Stanton, K N, Giri, J C, Bose, A.J. "Energy Management The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton CRC Press llc. 2000
Stanton, K.N., Giri, J.C., Bose, A.J. “Energy Management” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
67 Energy management 67.2 Power System Data Acquisition and Control 67.3 Automatic Generation Control Load Frequency Control. Economic Dispatch.Reserve Neil n y 67.5 Energy Management Cegelec ESCA Corporation 67.6 Security Control Anjan Bose 67.7 Operator Training Simulator Energy Control System. Power System Dynami Washington State University Simulation. Instructional System 67.1 Introduction Energy management is the process of monitoring, coordinating, and controlling the generation, transmission and distribution of electrical energy. The physical plant to be managed includes generating plants that produce energy fed through transformers to the high-voltage transmission network(grid), interconnecting generating plants and load centers. Transmission lines terminate at substations that perform switching, voltage transfor mation, measurement, and control. Substations at load centers transform to subtransmission and distribution levels. These lower-voltage circuits typically operate radially, i.e., no normally closed paths between substations through subtransmission or distribution circuits (Underground cable networks in large cities are an exception. Since transmission systems provide negligible energy storage, supply and demand must be balanced by either generation or load. Production is controlled by turbine governors at generating plants, and automatic generation control is performed by control center computers remote from generating plants. Load management, sometimes called demand-side management, extends remote supervision and control to subtransmission and distribution circuits, including control of residential, commercial, and industrial load Events such as lightning strikes, short circuits, equipment failure, or accidents may cause a system fault Protective relays actuate rapid, local control through operation of circuit breakers before operators can respond The goal is to maximize safety, minimize damage, and continue to supply load with the least inconvenience to customers. Data acquisition provides operators and computer control systems with status and measurement information needed to supervise overall operations. Security control analyzes the consequences of faults to establish operating conditions that are both robust and economical. Energy management is performed at control centers(see Fig. 67.1), typically called system control centers, by computer systems called energy management systems(EMS). Data acquisition and remote control is per- formed by computer systems called supervisory control and data acquisition(SCADA) systems. These latter systems may be installed at a variety of sites including system control centers. An EMS typically includes a SCADA"front-end"through which it communicates with generating plants, substations, and other remote Ices. Figure 67. 2 illustrates the applications layer of modern EMS as well as the underlying layers on which it is built: the operating system, a database manager, and a utilities/services laye c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 67 Energy Management 67.1 Introduction 67.2 Power System Data Acquisition and Control 67.3 Automatic Generation Control Load Frequency Control • Economic Dispatch • Reserve Monitoring • Interchange Transaction Scheduling 67.4 Load Management 67.5 Energy Management 67.6 Security Control 67.7 Operator Training Simulator Energy Control System • Power System Dynamic Simulation • Instructional System 67.1 Introduction Energy management is the process of monitoring, coordinating, and controlling the generation, transmission, and distribution of electrical energy. The physical plant to be managed includes generating plants that produce energy fed through transformers to the high-voltage transmission network (grid), interconnecting generating plants and load centers. Transmission lines terminate at substations that perform switching, voltage transformation, measurement, and control. Substations at load centers transform to subtransmission and distribution levels. These lower-voltage circuits typically operate radially, i.e., no normally closed paths between substations through subtransmission or distribution circuits. (Underground cable networks in large cities are an exception.) Since transmission systems provide negligible energy storage, supply and demand must be balanced by either generation or load. Production is controlled by turbine governors at generating plants, and automatic generation control is performed by control center computers remote from generating plants. Load management, sometimes called demand-side management, extends remote supervision and control to subtransmission and distribution circuits, including control of residential, commercial, and industrial loads. Events such as lightning strikes, short circuits, equipment failure, or accidents may cause a system fault. Protective relays actuate rapid, local control through operation of circuit breakers before operators can respond. The goal is to maximize safety, minimize damage, and continue to supply load with the least inconvenience to customers. Data acquisition provides operators and computer control systems with status and measurement information needed to supervise overall operations. Security control analyzes the consequences of faults to establish operating conditions that are both robust and economical. Energy management is performed at control centers (see Fig. 67.1), typically called system control centers, by computer systems called energy management systems (EMS). Data acquisition and remote control is performed by computer systems called supervisory control and data acquisition (SCADA) systems. These latter systems may be installed at a variety of sites including system control centers. An EMS typically includes a SCADA “front-end” through which it communicates with generating plants, substations, and other remote devices. Figure 67.2 illustrates the applications layer of modern EMS as well as the underlying layers on which it is built: the operating system, a database manager, and a utilities/services layer. K. Neil Stanton Stanton Associates Jay C. Giri Cegelec ESCA Corporation Anjan Bose Washington State University
