Phasor Measurement Units and Wide Area Monitoring Systems

Antonello Monti, Carlo Muscas, Ferdinanda Ponci

• Publisher: Academic Press
• Year: 2016
• eBook ISBN: 9780128031551
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Phasor Measurement Units and Wide Area Monitoring Systems presents complete coverage of phasor measurement units (PMUs), bringing together a rigorous academic approach and practical considerations on the implementation of PMUs to the power system. In addition, it includes a complete theory and practice of PMU technology development and implementation in power systems. - Presents complete coverage of the topic from the measurement to the system, bringing together a rigorous academic approach and practical considerations on the implementation of PMUs to the power system - Includes a complete proposal of implementation for a PMU platform that could be replicated in every laboratory - Covers PMU software compiled for National Instrument HW, a compiled monitoring platform to be used to monitor PMU data and developed custom solutions, and a compiled National Instrument schematic to be executed within a SmartPhone app

Chapter 1

Introduction

A. Monti*; C. Muscas^†; F. Ponci* ^* RWTH Aachen University, Aachen, Germany
^† University of Cagliari, Cagliari, Italy

Abstract

The goal of Chapter 1 is to give the reader a clear understanding of the goals of the book. The historical perspective should make evident why phasor measurement units are important. The guide for the reader should clarify how to use the book.

Keywords

Phasor measurement unit (PMU); Synchrophasor

Contents

1.1 Motivation for the Work

1.2 What is a PMU?

1.3 A Short History of the PMU

1.4 Structure of the Book

References

1.1 Motivation for the Work

Power systems all around the world are facing a period of significant change. Different drivers are pushing for a modernization of the infrastructure to better cope with the current operating conditions. One of the strongest drivers is the growing role of renewables, or more generally, decentralized energy sources. This is particularly true in Europe, where the targets set by the European Commission are promoting ambitious plans, in the member states, of renovation of the generation portfolio. Significant changes are also happening in other parts of the world, with wind now being one of the most important energy generation technologies.

The result of this change in the generation portfolio is a modification in the requirements in the monitoring and automation of the power infrastructure.

Until now, power grids have operated in a load-driven mode. The basic idea behind this is that loads are predictable, even though only in a statistical sense. Thanks to this statistical prediction, generation is scheduled. Given that predictions are never perfect, deviations are compensated at run-time. Such an approach is possible, assuming that the generation is fully controllable and possibly concentrated in large power plants, so that the scheduling problem is solvable.

In the new scenario, we are moving more and more towards a generation-driven system, where generation leads and the rest of the system follows. In fact, the output power of the renewable energy sources cannot be adapted as easily as in the case of traditional power sources.

This new scenario is creating the need for new solutions and technologies, among which, for example, storage, electrical, and also thermal technologies are expected to play a critical role in achieving the power balance.

In a nutshell, operating the power system is getting more complex and requires more sophisticated monitoring for automation technologies.

The first part of the automation that felt this impact was monitoring. All the operators currently recognize the need for a more accurate and extensive monitoring of the power grid.

Transmission systems have been the first to evolve in the direction of more sophisticated solutions, but distribution grids are expected to receive an even more clear impact.

In effect, while transmission grids have been already operating with quite advanced control rooms for quite some time, distribution grids are now becoming much more sophisticated than in the past.

The main game changer is the new role played by distribution grids in relation to generation. In a new scenario characterized by a large presence of distributed energy sources connected at low or medium voltage, distribution grids are now becoming the key infrastructure also for the purpose of generation.

This change of role required the definition of the new concept of active distribution grids.

More advanced monitoring, for both transmission and distribution, basically means two things:

• new algorithms,

• new measurement technologies.

Phasor measurement units (PMUs) fall in the second category. PMUs are a step up in technology when it comes to measurements for power systems. They introduce two new fundamental concepts that were not present at all in power systems:

• the concept of synchronized measurements characterized by precise time tags;

• the concept of a measurement that is beyond the simple idea of root mean square (RMS) and brings direct information about the phase.

These new aspects not only represent a significant enhancement in the concept of measurement of an AC quantity; they also unlock completely new scenarios and possible applications.

1.2 What is a PMU?

While a complete discussion of the definition and of the implementation of a PMU is presented in the following chapters, we would like here to introduce the nonexpert readers to the basic concepts.

Traditionally, devices utilized for power system monitoring have been designed to provide scalar information. While for power measurements this is obvious, for voltages and currents in AC systems this usually means reporting the RMS value of the quantity.

