WearCheck Ensures Transformer Health


WearCheck, a company specialising in condition monitoring, offers crucial services aimed at safeguarding the health and efficiency of transformers. Transformers play a vital role in electrical power systems, but they are not immune to failure, which can lead to costly consequences. Gert Nel, the manager of WearCheck’s transformer division, emphasises the importance of maintaining transformer health and focuses on the analysis of ester oils used as transformer lubricants in this article.

Transformer insulating oil: natural ester vs. synthetic

Ester oil serves as a critical insulating oil in electrical equipment. Two primary categories of ester oils exist: natural ester and synthetic ester oils. Natural ester oils are derived from vegetable oils, like rapeseed or canola, while synthetic ester oils result from the esterification of vegetable oils or animal fats. These ester oils can be blended with other chemicals to enhance their properties. Several advantages make natural ester oils preferable over traditional mineral oil:

  • Less Flammable: Ester oil has a higher flashpoint and fire point compared to mineral oil, making it safer for use in electrical equipment.
  • Biodegradability: Ester oils are environmentally friendly as they biodegrade.
  • High Dielectric Strength: Ester oil can withstand higher voltages without breaking down.
  • Wider Temperature Range Stability: Ester oils remain stable over a broader temperature range.
  • Superior Moisture Tolerance: They are more resistant to moisture.
  • No Corrosive Sulphur: Ester oils lack corrosive sulphur components.

Midel produces transformer insulating oils that are alternative to mineral oils. Below are the typical properties of these oils:

The choice between synthetic and natural ester oils depends on the specific application and type of transformer. Synthetic ester oils excel in free-breathing transformers due to their outstanding dielectric strength, thermal stability, and resistance to oxidation. These qualities make them suitable for transformers exposed to varying temperatures and operational stresses, ensuring consistent performance in demanding conditions.

In contrast, natural ester oils find their niche in sealed transformer units, where stability within confined spaces is crucial. Their excellent insulating properties and lower vapor pressure make them well-suited for sealed environments. Furthermore, their biodegradability aligns with the growing emphasis on environmentally conscious technologies.

The rise of ester oil transformers

WearCheck has observed a significant increase in the adoption of new ester oil transformers, along with retrofill transformer installations. This trend is exemplified by WearCheck’s transition from monitoring just two retrofilled transformers in 2018 to an impressive 418 in 2022. The industry’s recognition of the advantages of ester oils underscores their growing popularity.

Effective communication and testing

To optimise transformer performance, effective communication between clients and testing laboratories is crucial. While testing procedures for ester oils share similarities with those for conventional mineral oils, it is essential to analyse each oil variant separately. Accurate identification of the oil type serves as the foundation for interpreting test results and devising precise maintenance strategies.

When assessing an oil sample from a newly constructed transformer designed for ester oil usage, Dissolved Gas Analysis (DGA) indicators may not be immediately apparent. In such cases, the diagnostic approach closely resembles that of traditional mineral oil transformers. However, retrofilled transformers present a different scenario. Here, there is a noticeable increase in ethane and particularly ethylene levels due to the higher viscosity of ester oils. After the retrofill process, these gases experience a sudden surge, eventually stabilising over time.


The world of transformer oils is intricate, with the choice between synthetic and natural ester oils carrying significant implications. As WearCheck’s experience demonstrates, the industry is increasingly embracing these oil variants, recognising their pivotal role in enhancing transformer efficiency and longevity. Successful utilisation hinges on clear communication, precise testing, and a deep understanding of the unique characteristics that set ester oils apart in the realm of transformer technology.

To view the full article on ester oils in transformers, please view https://www.wearcheck.co.za/shared/TB86.pdf For more information, please visit www.wearcheck.co.za, email marketing@wearcheck.co.za or call head office on +27 31 700-5460

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