Showing papers in "IEEE Electrical Insulation Magazine in 2005"

Partial discharge monitoring of power transformers using UHF sensors. Part I: sensors and signal interpretation

Abstract: Determining whether power transformers are suffering from internal arcing or dangerous levels of partial discharge (PD) is important because failure without warning can result in damage to neighboring equipment, customer dissatisfaction, disruption to economic activity, and the imposition of regulatory fines. This paper provides an overview of the excitation of UHF signals by PD inside transformers. The use of externally mounted sensors has been outlined, and the structure of a dielectric window that can be constructed on an inspection hatch has been outlined. Attenuation of UHF signals propagating inside a transformer tank has been shown to be relatively low. A new approach to locating PD sources in three dimensions has been presented, based on using a numerical model of the transformer materials, which defines electromagnetic propagation velocities on a mesh of 5-cm sub-cells. The concepts of the propagation-velocity matrix (PVM) and the propagation-time matrix (PTM) were thereby introduced. Once the PD source has been located to a specific region of the transformer, the PVM might be used to index a database of physical descriptions of the transformer that could provide additional information to assist with diagnosis.

Effects of converter pulses on the electrical insulation in low and medium voltage motors

Abstract: A new IEC test specification is under development for the insulation systems used in converter-driven machines with the aim of providing manufacturers with a reliable means to quality insulation systems for use in converter-fed machines and users with confidence that the machines they purchase contain insulation that is fit for their intended purposes. Much of the qualification testing for insulation systems subject to converter voltages may be undertaken with existing 50/60 Hz equipment. However, for testing of complete coils in low voltage machines, an impulse generator and an impulse PD test set are required. For large machines, which have stress-graded endwindings, it will be necessary for a source of converter voltage to be available.

Partial discharge monitoring for power transformer using UHF sensors. Part 2: field experience

Abstract: Because of the large capital value of power transformers and their critical role in the electricity network, there is an increasing need for non-intrusive diagnostic and monitoring tools to assess their internal condition. The UHF technique is being applied increasingly to monitor partial discharges (PD) in power transformers and has been shown to be much more sensitive than acoustic techniques when the signal path passes through solid insulation. Field trials were performed on five power transformers ranging in size from 18 to 1000 MVA and in voltage from 25 to 400 kV. The results of PD tests on power transformers provide sufficient evidence to justify making provision for UHF sensors on new transformers to facilitate their monitoring when required during the service lifetime.

Modern outdoor insulation - concerns and challenges

Abstract: Because modern society is strongly dependent on a reliable power supply, there is an urgent need to develop more reliable high voltage insulators. While porcelain and glass have dominated insulator materials for a long period of time, there is renewed interest in polymeric and semiconducting glaze insulators. This paper provides a brief description of composite insulators, including a discussion of their benefits, aging of housing materials, and brittle fracture phenomenon. The paper also discusses the benefits of using insulators with semiconducting glaze surfaces, and identifies four broad issues that are being addressed by the present research on outdoor insulation.

Improving the reliability of transformer gas-in-oil diagnosis

Abstract: Dissolved gas analysis (DGA) is widely used to detect incipient faults in oil-filled electrical equipment There is always some degree of inaccuracy in laboratory dissolved-gas measurements, especially at low gas concentrations, which affects gas ratios and other diagnostic calculations This paper examines the measurement inaccuracy problem in detail using recent CIGRE data It shows how proper allowance for this inaccuracy can improve transformer condition assessment and diagnosis by DGA

Aging of cellulose at transformer service temperatures. Part 1: Influence of type of oil and air on the degree of polymerization of pressboard, dissolved gases, and furanic compounds in oil

Abstract: A power transformer is expected to operate up to 40 years. With current cost-benefit calculations, the user is forced to perform maintenance that will bring a further extension of transformer life. The maintenance can only be based on on-site and off-site monitoring with extended analytical and electrical tests, which can define the service condition of the transformer and predict its further life expectancy. The life of a transformer is mainly dependent on the life of its solid insulation, and the life-limit is determined by the thermal degradation of the winding paper. The Kraft paper decreases in tensile strength with the progress of aging and, at some point, can no longer withstand the short circuit stresses. The monitoring of the complex permittivity of transformer oils, as a function of frequency and temperature, may provide information concerning the state of the insulation. In this article, we show the influence of air and oil type on the aging of pressboard under the influence of a considerable amount of moisture. The rate of the degree of polymerization, the development of furanic compounds, as well as the gas-in-oil analysis in comparison with the aging of the pure oil under the same conditions have been investigated.