FIGURE 67.1 Central dispatch operation arena of Entergy Corporation's Beaumont Control Center(Beaumont, Texas) OPERATING SYSTEM DATAbASE UTILITIES, SERVICES APPLICATIONS Operations Training AUTOMATIC ERATION Supervisory CONTROL POWER SYSTEM Contro SECURITY ONTROL FIGURE 67.2 Layers of a modern EMS. 67.2 Power System Data acquisition and Control A SCADA system consists of a master station that communicates with remote terminal units(rTUs) for the purpose of allowing operators to observe and control physical plants. Generating plants and transmission substations certainly justify RTUs, and their installation is becoming more common in distribution substations as costs decrease. RTUs transmit device status and measurements to, and receive control commands and setpoint data from, the master station. Communication is generally via dedicated circuits operating in the range of 600 to 4800 bits/s with the RTU responding to periodic requests initiated from the master station(polling)every 2 to 10 s, depending on the criticality of the data The traditional functions of SCADA systems are summarized Data acquisition: Provides telemetered measurements and status information to operator Supervisory control: Allows operator to remotely control devices, e.g., open and close circuit breakers A"select before operate" procedure is used for greater safety Tagging: Identifies a device as subject to specific operating restrictions and prevents unauthorized operation. c2000 by CRC Press LLC
© 2000 by CRC Press LLC 67.2 Power System Data Acquisition and Control A SCADA system consists of a master station that communicates with remote terminal units (RTUs) for the purpose of allowing operators to observe and control physical plants. Generating plants and transmission substations certainly justify RTUs, and their installation is becoming more common in distribution substations as costs decrease. RTUs transmit device status and measurements to, and receive control commands and setpoint data from, the master station. Communication is generally via dedicated circuits operating in the range of 600 to 4800 bits/s with the RTU responding to periodic requests initiated from the master station (polling) every 2 to 10 s, depending on the criticality of the data. The traditional functions of SCADA systems are summarized: • Data acquisition: Provides telemetered measurements and status information to operator. • Supervisory control: Allows operator to remotely control devices, e.g., open and close circuit breakers. A “select before operate” procedure is used for greater safety. • Tagging: Identifies a device as subject to specific operating restrictions and prevents unauthorized operation. FIGURE 67.1 Central dispatch operation arena of Entergy Corporation’s Beaumont Control Center (Beaumont, Texas) which includes a modern EMS. FIGURE 67.2 Layers of a modern EMS
Alarms: Informs operator of unplanned events and undesirable operating conditions. Alarms are sorted by criticality, area of responsibility, and chronology. Acknowledgment may be required. Logging: Logs all operator entry, all alarms, and selected information Load shed: Provides both automatic and operator-initiated tripping of load in response to system emergencies. Trending: Plots measurements on selected time scales Since the master station is critical to power system operations, its functions are generally distributed among several computer systems depending on specific design. A dual computer system configured in primary and standby modes is most common SCADA functions are listed below without stating which computer has specific Manage communication circuit configuration Downline load RTU files Maintain scan tables and Check and correct message errors Convert to engineering units Detect status and measurement changes Monitor abnormal and out-of-limit conditions Log and time-tag sequence of events Respond to operator request Display information Acknowledge alarms Transmit control action to rtus Inhibit unauthorized actions Maintain historical files Log events and prepare reports Perform load shedding 67.3 Automatic Generation Control Automatic generation control (AGC) consists of two major and several minor functions that operate on-line in real time to adjust the generation against load at minimum cost. The major functions are load frequency control and economic dispatch, each of which is described below. The minor functions are reserve monitoring, which assures enough reserve on the system, interchange scheduling, which initiates and completes scheduled inter hanges, and other similar monitoring and recording functions. Load Frequency Control Load frequency control (LFC)has to achieve three primary objectives which are stated below in priority order 1. To maintain frequency at the scheduled value 2. To maintain net power interchanges with neighboring control areas at the scheduled values 3. To maintain power allocation among units at economically desired values The first and second objectives are met by monitoring an error signal, called area control error (ACE), which is a combination of net interchange error and frequency error and represents the power imbalance between c 2000 by CRC Press LLC
