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III
Power System
Operation and
Control
Bruce F. Wollenberg
University of Minnesota
17 Energy Management Neil K. Stanton, Jay C. Giri, and Anjan Bose ............................. 17 - Power System Data Acquisition and Control.^ Automatic Generation Control.^ Load Management.^ Energy Management.^ Security Control.^ Operator Training Simulator 18 Generation Control: Economic Dispatch and Unit Commitment Charles W. Richter, Jr. ....................................................................................................... 18 - Economic Dispatch.^ The Unit Commitment Problem.^ Summary of Economical Generation Operation 19 State Estimation Danny Julian ........................................................................................ 19 - State Estimation Problem.^ State Estimation Operation.^ Example State Estimation Problem.^ Defining Terms 20 Optimal Power Flow Mohamed E. El-Hawary ............................................................... 20 - Conventional Optimal Economic Scheduling.^ Conventional OPF Formulation.^ OPF Incorporating Load Models.^ SCOPF Including Load Modeling.^ Operational Requirements for Online Implementation.^ Conclusions 21 Security Analysis Nouredine Hadjsaid ............................................................................ 21 - Definition.^ Time Frames for Security-Related Decision.^ Models. Determinist vs. Probabilistic.^ Appendix A.^ Appendix B
FIGURE 17.1 Manitoba Hydro Control Center in Winnipeg, Manitoba, Canada. (Photo used with permission of ALSTOM ESCA Corporation.)
APPLICATIONS
UTILITIES, SERVICES
DATABASE
OPERATING SYSTEM
Operations
LOAD MANAGEMENT
Training
SCADA
ENERGY MANAGEMENT
AUTOMATIC GENERATION CONTROL EMS FUNCTIONS POWER SYSTEM SIMULATION INSTRUCTIONAL SECURITY SYSTEM CONTROL
Supervisory Control
Data Acquisition
TRAINING SIMULATOR
Supervisory Control And Data Acquisition
FIGURE 17.2 Layers of a modern EMS.
17.1 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 dedi- cated 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. . (^) Alarms: Inform 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
17.2 Automatic Generation Control
Automatic generation control (AGC) consists of two major and several minor functions that operate on-line in realtime to adjust the generation against load at minimum cost. The major functions are load
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. This ED algorithm generally applies to only thermal generation units that have cost characteristics of the type discussed here. The hydro units have to be dispatched with different considerations. Although there is no cost for the water, the amount of water available is limited over a period, and the displacement of fossil fuel by this water determines its worth. Thus, if the water usage limitation over a period is known, say from a previously computed hydro optimization, the water worth can be used to dispatch the hydro units. LFC and the ED functions both operate automatically in realtime but with vastly different time periods. Both adjust generation levels, but LFC does it every few seconds to follow the load variation, while ED does it every few minutes to assure minimal cost. Conflicting control action is avoided by coordinating the control errors. If the unit control errors from LFC and ED are in the same direction, there is no conflict. Otherwise, a logic is set to either follow load (permissive control) or follow economics (mandatory control).
17.2.3 Reserve Monitoring
Maintaining enough reserve capacity is required in case generation is lost. Explicit formulas are followed to determine the spinning (already synchronized) and ready (10 min) reserves required. The availability can be assured by the operator manually, or, as mentioned previously, the ED can also reduce the upper dispatchable limits of the generators to keep such generation available.
17.2.4 Interchange Transaction Scheduling
The contractual exchange of power between utilities has to be taken into account by the LFC and ED functions. This is done by calculating the net interchange (sum of all the buy and sale agreements) and adding this to the generation needed in both the LFC and ED. Since most interchanges begin and end on the hour, the net interchange is ramped from one level to the new over a 10- or 20-min period straddling the hour. The programs achieve this automatically from the list of scheduled transactions.
17.3 Load Management
SCADA, with its relatively expensive RTUs installed at distribution substations, can provide status and measurements for distribution feeders at the substation. Distribution automation equipment is now
available to measure and control at locations dispersed along distribution circuits. This equipment can monitor sectionalizing devices (switches, interruptors, fuses), operate switches for circuit reconfigur- ation, control voltage, read customers’ meters, implement time-dependent pricing (on-peak, off-peak rates), and switch customer equipment to manage load. This equipment requires significantly increased functionality at distribution control centers. Distribution control center functionality varies widely from company to company, and the following list is evolving rapidly.
