The July / August Issue of the Electrical Insulation Magazine has been released. Use the accordion headings below to explore this issue’s content, and visit the IEEE Xplore for full magazine access.
Dielectric Dissipation Factor Measurements on New Stator Bars and Coils—Results from a Global Survey
M. G. Krieg-Wezelenburg
Comparative Investigation on Fracture of Suspension High Voltage Composite Insulators: A Review—Part II: Chemical Properties and Criteria System
Yanfeng Gao; Xidong Liang; Yi Lu; Jiafu Wang; Weining Bao; Shaohua Li; Chao Wu; Zhou Zuo
Compatibility of Materials with Insulating Liquids—Why and How to Test
Howard W. Penrose, PhD, CMRP
President, MotorDoc® LLC
From Green Coils to a Green Economy
As global focus turns to the green economy, the challenges that are presented to manufacturers of rotating machinery and insulation material can be daunting. The application of traditional standards and materials to the new applications and rotating machinery designs will, and have, resulted in serious equipment reliability and monitoring problems. A whole new approach is required for practical applications and the use of those standards in everything from smart grid energy storage, hybrid-electric vehicles to Internet of Things (IoT) devices, and sensors to data interpretation.
When the average person is exposed to media reports about zero emissions and the elimination of fossil fuels, they rarely consider the challenges. From the rotating machinery and materials perspective, it can be exciting as new materials, applications, and aging systems develop, especially as the global focus is on the electrification of almost everything. What is the next generation of insulating materials if we lose access to oil? How do we process the materials? What are the impacts? How do we test them?What is the transition?
Right now, we are managing the impacts of traditional manufacturing and repair techniques in generators, transformers, cabling, and other electrical systems in wind power. As noted over the past decade, small deviations in slot wedges and coil design and stator manufacturing have significant effects on the reliability of wind generation. What would pass on a constant-load, ground-based generator has serious implications when the rotating machine is high on a platform with frequent starts and stops and rapidly varying load conditions.The effects of tolerances and thermal cycling becomes critical in large machines that are manufactured to be as cost effective as possible.
When we look to transportation and heavy equipment electrification, the design engineer must consider how the human operator will manage the system. What limitations must be put on a hybrid or electric vehicle to reduce the chaos associated with varying driver styles in order to achieve the perceived reliability of that vehicle on the market? What is the effect of the cooling medium, such as transmission fluid,especially as it ages and carries other contaminants? Although this may become a simpler issue as vehicles move toward driverless, the acceptance of the newer technologies by the public will determine how quickly we move in that direction.
Sensors and prognostics are also accelerating as machine learning and augmented intelligence (AI) techniques continue to advance.Challenges range from ethical use of AI to the reliability of the sensors themselves and how the data are managed. The effects of a faulty sensor, a defect in programming, or even cyber security failures must be considered as all these advances become more automated and connected. The engineer involved in the design of new or modification of existing rotating machinery systems must now understand the effect of connected faults in a far more complex system and consider the new term“resilience.”
As the renewable energy portfolio continues to expand in the areas of new technologies such as wind, solar, tidal, geo-thermal, battery storage, flywheel storage, and fuel cells and older technologies, the effects on transmission and distribution systems have increased. While new standards, agreements,and smart grid frameworks are being developed, the effect on existing systems is becoming apparent. At the local level, in such locations as wind farms, oil-filled and dry transformer failures are occurring well above the expected failure rates, as are failures in cable splices and generator components such as coils, wedges, and connections. On the larger scale, as electrification increases, regions struggle with meeting demand, and some systems are operating at peak capacity, leaving the systems vulnerable to weather and equipment failures. The component failures range from legacy systems being exposed to new aging systems and unplanned weather exposures to newer systems made from materials with shorter life-cycles. Maintenance and monitoring techniques have also evolved in an environment where cost reductions are a goal. Some Of the changes have been improvements, such as new monitoring systems, and some have been excessive and have had disastrous results, as have been noted since 2003 in the USA alone. As we add additional electrification, such as with electric vehicle charging stations, the effects on transmission, distribution, and generation will continue to increase even as smart grid and smart charging systems develop.
