Generator Overload Load Shedding LSC sheds load under two situations:GTG overload or When load shedding occurs,the LSC calculates total GTG trip.When GTGs are running and one generator is GTG power and deducts the total loads;the difference in overload,a warning signal is generated and a timer is the loads to be shed.The LSC then adds the loads in (preset time)is initiated concurrently.The loading con- the load-shedding priority list by sequence,until the dition is checked every second.If the overload condi- total loads are greater than the loads to be shed.It tion is rectified,the timer is cleared.If the overload issues commands to every load to be shed through the condition persists for the preset amount of time or programmable logic controller (PLC).With high-perfor- another GTG also trips,the loads are shed according to mance 10 Mbps Ethernet,the response time from the priority list. event trigger until all loads are shed is less than 200 ms in the worst situation. LSC System Architecture The LSC is part of the entire SCADA/EMS GTG- Information Bridge system that makes use of the SCADA LAN Substation Hub for load-shedding information transmis- ation HI止 sion.Figure 2 shows the basic network architecture of the power plant data trans- mission network. Switch Switch Local Area Network Topology High performance,low cost,and scalabili- ty make LANs a widely used data commu- Optical Fiber Optical Fiber IED nication backbone in many power system ED Iransceiver Iransceiver m control and monitoring systems.Among D ED different LAN systems,Ethernet is normal- ED ly chosen as physical/data-link layer net- ED work because of its predominance in the marketplace,the subsequent availability Figure 2.LSC LAN structure of low-cost implementation,and associat- ed network hardwire (bridge,router,and switch) During data transmission,Ethernet Real-Time Priority List uses carrier sense multiple access and Data Commands Load collision detection (CSMA/CD)as the GTG- collision-detection algorithm.When two Feeder IED Requests Report or more IEDs desire access to the LAN Gatewvav LSC Server SCADA Server DIU simultaneously,data collision may occur.When this happens,all colliding devices set a random delay time and try PLC the connection again.This raises a ques- tion:will Ethernet meet the timely requirement of the substation?For the substation environment,timely was Figure 3.LSC data stream defined as 4 ms in order to perform fast functions such as tripping. Several studies were undertaken by Generator Trip the Electric Power Research Institute (EPRD)to evalu- When one or two GTGs trip,the LSC gets the tripping sig- ate Ethernet performance under a worst-case sce- nal immediately through the DIBs.The total loads and nario.Results of these studies showed that either a GTG generation are calculated at the same time.If total 100 Mbps Ethernet on a shared hub or a 10 Mbps Eth- generation with reserve is greater than loads,a GTG trip- ernet connected via a switched hub could meet the 4 ping warning signal will be sent to the SCADA system;no ms network-communication time required. load shedding will occur.If the generation is lower than The substation LAN is based on the 10BaseT Ether- loads,load is shed according to the preset priority list net.As show in Figure 3,two LANs are set up for data Figure 1 shows the load-shedding logic used by the LSC. transmission.Data is sent to two LANs for redundan- 40 IEEE Computer Applications in Power40 IEEE Computer Applications in Power Generator Overload LSC sheds load under two situations: GTG overload or GTG trip. When GTGs are running and one generator is in overload, a warning signal is generated and a timer (preset time) is initiated concurrently. The loading con￾dition is checked every second. If the overload condi￾tion is rectified, the timer is cleared. If the overload condition persists for the preset amount of time or another GTG also trips, the loads are shed according to the priority list. Generator Trip When one or two GTGs trip, the LSC gets the tripping sig￾nal immediately through the DIBs. The total loads and GTG generation are calculated at the same time. If total generation with reserve is greater than loads, a GTG trip￾ping warning signal will be sent to the SCADA system; no load shedding will occur. If the generation is lower than loads, load is shed according to the preset priority list. Figure 1 shows the load-shedding logic used by the LSC. Load Shedding When load shedding occurs, the LSC calculates total GTG power and deducts the total loads; the difference is the loads to be shed. The LSC then adds the loads in the load-shedding priority list by sequence, until the total loads are greater than the loads to be shed. It issues commands to every load to be shed through the programmable logic controller (PLC). With high-perfor￾mance 10 Mbps Ethernet, the response time from event trigger until all loads are shed is less than 200 ms in the worst situation. LSC System Architecture The LSC is part of the entire SCADA/EMS system that makes use of the SCADA LAN for load-shedding information transmis￾sion. Figure 2 shows the basic network architecture of the power plant data trans￾mission network. Local Area Network Topology High performance, low cost, and scalabili￾ty make LANs a widely used data commu￾nication backbone in many power system control and monitoring systems. Among different LAN systems, Ethernet is normal￾ly chosen as physical/data-link layer net￾work because of its predominance in the marketplace, the subsequent availability of low-cost implementation, and associat￾ed network hardwire (bridge, router, and switch). During data transmission, Ethernet uses carrier sense multiple access and collision detection (CSMA/CD) as the collision-detection algorithm. When two or more IEDs desire access to the LAN simultaneously, data collision may occur. When this happens, all colliding devices set a random delay time and try the connection again. This raises a ques￾tion: will Ethernet meet the timely requirement of the substation? For the substation environment, timely was defined as 4 ms in order to perform fast functions such as tripping. Several studies were undertaken by the Electric Power Research Institute (EPRI) to evalu￾ate Ethernet performance under a worst-case sce￾nario. Results of these studies showed that either a 100 Mbps Ethernet on a shared hub or a 10 Mbps Eth￾ernet connected via a switched hub could meet the 4 ms network-communication time required. The substation LAN is based on the 10BaseT Ether￾net. As show in Figure 3, two LANs are set up for data transmission. Data is sent to two LANs for redundan￾Figure 2. LSC LAN structure Figure 3. LSC data stream