Monthly Archives: July 2014

Synthetic Transformer Oils

Ordinarily flash points of mineral transformer oils are around 150°C. Therefore, mineral oils are not so desirable for trans­formers in trains and indoor substations. For those uses it is desirable to use nonflammable or less-flammable transformer oils. PCBs are nonflammable and are the most desirable oils for such applications. However, PCBs are no longer environ­mentally acceptable. Since they were banned, no transformer oils have been found that have the desired nonflammability.

Silicone (polydimethylsiloxiane) liquids have been put into use. These liquids (described in IEC 60836) have high fire points and good oxidation resistance, and are classified as less-flammable liquids in the National Electrical Code in the USA. They are often used for transformers of trains, and in some countries they have been used for distribution trans­formers.

Some polyolester liquids (described in IEC 61099) are used for transformer oils on account of their good thermal stability and low hydrolysis in the presence of water. Midel 7131 (The Micanite and Insulators Co.) and Enviro Temp 100 (RTE Co.) are examples. Mixtures of flon 112 and tetrachloroethylene such as Formel. NF (ISC Chemicals Ltd.) have been developed for transformer use. This liquid has environmental problems because of the flon 112. However, tetrachloroethylene is non­flammable, and it and its mixtures with mineral oils have been classified as nonflammable by Factory Mutual.

As previously mentioned, high-molecular-weight hydrocar­bons with fire point higher than 300°C are classified as less- flammable oils in the National Electrical Code and are used for transformer oils. In Table 7 properties of some trans­former oils are shown.


Vegetable oils (castor oil, rapeseed oil, etc.) are basically tri­glyceryl esters of fatty acids, and the fatty acids can be satu­rated or unsaturated. They were once used for cables and ca­pacitors, and are now mostly used for the impregnation of dc capacitors and especially energy storage capacitors, as they have high permittivity. They have not been used for ac power capacitors, as they have poor dielectric dissipation factors. Re­cently, however, they have been tried for use with metallized polypropylene films, with which they have good compatibility, and their dissipation factor and gas-absorbing ability have been improved by blending them with aromatic hydrocarbon liquids.


Transformers were developed and began to be manufactured in the mid 1880s in Hungary, the USA, the United Kingdom, and France. In the years 1886 to 1891, manufacturers began to use oils for insulation transformers. Such oils (transformer oils) are specified in IEC 60296 and 60836 and in ASTM D3487 and D4652.

Transformer oils must have the following properties:

1. High dielectric strength and low dielectric losses

2. Good cooling power (mainly dependent on viscosity)

3. High chemical stability and high resistance to oxidation

4. Good compatibility with insulating materials

5. Low corrosive sulfur content

6. Low viscosity and good fluidity over a wide temperature range (low pour point)

7. Sufficient source of supply

8. High flash point

9. Nontoxicity

Of these, properties 1 and 2 are most important from the viewpoint of transformer performances.

Mineral Transformer Oils

Mineral oils have been used as transformer oils since the be­ginning of their manufacture. When properly refined, mineral oils have various excellent properties mentioned above. At present, mineral oils are used over wide range of transformer capacity, from distribution transformers to ultrahigh-voltage transformers.

Mineral oils are manufactured by refining crude oils. De­pending on the composition of the crude oils, there are two kinds of mineral oils: naphthenic and paraffinic. The pour points of paraffinic oils are generally higher than those of naphthenic oils.

Sometimes mineral oils are mixed with each other or with other oils. The mixtures may be between oils of the same type, between naphthenic and paraffinic oils, or with nonmineral oils. Some specifications can be found in IEC 60296. In some countries, mixtures of mineral oils and alkylbenzenes are used as transformer oils. Such oils have high resistance to oxidation, low corrosion, and low pour point. In the case of paraffinic oils, because of their relatively high pour points, pour-point depressants are added.

Because oils are oxidized under air, small amounts of anti­oxidants are added to some mineral oils, especially in Europe and North America. Such mineral oils are classified in IEC 60296 and ASTM D3487. However, in some countries mineral oils with antioxidants are not used.