Starting back with deregulation in the 1990s, in the USA, the power-generation landscape changed drastically. From the larger base-loaded plants with spinning reserves, we have moved to a more distributed network of peaking plants and energy storage systems. A natural gas plant, solar or wind farm, geo-thermal facility, or hydro plant can come online from a cold stop far faster than a coal-fired plant or nuclear power. With varying load requirements, the larger coal-fired plants are brought on and removed from production more than in the past, with load variations designed to match customer needs. Although the older plants are operating outside of their original design context, the newer types of facilities are operating with equipment and machines designed with the same standards used to develop the older rotating machines and transformers. The problems become more complex when you add in newer material technology in which limited application experience exists, especially in a variable-demand environment, coupled with the effects of power and home electronics on power quality and homeowners and businesses becoming power producers.
Because wind power is one of the larger utility-scale renewable technologies, and a significant area of material and reliability research, it should receive a little extra consideration. The materials that make up the blades,generator, controls, cabling, and transformers all require attention, with the blades beingsubjected to direct environmental conditions such as weather and location. While off-shore and on-shore wind power have some specific differences, there are some definite similarities. These include wedge issues in the stators and DFIG (doubly fed induction generators) rotors, connections, and coils, which are normally form wound. The ambient conditions are global to the components because the machine is suspended in air, resulting in more drastic thermal cycling with load changes and outages driven by power demand and wind conditions. This affects rotor connections as well as components such as magnetic wedges that are not keyed properly and coil fits in the slots, which can result in grounds. Depending on the design, these three conditions will often rank in the top five reasons for extended turbine outages, with blades and gearbox failures also ranking near the top.
Transportation is having a significant impact as plug-in hybrid-electric and electric vehicles increase in market share, in addition to fuel-saving techniques built into other personal vehicles. For instance, with the use of the autostop function for in-town driving, where a vehicle’s engine turns off at stops (traffic lights and stopped traffic), most of the previous engine and belt-drive components are now electric motor driven. The traction motors in hybrid and electric vehicles vary in type but are generally cooled with transmission fluid and are usually developed as motor generators with permanent magnet rotors. Theinsulation systems used in these technologies are not much different from those used in other commercial or industrial applications but are subjected to rapid changes in operating temperature and load, depending on driving conditions and the operator. Improvements in materials are often implemented, such as the increase in nano-dielectric materials and other specialty wire insulation systems.
The most severe mobile equipment application is hybrid-electric construction machinery with more constant changing loads and variable operation. The studies associated with the insulation system for one large loader with two generators and four switched reluctance machines went about two years. The reliability goals generated completely new materials testing techniques with the goal of accurately determining risk of failure within specific parameters and length of time. Resulting improvements had significant effects on the rotating machine manufacturer’s original designs and material selection. As this sector grows, opportunities for improved insulation material design will be critical, for chemical resistance and thermal aging.
Commercial and industrial electric machines are experiencing changes in design and materials applications. The US Department of Energy has been funding novel approaches to electric machine designs for higher efficiencies including high-speed and high-horsepower compressor motors to reduce powertrain losses. Smaller smart motors are being developed with variable frequency drives being built into the motor frame, with several European fan manufacturers moving from induction motors to brushless DC. A significant consideration in every case is the power quality changes in most facilities as electronics from personal computers to IoT sensors and systems are installed.The need to understand the effect of True Power Factor, in addition to the harmonic conditions and related heating, on the electrical insulation system is second only to the effect of ground“noise” on the insulation systems. Fast-rise-time faults on across-the-line machines that are seeing impulses in the ground system from nearby large variable frequency drives have been confounding technicians at a growing rate.
Possible Future State
Advances in insulating material sciences and research appear to be rising to the challenge. New concepts in self-healing and self-diagnostic systems are in various stages of research and development due to investments in hybrid technology and green-economy rotating machinery and the distribution systems that support them. As we look to a carbon-neutral future, greater political and public pressure will be put on research and development to identify solutions to the electrification of the economy. For rotating machinery there are several directions of research from MEMS (microelectromechanical systems) sensors to materials self-sensoring and new testing technologies to older technologies being applied in new ways.
Although some discuss self-maintaining and self-repairing technologies, tradespeople are working with decades old technology for the prognostics and quality control of in-place and manufacturing process equipment. The challenges relate to the application of these technologies, and their advancement, through the life-cycle of rotating machinery in new environments. For the growing number of smaller electric machines (<700 volts), which include wind turbine generators, traditional testing, such as vibration analysis, has yielded troublingly few findings prior to failure. Older testing methodologies, such as electrical signature analysis, have yielded greater prognostics than expected once they are viewed in a new way. For larger machines (>1,000 volts), the situation is similar in that the noisier environment during operation and the application of traditional technologies becomes more challenging by system. In all cases, offline testing will need to be tailored to the potential operating environment, including the power supply and grounding system.Overall, the ability for AI to assist reliability engineers in modeling and trending a specific machine location and its operation will be critical in providing remaining useful life, or time-to-failure, estimation.
