Dielectric Materials for Electrical Engineering

Chapter 16

Insulating Oils for Transformers 1

16.1. Introduction

Insulating oils are used in electrical engineering as impregnators of cellulose insulations or as a filling product for various electrical equipment: transformers (power, distribution, traction, furnace, instruments, etc.), reactors, capacitors, cables, crossings, circuit-breakers, tap-changers, etc. [BER 97]. The volumes of oil which are used vary from a few liters for capacitors to several tens of thousand liters for power transformers. The main role of this oil is to remove the air and other gases so as to improve the dielectric behavior of the solid insulation. However, in many applications, they are used to ensure both the electrical insulation and the heat transfer of a component or a system, as in the case of transformers.

Oils can also be used for their capability of extinguishing electric arc (in the arcing chambers of switches and some circuit-breakers), thanks to their lubricating power for materials containing pieces in motion (plug selectors, immersion pumps) and in certain cases to improve resistance to fire, as is the case for distribution transformers near populations. Finally, they are used to slow down direct oxidation of cellulose insulation.

Mineral oil is the most commonly used liquid in power transformers, both for its physico-chemical properties and for its low cost [ROU 98]. Originally, the electric insulation of transformers was ensured by non-impregnated natural products in direct contact ...