Aging of cellulose at transformer service temperatures. Part 2. Influence of moisture and temperature on degree of polymerization and formation of furanic compounds in free-breathing systems

Abstract: This paper examines the influence of moisture and temperature on the degree of polymerization (DP) and furanic compounds. At normal service temperatures, the average moisture content in the solid insulation of transformers has a much stronger influence on the development of furanic compounds and degradation of the insulation than temperature itself. The influence of the initial moisture in the solid insulation on aging under normal transformer service temperatures of 75, 85, and 95 /spl deg/C has been investigated experimentally. Wet transformers have usually higher levels of furans, inferior oil properties, and lower DP values of paper than dry ones under similar service conditions. The development of furan concentration and moisture content are very important aging indicators for the evaluation and life assessment of transformer in service. Further work is necessary to combine these important aging characteristics into a valuable tool for the assessment of a large aging transformer population.

Failure analyses of nonceramic insulators. Part 1: Brittle fracture characteristics

Abstract: Nonceramic insulators, also referred to as composite, polymer or polymeric insulators, are used in overhead transmission lines with line voltages in the range of 69 to 735 kV. Despite the many benefits that nonceramic insulators offer in comparison with their porcelain counterparts, they can fail mechanically in service by rod fracture. One of the mechanical failure modes of the insulators is a failure process called brittle fracture, which is caused by the stress corrosion cracking (SCC) of the GRP rods. The process is catastrophic and unpredictable, leading to the drop of energized transmission lines. The most important characteristics of the brittle fracture process, which can occasionally affect high voltage nonceramic transmission line insulators, leading to their catastrophic in-service failures, have been presented in this article. In addition, several experimental techniques were suggested for the simulation of brittle fracture under laboratory conditions. Only the most important aspects of brittle fracture process have been discussed here.

Partial discharge and dissipation factor behavior of model insulating systems for high voltage rotating machines under different stresses

Abstract: This article describes the change in partial discharge (PD) pattern of high voltage rotating machines and the change in the tan /spl delta/ as a function of the applied test voltage during the aging processes as caused by the application of different stresses on stator bars. It also compares the PD patterns associated with internal, slot, and end-winding discharges, which were produced in well-controlled laboratory conditions. In addition, the influence of different temperature conditions on the partial discharge activities are shown. The investigations in this work were performed on model stator bars under laboratory conditions, and the results might be different from those obtained for complete machines, as rotating machines are complex PD test objects, and for example, the detected PD signals in a complete machine significantly depend on the transmission path from the PD source to the measurement device.

Failure analyses of nonceramic insulators: part II - the brittle fracture model and failure prevention

Abstract: In this work, an improved version of a brittle fracture model, based on the formation of nitric acid in service through corona discharges, ozone, and moisture, is presented and is used to explain several different modes of brittle fracture. Similar to Part I, we refer throughout this article to the insulators as nonceramic insulators (NCIs). To prevent brittle fracture in-service, its causes must be first established. To prevent brittle fracture in-service, its causes must be first established.

Dielectric characterization of power transformer oils as a diagnostic life prediction method

Abstract: The monitoring of the complex permittivity of transformer oils as a function of frequency and temperature provides information concerning the state of the insulation. In this work, it has been investigated as a quality control method, providing service life estimations of power transformations. The formation of databases recording polarization effects in a wide frequency and temperature range may be used effectively as a working tool for service engineers. Reliable insulation monitoring reduces accidents in aged HV transformers, some of which have been in operation since the 1960s. These transformers always impose an environmental threat because of possible accidental leaks or fires, especially when containing traces of PCBs.