For the rotating machinery and insulating materials industry, the future is electrifying.Although standard technologies will continue, the general trend is toward specialized technologies for many new applications. This means a broader base of rotating machine technologies, the need for faster manufacturing of those technologies, rapid repair turnaround, and earlier fault detection will be required.
From The Editor
By now, most of us have probably gotten used to communicating with flat faces on screens. In the May-June issue of the Magazine, the Editorial focused on online meetings and conferences. There are definitely positive elements in these online meetings, and we arranged an online survey for DEIS members to share with us their thoughts on conferences, before, during, and after COVID-19. In one of the next issues, we will publish the results of the survey, together with an analysis of what could be the direction for future DEIS conferences.
This issue of the Magazine starts with the results from a global survey on dielectric dissipation factor measurements on new stator bars and coils and continues with the second part of an extensive review on composite suspension insulators. The third article discusses the compatibility between insulating liquids and various materials used in transformers.
The first article in this issue, authored by Monique Krieg-Wezelenburg of High Energy B.V. , the Netherlands, on behalf of Cigré WG A1.39, is titled “Dielectric Dissipation Factor Measurements on New Stator Bars and Coils—Results from a Global Survey.” Dielectric dissipation factor (DDF) measurement is a diagnostic test method used to assess the
manufacturing quality of individual stator bars/coils, and the dielectric behavior of the electrical insulation system of a stator winding is widespread and standardized in IEEE and IEC documents. The requirements specified in the new IEC 60034-27-3 document are viewed by some as too restrictive and by others as overly lenient. Therefore, a CIGRE working group organized an international survey with the aim of collecting worldwide information and data on DDF testing of newly manufactured stator bars and coils. The survey contained questions concerning the use of internal quality standards and limits, the use of international standards, used equipment, and the use of statistical evaluation of DDF results, including the request for measurement results of new bars or coils provided with information such as used guarding technique, applied stress grading, rated voltage, core length, and insulation system. A total of 167 responses were obtained, comprising 119 data sets containing more than 20,000 bar/coil dielectric dissipation factor records. The article provides the key findings of this global survey among owners/users and manufacturers of rotating machines.
The second article is part two of a review in two parts, titled “Comparative Investigation on Fracture of Suspension High Voltage Composite Insulators,” focusing on the chemical properties of all known fracture modes of suspension composite insulators and proposing a novel criterion for discerning fracture modes. It is authored by professor Xidong Liang, Shaohua Li, and Zhou Zuo from Tsinghua University; Yanfeng Gao and Yi Lu from the State Grid Jibei Electric Power Co.; Jiafu Wang from the National Institute of Metrology in Beijing; Weining Bao from China Electric Power Planning & Engineering Institute; and Chao Wu from the University of
Connecticut, USA. This second part of the review focuses on the use of chemical analytical techniques [i.e., Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and thermal gravimetric analysis (TGA)] to study the fracture mechanism. A comprehensive criteria system for abnormal fracture of composite insulators is proposed, based on macroscopic fracture features, microscopic fracture features, and chemical analysis. The authors claim that compared with existing criteria, their novel proposed system is more accurate and systematic, and lays the foundation for accurately discriminating between different fracture types.
The third article is titled “Compatibility of Materials with Insulating Liquids — Why and How to Test,” authored by Ivanka Atanasova-Höhlein of Siemens Energy, Germany. In this article, the need for rigorous compatibility testing of different construction materials and liquid insulation is stipulated, and the consequences are described when this is not being done correctly. Existing standards are discussed, as well as the requirements for compatibility testing. Recommendations are given on the practical compatibility testing, based on procedures that were used successfully for many years. It is concluded that compatibility testing after aging tests should not be restricted to the insulating liquid but also involve the construction materials. The author further suggests that the selected properties shall be chosen in relation to corresponding standards or to required design values. In certain cases, long-term testing can be necessary. Apart from the liquid insulation values, it is advised to also test the gassing behavior of the construction material, to prevent interference with applied diagnostic criteria.
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