上海交通大学：《电力系统自动化》课程教学资源（推荐论文）LAN-based control for load shedding

LAN-Based Control for Load Shedding Bin Qiu,Yilu Liu,Eng Kiat Chan,Lawrence L.J.Cao oad shedding takes place as an emergency measure used for generation units to avoid malfunction due to in cases of falling frequency conditions or loss of surges.This time delay may result in fast frequency drop power generation.Particularly in isolated (island) under severe overload conditions and cause under-fre- systems,due to lower inertia and limited reserves,the quency relay tripping in the generation unit.The load- rate of frequency decay due to loss of generation can be shedding strategy may,therefore,fail.Modern SCADA more pronounced.Therefore,a more carefully designed systems are designed as open and distributed systems. load-shedding scheme is required in an island system Industry-standard hardware and software,configured in than in a large interconnected system. local area networks (LAN),have become the basic sys- To restore the system operating frequency after seri- tem building blocks in SCADA systems.The intense com- ous disturbances,one of the main protection strategies petition among different SCADA vendors requires the is the under-frequency load-shedding scheme.It is sim- distributed computer systems to be low-cost,fast- ple and inexpensive;however,sometimes it has poor response,and high reliability.Low cost demands the use performance in coordination due to tedious steps and is of simple,standardized,open systems and easy installa- apt to result in over or under shedding.In addition, tion,configuration,and extension.Real-time is character- under-frequency relays with time delay are generally ized by high bandwidth,low latency,fast,and continuous update of the system status.Achievement of B.Oiu and Y.Liu are with Virginia Polytechnic Institute and State Uni- reliability is by means of structural redundancy and a versity,Blacksburg.Virginia,USA.E.K.Chan is with Power Automation fault-tolerant computer control system. Pte.Ltd.,Singapore.LL.J.Cao is with Foxboro Australia Pty.Ltd., Advances in network and communication technolo- Alexandria,NSW,Australia. gies open the door for fast load-shedding controller 38 IEEE Computer Applications in Power ISSN0895-0156/01/10.00©2001IEEE

38 IEEE Computer Applications in Power ISSN 0895-0156/01/$10.00©2001 IEEE L oad shedding takes place as an emergency measure in cases of falling frequency conditions or loss of power generation. Particularly in isolated (island) systems, due to lower inertia and limited reserves, the rate of frequency decay due to loss of generation can be more pronounced. Therefore, a more carefully designed load-shedding scheme is required in an island system than in a large interconnected system. To restore the system operating frequency after seri￾ous disturbances, one of the main protection strategies is the under-frequency load-shedding scheme. It is sim￾ple and inexpensive; however, sometimes it has poor performance in coordination due to tedious steps and is apt to result in over or under shedding. In addition, under-frequency relays with time delay are generally used for generation units to avoid malfunction due to surges. This time delay may result in fast frequency drop under severe overload conditions and cause under-fre￾quency relay tripping in the generation unit. The load￾shedding strategy may, therefore, fail. Modern SCADA systems are designed as open and distributed systems. Industry-standard hardware and software, configured in local area networks (LAN), have become the basic sys￾tem building blocks in SCADA systems. The intense com￾petition among different SCADA vendors requires the distributed computer systems to be low-cost, fast￾response, and high reliability. Low cost demands the use of simple, standardized, open systems and easy installa￾tion, configuration, and extension. Real-time is character￾ized by high bandwidth, low latency, fast, and continuous update of the system status. Achievement of reliability is by means of structural redundancy and a fault-tolerant computer control system. Advances in network and communication technolo￾gies open the door for fast load-shedding controller Bin Qiu, Yilu Liu, Eng Kiat Chan, Lawrence L.J. Cao B. Qiu and Y. Liu are with Virginia Polytechnic Institute and State Uni￾versity, Blacksburg, Virginia, USA. E.K. Chan is with Power Automation Pte. Ltd., Singapore. L.L.J. Cao is with Foxboro Australia Pty. Ltd., Alexandria, NSW, Australia. ©2000 Image 100 Ltd

