Figure 1.1 Typical Configuration of an Energy Management System Hardware In a modem energy management system, a computer (automatically or on dispatcher command) can issue commands which are transferred through the CIOCs, communication links, and RTUs to the survey points for execution. This configuration and function is illustrated in Figure 1.2. Figure 1.2 Basic SCADA System Configuration Figure 1.3 Schematic Representation of Control Schemes for a Generating Unit The Data Acquisition and Processing Subsystem comprises a set of software which process the data collected by the SCADA system. This data is utilized in two ways. Status data (circuit breaker status, interrupt switch status, transformer tap setting, etc.) are utilized to form the system configuration and model. The software which uses the status data for the purpose of computing the system configuration and model is known as system network configurator. Typically, this software is executed only when a change in status data occurs. All other data (analog measurements) is utilized to compute the best (in some sense) estimate of the operating state of the system. Two approaches are used in practice: (1) use of an on-line power flow, or (2) use of a state estimator. Figure 1.5. Network Configurator (a) Breaker Oriented Model (b) Bus Oriented Model The task performed by the system configurator is illustrated in Figure 1.5. The information received through the SCADA system determines the status of the breakers. The system configurator uses prestored information and the breaker/switches, etc., status to determine a ‘bus oriented model’, i.e., what circuits are connected to what bus and how much power is injected at a bus. This task is illustrated in Figure 1.5. The bus oriented model can be also utilized by other applications programs. Figure 1.6 Illustration of Measurements Necessary for an On-Line Power Flow With above definitions it is quite easy to explain how the data collected with the SCADA system are used to develop a mathematical description of the system operating state. This task can be achieved with procedures of varying degree of sophistication. In the early days of development, on-line power flows were used which were later replaced with state estimation models. A brief description of these models follows. Figure 1.7 Power System Operating States (after DyLiacco, Fink) Figure 1.8 Simplified View of aSCADA System (a) Survey Points (b) SCADA System Configuration Figure 1.9 Dual Computer Configuration Figure 1.11 Conceptual Hierarchical Structure of an Energy Management System Figure 1.11 illustrates in a conceptual manner a typical EMS structure for a vertically integrated electric power system. At the lower level, the Regional Dispatch Center (RDC) is the lower level decision maker. The RDC has the capability to send the following control commands, automatically or manually: (a) To power plants: Scheduled power and scheduled voltage or reactive power, and (b) To substations: Position of breakers, switches, transformer taps, etc.