High frequency properties of shielded power cable - part 1: overview of mechanisms

Abstract: This paper provides an overview of the mechanisms of high frequency (HF) loss in shielded power cables and discusses the challenges in the measurements of cable material properties at HF.

Defect tolerance of solid dielectric transmission class cable

Abstract: This paper addresses the issue of determining the level of defect that is likely to cause the failure of solid dielectric transmission class cables. It also proposes methods for predicting the level of defect that is likely to cause failure and to provide a simple analytic approximation for doing so in the case of conducting spheroids aligned with the electric field. A common assumption is that conducting particles > 100 /spl mu/m in length are likely to cause failure of extruded dielectric transmission cable. This analysis suggests that when the effects of operation at elevated temperature are included in the analysis, this is probably an appropriate criterion with a sound technical basis. For maximum background fields in the range of 15 kV/mm, as presently seen near the conductor shield of some transmission class cables, a worst-case particle length in the range of 0.1 mm is likely to be required to cause failure for the worst-case local polymer morphology in the range of the maximum operating temperature.

The continuing evolution of semiconductor materials for power cable applications

Abstract: Significant improvements of the semiconductive materials and the measurement techniques have been made in the last decades. This article has demonstrated that semiconductive materials with more robust processing properties can be developed. The cleanliness and surface smoothness have been dramatically improved because of continuous attention to raw materials, compounding, and techniques to monitor the quality of the semiconductive material. Both at distribution and transmission voltages, improving semiconductive materials have made vital contributions to the increasing dependability of XLPE cable systems.

Condition assessment of 12- and 24-kV XLPE cables installed during the 80s. Results from a joint Norwegian/Swedish research project

Abstract: This paper reports the results from the condition assessment of 12- and 24-kV cross-linked polyethylene (XPLE) cables using a technique based on dielectric spectroscopy initially developed at KTH in Sweden. The work aims to examine whether the method could detect water tree degradation for the second generation medium voltage (MV) cables with long, but not bridging, water trees. While the overall cable condition was better than expected for second generation XPLE cables, water trees were found in most of the selected cables. The diagnostic method based on the measurement of the dielectric response could only detect water tree degradation in the examined second generation cables when the water trees bridged the insulation wall. Condition assessment above service stress may, in some cases, be required to detect bridging water trees. The results indicate that there is a correlation between the voltage level and the breakdown voltage of the cable. This can be used as a diagnostic criterion for this group of cables.

Calibration of capacitive couplers for online PD detection in HV cables

Abstract: Possible online quantification and calibration methods for capacitive couplers have been investigated experimentally. The purpose of the calibration is to ensure that if two different systems are used to measure the same sample, they measure the same "apparent PD magnitude," which is the charge transfer at the sample electrodes. Evolved from the conventional electrical method, the terminal injection method for calibration of capacitive couplers has been accepted as a standard approach. However, for online, on-site calibration, alternative methods that approximate the terminal injection method have to be used, because the terminals generally either not accessible or are too distant from the sensor. This paper investigates possible online calibration methods used for capacitive couplers and compares the obtained results. The advantages and limitations as well as the sources of error are discussed in detail. Frequency response test have been undertaken on capacitive couplers to determine their performance over their potential bandwidth. According to linear systems theory, this is an efficient experimental method to determine system behavior when the theoretical model of the system is difficult to obtain. Approximated transfer function model can be obtained from analysis of the frequency response data.

The Practical Guide to Outdoor High Voltage Insulators [Book Review]

Abstract: IEEE Electrical Insulation Magazine damage models, and aging degradation testing and models. Today, many companies are stressing the importance of higher product reliability and have incorporated six sigma-training programs. Part of that training includes accelerated life testing, for which this book would be an excellent resource, especially as it also contains problems at the end of each chapter to reinforce learning. Although the fundamentals of accelerated life testing are not going to change, the tools used to make these calculations, graphing, and data analysis easier have advanced since the original writing of this book. Software such as minitab can greatly aid the user to quickly and efficiently arrive at conclusions after taking the test data. This book is a reprint of the original book published in 1990 and, as such, does not have any reference to models developed after this date or any information on software. However, this in no way detracts from the usefulness of this book. Researchers in insulation and engineers in general, especially those interested in six-sigma training or reliability engineers, will find this an outstanding reference book that will be used constantly.

