Abstract: Based on a survey of 500 kV substation automation in Zhejiang Province and a pilot project at Shuanglong Substation, this paper discusses the structure of a 500 kV substation automation system. Through comparison, it is clear that a distributed substation automation system has more advantages than a centralized system. With the rapid development of information and communication technologies, the reliability and performance of each component in the substation automation system have been greatly improved, laying a good technical foundation for simplifying the substation automation system. Based on the implementation of the substation automation system at Shuanglong Substation, suggestions are made to optimize the substation automation system structure, including simplifying the substation-level computer system, adopting a single Ethernet network for the station-level computer network, eliminating (reducing) protection management machines, adopting new network technologies, and using integrated devices in the low-voltage system. Keywords: Substation automation system; Distributed substation automation system; Centralized substation automation system 1 Introduction With the rapid development of the power system in Zhejiang Province, the 500 kV system will gradually become the main grid of Zhejiang Province. To improve the safe and economical operation of 500 kV substations, Zhejiang Province actively explored the structure of 500 kV substation automation systems, based on the development of modern microcomputer and network communication technologies, and conducted a pilot project at Shuanglong Substation. Zhejiang Province has over ten years of operational history with its 500 kV substations. Starting with the first 500 kV Pingyao Substation, the secondary system design has been significantly upgraded compared to the original 220 kV substations. For example, Lanting Substation introduced advanced RTU equipment and, based on this, implemented local monitoring functions using computer data interfaces, providing strong support for the safe operation of the substation and enabling dispatchers to promptly understand the system's operational status. However, this conventional monitoring system, combined with a computer monitoring system mainly composed of enhanced RTU equipment, also revealed some shortcomings during operation: (1) During normal operation, the conventional monitoring system is the main control system, and the computer monitoring system is only used as an auxiliary monitoring means; (2) The information collected is based on the needs of dispatching, and the amount of information collected is too small to meet the needs of substation operation monitoring; (3) Due to the limited technology at the time, the system failed to solve the problem of false remote signaling; (4) The lack of necessary control functions restricts the integration of substation automation functions, such as the need for external devices to realize the automatic voltage regulation function and the need for external wiring to realize the electrical operation interlocking of the substation. With the development of computer technology and communication technology, the computer monitoring system mainly composed of enhanced RTU equipment has added control functions for primary equipment. After integrating some substation automation-related functions, it can also form a substation automation system (referred to as a centralized system). At the same time, domestic and foreign countries have also launched substation automation systems (distributed systems) with a distributed structure arranged according to electrical bays. Centralized systems evolved from enhanced RTUs. To save hardware costs, modules are divided according to four remote functions (remote control, remote monitoring, remote control, and remote control), and the entire substation is monitored. A typical approach is to divide modules into digital input boards, digital output boards, and analog input boards. Taking a digital input board as an example, a single module can integrate 16 or even 32 input points, unifying the digital inputs of the entire substation. While this reduces investment, it creates difficulties for expansion and daily maintenance. A comparison of centralized and distributed systems is shown in Table 1. It should be said that the technological advantages of distributed systems are obvious, which is why some traditional RTU manufacturers have introduced distributed systems. During the construction of the Shuanglong substation, approximately 6.05 million yuan was saved by eliminating conventional RTU equipment, operating relay panels, and operating interlocking equipment, as well as saving on cables. After adopting a distributed system, the Shuanglong substation achieved comprehensive monitoring of the substation, becoming an effective means of real-time monitoring and control. It performed exceptionally well in equipment anomaly alarms and accident analysis, and was affirmed by the operating unit. 2 Implementation of the Automation System Structure of Shuanglong Substation The automation system of Shuanglong Substation is shown in Figure 1. The local status signals, alarm signals and AC inputs of each bay input/output unit are sent to the main unit of the small room via serial communication (some signals are directly connected to the main unit). They are sent to the station-level Ethernet via protocol converter. After data processing by the host, the relevant information is displayed on the human-machine workstation. Some information is sent to the