0 Introduction Communication plays a crucial role in substation automation. It includes communication between the local switchyard's data acquisition and control units and the substation's monitoring and management layer, as well as communication between the local substation and a remote dispatch center. Different substation automation system architectures imply different communication network configurations, each with varying performance indicators in terms of communication speed, reliability, and scalability. Substation automation systems are generally classified into two structural forms: centralized and hierarchical distributed. The centralized structure is the traditional form, typically consisting of a single front-end processor responsible for exchanging data between various acquisition and control devices and transmitting information to the host computer. This type of system suffers from excessive front-end processor workload, slow speed, poor reliability, and the impact of any acquisition or control device failure on the entire system's operation. These problems significantly limit the application of the centralized structure. A hierarchical distributed architecture typically consists of three layers: the first layer is the substation monitoring layer, responsible for tasks such as human-machine interface, monitoring, management, and control within the substation; the second layer is the communication processing layer, responsible for communication management of lower-level local devices and communication with the remote dispatch center, undertaking data acquisition, control, and remote control functions; the third layer is the local analog, digital, and pulse data acquisition, protection, and control operation output, usually in a distributed structure. The three layers are connected by communication. There are many schemes for hierarchical distributed systems; this paper discusses a fully distributed substation automation communication system. [b]1 Overall Communication System Scheme[/b] The system structure is shown in Figure 1. The system consists of a monitoring host, two communication management cards, one remote control management unit, and several local units. [img=333,132]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9913/image13/1601.gif[/img] Fig.1 Structure of communication system Two communication management cards access the corresponding local units to obtain various data information. The monitoring host completes the substation main control room monitoring function by accessing the communication management card. The remote control unit obtains data by accessing the communication management card, transmits it to the remote dispatch center according to the remote control protocol, and receives remote control and remote adjustment commands from the dispatch center, which are then handed over to the communication management card for execution, realizing the conversion between the local communication protocol and the remote communication protocol and the remote control function [4]. The local unit is designed according to the bay, and introduces a microcomputer device from France, which has measurement, protection, and control functions. It is equipped with a local human-machine interface for monitoring, can operate independently, and its commissioning or decommissioning does not affect the system operation. Two communication management cards are smart cards with built-in CPUs and dual-port RAM, inserted into the PC bus slots of the monitoring host. Each communication management card has four RS-485 interfaces leading to four channels, numbered 0-3 and 4-7 respectively. Each channel can connect up to eight local units via a 485 bus network. The number of communication management cards used depends on the number of local units required by the substation's scale. Both communication management cards operate simultaneously, with each card's four channels working in parallel. The communication speed is not reduced by the presence of multiple channels, but only depends on the number of units on each channel. When a channel is fully connected to eight units, the waiting time for empty units is reduced, resulting in the fastest communication speed. The communication management cards work in parallel with the monitoring host. Because the communication management cards are inserted into the monitoring host's bus, the host can access the communication management cards by accessing memory, making the monitoring host's data retrieval from the communication management cards very fast and simplifying the monitoring host's programming. The communication management cards have strong independent operating capabilities. They only need to be initialized by the program within the host computer upon power-up. After that, the communication management cards can work in parallel with the monitoring host without consuming host resources. Even if the monitoring host fails (unless the host power is interrupted), the communication management cards will still function normally, the remote control function will remain normal, information will not be lost, and the local units can perform local monitoring normally. The remote control unit is also a smart card with a CPU and RAM. It has two RS-485 interfaces and one RS-232 interface. As shown in Figure 1, the remote control unit uses one channel each from communication management cards 1 and 2 to communicate with them. It also communicates with the remote dispatch center via a modem through the RS-232 interface. All three interfaces operate in parallel. The local units are designed according to bays, using microprocessor-based devices. They connect to the secondary side power of TV and TA, as well as the auxiliary contact positions of switches and disconnectors. They have an output control loop and possess measurement, protection, and local monitoring and control functions. The local units communicate with the corresponding communication management cards via a 485 network. Its communication functions include: transmitting the measured quantities of this interval, switch and disconnector position signals and protection action status; controlling switch positions; modifying protection settings and soft enabling/disabling of protection. The local unit adopts a 485 bus socket wiring method, and adding or disabling lines will not affect the normal operation of the entire communication network. In summary, the communication management card, remote control unit and local unit are all intelligent independent operating devices, communicating with each other only through the 485 network. All communication links are carried out in parallel, and the data exchange speed is relatively fast. All intelligent devices, including the communication management card and the monitoring host, do not affect each other after a failure, and the system has good reliability and scalability. [b]2 Communication Software Design[/b] The design of the communication software involves the coordination of various devices. Here, only the working mechanism of the communication software is introduced. 