Oil Testing

Taking a sample of transformer oil (or other fluid), for analysis at a laboratory is a quick and inexpensive way to monitor the general health of a transformer. The collection and study of this data along with other testing will aid the transformer owner in making informed decisions in the care and maintenance of this valuable equipment. This article will discuss Standard Oil Testing . (Dissolved gas in oil tests and furan analysis are other types of oil tests) When practical, for routine annual testing, test the same week every year to normalize the test data.

Standard Oil Test – Taking an Oil Sample

Caution: When taking an oil sample from a sealed tank transformer, ensure that the transformer is not under vacuum by checking the vacuum/pressure gauge. A sample drawn while a transformer is under vacuum has the potential of pulling air into the transformer, possibly leading to failure. It is important to get a representative sample of the oil inside the transformer tank. Use a clean glass sampling bottle, free of dirt and moisture. Amber coloured glass is often used since sunlight will deteriorate the oil sample. Use the following procedure to take the oil samples for Standard Oil Tests (refer to ASTM D923).

  • From a bottom drain/sample valve, half-fill the sample bottle. Cap the bottle and swirl oil around. Pour out oil in a waste can.
  • Refill and dump two more times
  • Refill the bottle to the neck, leaving a small air space and cap tightly

The purpose of this test procedure is to get a sample of oil that is not contaminated either by the debris on the bottom of the tank or from the sample jar. The oil sample is then properly packaged for shipment to a lab for analysis.

Laboratory Analysis Performed

  • Dielectric Strength (kV) – measures insulating capacity of the oil
  • Interfacial Tension (dyne/cm) – a measure of the level of contaminants in the oil, moisture carbon, etc.
  • Power Factor (%) – another measure for contaminants
  • Neutralization number – a measure of the degree of oxidation of the oil
  • Water content (ppm) – measures water content in the oil

Water Content and Dielectric Strength

Water content and Dielectric strength correlate in that as the water content increases, the dielectric strength decreases. Also the amount of moisture oil can hold (stay in solution, does not evolve out of the oil as water droplets) increases with oil temperature. As the temperature increases water migrates from the insulation into the oil. As the oil cools, water returns to the insulation. The changing load and the changing seasons impact on varying the temperature. The movement of water from oil to insulation and back, forming an equilibrium as a function of temperature is a natural occurrence and generally does not cause problems.

Problems will arise if there is too much water in the system or if the temperature transition is too fast. This will result in water coming of solution in the form of water droplets, clouding the oil, dramatically dropping the dielectric strength and potentially leading to dielectric failure. A rule of thumb for water content in oil for industrial use transformers with a maximum voltage of 69 kV, is less than 30 ppm. The dielectric strength for this equipment should be greater than 30 kV (using ASTM D877 method)

Water Content in Silicone

Silicone absorbs water more readily than oil and can hold up to fours times as much at a given temperature. Water contents up to 90 ppm are normal, values between 90 and 150 ppm should be candidates for reclaiming and silicone with levels greater than 150 ppm are likely poor candidates for reclamation.