Accelerated Testing- Statistical Models, Test Plans, and Data Analysis [Book Review]

Abstract: 6 1 There are so many systems that rely on a continuous source of power that even minor interruptions of service can result in major and costly losses. Hospitals, traffic lights, computers that run financial systems, air traffic controls, nuclear power plants, military sites, water and sewage plants, and telephone and internet communications are all vulnerable. Loss of service is inevitable, especially from acts of nature (storms, wind, lightning, etc.), accidents such as a car hitting a telephone pole and knocking down transmission lines, failure of equipment, or intentional attacks meant to cause widespread outages and cause as much damage as possible. However, today’s system is designed to mitigate and isolate the damaged section. The author proposes, in seven short chapters, methods to change the existing system to one that is more secure and less likely to be able to cause a cascading blackout. Some of these are simple proposals, such as making substations more secure by adding barbed wire. Others are more involved such as sectionalizing the grid to eliminate massive blackouts. The focus of this book is primarily on transmission line security, as it is very exposed and difficult to protect. Chapters include proposed circuit configurations, electrical and mechanical criteria, substations, DC transmission, operations, and economics. Some of the proposed circuit configurations and descriptions are interesting. However, many of the drawings lack any explanation, and the text does not describe the diagrams in sufficient detail. Also, there are references to outdated material (e.g., references to Westinghouse Electric). Although interesting, most of the proposals lack any technical details, implementation, and costs. Appendices follow the main body (over two-thirds of the book) and include the pros and cons of polymer versus porcelain insulators and a study on the national transmission grid (233 pages). The study provides details for more capacity, planning issues, and advanced transmission technologies. This book would be useful for the technical or non-technical reader interested in learning more about how transmission lines are configured today, before and after deregulation, and to learn about the weaknesses in today’s transmission system. Simulation Methods for Polymers Edited by M. Kotelyanskii and D.N. Theodorou Marcel Dekker, Inc. 270 Madison Ave. New York, NY 10016 ISBN# 0-8247-0247-6 Phone: +800 228 1160 Fax: +212 685 4540 http://www.dekker.com 602 pp. -$195.00 (Hardcover), 2004

Transformer and Inductor Design Handbook, 3rd Ed. [Book Review]

Abstract: 6 1 and cost of the product. Not all materials can be scaled-up with the same mixing process. Frequently, scaling-up the mixing process from small research batches to large quantities, necessary for production, can lead to unexpected problems. This reference book is intended to help the reader both identify and solve mixing problems. It is a comprehensive handbook that provides excellent coverage on the fundamentals, design, and applications of current mixing technology in general. Although this book includes many technology areas, one of main areas of interest to our readers would be in the polymer processing area. This would include the first eight chapters in the book and a specific application chapter on polymer processing. These cover the fundamentals of mixing technology, important to polymer processing, including residence time distributions and laminar mixing techniques. In the experimental section of the book, some of the relevant tools and techniques cover flow visualization technologies, lab scale mixing, flow and torque measurements, CFD coding, and numerical methods. There is a good overview of various types of mixers used for polymer processing in a dedicated applications chapter on mixing high viscosity materials such as polymers. There are many details given on the differences between the mixing blades in various types of high viscosity mixers and suggestions for choosing the proper mixer for high viscosity applications. The majority of the book does, however, focus on the chemical, petroleum, and pharmaceutical industries that generally process materials with much lower viscosity than polymers. The reader interested in learning about the fundamentals of mixing in general as well as some specifics on polymer processing would find this book to be a useful reference.

A new capability to detect and locate insulation defects in complex wiring systems

Abstract: Pulse arrested spark discharge (PASD) is an effective means to detect and locate a variety of insulation defects in complex wiring geometries such as breached insulation, chafing, and physically small insulation cracks. It is highly immune to line impedance variations, an important property in aircraft wiring systems, and has been shown to be nondestructive to electrical insulation materials. Because of the simplicity of the PASD concept, the low energy PASD pulser and diagnostics can be readily implemented into a portable, briefcase-sized diagnostic system. Although this patented technique will likely evolve as it enters into field applications, it is capable of making a near-term impact on the ability of inspection and maintenance organizations to detect and locate potentially hazardous insulation defects.

