Alternative Liquid Dielectrics for High Voltage Transformer Insulation Systems. Группа авторов

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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 СКАЧАТЬ