third-level dispatch via the main and backup communication machines. The relevant information of the microcomputer protection is sent to the station-level system via the protection management machine. The electrical interlocking is completed using relatively independent hardware. The system has the following main shortcomings: (1) There are many conversion links for information transmission, which affects the real-time performance of information transmission to a certain extent; (2) Given the technical level at that time, the synchronization device and the switch operation interlocking system are independent devices. The system uses a lot of equipment and the price is high; (3) A distributed database is not used; (4) The number of signal input points of the bay layer input/output unit is relatively small. Many signals can only be connected to the main unit; (5) The station-level computer system is too complicated. 3. Discussion on the Structure of a Hierarchical Distributed Automation System With the development of substation automation system technology and related computer and communication technologies, it has become possible to solve the above-mentioned problems. The capacity of the input/output units at the bay level has greatly increased, meeting the input/output requirements of an electrical unit. Many input/output units provide large LCD screens as local operation interfaces. The devices integrate programmable logic controllers (PLCs), enabling electrical operation interlocking, and synchronization functions can also be implemented internally within the input/output units. The substation automation system proposed by the author is shown in Figure 2. At the bay level, input/output units are configured according to electrical bays. Each unit is connected to a local data processor via fieldbuses such as the (LON) network. The local data processor serves both as a hierarchical data processor, completing data exchange between bay level devices of different voltage levels, and as a protocol converter between the fieldbus and station-level Ethernet, ensuring the relative independence of the bay level devices. The main differences between this automation system and Shuanglong Substation are as follows: (1) The station-level computer network adopts a single network instead of a dual network. The use of a dual network for the station-level Ethernet is mainly to improve the reliability of the network and the availability of the system. It does not improve the transmission bandwidth and efficiency of the network. From the operation practice of Shuanglong Substation, the reliability of the network is very high. So far, there has been no network switching or network anomaly. However, the management of the dual network occupies a certain amount of system resources, which brings instability to the system operation and results in a higher initial investment. The dual network can improve the availability of the system to a limited extent. However, among the network equipment, except for optical cables, replacement is relatively convenient. Therefore, when using a single network, long optical cables can be reserved during the construction phase to facilitate emergency replacement in case of optical cable failure, thus ensuring the availability of the system. (2) Adjust the functions of the station-level computer to reduce the total number of station-level computers. In the substation, it is unreasonable for offline management functions to occupy host resources. During the commissioning phase of Shuanglong Substation, due to too many management processes being opened and not closed in time, host resources were occupied too much, affecting the stability of the system. Therefore, in the proposed station-level computer system, only real-time monitoring and control functions are retained, while offline management functions are isolated from the real-time system and uniformly considered within the substation's Management System (MIS). Information is transmitted to the MIS via a newly added gateway/engineer workstation, which is responsible for information forwarding and system maintenance. A maintenance interface is reserved on the station-level Ethernet, allowing external laptops to be connected for system maintenance when needed (e.g., periodic calibration of automated equipment), without occupying the operator's HMI workstation. In this way, the main task of the host computer is to coordinate the entire station and perform some automation functions, such as automatic voltage and reactive power adjustment, reducing the host computer's load. The host computer and HMI workstations can be merged to reduce the number of station-level computers. Simultaneously, the protection management computer and the protocol converter in the small room are eliminated, and a local data processor is used to handle data preprocessing and data distribution, further reducing the number of station-level computers. (3) Direct serial communication or independent networking between protection and input/output units, eliminating (reducing) the protection management unit. For 500kV substations, the domestic practice is to tender for protection and automation systems separately. Generally, protection and automation systems are not from the same manufacturer, causing difficulties in communication interfaces between them. To solve this problem, Shuanglong Substation uses a protection management unit to communicate with protection via RS232 port and performs protection communication protocol conversion to connect protection information to the station-level