Transformer Oil Testing

Transformer oil testing and analysis are often overlooked, but play a key role in ensuring the lifespan of transformers. Just as medical tests reveal details about our health, oil tests reveal the internal condition of transformers, helping to detect potential hazards at an early stage. This preventive approach not only saves costs associated with emergency repairs, but also minimizes the risk of unexpected outages.

The Role of Oil in Transformers

Transformer oil is a specially refined mineral oil intended to perform major functions in power distribution transformers. Essentially, it acts as both a cooling medium and an insulator. When the transformer windings are immersed in oil, the liquid dissipates heat and thus prevents overheating. Furthermore, it insulates the transformer’s active parts, safeguarding them from undesirable electrical discharges, or flashovers, which can cause significant damage.

However, over time, electrical stress, high temperatures, and exposure to solid particles can alter the oil conductivity. The insulating oil may become saturated with moisture and other impurities, potentially lowering its dielectric strength and affecting its performance. Regular transformer oil testing is a widely used practice in the industry. Experts recommend performing general tests twice a year, dissolved gas analysis — annually, and furan testing for equipment that has been in operation for at least five years — on a biennial basis.

Why Conducting Transformer Oil Testing?

Poor quality of oil, particularly the oil that contains moisture, poses a safety concern as it can cause deterioration to solid insulation and contribute to overheating or arcing. Regular transformer oil testing, recommended every one to three years for detection of contaminants, allows predicting potential malfunctions. During analysis, key factors are tested, including:

• Flash and pour points
• Neutralization number
• Dielectric strength
• Gas, moisture, and particulate matter levels.

When contaminant levels or degradation products exceed safe limits, oil filtration and purification may be required to restore oil quality and reduce the risk of equipment malfunction.

Approaches to Transformer Oil Testing

Over time, transformer oil degrades due to continuous loading and operating conditions such as elevated temperatures, as well as exposure to moisture and gases. Oil aging leads to a loss of dielectric and cooling properties, which adversely affects transformer reliability. Therefore, regular oil testing is not only a matter of maintenance, but also a strategic approach to avoiding unplanned shutdowns and potential complete transformer failures.

For transformers with power ratings up to 100 kVA (U < 10 kV), oil sampling is typically not carried out during routine operation, and oil quality is assessed during preventive testing. Oil quality provides insight into the transformer condition and is evaluated based on the amount and type of impurities present. For instance, when the moisture content reaches 0.01–0.02 %, the breakdown voltage of oil can decrease 4–5 times, compromising the transformer insulation.

In order to keep transformer oil within safe operating standards, it must remain clean and free of excess moisture. During operation, oil tends to accumulate water, particulates, and oxidation products, which is why chemical testing is essential. Samples are usually taken from oil-filled electrical equipment before initial operation, periodically during use, and after power interruptions caused by damage. Qualified personnel must take these samples under proper conditions, as weather changes can affect the results. Sampling containers must be carefully cleaned, dried, and kept at 80–100 °C for thorough preparation.

Transformer oil analysis combines electrical and chemical tests to assess the performance and suitability of the oil. Electrical tests focus on measuring the breakdown voltage and the dielectric loss tangent (tan δ). Chemical analysis includes determining the acid number, the presence of water-soluble acids and alkalis, and the water content. Breakdown voltage, one of the most important indicators of oil quality, reflects the ability of the oil to withstand electrical stress and serve as insulation. Timely monitoring of these parameters ensures that the oil continues to meet industry standards and supports reliable transformer operation.

Fluidex Technology for Breakdown Voltage Testing of Transformer Oil with the FLD 90T

The FLD 90T  offers a modern and user-friendly approach to measuring the breakdown voltage of transformer oil. Compared to competing devices, the FLD 90T stands out for its convenience, especially in laboratory environments. This unit is notably lighter by weight than many other models, which makes it easier to handle and transport. Furthermore, it automates several steps in the transformer oil testing process, improving both accuracy and efficiency. For example, the intervals between tests are automatically calculated, and the test results are saved digitally, allowing laboratory technicians to conduct tests with minimal operator intervention.

Fluidex Technology for Measuring Dielectric Loss Tangent (Tan Delta) with the FLD T Oil Tan Delta Tester

Transformer oil can degrade as it ages, with increases in the dielectric loss tangent (tan δ) and the acid number serving as common indicators. Routine laboratory analysis of transformer oil can reveal these changes; however, such tests are costly and time-consuming. In many cases, industrial facilities require rapid test results “right here, right now”— within a few hours of sample collection. The FLD T Oil Tan Delta Tester by Fluidex provides this capability, enabling accurate and rapid testing of transformer oil.

The FLD T tester is engineered to measure the dielectric loss tangent (tan δ) and the permittivity of mineral oil and other dielectric fluids in accordance with IEC 60247 specifications. This fully automatic tester can perform tests according to these standardized procedures, or users can specify custom settings through the setup menu, making it a versatile tool for various testing requirements. The automation and digital storage of results make this tester a highly efficient solution, allowing technicians to rapidly assess oil quality and make informed maintenance decisions.

Incorporating the Fluidex technology for transformer oil testing not only improves the accuracy of assessments, but also ensures faster and more reliable results. Both the FLD 90T and FLD T testers offer advanced solutions for maintaining consistent transformer oil quality, helping the industrial facilities avoid the costly repairs, enhance the reliability, and extend the service life of their equipment.