(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

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 Power

40 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

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 Juby200141

cy. 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

Load Shedding Controller-ILD Datibase Monogement 四型回w The refinery facility power plant is an isolated GTG S power plant with four GTGs.Each GTG's full load is D Infnrmation ED Numbor GTG1 I0 Type GTG Leading(MW)0 00 around 40 MW.The actual output and rated output FNr DIF IP Addre车s Tel N am Mask are determined by online monitoring of out-going CB Status 100 192320g100 0000011 CB status,output,and ambient air temperature. CB-Commend 2 12p 201 0000000 GAII Normal operation condition is such that there is Da Nr DiF IP Address Poit Nr Puwer (MW) 100123203100380 4 always one generator serving as spinning reserve Temp (Deg C) 10012320100300 576 0T0 About 44 loads at 33/6.6 kV in 16 substations can CCPrnt Pesition in Priority List IED Nimber be controlled by the LSC.Their CB statuses are GIGI reported to the LSC by exception reporting.The CB GTG Ut21■Ust61■Ut101wt141■ status is then registered by the LSC.Subsequent A中 L3■u7目■t1■t5 load-shedding logic will exclude those CBs that are S5-0166kV94PM211C L41■851■t21■m161 SS-0166KV 94PMZ21A currently in an open position. Hoip DIUs send a life-sign signal to the LSC at a preset interval (normally 10 s).This signal is used to moni- tor the health of the communication link.If more Figure 5.LSC IED database than 6 life-signs are missing,the LSC will treat this connection as "down."Load-shedding logic also network might be critical for the system control deci- excludes those CBs that indicate a failed communica- sion-making.Monitoring the operating status of the tion link whole network and reporting the network communica- When communication is reestablished,a general tion failure becomes an essential function. request will be sent to DIUs to request update for all relat- In the LSC design,the simple network management ed CB status.The same applies to all GTG parameters. protocol (SNMP)is employed to monitor the LAN communication status.Network management of the LSC Configuration LAN consists of network management stations (man- The LSC application is running on a industrial PC with agers)communicating with network elements (IEDs). dual power supply and dual NIC.The operating system is The software in the network element that runs the Windows NT.SQL Server is used as the backend sup- management software is called an agent.The manager porting database.The application on the LSC PC is com- can ask the agent for a particular value,or the agent piled using C++.The SCADA server is running on Unix can report to the manager that something important Solaris with Siemens Spectrum SCADA software. happened (e.g.,an IED has failed);through this two- way communication,the status of all LAN communica- LSC PMI tion can be monitored.When a network component is The LSC person-machine interface (PMD)consists of dis- down,an error or warning message is sent to the con- plays that give the operator a current system situation trol center immediately to avoid system malfunction. overview as required for load shedding.It enables inter- action by input of selected parameters,such as priority Database Operation numbers,deactivation of sequences. A local database is used by the LSC to store power plant Device parameters are stored in the LSC local data- equipment parameters as well as historical event data. base.New devices (such as GTG.load.feeder)can be When the LSC starts,power plant information (including dynamically added or deleted through the IED data- IED name,DIU IP address,data communication telegram, base management display shown in Figure 5.The data mask,etc.)is transmitted from the SCADA server to the contains loads and generations,IED number,associat- LSC database.Different load-shedding priority lists and ed gateway number,load value,telegram number,and the current active list is stored in the LSC database as mask for the signal control.System parameters are well.In case communication with SCADA server is lost, also set up in this display.Using this screen,opera- the LSC will shed loads according to the information in tors can manually set and change and IED parameters. the database.During operation,important messages LSC parameters (such as the life-sign checking inter- (such as system alarms,communication abnormal,load- val,overload response time,and overload scan inter- shedding events)will be stored for future historical data val)are also configured in this display.LAN retrieval.Open database connectivity (ODBC)is used as connection parameters can also be modified for differ- a bridge between the LSC and the local database. ent systems. Figure 6 shows the main load-shedding active pri- Implementation Experience ority list display.The load IED number,gateway num- The LAN-based LSC design has been successfully imple- ber,IP address,and load value are listed.The priority mented in an oil refinery facility power plant. list is downloaded from the main SCADA server and 42 IEEE Computer Applications in Power

