Two for Fall Reading
Transformers & Structure Preserving Energy Function
Two books are reviewed in this issue. The first book, Electric Power Transformer Engineering, Third Edition, the reviewer proclaims, is “an excellent volume” and “strongly recommended.” The second book, Structure Preserving Energy Functions in Power Systems: Theory and Applications, according to the reviewer, is “a good platform for further research in using SPEF for stability and control in real time.”
Electric Power Transformer Engineering, Third Edition
It is noted that the readership of the book is seen as “persons with a power transformer background experience, be they merely curious seasoned professionals or acknowledged experts.” This is basically the same group that I see as readers of my J & P Transformer Book (12th and 13th editions), certainly the first two of those categories; I have not really considered myself to be addressing “acknowledged experts.”
In his preface to this third edition, James Harlow, the editor, reflects on the way that this most extensive work, covering all aspects of transformers, has evolved from the single chapter he prepared for the Electric Power Engineering Handbook produced by Leo Grigsby in 2001. Nearly a dozen years have gone into its evolution from that single chapter, presumably produced by his own hand, to create a book of 25 chapters assembled from the contributions of no fewer than 47 acknowledged experts in their respective fields.
For a number of these chapters, as many as four joint authors are identified. One is inclined to wonder how well these teams worked together and what level of input was necessary from the editor to achieve such excellent finished efforts. It is certainly the case that “you can’t see the join,” so it must be concluded the various chapters have only benefited from having multiple inputs.
The work of the editor of a volume such as this must be very much a struggle to decide what can be left out rather than striving to gather all the information that is necessary to do justice to the multitude of aspects with which it is required to deal.
The editor has done well. Not only does this book provide so many of the answers required from an extensive reference work, but these have been assembled in a very logical and readable manner. The book can be of value to a student with a general power engineering background who has a wish to pursue a transformer-related research topic, a project manager responsible for the procurement and building of a new substation, or a plant manager with day-to-day responsibility for the operation of the substation. Perhaps there is a need to know whether the frequency of oil sampling is adequate or whether the insulation moisture level is getting a little higher than it should. Insurers may want to gain a feel for the level of risk associated with a client’s plant. All of these individuals will find, if not precise and total answers to their problems, a means of greatly increasing their understanding of what is involved.
As will be expected in a work that has been compiled by so many experts in the many aspects of transformers, there is a most extensive reference list at the end of many of the chapters so that, while a reading of the book alone might be enough to provide the level of knowledge that many readers will require, reading a single chapter or so can also provide a starting point for acquiring an answer to a particular problem. A significant proportion of the references are to IEEE standards and papers, but these are far from being the source for the majority, and it is clear that most of the chapter authors have relied on a wide spectrum of sources for their background information.
It is noted that the book has an additional chapter, compared with the 2004 edition, devoted to transformers for wind turbine generators and photo-voltaic applications. This is clearly very appropriate since these transformers do have particular features that are not always fully recognized and effectively catered for in so many transformers manufactured for this particular duty.
It is interesting too, while noting the inclusion of a chapter relating to transformers for wind generators, that the book does not have a stand-alone chapter devoted to main generator step-up transformers. These do have their own very specific requirements, operating, as they do, very nearly continuously at a high load factor and with a wide voltage ratio, so that the low-voltage winding carries tens of thousands of amps and the high-voltage winding has a modest level of current but generally a large number of turns.
Another omission too, certainly in the mind of this British reviewer, is the absence of a chapter dealing specifically with large high-voltage autotransformers. Is it that these are not widely used on a U.S. transmission network? These certainly have a few special requirements of their own. Perhaps it is a little soon to be thinking of a fourth edition, but a couple of thoughts to be put on file!
Overall an excellent volume strongly recommended for a spot of reading around the subject and also to have on the shelf as a very comprehensive reference work.
–Martin J. Heathcote
Structure Preserving Energy Functions in Power Systems: Theory and Applications
By K.R. Padiyar, ISBN 13:978-1-4398-7936-8
Energy function analysis for stability evaluation has been used in power systems for over six decades. Initially, it was confined to models with differential equations only using reduced network models at the internal nodes of the machines. In those cases, Lyapunov’s direct method was applicable. Two breakthroughs came from the researchers at the University of California, Berkeley, in the 1980s. First the concept of structure preserving energy functions (SPEF) and the other one in computing the region of attraction or stability boundary of the post fault equilibrium point through the concept of controlling UEP and the PEBS methods. This made the energy function method applicable to a wide variety of situations such as dynamic security assessment (DSA). SPEF enabled the modeling of loads as well as HVdc and FACTS devices. The PEBS method made it possible to quickly compute the critical clearing time for faults in the system.
The author, through his research spanning over 40 years, has captured all the above aspects in a systematic way. What is interesting is that through this work it has opened the door for more research using SPEF such as the inclusion of renewable energy sources as well as a possible application in the WAMS area by using the PMU data to develop useful metrics for stability of the system.
The book is divided into nine chapters. After the introductory chapter, the second chapter reviews the literature using a simplified model of the machine from equal area criterion, Lyapunov’s method, center of inertia formulation to the controlling UEP method. All the discussion is based on the reduced network model at the internal buses of the machines represented as classical models. Transfer conductances arising out of eliminating network buses are included. The third chapter introduces the structure preserving model to develop the SPEF. It uses the center of inertia formulation, a flux decay model of the generator, and voltage dependent nonlinear load models. Test results for a 17-generator and a 50-generator system are presented. Chapter 4 continues the discussion of SPEF with detailed generator models as well as nonlinear load models. A novel thing is the inclusion of dynamic induction motor load models.
Chapter 5 discusses SPEF with inclusion of HVdc and FACTS controllers. Both STATCOM and TCSC are considered. With the author being an expert on HVdc and FACTS with textbooks in the area, this chapter is particularly useful and authoritative. Results on a three-machine, ninebus system are presented.
Chapter 6 is devoted to a discussion of detecting instability based on identifying critical cut sets. It is shown that this technique is particularly relevant in online monitoring of stability in the DS A context. Chapter 7 deals with sensitivity analysis of the energy margin, both parametric as well as trajectory sensitivity based. The latter is useful in preventive control.
Chapters 8 and 9 are devoted to the practical applications of FACTS controllers for emergency control with detailed simulation. Motivation through a single machine infinite bus system backed up by good theoretical explanation makes these chapters very useful. Different types of controllers are considered such as STATCOM and UPFC, and results on the ten-machine system are presented
To summarize, the book is a thorough discussion of energy-based methods, starting from the equal area criterion to present-day complex structure preserving models with all types of devices included. At the end of each chapter there is good discussion of further research topics in the area. The bibliography is very comprehensive. The book on the whole provides a good platform for further research in using SPEF for stability and control in real time.