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Oil Testing of a High-Voltage Power Transformer

field-testingAn oil-insulated power transformer benefit from routine inspections and field tests. Over time, gradual chemical breakdowns can decrease the dielectric strength and insulating properties of the transformer oil. Routine maintenance helps generating companies and power distributors to protect valuable equipment, reduce operating costs and improve grid-wide reliability. Thanks to improved on-site testing systems, chemical analyses can be completed quickly and with minimal disruption to the power supply. Mobile testing units were a major advance. However, some testing units can now be used while the equipment is online, which saves time and money.

Transformer Oil Testing Standards

International organizations recommend annual or biannual tests to assess the integrity and dielectric strength of transformer oil. Oil testing standards and procedures have been developed by the American Society for Testing and Materials (ASTM), the British Standards Institution, the International Electrotechnical Commission and other organizations. Some tests rely on a visual analysis. For example, the density of suspended particles can be measured visually since contaminants darken the oil. The condition of the fluid is rated as good, fair or bad depending on the color and opacity. Clean oil is a clear or yellowish color. As contamination increases, the oil turns amber or dark brown.

Common Transformer Oil Tests

Mobile oil testing companies can perform a full range of inspections and analyses to determine the health of the transformer and its dielectric fluid. Test results are most valuable when they are charted over time. Sudden changes may indicate leaks and underlying problems. When issues are addressed promptly, the oil can be processed, dried and filtered. The cost of reconditioning is significantly lower than replacement. Major repairs to transformers and high voltage substations can also be avoided.

Field technicians can complete a full battery of tests that provide an accurate picture of the transformer’s performance. Diagnostic equipment can be used on all types of insulating fluids, including mineral oil, synthetic esters, natural esters, silicone liquids and high molecular weight hydrocarbon (HMWH) fluids. Results of all tests are compared to benchmarks established by the ASTM and other groups. These are a few of the most common and important oil tests.

  • Moisture content
  • Specific gravity
  • Interfacial tension
  • Sediment and sludge
  • Acid neutralization

Measuring Dielectric Strength

The dielectric strength and breakdown voltage of transformer oil is measured according to ASTM D877 or D1816 standards. According to this protocol, electrodes are inserted into an oil sample. Then, the AC voltage is increased until the current arcs between the electrodes. The dietetic strength is based on the voltage that the oil can withstand.

Dissipation and Characteristics

The characteristics of the transformer oil are analyzed at cool and very hot temperatures. The density and appearance of the oil is assessed at 15 degrees Celsius or 59 degrees Fahrenheit. The dissipation factor, also called the power factor, is measured at 100 degrees Celsius or 212 degrees Fahrenheit. This data set quantifies the electrical loss rate.

Dissolved Gas Analysis (DGA)

Dielectric breakdowns liberate gasses in the oil. The type and distribution of the gasses can be used to identify specific faults and the severity of the problem. Results from the DGA are useful for developing maintenance plans for each transformer. Testing units can detect the following gasses and provide measurements in parts per million.

  • Oxygen
  • Nitrogen
  • Hydrogen
  • Carbon dioxide
  • Carbon monoxide
  • Methane
  • Ethane
  • Acetylene
  • Ethylene

The Benefits of Maintenance

Transformer oil testing has many advantages for power transmission and distribution companies as well as industrial consumers. First, preventive maintenance can substantially increase the service life of a. Second, operating costs are reduced. Maintenance can prevent costly problems from developing while protecting the supplier’s valuable equipment and mobile substation.

For more information about mobile testing services, call 713-733-3999. Equisales Associates provides field services, mobile substations and equipment rentals to global high-voltage clients.





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