network might be critical for the system control deci￾sion-making. Monitoring the operating status of the whole network and reporting the network communica￾tion failure becomes an essential function. In the LSC design, the simple network management protocol (SNMP) is employed to monitor the LAN communication status. Network management of the LAN consists of network management stations (man￾agers) communicating with network elements (IEDs). The software in the network element that runs the management software is called an agent. The manager can ask the agent for a particular value, or the agent can report to the manager that something important happened (e.g., an IED has failed); through this two￾way communication, the status of all LAN communica￾tion can be monitored. When a network component is down, an error or warning message is sent to the con￾trol center immediately to avoid system malfunction. Database Operation A local database is used by the LSC to store power plant equipment parameters as well as historical event data. When the LSC starts, power plant information (including IED name, DIU IP address, data communication telegram, mask, etc.) is transmitted from the SCADA server to the LSC database. Different load-shedding priority lists and the current active list is stored in the LSC database as well. In case communication with SCADA server is lost, the LSC will shed loads according to the information in the database. During operation, important messages (such as system alarms, communication abnormal, load￾shedding events) will be stored for future historical data retrieval. Open database connectivity (ODBC) is used as a bridge between the LSC and the local database. Implementation Experience The LAN-based LSC design has been successfully imple￾mented in an oil refinery facility power plant. The refinery facility power plant is an isolated power plant with four GTGs. Each GTG’s full load is around 40 MW. The actual output and rated output are determined by online monitoring of out-going CB status, output, and ambient air temperature. Normal operation condition is such that there is always one generator serving as spinning reserve. About 44 loads at 33/6.6 kV in 16 substations can be controlled by the LSC. Their CB statuses are reported to the LSC by exception reporting. The CB status is then registered by the LSC. Subsequent load-shedding logic will exclude those CBs that are currently in an open position. DIUs send a life-sign signal to the LSC at a preset interval (normally 10 s). This signal is used to moni￾tor the health of the communication link. If more than 6 life-signs are missing, the LSC will treat this connection as “down.” Load-shedding logic also excludes those CBs that indicate a failed communica￾tion link. When communication is reestablished, a general request will be sent to DIUs to request update for all relat￾ed CB status. The same applies to all GTG parameters. LSC Configuration The LSC application is running on a industrial PC with dual power supply and dual NIC. The operating system is Windows NT. SQL Server is used as the backend sup￾porting database. The application on the LSC PC is com￾piled using C++. The SCADA server is running on Unix Solaris with Siemens Spectrum SCADA software. LSC PMI The LSC person-machine interface (PMI) consists of dis￾plays that give the operator a current system situation overview as required for load shedding. It enables inter￾action by input of selected parameters, such as priority numbers, deactivation of sequences. Device parameters are stored in the LSC local data￾base. New devices (such as GTG, load, feeder) can be dynamically added or deleted through the IED data￾base management display shown in Figure 5. The data contains loads and generations, IED number, associat￾ed gateway number, load value, telegram number, and mask for the signal control. System parameters are also set up in this display. Using this screen, opera￾tors can manually set and change and IED parameters. LSC parameters (such as the life-sign checking inter￾val, overload response time, and overload scan inter￾val) are also configured in this display. LAN connection parameters can also be modified for differ￾ent systems. Figure 6 shows the main load-shedding active pri￾ority list display. The load IED number, gateway num￾ber, IP address, and load value are listed. The priority list is downloaded from the main SCADA server and Figure 5. LSC IED database 42 IEEE Computer Applications in Power

