16-01-2013, 04:28 PM
Power System Modeling, Analysis and Control
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Introduction
The electrification of many processes through technological advances resulted in the continuous development and evolution of the electric power system over the last one hundred plus years. As an example, in the U. S. A. today, more than one third of energy consumption is in the form of electric energy. The modern day electric power system is responsible for generating, transmitting and delivering more than one third of the total consumed energy. With this progress, the complexity of the system has grown. To manage this complex system, monitoring, control and operation functions are computer assisted. The systems for computer control of electric power systems have evolved as computer and monitoring technology evolved. Throughout the years, these systems have been named “Control Centers”, “Energy Management Systems (EMS)”, “Independent System Operations”, etc. The names reflect the changing emphasis in the functions of these control centers. In this book, we will use the term Energy Management System.
Power System Control Functions
The operation of an electric power system is characterized with a number of control functions. Some of them are automatic and others require operator initiation. Consider, for example, a single unit power plant as in Figure 1.3. One can recognize a number of control loops:
(1) The Voltage Control Loop. The objective of this control loop is to regulate the voltage at the terminals of the generator. It consists of the voltage regulator and exciter system. Inputs to this control loop are the reference voltage Vref, which may be selected by the system dispatcher or automatically by computers (VAR dispatch), and the actual voltage at the terminals of the generator Vg.
(2) The Power System Stabilizer (PPS) Loop. The objective of this control loop is to slow down the oscillations of the generator following a disturbance. It consists of a feedback system which injects a stabilizing signal into the exciter system. Feedback quantities may be: frequency, f, real power, Pg, etc.
Data Acquisition and Processing Subsystem
The objective of the data acquisition and processing subsystem is to obtain an accurate (as much as possible) estimate of the operating state of the system. This is achieved with a large number of Remote Terminal Units (RTU) and associated communication network. The system is known as SCADA (Supervisory Control And Data Acquisition). The remote terminal units collect analog measurements (i.e. voltage magnitude, power flows, etc.) and status variables (i.e. status of breakers, switches, etc.) and transmit this data to the computers of the energy management system via the communication network. There, the topology of the network is formed (network configurator) and the state of the system is constructed (on-line power flow or state estimation). The results are displayed in whatever user interface media exist at the energy management system, i.e. computer monitors, mimic boards, projection systems, etc..
Security Monitoring and Control Subsystem
Security of an electric power system is loosely defined as the ability of the system to withstand major disturbances without losing synchronism. The security of the system is a very complex concept. Experience accumulated over the years indicates that the security of the system can be only insured by continuous monitoring and control of the system. Security control comprises the integration of a number of automated and manual control operations.
Summary and Discussion
An overview of a modern Energy Management System has been presented. Emphasis was placed on the basic structure of an EMS. The design of an EMS evolves as technology advances: computers become faster and more powerful, display hardware improve in resolution and capability, microcontrollers dominate the design of Remote Terminal Units, etc. In the past couple of decades, we have experienced a tremendous progress in SCADA and computer hardware. Yet, central to the success of an EMS is the software for the overall coordination and optimization of power system operation. The rapid advances in hardware technology have been mediated with slow progress in software development for coordination and optimization of large scale systems. Much work remains to be done in this area. One hopes that developments and progress will continue in both hardware and software in a coordinated way. This book is focused on the concepts and objectives which drive the developments of Energy Managements Systems.