2.1 Data Acquisition Mechanism The station communication adopts the Polling asynchronous communication protocol. As shown in Figure 2, in channels 0-2 and 4-6, the communication management card is the master and the local unit is the slave. The communication management card continuously collects data from each local unit via its RS-485 interfaces and stores it in its own data area, ensuring real-time data transmission. When the monitoring host or remote control unit accesses the communication management card, it directly obtains real-time data from its RAM without waiting for the card to access the local units. The communication management card sends and receives characters using timer interrupts. Because the CPU's processing speed differs significantly from the communication rate, the interrupt-driven transmission and reception of the four RS-485 serial communication ports provides ample time for CPU data management, allowing all four channels to operate in parallel. [img=312,237]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9913/image13/1602.gif[/img] Fig.2 Diagram of communication software mechanism Since the communication management card is plugged into the monitoring host bus, its data area can be mapped to the monitoring host's RAM. The monitoring host can obtain real-time data by accessing memory without occupying the CPU processing time of the communication management card. The communication management card communicates with the remote control unit (RTU) through channels 3 and 7. The RTU is the master, and the communication management card is the slave. The RTU continuously accesses the communication management card through two RS-485 interfaces to obtain real-time data and forwards it to the remote dispatch center according to the remote control protocol. The three interfaces can work in parallel, with the same mechanism as the communication management card. 2.2 Control Operation Communication Mechanism If control operations are performed in the main control room, the monitoring host sends control commands to communication management card 1 or card 2, which then forwards the commands to a local unit. After the local unit completes the execution, it sends the execution results and other data back to the communication management card. At this time, the monitoring host obtains the execution results from the data area of ​​the communication management card by accessing its memory. If remote control or remote adjustment is performed at a remote dispatch center, the remote control unit receives the control command according to the remote control protocol and forwards the command content to communication management card 1 or card 2 according to the local communication protocol. The communication management card then executes the command. After execution, the remote control unit reads the execution results and other data from the communication management card and sends them to the dispatch center according to the remote control protocol. 3 Engineering Practice The Changsha Mawangdui 110 kV/10 kV substation is designed as an unmanned substation. The communication network consists of 2 communication management cards, 1 remote control unit, and 48 local units. The communication management card uses the Applicom International programmable communication card manufactured by Merlin Gerin, a subsidiary of Schneider Electric. The local unit uses its Sepam series products, realizing functions such as measurement, line protection, component protection, circuit breaker opening and closing control, automatic Q-V regulation, and low-frequency load shedding. The local communication protocol adopts the Jbus communication protocol recognized by the local unit, and a communication rate of 9600 bit/s is sufficient to meet the requirements [5]. The remote communication protocol adopts the American 1801 Polling remote control protocol according to the requirements of the dispatch center, with a communication rate of 1200 bit/s. After the system was accepted, all indicators met the substation operation standards and it was officially put into operation on April 29, 1998. Since its commissioning, the communication has been normal, the information exchange speed is fast, and the reliability is high. Practice has proved that this fully distributed communication system has good performance. [b]4 Conclusion[/b] The fully distributed substation automation communication system discussed in this paper has the following characteristics: a. High speed. The upper-level monitoring host can read data from the communication processing layer by accessing memory. In the communication processing layer, local communication and remote control functions are handled in parallel by two communication management cards and one remote control unit, respectively. The communication management cards communicate with the lower-level local units using a multi-channel parallel operating mode. b. High reliability. The communication management cards, remote control units, and lower-level intelligent local units in the communication processing layer operate independently. They are only weakly connected through the communication network; a failure in one device does not affect the normal operation and communication of other devices. The communication management card can still function normally after a host failure unless power is interrupted. c. Strong scalability. When adding or decommissioning lines, local units designed according to intervals can be added to the communication network without affecting the normal operation of the network. Adding lines will not reduce communication speed. During large-scale expansion, more communication management cards can be added; adding one card can expand the number of lines by 32. [b]References[/b] [1] Liu Yuzhong, Wu Junyong, Liu Pei. Data communication in substation integrated automation. In: Proceedings of the 13th National Conference on Power System and Automation of Higher Education Institutions. Guangzhou: 1997 [2] Yang Qixun. Development trend of substation integrated automation technology. Automation of Electric Power System, 1995, 19(10): 7-9 [3] Jin Tao, Tang Tao, Que Lianyuan. Analysis and discussion of distributed substation automation system. Automation of Electric Power System, 1997, 21(10) [4] Booth C, McDonald J R. Substation Based Data Interpretation Techniques for Improved Power System Management. IEEE Trans on Power Delivery, 1997, 9(2) [5] Zhu Li'an, Shuai Junqing. Functional requirements of 110 kV and 35 kV substation integrated automation system. Power Grid Technology, 1997, 21(1)