saved to the local database.Up to 12 lists can be pre- tems (WSDS)for access via Internet,"IEEE Trans.Power Syst,pp.206- set by the operator.The active list is determined by 212.May2000. the SCADA system administrator as well.Through this ODBC Web site.Available:http://www.microsoft.com/data/odbc. display,an operator can move a specific load up or down manually as well as add or remove a load from Biographies the list. Bin Qiu is a Ph.D.candidate of Virginia Tech.He received an M.S.in electrical engineering from Xi'an Jiaotong University,China,and an When an event occurs,messages are sent to two M.S.in electrical engineering from Nanyang Technical University,Singa- displays.Common system messages are sent to the pore.He worked in Power Automation Pte.,Ltd,a joint venture alarm list for display.The event list is used for warn- between Singapore Power and Siemens,as a network and software ing or urgent messages.In the meantime,common engineer.His interests include power system automation and comput- er and network applications in the power systems.He may be reached messages are saved to log file and events saved to the by e-mail,qiubin@vt.edu. database for the historical data retrieval.Figure 7 Yilu Liu is an associate professor at Virginia Tech.Her main shows the event list display. research interests are Internet and IT applications related to power system monitoring and control.She may be reached by e-mail, yilu@vt.edu. Open to the Future Eng Kiat Chan is managing director of Power Automation Pte,Ltd, Power system communication internetworking,such Singapore,and may be reached by e-mail,chanek@pa.cpm.sg. as the interconnection of multiple independent utility Lawrence Cao is a senior engineer of Foxboro Australia Pty Ltd.He received his Ph.D.from Sydney University,Australia.He is currently systems,has become the norm in new SCADA systems. working on n EMS project as a software team leader.He may be With rapid development of broadband techniques reached by e-mail,ljcao@foxboro.com.au. such as giga/tera bps fiberoptic and quality of service (QoS),LAN-based load shedding can be extended to large complex systems to per- form a certain degree of WAN-based control Lood Shedding Scquenoe and protection functions,such as load shed- CB St IED 5t DIFNr DIF Status Load( No 1 SS 146.6KV29KM.N/A 1方3 DisconnectD...040 ding and emergency area separation. Acknowledgment SS-2666KVTR-26_ A 470 SS-316.6KV73PM. 200 This project was implemented and commissioned in Power Ha SS-316.6KV73PM. 1042 200 No933316.6KV13PM- NA 10482 Disconnect/D... Automation Pte.Ltd.The authors wish to thank all col- 200 A N 2 leagues involved in the project for their hard work and No. A 10590 cooperation. 0000 D. For Further Reading B K.Rajamani and U.K.Hambarde,"Islanding and load shed- GTG1 GTG2 GTG3 GTG4 GTG5 COMI Listening ding schemes for captive power plant,"IEEE Trans.Power A N/A NA N/A N/A COM Dennet rl Delivery,vol.14.pp.805-809,Jul.1999. C.Concordia,L.H.Fink,and Geroge Poullikkas,"Load shedding on an isolated system,"IEEE Trans.Power Syst,vol. 10,pp.1467-1472,Aug.1995. Figure 6.LSC load shedding main screen O.Moya,"Power system computer controlled load shedding,"Electric Power System Research,pp.165-171, April 1996. y图回到■ G.S.Grewal and J.W.Konowalec,"Optimization of load shedding sheme in an integrated process plant,"IEEE Trans Event List Continue from Previous Page Pag2at27 Ind.Appl,vol.35,Jul./Aug.1999. 08 Jun 18:4603 [Real LSC]Open Log Fie [e:qiubinprogramvrainerVoadshedLog Eventl L.L.Peterson,et al.,Computer Network:A System 08 Jun 18:4603 [Real LSC]Open Alarm Log File [e:qiubin'vrprogramraineroadshed Logv Approach,San Mateo,CA:Morgan Kaufmann,2000. 184600 A.Berson,Client/Server Architecture,New York,NY: McGraw Hill,1997. Jun 184604 (Real ELCS]S r2 Socket can not be set Jun 18:4604 [Real DIF]LSC Socket (DIF100)Created on LAN 1 D.Guilfoyle,E.Connolly,"Distributed SCADA systems Jun 18:4604 [Real DIF]LSC Socket (DIF105)Created on LAN 1 for electricity distribution control,"Power Technology Int, Jun 18:4604 [Real DIF]LSC Socket (DIF43)Created on LAN 1 Last Page pp.169-172,1994. n 18:4604 IFeal DiFl Net Page M.Adamiak,et al.,"The role of utility communication in a deregulated environment,"in Proc.Hawai Int.Conf.Syst. Jun 18:4604 (Real DiF]LSC Socket (DiF73)Created on LAN 1 Previous Page Jun 18:4604 [Real DIF]LSC Socket (DIF35)Created on LAN 1 Sciences,,Maui,Hawii,Jan.1999. Jun 18:4604 [Real DIF]LSC Socket (DIF38)Created on LAN 1 First Page M.J.Menz and B.Payne,"Servers in SCADA applica- tions,"IEEE Trans.Ind.Appl,pp.1295-1299.Sept.1997. "Test methodologies,setup,and result documenta- LSC Socket (D3C, LAN 1 Last Page Hlelp tion for EPRI sponsored benchmark of Ethernet for protection control."Available:ftp://sisconet.com/epri/ benchmrk/ethernet.zip. E BEALAOMAUE下U B.Qiu and H.B.Gooi,"Internet-based SCADA display sys- Figure 7.LSC event list screen Juy200143

