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Ester Impregnated Paper Transformer Bushings

The introduction of ester fluid as insulating media is the next step of development based on the well consolidated OIP technology, addressing the increasing demand for sustainable HV equipment as well as enhanced safety requirements. The Ester impregnated transformer bushings are the perfect solution for ester filled power transformers, bringing the unique solution for a full ESTER application, but can be installed also on the conventional mineral oil insulated power transformers.


Using CET bushings reduces the risk of environmental damage in case of spillage. The insulation liquid is classified as readily biodegradable according to OECD 301 unlike mineral oil which is non-biodegradable.

CET bushings have an increased eco-performance during the product lifecycle:

  • No pollution hazard during bushing manufacturing, testing and operation
  • Reduced containment measures for transformer installation or bushing storage
  • Recycling capabilities of bushings components at end of life


Fire safety is a key concern for users of insulating liquids, especially where they will be used in populated areas. Synthetic esters have a high fire point, significantly increasing the fire safety of the equipment.

Using a less flammable K class fluid such as synthetic ester brings the following advantages:

  • No fire risk in case of major failure: even if ester ignites, the resulting pool of liquid rapidly ceases to burn
  • Low density, non-toxic smokes
  • Lower costs for installation and maintenance of fire safety equipment
  • Associated insurance costs


CET products offer the characteristic of higher load capabilities at higher temperatures without reducing the insulation life expectancy, compared to conventional OIP insulation. Based on ageing tests, kraft paper has shown improved life characteristics when combined with ester liquid: as presented in IEC 60076-14, cellulose paper combined with synthetic ester can be considered as an insulating system of thermal class 120.


Ester impregnated transformer bushings from Trench: high performance products with increased sustainability

ain features

CET ester bushings are derived from the design of our COT transformer bushings, providing the same main features:

  • Technical characteristics as per IEC 60137-2017
  • Capacitive fine graded insulation made of kraft paper impregnated with liquid insulation (synthetic ester)
  • Available with porcelain or composite insulator
  • Effective solution also for cold climates thanks to the low pour point of synthetic ester (-56°C)
  • Increased eco-performance during the product lifecycle
  • Higher load capabilities at higher temperatures without reducing the insulation life expectancy
  • Fire safety: synthetic esters have a higher fire point, significative increasing the fire safety of the equipment
  • No miscibility issue: synthetic esters are miscible with mineral oils in all proportions
  • Perfect solution for ESTER transformers for a full ESTER technology
  • CET products are fully type tested
  • Cost-effective solution, with a price level comparable to OIP bushings

Mechanical and electrical data

These bushings are available from voltage range 72,5kV to 245kV and current range up to 1250A with a complete type test reports according to IEC 60137 in addition to special tests beyond Standard requirements, but the portfolio is subjected to the next step of ratings extension.

Porcelain or composite insulator can be defined according to the seismic requirements and pollution level.

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