World's largest 765 kV ester transformer for enhanced safety and environmental protection in electrical grids

765 kV class 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. Another major challenge is to achieve more sustainable power grids. In parallel with significant achievements in renewable electricity generation (Figure 1), there are strongly growing efforts to achieve more sustainable transmission network by utilizing advanced technology within its equipment.

Transformers traditionally for more than one century use mineral oil, well known for its excellent electrical properties, primary acting as a dielectric and cooling medium. More recently, transformer technology innovation has advanced the use of alternative liquids, such as natural ester, with enhanced fire safety and environmental performance features. The transformer presented here represents the latest achievement in more than 20 years of conglomerated progressive knowledge in research, design, manufacturing and operation of ester filled power transformers [2], [3], [4]. It is 250 MVA, 765/400/33 kV, KNAN, KDAF single-phase auto transformer, which makes it world’s largest and world’s first for this voltage classes combination of ester filled power transformer (Figure 2).

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). Transient voltage that are generated along windings during these tests may be critical for ester dielectric withstand capabilities. The transformer is also designed to withstand an AC induced voltage level of 970 kV, representing a significant milestone in the application of ester technology at ultra-high voltages.

Power density at highest rating required extraordinary efficient cooling system to maintain the windings temperatures within specified limits. Due to different viscosity dependence on temperature of esters compared to mineral oil, additional verifications at the design stage of thermo-hydraulic model were required (e.g. CFD simulations).

A complete FAT set of tests as per IEC 60076 requirements was performed successfully.

Fruitful deliverables of this project enable the technology for next level of sustainability in power transformers industry.