© 2000 by CRC Press LLC • Alarms: Informs operator of unplanned events and undesirable operating conditions. Alarms are sorted by criticality, area of responsibility, and chronology. Acknowledgment may be required. • Logging: Logs all operator entry, all alarms, and selected information. • Load shed: Provides both automatic and operator-initiated tripping of load in response to system emergencies. • Trending: Plots measurements on selected time scales. Since the master station is critical to power system operations, its functions are generally distributed among several computer systems depending on specific design. A dual computer system configured in primary and standby modes is most common. SCADA functions are listed below without stating which computer has specific responsibility. • Manage communication circuit configuration • Downline load RTU files • Maintain scan tables and perform polling • Check and correct message errors • Convert to engineering units • Detect status and measurement changes • Monitor abnormal and out-of-limit conditions • Log and time-tag sequence of events • Detect and annunciate alarms • Respond to operator requests to: Display information Enter data Execute control action Acknowledge alarms • Transmit control action to RTUs • Inhibit unauthorized actions • Maintain historical files • Log events and prepare reports • Perform load shedding 67.3 Automatic Generation Control Automatic generation control (AGC) consists of two major and several minor functions that operate on-line in real time to adjust the generation against load at minimum cost. The major functions are load frequency control and economic dispatch, each of which is described below. The minor functions are reserve monitoring, which assures enough reserve on the system, interchange scheduling, which initiates and completes scheduled interchanges, and other similar monitoring and recording functions. Load Frequency Control Load frequency control (LFC) has to achieve three primary objectives which are stated below in priority order: 1. To maintain frequency at the scheduled value 2. To maintain net power interchanges with neighboring control areas at the scheduled values 3. To maintain power allocation among units at economically desired values The first and second objectives are met by monitoring an error signal, called area control error (ACE), which is a combination of net interchange error and frequency error and represents the power imbalance between
tion and load at any instant. This ACE must be filtered or smoothed such that excessive and random ges in ACE are not translated into control action. Since these excessive changes are different for different systems, the filter parameters have to be tuned specifically for each control area. The filtered ACE is then used to obtain the proportional plus integral control signal. This control signal is modified by limiters, deadbands, and gain constants that are tuned to the particular system. This control signal is then divided among the generating units under control by using participation factors to obtain unit cor trol errors These participation factors may be proportional to the inverse of the second derivative of the cost of unit generation so that the units would be loaded according to their costs, thus meeting the third objective. However, cost may not be the only consideration because the different units may have different response rates and it may be necessary to move the faster generators more to obtain an acceptable response. The UCEs are then sent to the various units under control and the generating units monitored to see that the corrections take place. This control action is repeated every 2 to 6s In spite of the integral control, errors in frequency and net interchange do tend to accumulate over time. These time errors and accumulated interchange errors have to be corrected by adjusting the controller settings according to procedures agreed upon by the whole interconnection. These accumulated errors as well as aCe serve as performance measures for LFC. The main philosophy in the design of lFC is that each system should follow its own load very closely during normal operation, while during emergencies each system should contribute according to its relative size in the interconnection without regard to the locality of the emergency. Thus, the most important factor in obtaining good control of a system is its inherent capability of following its own load. This is guaranteed if the system has adequate regulation margin as well as adequate response capability. Systems that have mainly thermal generation often have difficulty in keeping up with the load because of the slow response of the units The design of the controller itself is an important factor, and proper tuning of the controller parameters is needed to obtaingood "control without" excessive"movement of units. Tuning is system-specific, and although system simulations are often used as aids, most of the parameter adjustments are made in the field using Economic Dispatch Since all the generating units that are on-line have different costs of generation, it is necessary to find the generation levels of each of these units that would meet the load at the minimum cost. This has to take into account the fact that the cost of generation in one generator is not proportional to its generation level but is function of it. In addition pendent on the generation Certain other factors have to be considered when obtaining the optimum generation pattern. One is that the generation pattern provide adequate reserve margins. This is often done by constraining the generation level to a lower boundary than the generating capability. a more difficult set of constraints to consider are the ansmission limits. Under certain real-time conditions it is possible that the most economic pattern may not be feasible because of unacceptable line flows or voltage conditions. The present-day economic dispatch(ED) algorithm cannot handle these security constraints. However, alternative methods based on optimal power flows have been suggested but have not yet been used for real-time dispatch. The minimum cost dispatch occurs when the incremental cost of all the generators is equal. The cost functions of the generators are nonlinear and discontinuous. For the equal marginal cost algorithm to