# RTE France Orders First 400 kV Biodegradable Ester-Filled Autotransformers for Ecodesign Substation Fleet

> French transmission system operator RTE has awarded a landmark contract for synthetic and natural ester-filled 400/225 kV, 600 MVA autotransformers, replacing conventional mineral oil to eliminate soil and groundwater contamination risks across environmentally sensitive river corridors.

- **Publish Date:** 2026-10-01
- **Category:** transformers-grid
- **Reading Time:** 5 min
- **Original Source Citation:** [Réseau de Transport d'Électricité (RTE France)](https://www.rte-france.com/en/actualites/rte-commissions-first-biodegradable-ester-filled-400kv-autotransformers-ecodesign-substations)
- **Tags:** RTE France, Ester Fluid, 400 kV Autotransformer, EcoDesign, Fire Safety, Substation Modernization, French Transmission Grid
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## Decarbonizing and De-risking Transmission Assets

As part of its 2035 Ecodesign Substation Roadmap, RTE announced on October 1, 2026, the procurement of high-capacity 400 kV autotransformers insulated with biodegradable ester fluid. The units are destined for strategic river-basin substations near the Rhône and Seine, where environmental regulations strictly prohibit uncontained mineral oil releases.

Synthetic and natural esters offer 100% biodegradability within 28 days and possess a fire point exceeding 300°C (K-class fluid), drastically reducing catastrophic fire hazards and permitting reduced separation distances between adjacent transformer bays.

## Engineering Challenges of Extra-High Voltage Ester Dielectrics

Operating ester fluids at 400 kV requires specialized winding geometries and enhanced insulation margins due to the liquid’s higher kinematic viscosity and distinct dielectric permittivity compared to standard mineral oils.

The autotransformers incorporate high-grade Nomex thermal barriers, forced directed liquid cooling (ODAF) with low-noise variable speed pumps, and advanced fiber-optic hotspot sensing integrated into the smart monitoring bay.

## Substation Footprint and Lifecycle Emissions Reduction

By utilizing ester fluids, RTE eliminates the requirement for heavy deluge firewalls and oversized concrete oil retention pits, lowering civil construction carbon emissions by 24% per substation bay while providing full compliance with European Water Framework Directives.

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