Situation According to the technical agreement and system design characteristics, problems were discovered when the Sepam integrated automation system entered the commissioning and operation phase. For example, the PLC chip in Sepam2000 burned out; during the switch's drive test, when the current increased from the primary and secondary sides to the overcurrent state, the switch tripped, and its tripping current response was slow; during instantaneous tripping, the current was not displayed on the monitoring computer at all, and information such as which segment or phase the fault belonged to could not be displayed; the power supply of the protection control circuit and the power supply of the switch mechanism control circuit were not independent; during voltage switching or during a very brief AC voltage loss, the monitoring computer would freeze. Based on these phenomena, after detailed and careful analysis, it was found that the PLC burnout in Sepam2000 was related to whether the switch's spring mechanism had reached the required energy storage level. The original design only considered the PLC board with ESB output in the Sepam2000 device, which includes anti-pumping relays and intermediate output relays. However, the actual switching mechanism uses a spring mechanism. Spring mechanisms are not widely used in my country, and domestic manufacturing processes do not meet the requirements for complete reliability, especially in terms of low energy storage sensitivity. There is a significant time difference between spring energy storage and full energy storage, causing the auxiliary normally open and normally closed contacts of the switch to not operate in time. Ultimately, the electronic contacts of the Sepam2000 PLC are used to interrupt the arc, resulting in the Sepam2000 PLC chip burning out. Two solutions were proposed: 1) Replace the spring mechanism of the switch with an ABB product. This method is too expensive and not feasible. 2) Use external relays, adding one integrated circuit intermediate relay to prevent pumping, one intermediate relay to detect whether the spring mechanism is charged, one trip relay, and one closing relay. A transmission test was conducted on each switch protection unit using a microprocessor-based relay protection integrated tester, and the results met the requirements of the protection verification procedure. The redesigned device, after more than a year of operation, has proven reliable. Other shortcomings were caused by issues with the communication channel and program, as well as unreasonable settings of various time parameters in the reclosing time counter of the Sepam2000 digital microcomputer protection device. AC voltage loss was restored after the UPS was installed. 3.2 System Performance Defects: No instantaneous alarm screen switching display. The system uses 1801 polling for communication with the dispatcher and JBUS protocol for communication with two local Applecom communication cards. Due to the capacity limitation of the 1801 protocol, not all change data information can be transmitted; a compromise is to use a hoisting method, with byte compression technology implemented in the program. The communication interface uses RS-485. Since only one master node can be connected to the RS-485 interface network, a multi-master redundant system cannot be formed, resulting in poor system reliability. The data communication method is command-response; slave nodes can only respond after receiving commands from the master node. Some important change information cannot be transmitted in a timely manner, resulting in poor system flexibility, poor real-time performance, and poor error correction capabilities. 3.3 Operational Status After more than a year of operation, the system is stable and flexible, achieving true unmanned operation and serving as a demonstration of 110kV unmanned substations in urban areas. It has accumulated experience for the future construction of substation automation and unmanned substations and trained operation and maintenance personnel. [b]4 Technological Development Trends[/b] Substation automation systems are changing rapidly with the development of computing technology. Based on the requirements and characteristics of power system technology, the development trend is: (1) For medium and low voltage substations, protection and control requirements should be as comprehensive and unified as possible; (2) For high voltage and ultra-high voltage central hub stations, protection and control must have relative independence; (3) The automation system should have strong software and hardware compatibility and upgradeability to adapt to the rapid development of computer technology and power electronics technology. The system architecture adopts a distributed and open system structure and has easy-to-expand functions. (4) Adopt and track ATM-based communication networks and switching technologies, and adopt intelligent electronic protocol converters to adapt to the development of internet/intranet network technology, and gradually realize an open communication architecture that is compatible with the international standard IEC870-5 series. 5 Conclusion Sepam automation system is a mature and worthwhile integrated automation system. However, several points should be noted at present: (1) At present, there are many manufacturers of integrated automation systems at home and abroad, and the quality and functions are uneven. The automation system used in the regional power system should be reliable and the number of models should not be too large. This is a good solution for operation and maintenance. (2) Solve the contradiction between the high starting point and high degree of automation of the automation system and the backwardness and mismatch of switches and mechanisms. (3) Solve the contradiction between the poor cultural quality of the operators and maintenance personnel in unmanned stations and the widespread application of new technologies. The correct solution to these problems is the key to giving full play to the advantages of integrated automation technology. (4) The reliability of the substation automation system is paramount. The interfaces between each device must be completely electrically isolated. The AC power supply protective ground and the device casing must be reliably grounded, and the lightning protection measures for the communication channel with the main station must be implemented. [b]References[/b] [1] Schneider Electric Company Sepam Data [2] Yang Qixun. Development Trend of Substation Integrated Automation Technology. Automation of Electric Power Systems, 1995, 19(10) [3] He Weijun, Chen Xiaochuan. Substation Integrated Automation System Architecture. Proceedings of the Centennial Anniversary of Southwest Jiaotong University, Electrical Engineering Volume, 1996