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Mineral oils

Mineral oils

Vol. 6 Issue 4

Not all mineral oils are equal


The life of a transformer is linked to the fundamental role of insulating liquids to dissipate heat and protect the transformer’s solid insulation. Historical developments affecting crude oil source and advances in refining technology mean that not all mineral insulating oils are made equal. Isoparaffinic oils in particular can offer enhanced oxidative stability, lower viscosity in colder temperatures, and superior heat transfer, resulting in improved reliability and utility for transformer operators.

Keywords: isoparaffinic, naphthenic, transformer oil, mineral insulating oil, liquid insulation


  1. Introduction

Before Elihu Thomson, an electrical engineer working for Westinghouse in the US, patented the use of mineral oil in transformers in 1887, the burgeoning transformer industry had a major problem to solve. As transformers operate, energy losses occur, generating heat. As higher loads are applied, losses increase exponentially, rapidly raising temperatures in a transformer’s core and windings. Without adequate cooling, this heat prematurely ages the transformer, ultimately leading to equipment failure. At the time, the only insulating material used was air, but because these first transformers generated high amounts of losses, they were quickly limited in size by the rapid generation of heat that air failed to properly dissipate. Any attempts at larger devices would fail. Once Elihu Thomson identified oil as a readily available solution, the history of oil as an insulating medium began. Today, several billion liters of mineral oil are used in electrical equipment worldwide.

This paper provides a review of the functions of mineral insulating oils, their chemical composition, and production techniques. Emphasis is placed on describing how differences in crude oil source and refining process can impact the final chemical composition of the oil, and therefore its performance in a transformer.

  1. Functions of a mineral insulating oil

Fulfilling four main functions, insulating liquids are a critical component of power equipment. Originally, the two primary functions of the liquid were to dissipate heat through convection (heat transfer), and to act as an insulating material providing dielectric strength. As testing methodologies and our understanding of oil in transformers developed, a third function grew in importance – that of a diagnostic tool (ex. Dissolved Gas Analysis). Like testing for cholesterol in an individual’s blood, measuring and monitoring specific chemical markers in a transformer’s oil allows for the early detection of warning signs in a transformer’s health.

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