saved to the local database. Up to 12 lists can be pre￾set by the operator. The active list is determined by the SCADA system administrator as well. Through this display, an operator can move a specific load up or down manually as well as add or remove a load from the list. When an event occurs, messages are sent to two displays. Common system messages are sent to the alarm list for display. The event list is used for warn￾ing or urgent messages. In the meantime, common messages are saved to log file and events saved to the database for the historical data retrieval. Figure 7 shows the event list display. Open to the Future Power system communication internetworking, such as the interconnection of multiple independent utility systems, has become the norm in new SCADA systems. With rapid development of broadband techniques such as giga/tera bps fiberoptic and quality of service (QoS), LAN-based load shedding can be extended to large complex systems to per￾form a certain degree of WAN-based control and protection functions, such as load shed￾ding and emergency area separation. Acknowledgment This project was implemented and commissioned in Power Automation Pte. Ltd. The authors wish to thank all col￾leagues involved in the project for their hard work and cooperation. For Further Reading K. Rajamani and U.K. Hambarde, “Islanding and load shed￾ding schemes for captive power plant,” IEEE Trans. Power Delivery, vol. 14, pp. 805-809, Jul. 1999. C. Concordia, L.H. Fink, and Geroge Poullikkas, “Load shedding on an isolated system,” IEEE Trans. Power Syst., vol. 10, pp. 1467-1472, Aug. 1995. O. Moya, “Power system computer controlled load shedding,” Electric Power System Research, pp. 165-171, April 1996. G.S. Grewal and J.W. Konowalec, “Optimization of load shedding sheme in an integrated process plant,” IEEE Trans. Ind. Appl., vol. 35, Jul./Aug. 1999. L.L. Peterson, et al., Computer Network: A System Approach, San Mateo, CA: Morgan Kaufmann, 2000. A. Berson, Client/Server Architecture, New York, NY: McGraw Hill, 1997. D. Guilfoyle, E. Connolly, “Distributed SCADA systems for electricity distribution control,” Power Technology Int., pp. 169-172, 1994. M. Adamiak, et al., “The role of utility communication in a deregulated environment,” in Proc. Hawai Int. Conf. Syst. Sciences, , Maui, Hawii, Jan. 1999. M.J. Menz and B. Payne, “Servers in SCADA applica￾tions,” IEEE Trans. Ind. Appl, pp. 1295-1299, Sept. 1997. “Test methodologies, setup, and result documenta￾tion for EPRI sponsored benchmark of Ethernet for protection control.” Available: ftp://sisconet.com/epri/ benchmrk/ethernet.zip. B. Qiu and H.B. Gooi, “Internet-based SCADA display sys￾tems (WSDS) for access via Internet,” IEEE Trans. Power Syst., pp. 206- 212, May 2000. ODBC Web site. Available: http://www.microsoft.com/data/odbc. Biographies Bin Qiu is a Ph.D. candidate of Virginia Tech. He received an M.S. in electrical engineering from Xi’an Jiaotong University, China, and an M.S. in electrical engineering from Nanyang Technical University, Singa￾pore. He worked in Power Automation Pte., Ltd, a joint venture between Singapore Power and Siemens, as a network and software engineer. His interests include power system automation and comput￾er and network applications in the power systems. He may be reached by e-mail, qiubin@vt.edu. Yilu Liu is an associate professor at Virginia Tech. Her main research interests are Internet and IT applications related to power system monitoring and control. She may be reached by e-mail, yilu@vt.edu. Eng Kiat Chan is managing director of Power Automation Pte, Ltd, Singapore, and may be reached by e-mail, chanek@pa.cpm.sg. Lawrence Cao is a senior engineer of Foxboro Australia Pty Ltd. He received his Ph.D. from Sydney University, Australia. He is currently working on n EMS project as a software team leader. He may be reached by e-mail, ljcao@foxboro.com.au. Figure 6. LSC load shedding main screen Figure 7. LSC event list screen July 2001 43