Safer, greener high voltage transformers

Our research has proved that ester liquids are safe and suitable for cooling and insulating high voltage transformers. International industry bodies have updated their specifications and standards for the use of these liquids; eco-friendly and safer transformers are now in use by utility companies.

Safer, greener high voltage transformers, School of Electrical and Electronic Engineering

Mineral oil is commonly used to cool and insulate  transformers, but this type of oil is far from ideal for the job; it is an environmental pollutant and as a flammable liquid with a low flash point (150°C), poses a substantial fire risk should the transformer malfunction or overheat.

In contrast, ester liquids – synthesised  when an acid and an alcohol combine and release water or produced using vegetable oils - remain safe up to a flash point temperature of 350°C. They are also biodegradable, so do not pose the same environmental risk as mineral oil. Natural ester liquids have been used in low voltage transformers for over 20 years, but before 2008 they had never been approved for use in high voltage transformers.


Oil capacity in a large HV transformer.
The cost of a single large HV transformer


The cost of a single large HV transformer.



The annual global transformer oil market.

School researchers examined how ester liquids perform within high voltage transformers. They concluded that esters could safely replace  traditional mineral oils for transformer cooling and insulation.

Based on our evidence, the International Electrotechnical Commission (IEC), which sets standards for the international electrical industry, are updating its standards  for power transformers, regarding  the use of esters. 

Our underpinning research has been highlighted across the global industry, for example in brochures published by the International Council on Large Electrical Systems (CIGRE) offering practical guidance for power engineers.

Alstom Grid, the third largest transformer manufacturer in the world, was the first company in the UK to design and build an ester-filled high voltage transformer. Today, the company offers a range of ‘eco-efficient’ ester-based transformer products. 

EDF Energy (now UK Power Networks) has applied our research; it is the first utility company to run a ‘green’ 132kV transformer containing only natural, biodegradable ester liquids from sustainable sources. 

National Grid, the UK power transmission system operator, operating at the 275kV and 400kV voltage levels, has revised its transformer oil policy to permit use of ester liquids. 

Since our work, the global market for ester liquids has expanded significantly. One company, M&I Materials, says its total  sales have increased from £15 million in 2008 to £29 million in 2012. 

Research background

Researchers in the School of Electrical and Electronics Engineering carried out extensive experimental tests on the performance of ester liquids under all the critical stresses of HV applications.

Main results:

  • Breakdown characteristics of ester liquids under homogenous electrical fields.
  • Full electrical characterisation of ester liquids under extreme inhomogeneous electrical  fields.
  • Development of breakdown voltage-gap distance equations for ester liquids.
  • Development of empirical equations to link liquid viscosity, temperature and vacuum level to the processing time required for ester liquids to impregnate solid insulating materials.
  • Demonstration of  dissolved gas analysis  for condition assessment of ester-filled transformers.
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