Underground cable specification advances and installation practices of the largest investor-owned utilities

Abstract: This article analyzes the technical specification trends for the design, installation, and use of 15 to 35 kV underground, insulated power cables by 60 of the largest investor-owned utilities. A distinctive 10-year trend has been established in insulation material preferences, and interesting new data are provided on the cable replacement and rejuvenation practices of these utilities. The results provide practical information that will enable all electrical utilities to compare their medium voltage underground cable specifications and installation practices with those of the largest utilities in their industry.

Overvoltage protection of low voltage systems, 2nd edition [Book Review]

Abstract: This tutorial style text on power distribution focuses on the practical application of medium voltage (1 to 36 kV) power distribution components. The book provides concise technical details, accompanied by well-written text explaining the details for understanding the basic behavior of switchgear, power cables, transformers, power factor correction, grounding, lightning protection, and network systems in electrical systems. Readers will gain practical technical know-how for becoming knowledgeable in some of the critical factors important in selecting switchgear, power cables, power transformers, power factor correction, and calculating short-circuit currents. However, those already familiar with medium voltage power distribution components and operation will find this book too basic. For switchgear, basic operation is reviewed as is recent technology (e.g., internal arc proofing) and advantages and disadvantages of different types of technology. The chapter on cables reviews calculations necessary for sizing cables, a description of various cable types, the practical aspects of pulling cables, and has a convenient listing of the electrical and physical properties of various cable sizes. I found the chapters on compensation and transformers to be very informative. Compensation included not only basic calculations for shunt and series capacitor values and location, but also discussed their affect on motor loads. The section on transformers has a very good review of various three-phase configurations with vector diagrams and lots of drawings, and the effects of faults and in-rush current. However, it was lacking explanations on positive and negative sequencing, thermal considerations, and per-unit calculations. Although not very broad in coverage, this book does give the reader an excellent overview of the more practical aspects of some medium voltage power distribution components in an electrical system. It would be best suited for engineers or technicians who may not design these components but still need a basic working knowledge about medium voltage systems.

Vehicular electric power systems [book review]

Abstract: In the automotive industry, the direction is toward increased use of electricity to improve fuel efficiency, emissions, reliability, and consumer demand for on-board electronics. Hybrid systems use electric motor assist for propulsion with battery storage energy from various possible sources, including inductive recharging from home power, regenerative braking, or fuel cells, among others. There was a great deal of research, begun by the automotive manufacturers, on increasing system voltage from the conventional 12 to 42 Vdc. This has subsequently been stopped because of the significant cost increases that would need to be passed on to the consumer. Presently, the direction is toward hybrid gas/electric motors for improved fuel economy. This book, Vehicular Electric Power Systems, presents a description of the latest systems being proposed for electric power conversion in automobiles, aircraft, boats, and spacecraft. The authors provide a good comparison between presently used conventional systems and new systems along with the fundamental issues holding back some of the proposed technologies. The book, consisting of 13 chapters, begins with a good review of conventional and advanced power electronics fundamentals and electric machines. These cover AC/DC, DC/DC, DC/AC, and AC/ AC converters and advancements in power electronics and motor drives. The next two chapters cover automotive power systems, including a comparison of conventional drive systems to hybrid electric cars. By illustrating the various drive distribution systems being proposed and their design tradeoffs, the reader gets a good understanding of the road blocks presently facing designers. Descriptions not only include propulsion systems themselves but also cover ancillary systems, such as power-assist steering and braking systems, needed to improve efficiency. Other chapters cover advanced power generation systems for aircraft and electric ships. Regarding the international space station, the primary technology discussed is photovoltaic (PV) systems with batteries and flywheel technologies as secondary energy sources. There is an extensive review of spacecraft power system architecture with emphasis on PV power conversion techniques and system stability analysis. Sea and undersea power systems cover various power system architectures along with power management centers and distribution, new motors. One chapter is devoted to descriptions of the fuel cell technologies along with typical current-voltage curves and performance for the various types of cells. The end of the book contains some of the more practical and more specific information on modeling batteries, PV cells, ultracapacitors, and fuel cells along with detailed motor drive descriptions. The authors present excellent reviews of proposed technology, which is particularly useful, as it is compared with conventional technology. One potential issue with presenting up-and-coming technology is the potential that it will not be completed. This appears to be the case with 42Vdc conversion. So, although there is some lack in being up-to-date, as 42V power net systems are no longer emphasized in automotive, the book is still an excellent way for an engineer, manager, or student in the area of electric vehicle propulsion and power systems to learn about new technologies and road block issues.

