We are a US contractor that performs HV maintenance for companies. We use the NETA testing standards. NETA has a testing frequency matrix based on the type of equipment and the reliability required.Transformer oil sampling has a base frequency of 12 months . Now depending on how reliable and the condition of the equipment multiply this number by a number in the range of .25 to 2.5. Most industrial plants use the number 2.5 which means the sampling should be performed every 30 months. But depending on your equipment condition and reliability requirements you may want to use a different multiplier.
NETA recommends performing the following tests of the oil sample:
Dielectric breakdown voltage - This determines the dielectric breakdown voltage. There are two standards for this test ASTM D877-87 and ASTM D1816-84a. If ASTM D877-87 is performed the minimum nuber should be
26KV for equipment under 69KV. If ASTM D1816-84a is performed the number should be 23KV for equipment under 69KV.
Acid neutralization number - This number is an indication of sludge buildup in the transformer. New oil should have a number of 0.03. This number should not exceed 0.2.
Specific Gravity(Optional) - This is the ratio of oil to water. Water has a specific gravity of 1, oil should have a specific gravity of 0.84
Interfacial Tension - This is an indication of the remaining useful life of the oil. As the ACid neutralization number goes up this number goes down. New oil in new equipment should have a number of 35 dynes/cm, min. This number should not be below 24 dynes/cm, min.
Color -This is an indication of oil contamination. Ths numbers range 0
- 8.5. New oil (Clear) is 0 - 0.5. Sevice aged oil (Yellow) is 1.0 -
2.5 and Marginal oil (Bright Yellow) is 2.5 -4.0.
Visual Condition - This test is to determine if more testing is required. The oil should be sparkling, bright and clear. If not you may want to perform other testing depending on the appearance.
Water in insulationing fluids(Required on 25KV or High Voltages) - Moisture greatly affects the paper insulation in a transformer. So it is important to know the moisture content in the transformer. These numbers are an indication of the amount of moisture in the insulation. Dry insulation will have a number of 0 - 5, moderate to wet insulation will have a number of 6 - 20.
Measure Power Factor or Dissipation Factor (Optional) - Power Factor test is the indication of power loss through the oil. New oil in new equipment (Equipment rated under 69KV) should have a %Power factor at
25C of 0.15. This number should not exceed 0.5.
All this information comes from the NETA testing standard and IEEE
Mike McCowan Industrial Automated Systems Davenport, IA