A Study On The Communication Network Structure Of Digital Substation

The general communication network of digital substation is divided into three levels by IEC61850 protocol:station level, bay level and process level. Switch network is adopted to execute the communication function. The abstract modeling method, which is to combine logical nodes according to specific function, is applied to build the models of physical devices. IEC61850 protocol has laid the foundation for the data sharing and the interoperation in digital substation by unifying data structure and substation automation system configuration language. Two kinds of messages are defined:GOOSE and SAV, which demand real-time transmission and high reliability of the network. In order to deliver these two kinds of messages as soon as possible, they are directly mapped from application layer to link layer in the OSI model, without TCP/IP headers. Further research and development could proceed on the basis of this characteristic. The transmission of GOOSE and SAV adopts the publisher/subscriber mechanism and takes advantage of group multicasting technique. VLAN should be reasonably organized so as to confine the multicast messages in their necessary and effective parts of the network. By this way, the potential broadcasting storm could be prevented, the unnecessary impact on switch ports relieved and the utilization of data improved.Three kinds of network structure could be used in digital substation:bus, star and ring topology. Considering the redundancy of the network, the ring topology is adopted between the station level and the bay level communication and the star topology is adopted between the bay level and the process level communication, network devices doubled when necessary. The major pattern in operating digital substations is to physically double the transmission network and to choose one running, the other staying backup.IEC62439 protocol defines four kinds of redundancy protocol, among which MRP based on ring topology is suitable for the communication between the station level and the bay level. PRP uses parallel transmission network and handles the duplicated packets in the receiving end devices, which is against packet loss in theory. Furthermore, PRP is compatible with MRP. So it is convenient to build communication network of digital substation applying IEC62439:using double MRP ring topology running PRP between the station level and the bay level, and using proper double star topology running PRP between the bay level and the process level. Gateway or agent could run on station level in case of remote control and station to station communication.Focusing on the characteristic that GOOSE and SAV messages are directly mapped to the link layer, and taking the redundancy requirement into account, a scheme of communication network structure based on the link layer switching node is proposed. A link layer switching node is a transfer unit which consists of three switches. According to different functions in the network, switching nodes are divided into two types:access switching node and transfer switching node. The former type is used to provide accessories to end devices, the latter to connect other switching nodes. For each switching node, only the topology information of the end devices or other switching nodes that relate to switching node in question is maintained. When topology changes occur due to device failure or device maintenance, the background flow of topology information is confined to each relative area so that the influence is reduced. Redundant physical links and proper transfer logic could still guarantee sufficient network redundancy even during switch failure, which is convenient for the maintenance of network devices.