work it is necessary for them to be convex. These incremental cost curves are often represented as monotonically increasing iecewise-linear functions. A binary search for the optimal marginal cost is conducted by summing all the generation at a certain marginal cost and comparing it with the total power demand. If the demand is higher, t higher marginal cost is needed, and vice versa. This algorithm produces the ideal setpoints for all the generators for that particular demand, and this calculation is done every few minutes as the demand change The losses in the power system are a function of the generation pattern, and they are taken into account by multiplying the generator incremental costs by the appropriate penalty factors. The penalty factor for each generator is a reflection of the sensitivity of that generator to system losses, and these sensitivities can be obtained from the transmission loss factors(Section 67.6 c 2000 by CRC Press LLC
© 2000 by CRC Press LLC generation and load at any instant. This ACE must be filtered or smoothed such that excessive and random changes in ACE are not translated into control action. Since these excessive changes are different for different systems, the filter parameters have to be tuned specifically for each control area. The filtered ACE is then used to obtain the proportional plus integral control signal. This control signal is modified by limiters, deadbands, and gain constants that are tuned to the particular system. This control signal is then divided among the generating units under control by using participation factors to obtain unit control errors (UCE). These participation factors may be proportional to the inverse of the second derivative of the cost of unit generation so that the units would be loaded according to their costs, thus meeting the third objective. However, cost may not be the only consideration because the different units may have different response rates and it may be necessary to move the faster generators more to obtain an acceptable response. The UCEs are then sent to the various units under control and the generating units monitored to see that the corrections take place. This control action is repeated every 2 to 6 s. In spite of the integral control, errors in frequency and net interchange do tend to accumulate over time. These time errors and accumulated interchange errors have to be corrected by adjusting the controller settings according to procedures agreed upon by the whole interconnection. These accumulated errors as well as ACE serve as performance measures for LFC. The main philosophy in the design of LFC is that each system should follow its own load very closely during normal operation, while during emergencies each system should contribute according to its relative size in the interconnection without regard to the locality of the emergency. Thus, the most important factor in obtaining good control of a system is its inherent capability of following its own load. This is guaranteed if the system has adequate regulation margin as well as adequate response capability. Systems that have mainly thermal generation often have difficulty in keeping up with the load because of the slow response of the units. The design of the controller itself is an important factor, and proper tuning of the controller parameters is needed to obtain “good” control without “excessive” movement of units. Tuning is system-specific, and although system simulations are often used as aids, most of the parameter adjustments are made in the field using heuristic procedures. Economic Dispatch Since all the generating units that are on-line have different costs of generation, it is necessary to find the generation levels of each of these units that would meet the load at the minimum cost. This has to take into account the fact that the cost of generation in one generator is not proportional to its generation level but is a nonlinear function of it. In addition, since the system is geographically spread out, the transmission losses are dependent on the generation pattern and must be considered in obtaining the optimum pattern. Certain other factors have to be considered when obtaining the optimum generation pattern. One is that the generation pattern provide adequate reserve margins. This is often done by constraining the generation level to a lower boundary than the generating capability. A more difficult set of constraints to consider are the transmission limits. Under certain real-time conditions it is possible that the most economic pattern may not be feasible because of unacceptable line flows or voltage conditions. The present-day economic dispatch (ED) algorithm cannot handle these security constraints. However, alternative methods based on optimal power flows have been suggested but have not yet been used for real-time dispatch. The minimum cost dispatch occurs when the incremental cost of all the generators is equal. The cost functions of the generators are nonlinear and discontinuous. For the equal marginal cost algorithm to work it is necessary for them to be convex. These incremental cost curves are often represented as monotonically increasing piecewise-linear functions. A binary search for the optimal marginal cost is conducted by summing all the generation at a certain marginal cost and comparing it with the total power demand. If the demand is higher, a higher marginal cost is needed, and vice versa. This algorithm produces the ideal setpoints for all the generators for that particular demand, and this calculation is done every few minutes as the demand changes. The losses in the power system are a function of the generation pattern, and they are taken into account by multiplying the generator incremental costs by the appropriate penalty factors. The penalty factor for each generator is a reflection of the sensitivity of that generator to system losses, and these sensitivities can be obtained from the transmission loss factors (Section 67.6)