Purification of transformer oils

Transformer oils are used as liquid insulation and cooling media in electric equipment, such as transformers, oil filled switches, high voltage capacitors and power cables. In oil filled switches oil is used for arc suppression. 

Transformer oils are produced by various processes of deep purification of petrochemical oils. Oil from different sources are different in chemical composition, which significantly influences physical and chemical properties and carbon composition of transformer oils. As equipment technology advances, so are the requirements to purity of transformer oil. Transformer lifetime is, in fact, defined by the lifetime of the transformer’s insulation system (i.e. the transformer oil). In the process of operation transformer oil accumulates various oxidation products, contaminants, etc.

If the oil contains oxygen and water, the oil oxidizes even in the ideal conditions. Condition of transformer oil is also influenced by contamination caused by transformer solid materials, which slowly dissolve in the oil. Acids formed in the process of oil oxidation, react with carbon and metals and create soapy metal, aldehyde and alcohol, which form acidic sediment on insulation. Sediment accumulates faster in a heavily loaded and incorrectly operated transformer. Sediment increases oil viscosity, thereby reducing it’s cooling ability, which in turn leads to reduction of transformer lifetime.

Contamination in fuels and oils have been classified as follows.

Classification of oil and fuel contaminants:

• Contaminants

•   Organic

  • Metalloid compounds

  • Metal oxides

  • Metals

•   Mineral

  • Crystal hydrocarbons

  • Sulfuric and nitric compounds

  • Carbon acid salts

  • Adsorption resins

•   Microbiological

  • Fungi

  • Pyrogenic agents

  • Bacteria

• Other
  • Petrol product vapors
  • Air and gases
  • Water

The purpose of purification (regeneration) of transformer oil is the extraction of moisture, acids, solid particles and such undesired substances as unsaturated hydrocarbons, tar, sulfur and nitrogen compounds. Used oil is regenerated by various methods, based on physical or chemical methods, or a combination of both.

The process usually includes the following methods in order:

• Mechanical – to remove free water and solid particles from oil (filtration, centrifuge, settling);

• Heat – evaporation, vacuum distillation;

• Physicochemical – coagulation, adsorption;

• Chemical – if the former three are not sufficient, this method involves more complex equipment and significantly higher costs.

Operational transformer oil contains water formed by aging of oil and insulation, as well as water entering the oil from the outside. Water is the most dangerous contaminant in the oil, as even small amounts of water significantly decrease the oil’s breakthrough voltage. Water contained in oil exists as drops, solution or emulsion. There is a certain equilibrium between the three states of water in oil, depending on temperature, pressure and the amount of stabilizers in the oil.

Removal of free and dispersed water should be done by centrifuge and filtering. Molecular solutions, which cannot be separated from the oil by the above methods, can be removed by adsorption and vacuum methods. Adsorption, i.e. purification of oil by solid porous media referred to as adsorbents, is the most effective. Contaminants are captured by the surface and internal cavities of adsorbents.

Adsorption may occur in of the three ways:

1. Percolation;

2. Contact;

3. Counter-current.

Percolation by definition implies that the oil percolates through a layer of granulated adsorbent (usually silica gel), loaded into a vertical cylindrical vessel.

Contact method involves mixing of oil with powder adsorbent at 70-75oC for a duration of time, with subsequent separation of adsorbent by a press-filter. Fine adsorbent powder and temperature promote rather fast mass exchange, making the process quick. However, the disadvantage of this method is the need to dispose of large amounts of environmentally hazardous adsorbent.

The third method involves oil and adsorbent moving towards each other. The most promising  method is adsorption in a layer of moving sorbent, with continuous process without the need to stop for sorbent regeneration or replacement, however, such process requires complex equipment.

Some of the commonly used adsorbents for oil drying and purification are: silica gel, alumina oxide, alumina silicates, bleaching clays and synthetic zeolites.

As a rule, transformer oil purification lines feature adsorber sections, containing of four 50 kg vessels each. One vessel is filled with NaA type zeolites, which are particularly effective at removing solved water from the oil. Drying of oil by zeolites should be done when the breakthrough voltage of the oil is 10 kV or above. Dynamic efficiency of A type zeoilite in electric insulation oil purification is 7 – 16%.

The three remaining vessels are filled with silica gel, which removes oil oxidation products, unsaturated hydrocarbons, tar, sulfuric and nitrous compounds. Beside the above, silica gel is able to accumulate solved water, hence the pre-treatment by zeolite, to extend silica gel life.

Adsorbent sections may contain varying number of vessels filled with zeolites and silica gels in various propotions. Such adsorbent units are installed in many plants, stands and lines for transformer oil purification. The most effective is one-pass purification, when the oil from oil regeneration machines is discharged into a clean vessel. The oil’s breakthrough voltage increases several times depending on the unit’s capacity. If the oil cannot be purifying in one pass, it is cycled through the system, i.e. the oil returns to the source vessel after processing. In this case the resulting breakthrough voltage of the oil depends on the number of passes of the total oil volume through the system, increasing gradually to the limit for the given system capacity.

For moderately contaminated oils, two or three cycles suffice, while highly contaminated oil may require 5 to 7 passes.

A typical oil processing system may be able to reach the following parameters of oil:

• Reduction of water content to 10 – 20 grams per 1 ton of oil;
• Reduction of acidity by 90%;
• Removal of solid contaminants;
• Increase of breakthrough voltage from 10 – 15 kV to 70 kV.

Since adsorbents require regeneration, sorbent regeneration section are usually connected to purification systems; such units are capable of regenerating and adsorbent without the need to unload them from the vessels. Such units can use different methods, such as:

• Regeneration of synthetic and natural adsorbents by high frequency current in vacuum, without the need to unload the sorbent;
• Regeneration of synthetic and natural sorbents by heating and vacuum;
• Purging of sorbents by hot air at 400oC.