chapter gives a general introduction to the key topics of the book authored by the editors (U. Mohan Rao and I. Fofana) and co‐authored by Dr. Rodriguez Mariela, Institut de recherche d'Hydro‐Québec, Canada. This chapter covers the fundamentals of the transformer oil–paper insulation and introduction to various insulating liquids. The second chapter contribution from the Indian Institute of Technology Guwahati, India, and Xi’an Jiaotong University, Shaanxi, China, provides the details on the development and evaluation of natural ester liquids. Given the fact that the insulating liquids should have a high degree of compatibility with solid insulating liquids, the third chapter originating from the University of Cantabria, Spain, and BEST Transformer, Turkey, addresses the compatibility of the ester liquids with cellulose insulating materials. Authors affirm that the esters are not only compatible with cellulosic materials, but they also protect the cellulose, reducing its aging rate in comparison with traditional oil. The fourth chapter, authored by researchers from the high voltage laboratory at Indian Institute of Technology, Madras, India, and HVDM Research Group at Khalifa University, UAE, presents the details on the degradation and characterization of ester liquids.
The fifth chapter is authored by researchers from the Research Chair on the Aging of Power Network Infrastructure from the University of Quebec at Chicoutimi, Canada, which emphasizes the monitoring of the decay products of the new insulating liquids. The authors have demonstrated the monitoring of soluble and colloidal particles in ester liquids and spanned the discussion to the feasibility of using fuller’s earth for regeneration of ester liquids. The sixth chapter arises out of a research collaboration between Lodz University of Technology, Poland, the University of Quebec at Chicoutimi, Canada, and the Institut de recherche d'Hydro‐Québec, Canada. This chapter is an extended and updated version of the International study group article published in the IEEE Transactions on Dielectrics and Electrical Insulation in Volume: 27, Issue: 5, October 2020, with a primary focus on the pre‐breakdown phenomenon of the ester liquids.
The seventh chapter presented by the research group from State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, China, discusses ester liquids' miscibility behavior and mainly focuses on the engineering application of the new mixed liquid. The eighth chapter, focused on the ester‐based nanofluid, is a contribution from the National Institute of Technology Calicut, India. The potential areas of future research related to the feasibility of natural ester nanofluids in transformers are discussed. The ninth chapter contributed by researchers from the high voltage laboratory at the Indian Institute of Technology, Madras, India, and KTH ‐ Royal Institute of Technology, Sweden, presents the details of a new stable synthetic ester silica nanofluid with better dielectric strength compared to the synthetic ester fluid.
The tenth chapter, authored by researchers from the Research Chair on the Aging of Power Network Infrastructure from the University of Quebec at Chicoutimi, Canada, details the gassing behavior of ester liquids under corona discharging, arcing, and hotspot conditions. Dissolved gas analysis and diagnostic characterizations have been reported in the sections of this chapter. The eleventh chapter is a contribution from ETEL Transformers, New Zealand, and Essential Energy, Australia, on the in‐service experience of natural ester‐filled transformers. An overview of different parameters and fluid measurements is presented to benefit transformer owners and utility engineers.
1 Liquid Insulation for Power Transformers
U. Mohan Rao1, I. Fofana1, and E. Rodriguez Celis2
1 Research Chair on the Aging of Power Network Infrastructure (ViAHT), University of Quebec at Chicoutimi, Chicoutimi, QC, Canada
2 Institut de Recherche d’Hydro-Québec, Varennes, QC, Canada
1.1 Background of Liquid‐Filled Transformers
Increasing requirements of electricity at an alarmingly rapid rate due to population and industrial growth have caused severe energy crises throughout the world. The shortage of fossil fuels (such as coal, crude oil, and natural gas) amplifies these crises, by progressively diminishing the availability of conventional methods of power generation [1–9]. In addition, emission of harmful greenhouse gases (by‐products of fossil fuels) discourages further consideration of conventional power generation as a long‐term future solution for increasing electricity demands [8–17]. The growing concern over issues related to energy security and global warming has resulted in the evolution of renewable energy resources (solar, wind, geothermal, and tidal power generation) as potential alternatives because of their environmental and economic benefits [12–15]. Distributed generation (DG) and high‐voltage direct current (HVDC) transmission systems are promising solution for renewable power production and usage. The integration of DG and HVDC to the existing grid involves a significant difference in the voltage magnitudes. Power and distribution transformers handle these voltage magnitudes and ensure a reliable operation of the power grid [16, 17]. It is to be mentioned that, a few millions of transformers are connected across the global electric power network. In addition, transformers contribute the major segment of the economy involved in generation, transmission, and distribution of electricity. Therefore, transformer technology is always a high engineering importance to the researchers and utilities.
In a typical liquid‐filled transformer, windings are wound on an iron core and the whole assembly is immersed in the insulating oil. Based on the assembly of the core and windings, transformers are classified into Core‐ and Shell‐type transformers. In core‐type transformers, windings surround a considerable part of the core whereas in shell‐type transformers, core surrounds a considerable portion of the windings [18]. Transformers are also classified on the basis of their purpose of the application as a step‐up transformer to increase the voltage level at secondary terminals and step‐down transformer to decrease the voltage level at secondary terminals [19]. Insulating papers and pressboards are used for insulating windings within the core assembly. In oil‐filled transformers, insulating oil is allowed to circulate for dissipating heat through the cooling tubes mounted on the body of a transformer tank.
Generally, main parts of a power transformer are transported from manufacturers separately and are assembled at the site. Some preinstallation tests like winding resistance tests, sweep frequency response analysis to ensure mechanical integrity of core and windings, and heat oil circulation will be performed to ensure effective operation. Initially, in oil‐filled power transformers, main body of the transformer is circulated with hot oil through it in order to remove any ingressed moisture and is filled with insulation oil. After the installation process, the transformer is connected across the supply mains. The successful operation of a transformer is dependent on the proper installation. When the primary winding is connected to ac mains supply, a current flows through it. Since this winding is magnetically linked with the core, current flowing through the primary winding will produce an alternating flux in the core. This alternating flux links with the secondary windings and an EMF is induced in the secondary winding due to mutual inductance. When the load is connected across the secondary terminals forming a closed path, a secondary current is circulated in the secondary windings through the load. In oil‐filled transformers, insulation oil and insulation paper together form a composite dielectric medium often called as oil–paper insulation. The aging performance of oil and paper are closely interrelated; deterioration of either of the two leads to premature aging of the other one. Oil–paper insulation in transformers is expected to last for three to four decades in operation. Owing to variable thermal excursions, the degradation rate of oil–paper insulation gets accelerated and thus leading to premature aging. Hence, there is a great need for continuous condition monitoring of in‐service transformers to avoid catastrophic failures and prevent subsequent capital as well as human loss in certain situations.
Insulation technology in transformers plays a critical role in judging the performance of the transformer. In oil‐filled transformers, insulation oil along with insulation paper is used as an insulating medium. Insulation