computer system. With the development of international standardization work, protection has standard communication protocols, such as IEC870-5-103. In this way, protection can be directly connected to the input/output units of the corresponding electrical bay. Direct connection of protection to the input/output units of the corresponding bay eliminates the need for a protection management unit, simplifies the station-level computer system, and clarifies the concept of electrical bays. For protection devices that support networking, they can also be networked separately, with a unified protection management unit configured throughout the station, instead of being configured in small rooms, thus reducing the number of protection management units. If the input/output units of the protection device and the automation system can be integrated into a single panel, it can save cables and panel positions, simplify the design of some secondary circuits, and reduce the project cost. (4) The use of integrated protection and input/output units in 35kV has proven to be feasible in low-voltage systems. The 35kV system of the 500kV substation is equipped with only capacitors and reactors, and the protection is simple voltage and current protection. The use of integrated protection and automation units is technically feasible and can also save investment. (5) Simplification and improvement of electrical interlocking system Shuanglong substation uses independent hardware to realize the electrical interlocking of the entire station, which ensures the operation safety and reliability of Shuanglong substation. However, due to the limitation of the operation mode, the grounding switch of the 220kV system that is manually operated can only use hard-wired logic to realize electrical interlocking. With the development of substation automation systems, many manufacturers have integrated PLC functions into the input/output units and directly used the input/output units to realize electrical interlocking. From the perspective of the correlation of primary equipment status realized by electrical interlocking, the primary equipment status correlation of this electrical bay is the greatest, followed by the primary equipment status of electrical units of the same voltage level. For the main transformer bay, the grounding switch status of the three sides of the main transformer needs to be mutually transmitted. According to the above information flow, it is recommended that the input/output units of the same voltage level be interconnected by fieldbus. In this way, the voltage interlocking of the same voltage level can completely exchange information, and the data processor should not be configured in a small room. Since the number of status quantities related to electrical interlocking on the three sides of the main transformer is not large, the input points of the input/output units can also be used for access. The latter has stronger operational independence and has its advantages. Using PLC, it is convenient to output permissive contacts and use electromagnetic locks to realize the operation interlocking of manually operated primary equipment; it is also possible to use the line voltage input of the input/output unit to realize line no-voltage check, which has its advantages when integrated into the electrical interlocking logic. (6) Adopting new network technologies to improve the real-time performance of the system From the perspective of data flow, since a local data processor is used, the data to the substation and the data to the dispatch can be split here. The primary purpose of utilizing a single substation-level Ethernet network is to reduce investment, especially given the advancements in network technology. Fast Ethernet technology, with its 100MB/s bandwidth, offers ten times the bandwidth of conventional Ethernet; conventional Ethernet, for each node, provides far less than 10MB/s of effective bandwidth, while new switched Ethernet technology provides 10MB/s bandwidth per node. Employing virtual network technology allows the construction of several virtual logical networks on the same physical network, separating internal and dispatch information and improving network efficiency. By comprehensively utilizing these new network technologies, although physically it appears to be just one Ethernet network, the actual network transmission efficiency can significantly exceed that of conventional Ethernet, improving system real-time performance. According to the proposed structure, for the dispatch communication function, the local data processor deals with a single dispatcher. The primary and backup communication units process the data according to the different requirements of the three-level dispatch system and send it to the third-level dispatcher. Alternatively, the local data processor can communicate directly with the dispatcher via serial port. In conclusion, while ensuring the safe operation of the substation, the substation automation system should be simplified as much as possible to reduce costs and increase system reliability. 4. Conclusion From the perspective of existing technical conditions, the proposed structure of the substation automation system is feasible. The system structure is simple and clear, ensuring the safe and reliable operation of the substation. However, the construction of a substation automation system is not merely a technical issue. Because the system involves all aspects of the substation, comprehensive consideration of various requirements is necessary to arrive at an implementable solution.