The Use of Transformer Oil in Oil-Filled High-Voltage Electrical Equipment

The reliability of high voltage transformers in severe operation conditions is mainly defined by the quality of insulation oil.

Mineral transformer oil is widely used for cooling and electrical insulation in oil-filled electrical systems.

Transformer oil is a product of crude oil refining, and its chemical composition is largely determined by the hydrocarbon composition of the crude oil and the refining method. It is known that oil performance is influenced by the crude chemical composition and production technology. The chemical composition of transformer oil has a significant impact on its performance.

The oil fractions of crude and the transformer oil produced contain a complex combination of paraffinic, naphthenic, aromatic and naphthenic-aromatic groups, regardless of the crude composition and refining technology.

Besides, transformer oil contains oxygenic, sulfuric and nitric derivatives of these hydrocarbons, which usually reduce its performance.

Mineral transformer oil are usually of low viscosity, easily impregnate the solid insulation, improving its dielectric strength significantly. Besides, mineral oils are characterized by high heat capacity. The combination of low viscosity with high heat capacity allows to use mineral transformer oil is a coolant. Heat is dissipated from oil-filled equipment mostly by convection of the liquid. This process is largely determined by oil viscosity, so the lower the viscosity, the better the heat dissipation, i.e. improved cooling.

An important condition of growth for the energy sector is the increase of production and the use of high quality of oil high voltage bushings and oil filled electrical systems.

To satisfy the demand of the industry, heat and power generation plants should use oil made from sour crude, alternately improving its performance on site.

The most important part of transformer oil are hydrocarbons. Hydrocarbon components, constituting the largest part of oil, are divided into three categories: paraffinic, naphthenic and aromatic.

Beside the hydrocarbons, the oil contains non-hydrocarbon compounds, which contain the corresponding hydrocarbon frame with one, two, three or more atoms of sulfur, oxygen or nitrogen.

Some of the non-hydrocarbon compounds in transformer oils are asphalts and resins, sulfuric and nitric organic compounds, naphthenic acids, esters, alcohols and metal compounds.

Asphalts and resins

Refining reduces the amount of asphalts and resins in the market transformer oil to 2.0-2.5 %. Even with this relatively low concentration, some of the substances of this type have a significant impact on transformer oil performance. Asphalts and resins give the oil its color, some have inhibiting effects, while others passivate antioxidation additives. In the process of oxidation, resins form a part of the resulting sludge.

Asphalts and resins include the following substances:

• neutral resins, soluble in petroleum ester, liquid and semi-liquid oil fractions; the density of these is about 1.0;
• asphaltenes, solid substances, insoluble in petroleum ester, but well soluble in benzol, chloroform and carbon disulfide; their density is above 1;
• polycarbons, insoluble in regular solvents and partially soluble in pyridin and carbon disulfide;
• asphaltogenic acids and their anhydrides, insoluble in petroleum ester, but soluble in alcohol;
• asphalthenes and neutral resins, which are oxygenic polycyclic compounds with no more than one double bond; specific reactions allowed to determine the presence of aromatic nuclei, sulfur and nitrogen in the resins, so these are classified as neutral polycyclic hetero compounds.

Structural analysis of resins identified presence of polycyclic structures, mostly aromatic.

Sulfuric compounds

Sulfuric compounds are present in all crude oil, but the concentration varies widely from tens of a percent to 20% by weight. Sulfur has a pronounced effect on crude oil properties and the selection of the refining method.

While low-boiling fractions, (gasoline, kerosine etc) include almost all kinds of sulfuric compounds, fractions which boil at 200°С and above contain cyclic sulfuric compounds, mainly aromatic.

During distillation, most of sulfuric compounds concentrate in the oily fractions.

The content of sulfur in transformer oil is reduced to 0.6-0.8% by desulfurization. Sulfuric compounds reduce the thermal stability of transformer oil and increase the amount of sediment during use.

The most common sulfuric compounds in oil are:

• mercaptans;
• sulfides;
• disulfides;
• thiophene.

Sulfuric compounds in transformer oil are corrosive.

Due to the high chemical reactivity of sulfuric compounds, their content in transformer oil is regulated by standards.

Nitric compounds

The amount of nitric compounds in crude oil is usually no more than 0.8%. These are mostly the derivatives of pyridine and benzazine. There are also pyrrols, benzopyrroles and dibenzopyrroles. Nitric compounds on transformer oil accelerate hydrocarbon oxidation. Thus, heterocyclic nitrogen compounds catalyze oxidation and are undesirable in transformer oil.

Naphthenic acids and oxygen compounds

Naphthenic acids are contained in oil distillates in the amounts from 0.02% to 2.6%. It has been determined that naphthenic acids accelerate the formation of insoluble sludge in the oil, which interact with transformer construction materials.

Naphthenic acids include cyclic (cyclopentane and cyclohexane) and bicyclic monoacids.

The carboxylic group of naphthenic acids is connected with the cycle by one or several methylene groups. In the process of transformer oil distillate purification, naphthenic acids are largely removed, so their content in the finished product does not exceed 0.02% by weight, corresponding to oil acidity of 0.05 mg KOH/g. Naphthenic acids easily react with alcohols, amines, bases and such metals as copper and lead.

Naphthenic acids are also formed by oxidation of naphthenic hydrocarbons during transformer oil use.

Beside naphthenic acids, oils contain fatty-aromatic acids, as well as esters, alcohols, ketones and peroxides.

Metal compounds

Transformer oil always contains organic acid salts and metal complexes. It has been determined that Germanium compounds are present in all crude oil fractions in concentration of 19 grams per ton. Iron and copper are present in trace quantities in transformer oils. Oils also contain other metals, such as aluminum, silver, chrome, molybdenum, calcium etc in trace quantities. Metals form salts with acids in the oil, increasing the dissipation factor.

The main transformer oil requirement is high dielectric strength. For voltages of 110-1150 kV, air tight bushings with oil and paper insulation are mostly used now. Therefore, to ensure reliable operation of oil-filled bushings, such factors as transformer oil stability to partial discharges, are important. Besides, the oil must not react with the electrical equipment construction materials.

Transformer oil in modern electrical equipment is used in strong electric fields; the strength of the field near the high voltage winding may reach 70 kV/cm. The temperature of the top layers of the oil may reach 95°С. The high field strength and high temperature combined with small insulation clearances are the severe conditions the oil is used in.

Therefore, beside high dielectric strength, the oil must also be highly stable to oxidation, low gas solution in strong electric fields and low dielectric loss.

Besides, transformer oil in electrical equipment comes in contact with copper and iron, which catalyze oxidation of hydrocarbons in the oil. These factors accelerate oil aging, reducing its performance and the efficiency of electrical systems in general.

The oil is subjected to thermal, chemical and electrical aging influences in the strong electric fields and high temperatures of electrical systems, reducing its performance due to the changes of its molecular composition.

Aging of oil causes sedimentation of sludge in the windings and the core. The solid sediment impedes cooling, accelerates aging of paper insulation and reduces its dielectric properties significantly.