Mineral insulating oils have a long history and have been used for transformers, cables, capacitors, and circuit break­ers. They are manufactured by refining crude oils. The main compounds in crude oils are naphthenic hydrocarbons, paraf – finic hydrocarbons, and aromatic hydrocarbons. Small amounts of sulfur compounds, nitrogen compounds, and oxy­gen compounds also occur. The naphthenic hydrocarbons in­clude dicyclic, tricyclic, and alkyl-substituted hydrocarbons; the paraffinic hydrocarbons include normal paraffinic and iso – paraffinic hydrocarbons; and the aromatic hydrocarbons in­clude dicyclic, tricyclic, and alkyl-substituted hydrocarbons.

The composition of crude oils depends on the area where they are produced. There are three kinds of crude oils: naph – thenic, paraffinic, and mixed. Naphthenic crude oils contain a large amount of naphthenic hydrocarbons, and paraffinic crude oils contain a large amount of paraffinic hydrocarbons. Mixed crude oils are intermediate between naphthenic and paraffinic. Naphthenic crude oils are produced in South America, North America, and southern Asia. Paraffinic crude oils are produced in some areas of North America and north­ern Asia. Mixed crude oils are produced in the Middle East.

The composition of mineral oils depends on that of the crude oils from which they are manufactured. There are two kinds of mineral oils: naphthenic and paraffinic.

Refining Process

Mineral oils are manufactured from distillate of heavy light oil and light lubricant oil by the process shown in Fig. 7. Where naphthenic oils are refined, acid treatment followed by clay filtration is also used. In the case of paraffinic oils dewax – ing is part of the refining process. Examples of the composi­tion of a naphthenic oil and a paraffinic oil are shown in Table

5. It is seen that in both paraffinic and naphthenic oils the amount of paraffinic compounds is greater than the amount of naphthenic compounds.

Naphthenic oil

Paraffinic oil

To obtain good dielectric characteristics, the amounts of nitrogen compounds and sulfur compounds should be as small as possible. However, excessive refining also decreases the amount of aromatic hydrocarbons. Decrease of the amount of aromatic hydrocarbons means a decrease in hydrogen absorp­tion, and decrease of the amounts of both aromatic hydrocar­bons and sulfur compounds means a decrease in oxidation stability. It is known that hydrogen adsorption relates to the partial discharge characteristics of oil (11,12) and that aro­matic hydrocarbons have high hydrogen absorption. It is also known that coexistence of aromatic hydrocarbons and sulfur compounds is effective for oxidation stability. Therefore, re­fining must be performed so that the insulating oils maintain balanced characteristics. The optimum amount of aromatic hydrocarbons is 10 wt% to 20 wt%. In this case the amountof sulfur compounds is at least 0.5 wt%. Among them there are some corrosive sulfur compounds, but the amount of those is very small.

In some cases, oxidation stability can be corrected through the addition of oxidation inhibitors.

Characteristics of Mineral Oils

In Table 6 characteristics of some naphthenic and paraffinic transformer oils are shown. The higher pour point of the par – affinic oil is owing to the larger amount of paraffinic hydrocar­bons in it. In the 1970s and 1980s the dielectric properties, compatibility with insulating materials, thermal stability, and other properties of paraffinic oils were investigated, and it was proved that various properties of paraffinic oils are by no means inferior to those of naphthenic oils, except for their higher pour points. To lower the pour points of paraffinic oils, pour point depressants are added. Mixing of alkylbenzenes with paraffinic oils is also effective for this purpose. An exam­ple of mixture of alkylbenzene and paraffinic oil is shown in Table 6. In this mixture a pour point depressant has also been added. The lowering of the pour point is seen.

Applications of Mineral Oils

Mineral oils are used for transformers, oil-filled (OF) cables, pipe-type oil-filled (POF) cables, capacitors, and circuit break­ers. The greatest use is in transformers. Specifications for mineral oils are given in IEC 60296, 60465 and ASTM D3487, D2297, D1818.

Table 5. Examples of Composition of Insulating Oils

Type of Oil



Proportion of C (%)






























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