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Quality Assurance: Methods of Transformer Oil Testing

The performance characteristics of transformer insulating oil are monitored and tested at several stages during its service life.  Regular monitoring of the oil’s quality and condition is a part of the process of servicing electric power equipment. The condition of the oil, its purity and contamination level will greatly help in indicating the condition of the transformer’s solid insulation.  It is therefore, essential to the life of the transformer to periodically monitor the condition of the insulating oil.  The heart of the transformer is the solid insulation, but the insulating oil is the life blood of the transformer.  Without the life blood, the heart will die.

The ability of insulating oil to maintain its original performance characteristics during long term operation of electric equipment is referred to as “oil stability.”  If the electric power equipment has no defects and operates in accordance with design and expectations, the performance characteristics of new oil will change and degrade slower.  When new, transformer oil has a very light color and complies with performance standards that include dielectric strength and other important characteristics.  During the course of the oil’s service life, the stability of the oil decreases and visible changes occur and oil’s color slowly becomes darker and darker.

Contaminated oil usually has a high ash content, increased acidity and presence of low molecular acids.  Acidic sludge forms in contaminated oil and aggressively attacks the cellulose insulation and reacts with the metals of the transformer’s other internal components.

Timely oil monitoring and oil analysis programs can identify when the oil needs to be changed or serviced through an oil purification and/or oil regeneration process.  Servicing the oil before it becomes aggressive against the solid insulation is the key to extending the service life of your transformers.

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

Dielectric strength is one of the most important indications of oil stability and this is often the first test performed.  The “dielectric breakdown test” is calculated as an average of five breakthroughs achieved in a standard discharger with two electrodes submersed in the oil at 2.5 mm distance from each other.  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 meets the minimum operating standard.

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

A similar process is used to calculate the oil’s “dissipation factor.”  It is the the oil’s ability to neutralize energy, prevent breakthroughs and cool the transformer.   It is a characteristic of the oil’s quality and purity and acidity.  In general, an 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 electric current.  The color is not in itself an indication of any specific problem, but a dark color is usually an indication of aged and/or contaminated oil.

The presence of solid particles in the oil and the acid number of the oil are two related oil purity characteristics.  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 that 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.  A normal and acceptable acid number is 0.25 mg KOH/g, while the limit of contaminant content is 515 parts per million (ppm).

Moisture and gas content in transformer oil must be tested for thoroughly.  Water and gases are very damaging to your transformer insulation system and are two main cause of the oxidation process and oil aging process.

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

The oil’s flashpoint and the oil’s setting point are two indications of the general fire safety of the oil and the oil’s ability to operate in adverse temperature conditions both hot and cold.

The are distinct advantages of testing and analyzing transformer oil before starting your electric power equipment and during scheduled maintenance events.  Oil testing allows the operator to determine the equipment’s operating efficiency, conditions and the possibility of future malfunctions.  If the purity and quality standards are followed, the equipment will be less likely to experience failures and downtime and unscheduled maintenance and repair costs.

Classification of New Transformer Insulating Oil Testing

The conditions in which insulating oil is used in transformers, switches, and other high voltage equipment are rather demanding on the performance characteristics of the insulating oil.  During the service life of the oil, it is heated by currents, contaminated by particles of solid insulation and chemical reactions with the internal surfaces of the equipment.  Each of these separately and together significantly accelerate the aging of transformer oil and can make the oil harmful to the equipment.  This is especially true with regards to the transformer insulating paper.

A sample of transformer oil is taken from the lower part of the transformer oil tank after rinsing the drain opening with oil.  The sampling vessel must be clean and very dry or the testing results will not reflect the true condition of the oil being tested.

Local and international standards define certain parameters by which the oil must comply.  Oil analysis and testing should be performed directly before placing oil into the electric power equipment.

New transformer oil is also tested before being placed into transformers.  The purity and quality guaranteed by the manufacturer of the oil does not prevent water, air and solid particles from entering the oil during handing and storage.  You cannot assume that the new oil is free from contaminates and moisture.  The oil must be properly analyzed immediately prior to use to ensure the proper operation of your electrical equipment

Before a transformer is energized, the oil in the transformer must be briefly tested for minimum dielectric strength levels, solid particulate matter, acidity and flashpoint.  If different brands of oil are used in the transformer, the final blend must be tested for stability.  The blended oil fill should not be inferior in stability to any of the separate oils with the lowest stability value.

Used oil must be tested according to standards that have been set for the operation of industrial equipment that it will be used in.

Before testing, the vessel, containing the oil sample, is inverted and turned slowly around several times to eliminate air bubbles from the oil sample.  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 electrodes.

Transformer oil is allowed to settle in the vessel for 15-20 minutes for air bubble removal.  The voltage is then gradually increased at the rate of 1 – 2 kV per second.  After a breakthrough is achieved (spark between the electrodes), voltage is recorded and then dropped to zero.  Six tests are performed and the average of the last five tests is taken.  The first breakthrough is not averaged in since it was only performed as a guide.

After each breakthrough, carbon particles are removed from the space between the electrodes by clean glass or metal rods.  The liquid is then allowed to settle for 10 minutes before performing the next test.

The transformer oil sampling process will vary from facility to facility depending on the equipment used and purity requirements of the equipment and oil being tested.

At any rate, a timely oil analysis program may well prevent significant costs and losses for your company.

Tests and Trials: Sport, Art or Industry? Analysis of Transformer Oil Purification

The correct use of oil guarantees the reliability of industrial equipment and the prevention of equipment failures.  Transformer oil is a dielectric insulator and a coolant.  It also serves as an arc extinguisher preventing shorts and internal fires.

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

Sludge accumulates in the solid insulation, the core, in cooling channels and other places inside the transformer.  Sludge is a dangerous enemy that degrades the oil’s cooling and heat transfer capabilities and damages the solid insulation causing it to disintegrate.  A short-circuit in the transformer windings becomes a much higher possibility due to oil degradation and sludge build up.

The acid level of the oil, as measured by the “Acid Number is the main indication of oil degradation and oil aging.  Acidity is measured by special litmus paper that changes color when subjected to contaminants.  Acids can damage cellulose insulation and metal parts of the transformer.  Acid levels can also measured through an oil analysis program.

The Acid number, as 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.  The Acid number indicates how much the oil has aged and whether it should be regenerated using the GlobeCore Process.  Once the Acid number reaches 0.08 and higher, the transformer oil becomes aggressive and begins to destroy the solid insulation of the transformer.  The Acid number therefore, is the best indicator of when to service your transformer before irreversible damage is done to the insulating paper.

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

For instance, the oil’s flashpoint must be high enough to ensure fire safety in overload conditions and high temperature increases inside the transformer.

Dielectric strength of transformer oil is important for the prevention of breakthroughs in the transformer insulation.  This parameter is measured regularly by high voltage breakthrough indicators.  Transformer oil is tested six times with 10 minute intervals between each test.  The average of six tests is used to determine the dielectric strength.  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 being placed 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.

These measures are designed to identify and resolve problems before the they can cause a transformer failure shutting down the transformer.

Transformer oil is purified of contaminants, acids and gases in purification and regeneration equipment specifically designed for transformer maintenance.  Complete preventive maintenance systems, such as the GlobeCore CMM-R line of equipment will reduce costs and downtime while increasing operating profits.