Alternative liquid application in power transformers

Author: Paloma Mendoza


Natural ester fluids

First introduced in the early 21st century, the use of alternative liquids, especially synthetic esters, is a well-established practice with distribution transformers. Similarly, retrofit transformers that are refilled with alternative liquids are also an accepted practice. 

The story of alternative liquids began in 2004 when a medium power transformer with a rated voltage of 238kV and a rated power of 135MVA was filled and tested successfully with a synthetic ester for the first time at Siemens. Several projects followed throughout the years. Then, in 2008, a medium power transformer with a rated voltage of 110kV and a rated power of 40MVA was filled and tested successfully for the first time with a natural ester.

In April 2013, the world’s first large power transformer filled with natural ester was successfully tested. This transformer was developed and built by Siemens at their power transformer factory in Nuremberg, Germany. The transformer used new design criteria and is the largest unit using a renewable resource liquid. The operation of this unit will guide further development and testing and is a pioneering feat for the future of large transformer development.


Ester fluids


In regard to the requirements for increased biodegradability and sustainability, the most important alternative liquids are ester based. These liquids are the only alternative liquids that are classified to be fully biodegradable. 

Esters are divided into two main classes: synthetic and natural. Synthetic esters are derived from crude oil-based chemicals. Natural esters are derived from vegetable oils.

Insulation liquids within electrical power equipment have to meet several standards. The ideal liquid represents the optimum electrical insulation and cooling performance and will be resistant to oxidation and aging. Aside from these important features, there are a lot of other factors that must be considered irrespective of liquid type. An evaluation will need to be completed to determine which liquid characteristics will be sufficient for the type of electrical equipment being used. 

The following is a partial list of challenges that need to be considered for use in power transformers:

  • Electrical insulation

  • Cooling and transport of thermal energy

  • Impregnation of solid insulations material (most likely based on cellulose)

  • Oxidation stability in case of thermal service stresses

  • Aging resistivity

  • Material compatibility and low chemical reactivity

  • Gassing tendency

  • Flash point and reaction to fire

  • Corrosion protection.

Depending on customer requests, additional issues may need to be considered (e.g., bio-degradability, low temperature characteristics, sustainability, and gas absorption characteristics). None of the existing liquids can be considered the ideal solution for all of the given challenges. Therefore, it must be clearly determined which challenges have to be tackled and which weak points of the liquid shall be balanced.


Characteristics with impact to environment and facilities


The most important characteristics of alternative fluids are the environmental properties. Biodegradability is one of the most significant benefits of natural, as well as synthetic, esters. Natural esters are known to be up to 100 percent biodegradable, depending on the composition of the natural esters. Synthetic esters are up to 90 percent biodegradable. Mineral oil is non-degradable within an acceptable time period, and just 10 percent of the composition of mineral oil will be naturally defragmented. The table below shows the most important characteristics of the liquids in regard to environmental health and safety.

Table: Typical Environmental Characteristics of Liquids

*1 Non biodegradability within an adequate time period, which should be below several decades.
Property (mean values)
Mineral oil (IEC 60296)
Synthetic ester (IEC 61099)
Natural ester
Fire point (°C)
Fire point (°C)
Pour point (°C)
Non biodegradability *1
Up to 100%
Water hazard class
Raw material
Crude oil
Chemical intermediate good
Renewable resource

The high degree of biodegradability of esters enables a financial benefit for facility investment, because no separate containment facility needs to be installed to prevent pollution of the environment in the event of leakage.

The increasing demand for electric energy leads to huge investments to develop new primary resources and techniques for electric energy generation and transmission. Simultaneously, the demand for environmentally friendly energy generation and supply becomes more important for all of us.

It is also becoming increasingly important to develop and install electrical equipment that provides a high level of environmental safety and reliability. Especially in regions of great infrastructure density, where newly interlinked systems become an ideal solution, this is a major target. A high degree of health and environmental safety will enable the installation of power equipment in areas of high population density without increasing risk level.

In the case of power transformers, insulating oil is the most critical conventional material. Consequently, the focus is to replace transformer oil with alternative liquids that enable a higher level of sustainability and environmental safety.


Case study


In July 2015, the U.S. Bureau of Reclamation awarded $37 million to Yellowstone Electric Co. out of Billings, Montana to replace 50-year-old single-phase transformers at the Glen Canyon Power plant. The 12 transformers, having reached the end of their service life, provided the Bureau and Yellowstone Electric the opportunity to ensure the sustained power quality and delivery of the hydroelectric plant for millions of customers, but also called for environmentally-friendly ester-insulated technology, a first for Reclamation to use transformers of this size filled with natural oils. In late 2015, Siemens was named as the transformer technology provider for the project and has been contracted to deliver 14 transformers for the hydroelectric dam, 12 for immediate operation and two spares. 

The ester fluid transformers, rated 125MVA (10 units designed for 345kV and 4 units for 230kV), will be supplied by the Siemens factory in Linz, Austria, and will connect Glen Canyon’s generation units to the transmission grid and increase the transmission power capacity to provide stable and reliable electricity to customers. This is the first time the Bureau of Reclamation will use ester fluid transformers of this size for a hydroelectric power project. Siemens will begin delivering the transformers in late 2017. The units are expected to be installed and begin operation in 2018 and 2019. 

Ester oil transformer technology is becoming more common in the U.S., but it already has a significant footprint in the global power marketplace. For example, Siemens partnered with the Letsi hydro power plant in Sweden to replace 40-year-old transformers to improve reliability and ensure environmentally-friendly operation. With Siemens eco-friendly transformers, the power plant, one of the most important hydro plants in Sweden, can continue to be relied upon to restore the Swedish national grid in case of major disturbances.




Prototype 420 kV Power Transformer using Natural Ester Dielectric Fluid, by Ronny Fritsche, Uwe Rimmele, Frank Trautmann and Dr. Michael Schäfer.

Ronny Fritsche*, Uwe Rimmele*, Frank Trautmann*, and Michael Schäfer** 
* Siemens AG – TR Nuremburg 
** TransnetBW GmbH – Germany