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Quality Assurance: Methods of transformer oil testing

Quality of transformer oil is tested in several stages of its use. Regular control of the oil’s quality is a part of the process of servicing electric equipment. The main parameters of oil and its purity indicate transformer condition.

The ability of the oil to maintain its original properties during operation of electric equipment is referred to as oil stability. If the equipment has not defects and operates without failures, the parameters of new oil change very slowly. New transformer oil has light color and complies with regulations, which define the oil’s dielectric strength and other properties. In use, stability of the oil decreases and notable changes occur, the color changes to dark.

Poor oil quality is indicated by high ash content, increased acidity and presence of low molecular acids. Acidic sludge forms in contaminated oil, which disrupts cellulose insulation and reacts with the metals of the transformer’s internal components.

Oil tests can identify the beginning of oil degradation.

The main physical and chemical properties tested are the oil’s dielectric strength, dissipation factor, flashpoint, color, solid, water and gas content as well as acid number.

Dielectric strength is one of the most important indications of oil stability and this is often the first test performed. It is calculated as an average of five breakthrough achieved in a standard discharger with two electrodes at 2.5 mm distance. Six breakthroughs are achieved in the test and the last five are averaged. If the oil is fresh, the lowest allowable breakthrough voltage is 30 kV. In some transformers, that is as good as needed.

Decreasing dielectric strength indicates contamination of the oil by gas, moisture, cellulose fibers or other particulate matter.

A similar process is used to calculate dissipation factor. The oil’s ability to neutralize energy, prevent breakthroughs and cool the transformer is characterized by the oil’s quality and purity, or acidity. In general, increased dissipation factor means degradation of the oil’s dielectric capabilities.

The color of transformer oil changes from light yellow to cloudy brown under the influence of temperature, contaminants and current. The color is not in itself an indication of any specific problem, but dark color is usually an indication of aged oil.

The presence of solid particles and the acid number of the oil are related. Unsolved materials accumulated in the oil in the form of sludge or suspended particles (fibers, dust, solved paint, metal particles, ash etc) degrade the oil’s dielectric properties and promote oil oxidation. The more particles are present in the oil, the faster the oil ages. The acid number is expressed as milligrams of KOH required to neutralize all acids in a gram of oil and indicates the degree of oil aging. Normal acid number is 0.25 mg KOH/g, while the limit of contaminant content is 515 ppm.

Moisture and gas content in transformer oil is tested thoroughly, because water and air are some of the main “catalysts” of oil aging process.

Moisture content is measured as amount of hydrogen when reacting the oil with calcium hydride in a certain time. Gas content is measured by an absorptiometric analyzer or a chromatographer.

Flashpoint and setting point are indications of general fire safety of the oil and its ability to operate in adverse temperature conditions.

Therefore, the advantages of testing and analyzing transformer oil before starting electric equipment and during regular maintenance, allow to determine the equipment’s efficiency, operation conditions and possibility of malfunctions. If the purity and quality regulations are followed, the facility is safe from failure and downtime of equipment and related repair costs.

Classification of tests on new transformer oil

The conditions in which transformer oil is used in transformer, switches and other high voltage equipment are rather demanding on the quality and purity of the oil. In the process oil is heated by currents, contaminated by particles of solid insulation and chemical reactions with the internal surfaces of the equipment, solves water and gas from the outside. Each of these separately and all together significantly accelerate aging of transformer oil and can make it harmful for the equipment.

A sample of transformer oil is taken from the lower part of transformer oil tank after rinsing of the drain opening by oil. The sampling vessel must be clean and very dry.

Local and international standards define certain parameters which the oil should comply with.

Oil analysis and testing is performed directly before filling of oil into power equipment.

New transformer oil is tested before filling into transformers. Purity and quality guaranteed by the manufacturer of oil does not prevent water, air and solid particles form entering the oil during handing and storage. Even new transformer oil must be tested and purified to extend its service life and ensure reliable equipment operation.

Before a transformer is energized, the oil in the transformer must be briefly tested for minimum dielectric strength, solids, acidity and flashpoint.

If different brands of oil are used in the transformer, the blend must be tested for stability, which should not be inferior to stability of one of the oils with the lowest stability value.

Used oil must be tested according to regulations on operation of industrial equipment.

Before testing the vessel containing the oil is inverted and turned slowly around several times to eliminate air bubbles from the oil. A ceramic oil test vessel with electrodes is filled three times. Oil is poured on the walls of the vessel in a thin stream so as to prevent formation of air bubbles. The level of oil in the vessel must be at least 15 mm higher than the top of the electrode. After each rinsing, the oil is completely drained.

Transformer oil is allowed to settle in the vessel for 15-20 minutes for air bubble removal. Then voltage is gradually increase at the rate of 1 – 2 kV per second. After a breakthrough is achieved (spark between the electrodes), voltage is dropped to zero. Six runs are done, and the average of the five subsequent tests is taken, the first breakthrough is not considered.

After each breakthrough carbon particles are removed from the space between the electrodes by clean glass or metal rods. The liquid then settles for 10 minutes.

Transformer oil sampling process may vary from facility to facility, depending on the equipment used and purity requirements.

At any rate, a timely analysis may well prevent significant costs and losses.

Tests and trials: sports, art or industry? Analyse of transformer oil purification.

Correct use of oil guarantees reliability of industrial equipment and prevention of failures. Transformer oil is a dielectric and a coolant. It also serves as arc extinguisher.

Transformer oil parameters degrade over time as the oil ages. Oil oxidation is mostly caused by external elements, such as water, air and heat. Aging of oil is indicated by its acidity, water reaction, sediment and sludge formation.

Sludge is accumulated on solid insulation and core of transformers, in cooling channels etc. Sludge degrades cooling and damages solid insulation, which begins to disintegrate. A short-circuit in transformer windings becomes a possibility.

Acidity of oil is the main indication of aging. Acidity is measured by special litmus paper, which changes color when subjected to contaminants. Acids can damage cellulose insulation and metal parts of the transformer.

Acid number, the main indication of transformer oil quality, is the amount of grams of KOH required to neutralize all free acids in one gram of oil. Acid number indicates how much the oil has aged and whether it should be replaced. After a certain degree of aging, transformer oil begins to destroy solid insulation of the transformer.

Beside chemical parameters, transformer oil must comply with certain physical and electric parameters. Degradation of these may indicate problems inside power equipment.

For instance, oil flashpoint must be exceedingly high to ensure fire safety in overload conditions and temperature increase inside the transformer.

Dielectric strength of transformer oil is important for prevention of breakthroughs in the transformer. This parameters is measured regularly in high voltage breakthrough indicators. Transformer oil is tested by six trials with 10 minute intervals. The average of six trials is considered. If the results are unsatisfactory, another sample is tested and a decision on the course of action is made.

Fresh or used transformer oil must be tested before filling into the transformer. Some parameters tested are particulate matter content, general oxidation stability, transparency, dissipation factor, flash and setting points, viscosity, acidity and water content. If the oil is in the transformer, sludge is also examined.

These measures are designed to identify and resolve a problem before the problem can shut down a transformer.

Transformer oil is purified from contaminants, acids and gas in transformer oil purification and regeneration plants. Complete preventive maintenance systems reduce costs and downtime, while increasing profits.