In 1970s some flashover faults were found in ultrahigh – voltage transformers due to flow-induced electrification (streaming electrification). Factors that affect this phenome­non are transformer design (especially the flow rate of the oil), oil temperature, and properties of the insulating oils such as the volume resistivity and electrostatic charging tendency. Flow rates of oil have been controlled in some transformers to suppress this phenomenon. It is said that 1,2,3-benzotriazol (BTA), which has been known as a deactivator agent for met­als, suppresses this phenomenon. In some countries a small amount of BTA has been added to mineral oils for high-volt­age and high-power transformers for that purpose.


PCBs were among the best and most widely used synthetic insulating liquids for electrical machines, such as power ca­pacitors and transformers, due to their superb electrical char­acteristics and nonflammability, until a total ban on their use and production was imposed, first in Japan in 1972, then in the USA in 1976, and then in Europe in 1985.

In the 1960s alkylbenzenes were initially developed for high-voltage cables in view of their superior gassing proper­ties under high voltage stress, and especially for use with syn­thetic paper.

At the time PCBs were banned, other kinds of synthetic aromatic hydrocarbons such as alkylnaphthalenes and alkyl- diphenylethanes had been developed as candidates for im­provements on mineral oils, but because of their higher cost, they had not been put into practical use. PCBs were then re­placed mainly by these new aromatic hydrocarbons.

Aromatic Hydrocarbons

Alkylbenzenes consist of a benzene ring and an alkyl group of the straight-chain or branched-chain type. Alkylbenzenes are

Table 6. Characteristics of Mineral Transformer Oils


Paraffinic Oil 1

Paraffinic Oil 2b



Flash point (°C)




Kinematic viscosity



7.8 X 10—6

9.6 X 10—6

7.9 X 10—6


2.2 X 10—6

2.5 X 10—6

2.1 X 10—6

Pour point (°C)

— 25








tan S (%)





p (П • m)





a tan S = dissipation factor; p = volume resistivity. b Containing azkylbenzene and pour-point depressant.

mainly used for high-voltage cables, including cables with synthetic paper, as they have excellent compatibility with plastics. They can be blended with mineral oil to improve its gassing properties and thermal stability.

In IEC 60867, alkylbenzenes are divided into three classes according to their viscosity and flash point.

Alkyldiphenylethane (phenylxylylethane), alkylnaphtha – lene (diisopropylnaphthalene), and methylpolyarylmethanes (blends of benzyl – and dibenzyltoluene and blends of benzylto – luene and diphenylethane) are mainly used for high-voltage power capacitors and also used for instrument transformers. Because all of these liquids consist of two benzene rings with much shorter-chain alkyl groups than in alkylbenzene, their aromatic contents are higher than those of alkylbenzene, and their resistance to partial discharge is very high. They have excellent dielectric properties and also good compatibility with plastic film, especially with the polypropylene film cur­rently used for capacitors; but all of them have lower per­mittivity (2.2 to 2.5) and flash point (130°C to 150°C) than PCBs. In some cases, they are stabilized by epoxiside or anti­oxidant. Their properties are also specified in IEC 60867.

Silicone Liquids

The silicone liquids currently used for electrical machines are polydimethylsiloxanes and have a variety of viscosities and flash points. Properties of a silicone liquid with a kinematic viscosity of 40 mm2/s at 40°C are specified in IEC 60836. They are used mainly for special transformers, due to their good thermal stability at higher temperature and better flow at lower temperature than mineral oil and because they are not very flammable. They are also sometimes used for capacitors and cables.

Organic Esters

Dioctylphthalate (DOP) and diisononylphthalate (DINP) have been used as substitutes for PCBs, especially for capacitors, because they have higher permittivity (4.5 to 5.5) and flash point (200°C to 240°C) than aromatic hydrocarbons. Di-2- ethylhexyl orthophthalate (DOP) is specified in IEC 61099 as a capacitor ester (type C1). As not easily flammable liquids, phosphoric acid esters such as tricresyl phosphate (TCP) and trixylenyl phosphate (TXP) are used as blends with aromatic hydrocarbons. Generally speaking, these esters have high permittivity and high inherent resistance to electrical stress, but as manufactured they contain much water and impurities and their dielectric dissipation factor is very high, so they must be carefully dehydrated and purified before impregna­tion and often need an antioxidant or scavenger.

