Dejan Vuković

The 765 kV AC high-voltage transmission is part of power grids in various regions worldwide, including North America, South America, South Africa, and Asia (e.g. China, India). 765 kV transformers are part of utilities’ strategic transmission expansion plans to meet growing energy needs, thanks to their ability to transmit large amounts of electricity over long distances, improving grid efficiency, stability, and reliability. At the same time, there are strongly growing efforts to achieve more sustainable power grids. Transformers traditionally use mineral oil, well known for its excellent electrical properties, as an insulation and cooling medium. More recently, transformer technology innovation has advanced the use of alternative fluids, such as natural ester, with additional fire safety and environmental performance features. The transformer presented here represents the latest achievement in more than 20 years of cumulated progressive knowledge in research, design, manufacturing and operation of ester filled power transformers. It is 250 MVA, 765/400/33 kV, KNAN, KDAF single-phase auto transformer, which makes it world’s largest ester filled power transformer. The paper presents an insight in design, manufacturing and testing of such unique transformer. One of the biggest challenges was to achieve resilient dielectric design that can withstand severe impulse voltage tests going up to 2100 kV for full lightning impulse (BIL) as per IEC 60076 requirements. Transient voltage that are generated along windings (especially considering that it is an autotransformer design) during these tests are critical for ester dielectric withstand capabilities. Calculated temperatures at the most relevant positions have been verified during FAT by means of installed temperature and pressure sensors, demonstrating excellent thermal design reliability. Several manufacturing processes and practices require ester liquids dedicated approach and solutions. Especially transformer drying and impregnation during production need to be done with special care in order to avoid potential issues in following production steps. Final acceptance tests (FAT) require in several aspects also additional attention to fit testing process more suitably to the properties of esters and revisit the expectations about reference measured values. For example, due to significant difference in resistivity compared to mineral oil, DC resistivities measured on transformer terminal will have in general lower values. Successful execution of this project enables the technology for next level of sustainability in power transformers industry.

Keywords: Power Transformer, Innovation, Sustainability, Alternative insulating liquids