Über die Autorin bzw. den Autor

Savu C. Savulescu, PhD, has more than thirty years of experience in computer engineering, utility operations, planning, and control. Currently CEO of Energy Consulting International, Inc., he has worked predominantly in the design and implementation of utility information systems, such as SCADA/EMS, and developed stability assessment software that is being used in real-time and off-line in the U.S., Europe, Latin America, and Asia. Dr. Savulescu has taught electric power systems and computer sciences at major universities in Belgium, Brazil, and the U.S.

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Practical, hands-on techniques for assessing power system stability in real-time

In response to the growing trend for using online stability assessment to quickly tell how far a given operating state is from instability, this book presents in a single volume the state-of-the-art in this rapidly advancing field. It begins with a SCADA/EMS primer aimed at familiarizing readers with the real-time and study-mode data environments in modern control centers. These installations are quite sophisticated and offer superb application integration opportunities that were not available just a few years ago. This background is complemented with a brief review of the stability landscape from the real-time implementation perspective.

The subsequent material is clustered along the lines traditionally recognized in the industry from steady-state stability, to transient stability, and to voltage stability. Within these clusters, each chapter describes actual solutions, emphasizes the particular challenges that were faced, shows how the problems were solved, and sheds light on the experimental results.

Because the book aims primarily at the practical aspects of implementing stability assessment in real-time, the space for theoretical background in each chapter was reduced to the strictest minimum. For the benefit of readers who may not be quite familiar with the underlying theoretical techniques, appendices that describe the key algorithms and theoretical issues directly related to the subject matter of the book are included. This is a valuable resource for students, researchers, and practitioners who are directly involved in the operating reliability of modern transmission systems.

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Practical, hands-on techniques for assessing power system stability in real-time

In response to the growing trend for using online stability assessment to quickly tell how far a given operating state is from instability, this book presents in a single volume the state-of-the-art in this rapidly advancing field. It begins with a SCADA/EMS primer aimed at familiarizing readers with the real-time and study-mode data environments in modern control centers. These installations are quite sophisticated and offer superb application integration opportunities that were not available just a few years ago. This background is complemented with a brief review of the stability landscape from the real-time implementation perspective.

The subsequent material is clustered along the lines traditionally recognized in the industry—from steady-state stability, to transient stability, and to voltage stability. Within these clusters, each chapter describes actual solutions, emphasizes the particular challenges that were faced, shows how the problems were solved, and sheds light on the experimental results.

Because the book aims primarily at the practical aspects of implementing stability assessment in real-time, the space for theoretical background in each chapter was reduced to the strictest minimum. For the benefit of readers who may not be quite familiar with the underlying theoretical techniques, appendices that describe the key algorithms and theoretical issues directly related to the subject matter of the book are included. This is a valuable resource for students, researchers, and practitioners who are directly involved in the operating reliability of modern transmission systems.

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Real-Time Stability Assessment in Modern Power System Control Centers

John Wiley & Sons

Chapter One

Sudhir Virmani and Savu C. Savulescu

1.1 INTRODUCTION

1.1.1 General Background

Most large industrial control systems need to collect data at a central location, or at distributed sites, from a range of equipment and devices in the field, and to process this data in order to make a decision regarding any action required. Electric power control systems work basically in the same way but impose particularly stringent requirements on remote data acquisition and related processes because:

Power systems may encompass large geographical areas as almost all electric utilities have strong electrical interconnections with neighboring systems, which are generally owned and operated by different entities. Examples include: the interconnected systems in North America, such as the Western Interconnection, which consists of the power systems in all the western U.S. states plus the provinces British Columbia, Alberta, and Manitoba in Canada; and the large interconnection in mainland Europe, covering all the mainland European Union member countries plus some nonmembers.

Interconnected power networks are therefore very large, with potentially tens of thousands of nodes and branches and thousands of generating units.

Power systems in general must operate synchronously and this requires that all the interconnected systems must operate cooperatively in order to maintain reliability of the entire system.

Because of these strong interconnections, any disturbance in one part of the large network can affect the rest of the network.

Power system disturbances can propagate very rapidly (milliseconds to seconds) and this requires high-performance control systems, some of which are local, such as protective relays that operate in milliseconds, and some at central sites such as SCADA/EMS systems, which typically operate on the time frame of seconds (monitoring and control) to several days (scheduling).

Power system operations typically entail control requirements that can be met only by implementing complex hierarchies of regional and central/national control systems.

Power system operations in the context of large regional or subcontinental electric markets typically require exchange of information and coordination of control actions among various entities, such as independent system operators, security coordinators, and transmission system operators, thus leading to a higher degree of coordination and control systems.

1.1.2 Anatomy of a SCADA System

The data acquisition systems that are implemented in the utility industry therefore have to be able to support these needs. Furthermore, in order to make sure that the control actions being taken are correct and safe, certain control actions performed centrally require a positive confirmation, that is, they must be supervised. This is the supervisory control function and, therefore, the overall system is called supervisory control and data acquisition or SCADA.

The basic elements included in, and the minimum capability of, a typical SCADA system, consist of:

Interfaces in the field (substations) to equipment and devices located within the substation.

Ability to scan these interfaces to obtain the values of various quantities such as real and reactive power, current, voltage, and switch and circuit breaker position. The data are either reported by exception or scanned periodically. Typical scan rates are every 1-2 seconds for generation and interchange data and circuit breaker status indications; every 2-15 seconds for line flow and voltage measurements, and every 15 minutes to one hour for energy values.

Transmission of these data items to a central location known as the SCADA (or SCADA/EMS, as shown in Section 1.4) center.

Processing and analyzing this information at the SCADA center and displaying it to the operator.

Determining any control action to be taken either automatically or by operator request. The control actions required can be for controlling real power, reactive power, voltage, circuit breakers, and power flows.

Transmitting the request for control to the field equipment.

Monitoring the completion of the control request.

Building the real-time database and periodically saving real-time information for archival purposes.

1.1.3 Real-Time Versus Study-Mode Processes

Most of the SCADA functions are executed in real-time. By real-time we mean that the:

Input data reflect the most recent picture of the system conditions. In the field (substation), they come directly from devices that capture analog values and status indications; at the SCADA center they are stored into, and retrieved from, the real-time database.

Processing is performed within very short delays typically not exceeding a couple of seconds.

Output is usable almost instantly; again, "instantly" in this context means approximately one to two seconds.

The monitoring of data generated by a real-time process is a typical example of real-time activity. But the information generated in a SCADA system can be used in many other ways that do not qualify as real-time. For example, statistics can be built to record how many times the taps of a tap changing under load (TCUL) transformer have moved during a specified period of time. The tap changes were recorded in the real-time database immediately after they occurred, and then they were exported to some archival system and became historical data. The calculations entailed in building the statistic constitute a "study" performed with "real-time" data and, perhaps, some additional information; thus we will say that this is a "study-mode" calculation.

In the computational environment of a modern power system control center, some functions are performed only in real-time, whereas some others are performed only in study mode. However, as we will see in the next section, there are functions that can be used both in real-time and in study mode.

Let us say in passing that real-time and online are not necessarily interchangeable attributes. On line implies that the calculations are available to the operator in the SCADA/EMS system itself, hence they are online as opposed to being available on some other separate system. However, there is no guarantee that the online computational process will be fast enough to produce results that can be labeled real-time. These considerations should help the reader understand the difference between the real-time stability assessment, stability monitoring, and online stability assessment concepts that are often mentioned throughout this book.

1.1.4 Next Level of Functionality: The EMS

In order to determine the control actions required, it is necessary to simulate the operation of the power system in close to...