1.1 Background of LiquidâFilled 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 [1â9]. In addition, emission of harmful greenhouse gases (byâproducts of fossil fuels) discourages further consideration of conventional power generation as a longâterm future solution for increasing electricity demands [8â17]. 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 [12â15]. Distributed generation (DG) and highâvoltage 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 liquidâfilled 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 Coreâ and Shellâtype transformers. In coreâtype transformers, windings surround a considerable part of the core whereas in shellâtype 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 stepâup transformer to increase the voltage level at secondary terminals and stepâdown transformer to decrease the voltage level at secondary terminals [19]. Insulating papers and pressboards are used for insulating windings within the core assembly. In oilâfilled 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 oilâfilled 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 oilâfilled transformers, insulation oil and insulation paper together form a composite dielectric medium often called as oilâpaper 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. Oilâpaper insulation in transformers is expected to last for three to four decades in operation. Owing to variable thermal excursions, the degradation rate of oilâpaper insulation gets accelerated and thus leading to premature aging. Hence, there is a great need for continuous condition monitoring of inâservice 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 oilâfilled transformers, insulation oil along with insulation paper is used as an insulating medium. Insulation paper (mostly cellulose fibers) is used to isolate live conductors. Insulation oil (traditionally, mineral insulating oil) is used preliminarily for insulation of the core. The main functions of the insulation oil in an oilâfilled transformer are as follows:
- Oil acts as an insulating medium: Provides insulation for the conductors.
- Oil serves as a coolant by dissipating internally generated heat: Heat dissipation is done in conduction, convection, and radiation modes with the help of solid insulation and cooling tubes.
- Oil acts as a diagnostic medium for prognosis of aging characterization of inâservice oilâfilled transformers: Small volume of the liquid is collected from the transformer to test according to the standards for diagnosis and prognosis in understanding the situations prevailing in the transformer (similar to blood in human body).
- Oil acts as a barrier to the laminated sheet steel core by preventing direct contact with atmospheric air.
Mineral insulating oils extracted from crude petroleum are being used in transformers successively since decades. However, due to various technical demands/benefits, health/safety aspects, and environmental concerns, alternatives for mineral oils are of high demand by the industry. There is a need to shift the transformer insulation technology to a suitable new alternative and biodegradable liquids. Esterâbased dielectric fluids, both natural and synthetic fluids, have been subjected to extensive research since decades. The performance of these new insulating liquids is found to be affirmative and comparable to mineral insulation oils. Despite their overwhelming technical benefits, very few utilities started using these alternative insulating fluids. One of the reasons includes the availability of limited diagnostic and prognostic information in the existing knowledge. The reclamation aspects and service experience of these new insulating liquids also remain as a pertinent challenge for the utilities.
Meanwhile, there are some significant aspects of ester fluids that need to be further investigated to improve the existing knowledge. This book aims at discussing the critical aspects that will act as a guide to the industry and researchers. The critical concepts with the significant research findings over the years and the topics that will need to be further emphasized will be digested in the chapters of this book. The knowledge embodied in this book will also act as a quick reference for utility engineers, transformer owners, academia, and researchers those interested in alternative and biodegradable insulating fluids for liquidâfilled transformers. The scope of this book will aid to the focus of the IEEE DEIS technical committee on liquid dielectrics and lies within the interests of the IEEE DEIS community and transformer industry across the world. This book will be a unique sample of its kind and will act as a reference pertaining to the biodegradable insulating liquids. The contents of this book are framed in a way to cover the pertinent orbits of ester liquids, key aspects that need to be em...
