(LSC)designs with low cost,convenience,scalability, output power is determined by looking it up on the and remote access capability.This article describes a curve.Therefore,during a load-shedding operation,only LAN-based LSC that makes use of these technologies for the excessive loads that exceed the GTG maximum out- the isolated power system. put power will be shed.If any of the GTGs are tripped and the total load is less than the maximum output of Load-Shedding Schemes the remaining GTG,the LSC will start counting the preset The LSC system consists of an industrial PC that per- timer (around 20 s)and expect the GTG generation con- forms trigger signal processing,determines load-shed- troller to adjust the GTG generation. ding sequences,and issues commands to RTUs in real-time.The evaluation process within the LSC is kept Administration of Priority List to a minimum to speed up the load-shedding actions, When an event triggers the LSC,it will shed the loads since most of the system calculations are performed in according to the load priority list,with lower priority the SCADA server,only the required load-shedding cal- loads being shed first.A predetermined load-shedding culation and decision-making are done in the LSC.All list contains all loads in rising order of priority. load-shedding information acquisition,transmission. Load-shedding sequences can be modified via the and commands are issued over the LAN.Moreover,the load-shedding-operating display by selecting one of operator is allowed to share control of the load-shedding policy.This gives more flexibility for the load-shedding High performance,low cost,and scalability strategy design. During power plant operation,vari- make LANs a widely used data communication ous conditions exist.Load-shedding backbone in many power system control logic is designed to cover all possible emergency conditions.The actual loads and monitoring systems and rated generation outputs are deter- mined by online monitoring of the gas turbine generator (GTG)circuit breaker (CB)status,output power,and ambient air temperature.Emergency-case recog- nition is activated through trigger sig- nals.An emergency case applies if GTG Trip Load Power Power>Load? 1.Calculate trigger signals are received. Calculation Power Loads 2.Determine Load Shedding System Status Assessment Sequence Generator CB status changes will trigger 3.Check Loads Overload Scan Overload? Set Timer Status load shedding.The status of GTG CBs is reported by hardwired digital interface boards (DIB)connected directly to them The updated CB status can reach the LSC within 10 ms to trigger the load-shedding logic.Other GTG information,such as Shed Loads ambient air temperature and output power,are reported through a data inter- face unit (DIU).Intelligent electronic devices (IED)will report to the LSC when Figure 1.LSC flowchart a CB status is changed.The LSC issues commands to ask for data from all IEDs periodically:if no the predetermined load-shedding lists in the emer- reply received from an IED,the LSC keeps polling to test gency-case-overview screen.The operator can its living status,and the load-shedding priority list is change the load-shedding priority list dynamically updated accordingly.In this way,the LSC keeps monitor- and shed,block,or reconnect single or multiple ing the whole system status and changes. loads at the control center.Special strategy is used to keep the priority list consistent at the control cen- Power Reserves Determination ter and the LSC.When the LSC starts or the priority Total available power generation is calculated through lists are modified by the operator,new priority lists the GTG ambient air temperature versus output power are downloaded from the SCADA server to update curve.The GTG temperature is acquired and maximum the LSC's database. Juy200139(LSC) designs with low cost, convenience, scalability, and remote access capability. This article describes a LAN-based LSC that makes use of these technologies for the isolated power system. Load-Shedding Schemes The LSC system consists of an industrial PC that per￾forms trigger signal processing, determines load-shed￾ding sequences, and issues commands to RTUs in real-time. The evaluation process within the LSC is kept to a minimum to speed up the load-shedding actions, since most of the system calculations are performed in the SCADA server, only the required load-shedding cal￾culation and decision-making are done in the LSC. All load-shedding information acquisition, transmission, and commands are issued over the LAN. Moreover, the operator is allowed to share control of the load-shedding policy. This gives more flexibility for the load-shedding strategy design. During power plant operation, vari￾ous conditions exist. Load-shedding logic is designed to cover all possible emergency conditions. The actual loads and rated generation outputs are deter￾mined by online monitoring of the gas turbine generator (GTG) circuit breaker (CB) status, output power, and ambient air temperature. Emergency-case recog￾nition is activated through trigger sig￾nals. An emergency case applies if trigger signals are received. System Status Assessment Generator CB status changes will trigger load shedding. The status of GTG CBs is reported by hardwired digital interface boards (DIB) connected directly to them. The updated CB status can reach the LSC within 10 ms to trigger the load-shedding logic. Other GTG information, such as ambient air temperature and output power, are reported through a data inter￾face unit (DIU). Intelligent electronic devices (IED) will report to the LSC when a CB status is changed. The LSC issues commands to ask for data from all IEDs periodically; if no reply received from an IED, the LSC keeps polling to test its living status, and the load-shedding priority list is updated accordingly. In this way, the LSC keeps monitor￾ing the whole system status and changes. Power Reserves Determination Total available power generation is calculated through the GTG ambient air temperature versus output power curve. The GTG temperature is acquired and maximum output power is determined by looking it up on the curve. Therefore, during a load-shedding operation, only the excessive loads that exceed the GTG maximum out￾put power will be shed. If any of the GTGs are tripped and the total load is less than the maximum output of the remaining GTG, the LSC will start counting the preset timer (around 20 s) and expect the GTG generation con￾troller to adjust the GTG generation. Administration of Priority List When an event triggers the LSC, it will shed the loads according to the load priority list, with lower priority loads being shed first. A predetermined load-shedding list contains all loads in rising order of priority. Load-shedding sequences can be modified via the load-shedding-operating display by selecting one of the predetermined load-shedding lists in the emer￾gency-case-overview screen. The operator can change the load-shedding priority list dynamically and shed, block, or reconnect single or multiple loads at the control center. Special strategy is used to keep the priority list consistent at the control cen￾ter and the LSC. When the LSC starts or the priority lists are modified by the operator, new priority lists are downloaded from the SCADA server to update the LSC’s database. July 2001 39 Figure 1. LSC flowchart High performance, low cost, and scalability make LANs a widely used data communication backbone in many power system control and monitoring systems