The main evolution in this field has been the transition from analog and electromechanical devices to digital implementation. This digital implementation allowed the development of more sophisticated measurement options which are also useful for power quality purposes (eg, total harmonic distortion).

On the other hand, the phase plays a critical role in the operation of power systems. Phase information has been usually extracted only at the control room level by means of the state estimation (SE) process. This process uses a large amount of measurement to extract a coherent picture of the operation of the grid: this picture is in many cases represented by the voltage profile in terms of amplitude and phase.

Such an approach has three main limitations:

– The information phase is always the result of a numerical process and it is never directly measured.

– The information phase is only available at the central level and it cannot be used for local processing.

– The refresh rate of SE is rather slow and even then, the information is available only with a coarse detail.

With an increasing role of dynamics and the need to operate grids closer to their limit, it has become progressively clear that it is critical to improve the knowledge of the phase quantity as critical assessment of the stability of the whole system.

Depending on the applications, as described in Chapters 8 and 9 of this book, this means

– use of the information for more local process;

– a faster and more accurate system-level process.

This awareness is the root cause of the development of PMUs.

PMUs are measurement devices able to extract not only the amplitude but also the phase of a sinusoidal quantity.

The phase is estimated with reference to a global time reference, which is usually selected to be based on the Global Positioning System, which provides an available and reliable definition of time everywhere.

While the definition may sound quite simple, its implementation is not.

The main hurdle is the formal definition of the quantity we want to measure and correspondingly of the algorithm needed to extract this quantity from a sequence of samples.

In a perfectly sinusoidal system operating in steady state, such a definition is quite simple and immediate. When we move to a real-life scenario, many challenges arise:

• What is the meaning of a phasor in a normal operation when a real steady state can never be reached?

• How do we extract a phasor quantity from a signal that is typically changing its frequency over time?

• How do we treat the presence of harmonics that determine a nonideal sinusoidal behavior?

These questions have been and remain at the heart of significant research efforts in many universities and research centers. It should be clarified that over time, this research work has brought an evolution of the vision of what a PMU really is. This book aims at explaining the more recent conclusions, but also the fundamentals.

All in all, the availability of this new type of measurement has also pushed the development of completely new infrastructure such as the idea of a wide area monitoring system: a network of PMUs working together to assess the status of a network.

These concepts have already found significant applications in transmission networks, but are expected to find more and more applications in distribution networks as well, for the reasons mentioned above.

1.3 A Short History of the PMU

The concept of a PMU was introduced in the 1980s [1]. A research group, led by Prof. Phadke, performed most of the original work at Virginia Tech.

The need for synchronized sampling first appeared in the design of protection systems: data samples were used in different substations far apart. This work resulted in the invention, at Virginia Tech, of the symmetrical component distance relay.

After this groundwork, the first idea of a PMU was introduced in 1988. The work had then its first industrial application at Macrodyne Co.

After this pioneering work, a lot of demo projects were developed, focusing on the applications of PMU at a transmission level. Correspondingly, the number of manufacturers grew in time up to tens of producers now. Furthermore, modern intelligent electronic devices used in electric substations include now PMU functionalities [2].

In parallel, the IEEE started a long and complex process of standardization. The first version of a PMU standard was published in 1995. The work went through further revision, up the current version released in 2014.

The IEEE standard leaves PMU manufacturers the choice of design solutions, giving only specifications under steady state and dynamic test conditions. It defines the indices, in particular the total vector error, for PMU accuracy evaluation and comparison. The standard IEEE C37.118.1 introduces two performance classes: a P-class, particularly intended for applications requiring fast responses, as the protection ones, and an M-class, requiring higher accuracy for measurement applications.

Another important standardization milestone is given by the IEEE standard C37.242, released in 2013, as a guide for PMU calibration, testing, and installation.

1.4 Structure of the Book

The idea behind this book is to provide a comprehensive view (from the sensor to the system) over the complex topic of PMUs, The rigorous methodological studies and the technological considerations that are necessary to design the software and hardware ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Acknowledgment
• Chapter 1: Introduction
• Chapter 2: Basic Concepts and Definitions: Synchrophasors, Frequency, and ROCOF
• Chapter 3: Algorithms for Synchrophasors, Frequency, and ROCOF
• Chapter 4: Sensors for PMUs
• Chapter 5: Hardware for PMU and PMU Integration
• Chapter 6: International Standards for PMU and Tests for Compliance
• Chapter 7: State Estimation and PMUs
• Chapter 8: Wide Area Measurement Systems: Applications
• Chapter 9: Real Life Examples of Wide Area Measurement Systems
• Author Index
• Subject Index