cy.Logical bus topology was used in the LAN design. to support mis- Table 1.DDEP LSC commands The LSC server and SCADA server connect to the sion-critical appli- LANs via switches.All IEDs are connected to the LAN cations throughout Command Description 0x0001 Request for a telegram via DIUs.For the connection within a short distance, the enterprise 0x0002 Set a telegram value serial port and cable are used to connect the IEDs to comprising thou- 0x0003 Request for all telegrams DIUs.For remote substations,a fiberoptic network is sands of users. used for the DIU connection.Different IED transmis- Communication sion protocols from different vendors are used for among IEDs,the Table 2.DDEP DIU data report data transmission.DIUs act as a protocol converter LSC.and the Usage Bytes among IEDs.Different parts of the network and differ- SCADA server are 0x0002 2 Data length 2 ent networks are connected through bridges,routers, through client/ Telegram number or switches.Through the IED and DIU connection,all server architec- Telegram value substation equipment data is available to any other ture,as shown in network device. Figure 4. When the LSC accesses information in an IED,the Load-Shedding Communication Structure IED acts as a server providing services and the LSC The LSC has two external interfaces connecting to the acts as a client requesting services.The LSC also acts SCADA system.Through network interface cards (NIC), as a server when the substation main server connects the LSC is connected to both LANs.Another interface with the LSC and asks for reports.Two data transmis- (DIB)connects the LSC to the GTG relay directly to sion modes are used in IED information transmission. acquire GTG CB status.System information,including In the polling mode,the LSC client sequentially polls the load-shedding priority list and status checking infor- IEDs,and each IED responds in an orderly fashion.In mation,is downloaded from the SCADA server to the LSC this approach,the polling cycle may be long if many via LAN.GTG trip information is transmitted through a IEDs have something to report.Another data transmis- DIB board.Loading statuses,GTG ambient temperature, sion mode is exception reporting;when the compo- and power will be transmitted via LAN also through dif- nent's information changes,such as changes of status ferent DIUs.Figure 3 shows the data stream of the LSC. or a fault,or to pass data to the LSC and SCADA serv- er,an exception report will be generated and submit- DIU Data Exchange Protocol ted.Estimating the traffic level is relatively difficult in The LSC can send various request commands to collect this case. real-time IED data as well as control commands to get IED points.All points in the IED are addressed as "bit in a Network Management telegram"with Modbus or other protocol addresses hid- As the number of networks within a power system den within the DIU driver software.The LSC addresses grows,along with the diversity of IEDs from various ven- all equipment using telegrams.Request commands exist dors comprising the network,managing all of these sys- to facilitate handling of disturbance;e.g.,when a DIU tems within a coherent framework becomes important. becomes "ready"after the disturbance,a request for all Especially in the substation,every datagram from each of the telegram is sent to update all changes lost during disturbance.The communication Presentation Service between the LSC and DIU is through DIU data exchange protocol(DDEP)runs on top of the Client Tier TCP/IP protocol (Over IEEE 802.3 Ethernet net- work).Different codes and different telegram ranges represent different commands and digital or analog values.Tables 1 and 2 show the LSC Application Logic and Service commands and DIU data report format. Service Tier Client/Server Architecture in the LSC Design Server Server The evolution of distributed computer architec- ture resulted in the birth of client/server Database Service architecture.Unlike traditional enterprise appli- Database cations that are self-contained monolithic pro- Tier grams with limited access to one another's procedures and data,the client/server structure Mainframe Server RDBMS Database provides the scalability and robustness required Figure 4.Three-tier client/server applications architecture Juby200141cy. Logical bus topology was used in the LAN design. The LSC server and SCADA server connect to the LANs via switches. All IEDs are connected to the LAN via DIUs. For the connection within a short distance, serial port and cable are used to connect the IEDs to DIUs. For remote substations, a fiberoptic network is used for the DIU connection. Different IED transmis￾sion protocols from different vendors are used for data transmission. DIUs act as a protocol converter among IEDs. Different parts of the network and differ￾ent networks are connected through bridges, routers, or switches. Through the IED and DIU connection, all substation equipment data is available to any other network device. Load-Shedding Communication Structure The LSC has two external interfaces connecting to the SCADA system. Through network interface cards (NIC), the LSC is connected to both LANs. Another interface (DIB) connects the LSC to the GTG relay directly to acquire GTG CB status. System information, including the load-shedding priority list and status checking infor￾mation, is downloaded from the SCADA server to the LSC via LAN. GTG trip information is transmitted through a DIB board. Loading statuses, GTG ambient temperature, and power will be transmitted via LAN also through dif￾ferent DIUs. Figure 3 shows the data stream of the LSC. DIU Data Exchange Protocol The LSC can send various request commands to collect real-time IED data as well as control commands to get IED points. All points in the IED are addressed as “bit in a telegram” with Modbus or other protocol addresses hid￾den within the DIU driver software. The LSC addresses all equipment using telegrams. Request commands exist to facilitate handling of disturbance; e.g., when a DIU becomes “ready” after the disturbance, a request for all of the telegram is sent to update all changes lost during disturbance. The communication between the LSC and DIU is through DIU data exchange protocol (DDEP) runs on top of the TCP/IP protocol (Over IEEE 802.3 Ethernet net￾work). Different codes and different telegram ranges represent different commands and digital or analog values. Tables 1 and 2 show the LSC commands and DIU data report format. Client/Server Architecture in the LSC Design The evolution of distributed computer architec￾ture resulted in the birth of client/server architecture. Unlike traditional enterprise appli￾cations that are self-contained monolithic pro￾grams with limited access to one another’s procedures and data, the client/server structure provides the scalability and robustness required to support mis￾sion-critical appli￾cations throughout the enterprise comprising thou￾sands of users. Communication among IEDs, the LSC, and the SCADA server are through client/ server architec￾ture, as shown in Figure 4. When the LSC accesses information in an IED, the IED acts as a server providing services and the LSC acts as a client requesting services. The LSC also acts as a server when the substation main server connects with the LSC and asks for reports. Two data transmis￾sion modes are used in IED information transmission. In the polling mode, the LSC client sequentially polls IEDs, and each IED responds in an orderly fashion. In this approach, the polling cycle may be long if many IEDs have something to report. Another data transmis￾sion mode is exception reporting; when the compo￾nent’s information changes, such as changes of status or a fault, or to pass data to the LSC and SCADA serv￾er, an exception report will be generated and submit￾ted. Estimating the traffic level is relatively difficult in this case. Network Management As the number of networks within a power system grows, along with the diversity of IEDs from various ven￾dors comprising the network, managing all of these sys￾tems within a coherent framework becomes important. Especially in the substation, every datagram from each Command Description 0x0001 Request for a telegram 0x0002 Set a telegram value 0x0003 Request for all telegrams … … Table 1. DDEP LSC commands Table 2. DDEP DIU data report Usage Bytes 0x0002 2 Data length 2 Telegram number 2 Telegram value 1 … … Figure 4. Three-tier client/server applications architecture July 2001 41