Polymeric foams: mechanisms and materials [Book review]

Abstract: This 8th edition of this classic reference book is still an indispensable resource for metallurgists and engineers. With the previous edition over 10 years old, this new edition has had every chapter revised and updated to reflect the latest advances in materials. For example, I have found the updated section on sintered materials of particular use for identifying dispersion strengthened copper alloys, along with electrical and mechanical properties of the material. In addition to the revisions, there are four new chapters pertaining to new metallic materials (foams, bulk metallic glasses, nanomaterials), techniques for modeling and simulating metals, new processing technologies, and a new bibliography that has sources for databases, books, and search tools. Those familiar with previous editions know how extensive and broad is the coverage. Some of the materials properties provided, for a wide range of metals and minerals, include thermo-chemical data, metallography, phase equilibrium diagrams, diffusion, physical properties, radioactive properties, electron emission, and electrical, magnetic, and mechanical properties. There are chapters focused on materials made by specific processes such as sintered materials, metal-matrix composites, and vapor deposition/thermal spraying. Some of the other chapters cover heat treatment, friction and wear, lubricants, ceramics and refractory materials, corrosion and electroplating, welding, and brazing. The book is loaded with extensive materials data in the forms of graphs and tables as well as many chapters of practical information for processing materials. For instance, there are extensive tables and descriptions detailing electroplating formulations for various materials along with critical process information. There are many ways in which this reference book might be useful—identifying new materials, determining material properties, learning about a process technique. However it is used, metallurgists and engineers in industry or academia, whose work involves metals and metals processes, will find this book an invaluable, musthave reference.

Programming the PIC microcontroller with Mbasic [Book Review]

Abstract: IEEE Electrical Insulation Magazine The second half of the book describes the system and control theory used to design these systems. This includes transfer functions, system models, Laplace transforms, system response, frequency response, and Nyquist criteria. The authors’ presentation of Laplace transforms and partial fractions can help explain this theory to anyone struggling with these concepts. The theory covered is necessary to develop system controllers. The book is suitable for undergraduate study in control engineering, as it also has questions and answers at the end of each chapter. The appendices cover error in measurements, differential equations, and more Laplace transforms.

Partial discharge factory test requirements and in-field partial discharge testing of medium voltage cables

Abstract: This article presents partial discharge testing of medium voltage cables. Utility engineers are faced with decision on whether to replace some of the older cables in their underground distribution systems. Today, measurement of PD at voltages above 1 Vg on extruded cables more than fifteen years old may not indicate that the cables have developed PD due to in-service aging. Removal of these cables based solely on PD measurements may not be warranted. Failure history of the cable, in conjunction with an analysis of the PD site, should be part of the utility engineer's decision-making process.

Rating of Electric Power Cables in Unfavorable Thermal Environment [Book Review]

Abstract: IEEE Electrical Insulation Magazine The second half of the book describes the system and control theory used to design these systems. This includes transfer functions, system models, Laplace transforms, system response, frequency response, and Nyquist criteria. The authors’ presentation of Laplace transforms and partial fractions can help explain this theory to anyone struggling with these concepts. The theory covered is necessary to develop system controllers. The book is suitable for undergraduate study in control engineering, as it also has questions and answers at the end of each chapter. The appendices cover error in measurements, differential equations, and more Laplace transforms.