. (^) Data acquisition: Acquires data and gives the operator control over specific devices in the field. Includes data processing, quality checking, and storage. . (^) Feeder switch control: Provides remote control of feeder switches. . (^) Tagging and alarms: Provides features similar to SCADA. . (^) Diagrams and maps: Retrieves and displays distribution maps and drawings. Supports device selection from these displays. Overlays telemetered and operator-entered data on displays. . (^) Preparation of switching orders: Provides templates and information to facilitate preparation of instructions necessary to disconnect, isolate, reconnect, and reenergize equipment. . (^) Switching instructions: Guides operator through execution of previously prepared switching orders. . (^) Trouble analysis: Correlates data sources to assess scope of trouble reports and possible dispatch of work crews. . (^) Fault location: Analyzes available information to determine scope and location of fault. . (^) Service restoration: Determines the combination of remote control actions that will maximize restoration of service. Assists operator to dispatch work crews. . (^) Circuit continuity analysis: Analyzes circuit topology and device status to show electrically connected circuit segments (either energized or deenergized). . (^) Power factor and voltage control: Combines substation and feeder data with predetermined operating parameters to control distribution circuit power factor and voltage levels. . (^) Electrical circuit analysis: Performs circuit analysis, single-phase or three-phase, balanced or unbalanced. . (^) Load management: Controls customer loads directly through appliance switching (e.g., water heaters) and indirectly through voltage control. . (^) Meter reading: Reads customers’ meters for billing, peak demand studies, time of use tariffs. Provides remote connect=disconnect.
17.4 Energy Management
Generation control and ED minimize the current cost of energy production and transmission within the range of available controls. Energy management is a supervisory layer responsible for economically scheduling production and transmission on a global basis and over time intervals consistent with cost optimization. For example, water stored in reservoirs of hydro plants is a resource that may be more valuable in the future and should, therefore, not be used now even though the cost of hydro energy is currently lower than thermal generation. The global consideration arises from the ability to buy and sell energy through the interconnected power system; it may be more economical to buy than to produce from plants under direct control. Energy accounting processes transaction information and energy measurements recorded during actual operation as the basis of payment for energy sales and purchases. Energy management includes the following functions:
. (^) System load forecast: Forecasts system energy demand each hour for a specified forecast period of 1 to 7 days. . (^) Unit commitment: Determines start-up and shut-down times for most economical operation of thermal generating units for each hour of a specified period of 1 to 7 days.
. (^) Bus load forecasting: Uses real-time measurements to adaptively forecast loads for the electrical connectivity (bus) model of the power system network. . (^) Transmission loss factors: Determines incremental loss sensitivities for generating units; calculates the impact on losses if the output of a unit were to be increased by 1 MW. . (^) Short-circuit analysis: Determines fault currents for single-phase and three-phase faults for fault locations across the entire power system network.
17.6 Operator Training Simulator
Training simulators were originally created as generic systems for introducing operators to the electrical and dynamic behavior of power systems. Today, they model actual power systems with reasonable fidelity and are integrated with EMS to provide a realistic environment for operators and dispatchers to practice normal, every-day operating tasks and procedures as well as experience emergency operating situations. The various training activities can be safely and conveniently practiced with the simulator responding in a manner similar to the actual power system. An operator training simulator (OTS) can be used in an investigatory manner to recreate past actual operational scenarios and to formulate system restoration procedures. Scenarios can be created, saved, and reused. The OTS can be used to evaluate the functionality and performance of new real-time EMS functions and also for tuning AGC in an off-line, secure environment. The OTS has three main subsystems (Fig. 17.4).
17.6.1 Energy Control System
The energy control system (ECS) emulates normal EMS functions and is the only part of the OTS with which the trainee interacts. It consists of the supervisory control and data acquisition (SCADA) system, generation control system, and all other EMS functions.
17.6.2 Power System Dynamic Simulation
This subsystem simulates the dynamic behavior of the power system. System frequency is simulated using the ‘‘long-term dynamics’’ system model, where frequency of all units is assumed to be the same.
Real-time Network Analysis Sequence
Study Network Analysis
SCADA (^) TopologyNetwork EstimatorState ContingencyAnalysis EnhancementSecurity
Power Flow
Contingency Analysis
Bus Load Forecast
Transmission Loss Factors
Preventative Action
Optimal Power Flow
Optimal Power Flow
Short Circuit Analysis
FIGURE 17.3 Real-time and study network analysis sequences.
The prime-mover dynamics are represented by models of the units, turbines, governors, boilers, and boiler auxiliaries. The network flows and states (bus voltages and angles, topology, transformer taps, etc.) are calculated at periodic intervals. Relays are modeled, and they emulate the behavior of the actual devices in the field.
17.6.3 Instructional System
This subsystem includes the capabilities to start, stop, restart, and control the simulation. It also includes making savecases, retrieving savecases, reinitializing to a new time, and initializing to a specific real-time situation. It is also used to define event schedules. Events are associated with both the power system simulation and the ECS functions. Events may be deterministic (occur at a predefined time), conditional (based on a predefined set of power system conditions being met), or probabilistic (occur at random).
References
Application of Optimization Methods for Economy=Security Functions in Power System Operations, IEEE tutorial course, IEEE Publication 90EH0328-5-PWR, 1990. Distribution Automation, IEEE Power Engineering Society, IEEE Publication EH0280-8-PBM, 1988. C.J. Erickson, Handbook of Electrical Heating, IEEE Press, 1995.
Instructor
Instructor Applications
Power System Simulation Applications
Trainee
Events
Prime Movers
Simulation Control
EMS Applications
Relays
SCADA ECS
data retrieval controls
Power Flow Solution
Load Model Topology Processing
FIGURE 17.4 OTS block diagram.
