Transformer Oil Analysis

During the operation of transformers, it is a common practice to conduct an oil analysis of dielectric liquid in use. The collected sample provides insight into how efficiently and reliably the transformer is operating and, most importantly, whether immediate repairs are necessary to ensure continued functionality. Routine maintenance is a natural part of the long-term operation of any equipment, and the use of a transformer begins with evaluating its technical parameters, including the oil composition.

Transformer Oil Sampling

Specialists choose the appropriate and effective method for collecting oil samples from transformers. Only technically trained personnel who are well versed in the specific characteristics of a transformer and the rules for its operation and sampling can ensure accurate results. However, there are fundamental guidelines that must always be followed:

• Use special containers for sample collection (a glass jar with an airtight lid can also be used as an alternative).

• Wrap the container in a thick layer of aluminum foil for protection.

• Prevent any air, dust, water, or other substances from entering the container, as these can alter the sample’s properties.

• Use specialized equipment, such as an oil sampler, during the sample collection process.

It is preferable to collect transformer oil samples during dry summer weather or under frosty winter conditions. Other weather conditions can affect the oil composition and potentially distort the oil analysis results. Specialists recognize that even minimal moisture condensation should be avoided to ensure the validity of the analysis.

Several effective methods for transformer oil sampling have been developed in practice. However, each transformer has unique characteristics depending on its degree of wear and specific operating conditions; therefore, the sampling method may vary.

Recommended Tools and Post-Sampling Precautions

A glass syringe is often used for sampling, and after the procedure, the following steps should be taken:

• Keep the syringe in a fixed position.
• Avoid contamination.
• Prevent exposure to moisture.
• Place the sample in a sealed container.

These precautions are necessary to prevent contact between transformer oil and air during the sampling process.

Frequency of Oil Sampling

The frequency of sampling depends on the specific transformer and its operational requirements. Only a qualified specialist should carry out this process, assessing the situation to perform it correctly. Sampling schedules often depend on the type of equipment and may also be implemented in response to emerging problems that require emergency sampling. For large-scale industrial facilities, a regular sampling schedule is usually established, typically every six months.

Parameters Analyzed in Oil Samples

In order to determine the current quality of transformer oil, a range of criteria is evaluated through physical and chemical analyses and categorized into basic and advanced oil analysis parameters.

Basic oil analysis:

• Acid Number: indicates the level of oil oxidation, as increased oxidation can degrade insulation and cause sludge/sediment formation.
• Water-Soluble Acids and Alkalis: excessive levels can corrode metal components and accelerate insulation aging.
• Moisture Content: high moisture levels reduce dielectric properties and the dielectric strength of the insulation.
• Dielectric Loss Tangent: a measure of oil conductivity; impurities increase conductivity and adversely affect insulation.
• Mechanical Impurities: detects contaminants and helps locate potential sources of damage within the transformer.
• Dielectric Strength: measures the breakdown voltage of the oil and assesses the overall performance of the insulation.
• Flash Point in a Closed Test Cup: indicates the presence of volatile substances that may result from oil degradation or contamination.

Advanced oil analysis:

• Oxidation Stability: evaluates oil aging by artificially oxidizing the oil to measure sludge/sediment and acid content.
• Furan Compounds: detects insulation aging by measuring secondary products dissolved in the oil.
• Gas Content: detects air or gas dissolved in the oil due to compromised transformer seals.
• Antioxidant Additive Content: determines the remaining concentration of additives such as Ionol or Agidol-1 to assess oil degradation.
• Silica Gel Monitoring: assesses moisture levels in transformers with the use of color-changing silica gel in desiccant filters.
• Oil Color: indicates aging or contamination; new oil is light yellow, while darker oil suggests degradation.
• Polarization Index: measures insulating paper degradation, providing insights into its tensile strength and operational limits.
• Oil Turbidity: detects colloidal particles that reduce clarity and indicate aging.
• Dissolved Sludge: detects deep oil aging products that could settle on transformer components and cause insulation damage.

Transformer Oil Analysis –  Laboratory vs. On-Site Testing

Currently, monitoring of ten physical and chemical parameters of transformer oils in service is performed, providing critical diagnostic information. These parameters are typically determined in fixed-site industrial chemical laboratories. However, oil sampling is carried out by substation maintenance personnel, who often lack modern equipment for collecting transformer oil samples and transporting them to the laboratory. In some cases, these personnel may not have the necessary expertise to properly collect samples.

Oil sample preparation, analysis, and testing are conducted by specialists in industrial laboratories who are not involved in the sample collection or transportation process. As a result, the sample may not accurately represent the oil condition. If the sample is unrepresentative, the analysis results will not reliably reflect the condition of transformer oil. Consequently, the diagnostic conclusion regarding the operational state of the equipment, which informs decision-making, may also be inaccurate.

Considering the specific features of power substations located in remote areas, far from industrial chemical laboratories, another challenge is ensuring that the oil samples are representative not only during collection, but also during prolonged transportation to the laboratory. Resampling may result in considerable additional costs. For example, the costs of resampling and repeat analysis after unsatisfactory initial analysis may exceed 200%. Therefore, the need for operational monitoring of transformer oil condition, including a complete diagnostic cycle — from collecting oil samples to issuing a diagnostic evaluation report and automated data transmission to asset management systems — has become particularly important in the current trend toward minimizing on-site personnel, for instance at minimally serviced or unmanned substations.

Fluidex Devices for Laboratory and On-Site Transformer Operation

The FLD 90T and FLD 75T units are designed to measure the breakdown voltage of insulating (transformer) oils in fully automated mode. Transformer oil breakdown voltage analysis is used to assess the degree of contamination by impurities and water.

Key Features of the FLD 90T and FLD 75T Breakdown Voltage Testers:

Programmed in accordance with global testing standards such as IEC 60156, ASTM D877, ASTM D1816 and designed for mineral, organic (silicone), and ester-based liquids.

Fast breakdown voltage testing of oil.

Compact design with a built-in LCD screen, printer, and USB port, making the units space-efficient and easy to use in the laboratory.

The devices are portable and designed for field use due to their internal battery, but they can also be used in laboratory settings when connected to the electrical grid. The battery can last up to one full operating day.

Typically, there is one laboratory for several hundred pieces of oil-filled equipment. This means that methods for determining oil parameters should provide rapid results, be cost-effective, and ensure sufficient accuracy. Portable devices such as the FLD 90T and FLD 75T offer an alternative to conventional laboratories, enabling on-site oil analysis.