Rating Of Electric Power Cables in Unfavorable Thermal Environment By Mr.George J Anders free pdf download.
The focus of this book is the calculation of the current-carrying capabilities of the cables crossing unfavorable thermal environments. However, in order to make this book self-contained and more accessible to a wide group of interested readers, a comprehensive review of the rating methods for standard installation conditions is also included. Cable rating standards deal with uniform laying conditions only; however, in modern cable installations, such conditions are encountered very seldom. Cable routes often cross heat sources, including other cables, or pass through regions of high soil thermal resistivity, for example, in the vicinity of trees or shrubs. Air, walls, ceilings, or floors form an impediment to heat flowing away from the cables. In all such situations, the rating of the cables should be reduced to avoid overheating There has been little attention devoted in the past to the requirement of ampacity derating in such cases. The fact that there have been relatively few failures attributed to cable overheating is a result of the conservative design procedures used by cable engineers. With the economic pressure arising from the restructuring of the electric power industry around the world, the transmission circuits are becoming more heavily loaded and hence more prone to thermal overloads.
Better understanding of the heat transfer phenomena around loaded electric power cables will not only help to establish correct transmission line limits, but also may help the circuit owner in the implementation of corrective measures needed to increase cable ratings. Thus, in addition to improved computational procedures, including probabilistic analysis, optimization of thermal backfill design will become more common. This book is aimed at providing cable design engineers and power network analysts and operators with the computational tools and techniques to address challenges arising from installation of power cables in a complex thermal environment.
Different readers may read the book in different ways. The readers not very familiar with power cable heat transfer theory may wish to consult my earlier book, ‘We will use the term cable rating or ampacity to denote current-carrying capability Rating of Electric Power Cables: Ampacity Computations,for Transmission, Distribution and Industrial Applications, published by IEEE Press in 1997 and McGraw Hill in 1998. The present book can be viewed as a continuation of the earlier work and it makes many references to it. However, in order to make the present book self-contained, the most important rating equations developed in the earlier book are quoted here but without the background mathematical developments. The new formulae presented in this book are, however, always supported by a rigorous mathematical thought. The readers familiar with the theory can follow all the developments presented in this text, whereas some engineers may wish to use only the final results. For those readers, I added at the end of each chapter a summary of the salient findings developed in the text.
The book is organized as follows. Chapter 1 summarizes the standard methods of rating calculations. Some background information on heat transfer theory is offered, followed by a brief description of the lump parameter method used in most standard applications. Continuous and time-dependent rating equations for underground and aerial cables are introduced in this chapter, together with a summary of the methods for evaluation of the parameters appearing in these equations. After reading Chapter 1, the reader should be able to perform ampacity calculations for the majority of cable constructions and installation conditions covered in the international and national standards. Chapters 2 and 3 address the main topics of this book, namely, cables crossing an unfavorable thermal environment. Chapter 2 looks at short sections of the cable right-of-way where either the thermal resistivity of the surrounding medium or the soil ambient temperature are higher than in the rest of the route. Chapter 3 addresses the issue of cables crossing other heat sources. In both chapters, the possibility of a soil dryout is considered and the issue of transient rating calculations is also addressed. Chapter 4 is devoted to the topics related to the application of the thermal backfill around power cables. Advanced computational techniques presented in this chapter include nonlinear optimization of backfill design and the probabilistic analysis of cable ampacity.
The daily variations of cable loading are addressed in transient rating calculations. These are examined in several chapters. The simplest way to include load variability is to consider the load-loss factor in thermal calculations. A fresh look at this issue is offered in Chapter 5. Especially, analysis of weekly and monthly load cycles, not considered generally in practice, sheds new light on the cable rating capabilities. An application of the theory presented in this chapter to cables buried in deep tunnels or using a horizontal direct drilling technique might be of particular importance in the future since more and more installations follow this method of laying.
Chapter 6 deals with cables in air. In particular, rating issues for bundled cables found mostly in the telecommunication industry are discussed. However, the mathematical theory applied to rating these cables is also applicable to any other group of cables in air bundled together. In addition, derating factors for some medium voltage cable installations in an unfavorable thermal environment are given. These are mostly based on the work reported in the IEC standards.
The final chapter presents mathematical models for rating calculation of pipetype cables with slow oil circulation. The classical model by Buller is thoroughly reviewed and compared with a new approach proposed in this chapter.
The book contains a large number of numerical examples that explain the vari ous concepts discussed in the text. Each new concept is illustrated through examples based on practical cable constructions and installations. To facilitate the computational tasks, I have selected six model cables that will be used throughout the book. Five are transmission-class, high-voltage cables and one is a distribution cable. The model cables were selected to represent major constructions encountered in practice and are described in Appendix A. Chapter 6 considers a special type of cable, found mostly in the telecommunication and aircraft industry, namely a cable composed of many cores bundled together.
Appendix B contains a summary of a method, due to Thorntwaite, to calculate moisture content in the soil. Moisture content is a critical parameter determining soil thermal resistivity. An algorithm to calculate the probability distribution of this resistivity based on the distribution of the moisture content in the soil is discussed in Appendix C.
There are literally hundreds of symbols used in the book in the derivations of the mathematical expressions. Even though it might be difficult to create a completely consistent set of variables, the author’s aim was to apply as much as possible the notation used by the International Electrotechnical Commission (IEC), which is a publisher of the international cable rating standards. Several topics covered in this book go beyond the material discussed in the standards. In such cases, generally ac cepted terminology was applied.
Several of the topics addressed in this book originated with the work of my colleague, Prof. Heinrich Brakelmann of the University of Duisburg in Germany. I am particularly indebted for the encouragement and constant support that I received from him while working on this book. Several complex mathematical derivations presented in the text were developed jointly with another colleague of mine, Mr. Eric Dorison of Electricte de France. His brilliant insight into the complex issue of thermal rating calculations is reflected in several parts of this work. A part of the material covered in this book was derived from various IEC reports dealing with the thermal rating of power cables. These publications are being prepared by Working Group 19 of Study Committee 20A (High Voltage Cables) of the IEC as an ongoing activity. The author is indebted to Mr. Mark Coates from ERA Technology in Britain, the Convener of Working Group 19, who reviewed several chapters of the book and offered his valuable comments. In addition, I could have not written this book without the involvement and close association of Dr. John Endrenyi from Kinectries Inc., who reviewed the entire manuscript and provided several helpful comments. I would also like to acknowledge the financial assistance of the Standards Council of Canada and Kinectrics Inc. in supporting my participation in the activities of Working Group 19 of the IEC over many years.