Recently, organic tetraester liquids have been introduced in transformers because they are less flammable. Their fire point is higher than 300°C, but their viscosity is low compared with that of currently used mineral oils. The same precau­tions should be followed as mentioned above, and additives are effective as in the case of other organic esters.

Tetrahydric alcohol and a mixture of monocarboxylic acid with suitable stabilizing additives are also specified in IEC 61099 (type T1).


Polybutenes can have a large range of viscosity (1 mm2/s to 105 mm2/s at 40°C), depending on polymerization. These liq­uids are now used mainly in hollow cables (pipe-type cables) and to some extent in low-voltage capacitors. Polybutenes are specified in IEC 963 and classified into three classes, mainly depending on their viscosity. They are selected according to the specific application.


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 (%)































Insulating oil, like petroleum system liquids such as gasoline, toluene, and kerosene, has high volume resistivity. In such insulating liquids, ionic compounds A+B— dissociate into A+ and B—, and the ions become charge carriers:

A+B – ^ A+ + B-

There is no charging when the numbers of A+ and B— are equal. However, for example, static electrification is observed when the number of positive ions is increased by the flow of liquid. This static electrification is called streaming electrifi­cation.

Mechanisms of Streaming Electrification

The streaming electrification process arises from electric charge motion, separation, and relaxation (Fig. 5). These pro­cesses happen simultaneously:

1. Electric Charge Motion. Certain ions in the liquid are adsorbed on a solid in contact with it.

2. Electric Charge Separation. Other ions are carried off by flow, and an imbalance of positive and negative ions occurs.

3. Electric Charge Relaxation. Surplus ions are neutral­ized, and the imbalance is canceled.

Properties of Streaming Electrification

When the following three conditions coexist, large streaming electrification is observed:

1. It is easy to move an electric charge, because a solid surface is active.

2. Charges are easily separated.

3. Surplus electric charge is not easily canceled.

When any one of the three processes is hindered, stream­ing electrification can be prevented. The choice of insulating oil can also affect streaming electrification.

Polarity of Streaming Electrification

In streaming electrification between insulating paper and in­sulating oil, the oil becomes positively charged and the insu­lating paper negatively charged. A possible reason is a pecu­liarity of the oxygen of the hydroxyl group (-OH) in the insulating paper (cellulose).

Oxygen, having high electronegativity (that is, ability to attract electrons), attracts the electron of hydrogen. It thereby becomes negatively charged, and the hydrogen becomes posi­tively charged. The cellulose molecule surface is covered with positively charged hydrogen, which adsorbs negative ions in oil selectively. Therefore, the insulating oil becomes positively charged, and the insulating paper is negatively charged.

Streaming Electrification and the Deterioration of Insulating Paper

The hydroxyl group (-OH) of cellulose is changed to the alde­hyde group (-CHO) or the carboxyl group (-COOH) by oxida­tive deterioration. The extent of polarization due to electron transfer from hydrogen to oxygen, mentioned above, is in the following order:

hydroxyl group < aldehyde group < carboxyl group

Accordingly, streaming electrification increases as the insu­lating paper deteriorates.

Oil temperature (°C)

streaming electrification, and also direct observation of streaming electrification (9) in transformers have been re­ported. A good measure of the streaming electrification in a transformer is the coil leakage current.

Figure 6 shows the oil temperature dependence of the coil leakage current, and reveals that it is influenced by differ­ences in the insulating oil.

Coil leakage current is significant at all operating temper­atures in large-capacity transformers. Therefore, measure­ment of the streaming electrification is essential in trans­former design with respect to internal structure, oil flow rate, and insulating material. One may also select an oil that re­sists charging and charge separation.