Cool new standard for insulating oils

Cooling transformers and switchgear

By Morand Fachot

For most people, mention oils in terms of an industrial environment and they immediately think of the lubrication of machines. However, the public at large is less aware that oils are also used in the electrotechnical domain as insulating elements in a wide range of equipment. IEC TC (Technical Committee) 10: Fluids for electrotechnical applications, has just published the latest version of an International Standard that covers unused mineral insulating oils for transformers and switchgear.

Oil-insulated transformer (Photo: Siemens) Oil-insulated transformer (Photo: Siemens)

Wide range of applications

Oils have been used in electrotechnology for a long time, leading to the creation of IEC TC 10, initially called "transformer oils", as early as 1924. The scope of this TC has been extended a number of times, first in1926 to include "insulating oils" and later to cover additional applications and equipment, including lubricants for steam turbines and substances like gases.

Some electrical equipment and systems, such as power transformers and switchgear, use mineral insulating oils. The functional requirements of these oils are insulation, heat transfer and arc quenching.

The standards issued by TC 10 in this domain provide guidelines to insulating fluids producers and are of interest to manufacturers of electrical transformers, switchgear and other equipment, and to engineers in charge of plant operation and control.

Quality matters

Certain properties of mineral insulating oils, such as viscosity, pour point or water content, may deteriorate with use and as a result of the influence of other elements during their service life. This may in turn affect equipment performance and even lead to failure.

Defining quality criteria for insulating oils is essential, as they deteriorate with time and contact with external factors and contaminants. The quality of unused oils is also important as, once initially spent, they can be recycled and reused. TC 10 works on standards that allow this.

With use and when in contact with air, insulating oils are affected by oxidation, a phenomenon that can be accelerated by high temperature of the oil and the presence of catalysts, such as small metal particles. Oxidation can produce water and acids and affect equipment.

Insulating oils can also be contaminated by moisture (through oxidation, water ingress or other factors) and by substances such as solid particles that the oil causes to be removed from equipment or that infiltrate during maintenance or repair.

Following strong market demand, TC 10 has worked on the revision of its 2003 edition of IEC 60296, Fluids for electrotechnical applications – Unused mineral insulating oils for transformers and switchgear. This revised International Standard was released in February 2012.

Expanded definitions

For the purpose of this standard, mineral insulating oils are classified into two categories: transformer oils and low temperature switchgear oils. To maintain the quality of mineral insulating oils as long as possible, chemicals such as oxidation or corrosion inhibitors need to be added to them to improve some of their properties. IEC 60296 expands significantly on the previous version as regards the definition of some of these additives. 

The revised standard also includes a comprehensive note, lacking in the previous edition, on specifications for corrosive sulphur compounds that can lead to the build-up of certain substances in transformers and on their potentially corrosive impact.

Other International Standards prepared by TC 10 concern different aspects of the use of mineral insulating oils in electrotechnical equipment.

Redline version

Edition 4 of this International Standard is available as an official publication and Redline version with track changes. Redline versions (available in English only) provide users with a quick and easy way of comparing all the changes between standards and their previous edition.

When changes introduced in a new International Standard are as extensive as in the case of IEC 60296, Redline versions are very helpful and highly valued by users.

S+ IEC 60296 released in February is the 16th addition to IEC Redlines. It highlights all additions and changes, including new relevant International Standards in the list of normative references. It underlines a large number of changes in applicable tests as well as new standards for mineral insulating oils, including for maintenance of in-service products and recycling of used products. This revised standard also contains detailed descriptions and properties of certain additives in oil.

Benefitting producers and users

This new edition of IEC 60296, and its S+ IEC 60296 Redline version in particular, should prove essential for manufacturers and engineers alike, giving better and updated guidelines for the production and use of such oils, and, in the case of the redline version, a clear view of the changes introduced in this new edition.

Oil-insulated transformer (Photo: Siemens) Oil-insulated transformer (Photo: Siemens)
Oil tank Oil tank
ABB Switchgear (Photo: ABB) ABB Switchgear (Photo: ABB)