Dielectric Breakdown Voltage Testing - Part 6

Dielectric Breakdown Voltage Testing Part 6 – ASTM D1816 for New Insulating Liquids

In Part 5, we discussed what to do when there is bad D1816 data for in-service transformer oil. Now, we will discuss using the same test, D1816 Dielectric Breakdown Voltage Using VDE Electrodes, to evaluate new oil. For guidance, we are going to use Standard C57.106-2006, IEEE Guide for Acceptance and Maintenance of Insulating Oil in Equipment.

Table 1 of the Standard lists the test limits for shipments of new mineral insulating oil as received from the supplier. The minimum D1816 values that are acceptable are 20 kV for the 1 mm gap setting and 35 kV for the 2 mm gap setting. D1816 oil results that do not meet the minimum for the selected gap should be rejected. D1816 dielectric breakdown voltage is an important acceptance for new oil, particularly when received in bulk containers.

Table 2 of C57.106-2006 lists the test limits for new mineral insulating oil received in new equipment of voltage class below 230 kV, prior to energizing. The listing is further broken down into voltage class up to and including 69 kV, and voltage class between 69 and 230 kV. For ≤ 69 kV, the minimum is 25 kV for the 1 mm gap, and 45 kV for the 2 mm gap. For > 69 kV and < 230 kV, the minimum is 30 kV for the 1 mm gap and 52 kV for the 2 mm gap. These values are higher than values for new oil because the new oil is processed using vacuum and filtration to remove suspended particles, moisture, and dissolved gases prior to filling the electrical equipment. It is also important to consider that when filling equipment of these voltage classes in the field, the dielectric breakdown voltage test values will decline somewhat when new oil is added. When testing the oil in the field, after oil processing, for electrical equipment <230 kV, higher values (more than those listed) will be needed in order for the filled equipment to meet the Standard.

Table 3 lists the test limits for new mineral insulating oil processed for equipment 230 kV class and above, prior to energizing. The listing is further broken down into voltage class at least 230 kV but less than 345 kV and voltage class 345 kV and above. For ≥ 230 kV and < 345 kV, the minimum is 32 kV for the 1 mm gap and 55kV for the 2 mm gap. For 345 kV and above, the minimum is 35 kV for the 1 mm gap and 60 kV for the 2 mm gap. As with lower voltage class equipment, when testing the oil in the field, after oil processing, for electrical equipment ≥ 230 kV, higher values (more than those listed) will be needed in order for the filled equipment to meet the Standard.

Note also that dielectric breakdown voltage values that are required for newly added oil are higher for high-voltage class equipment because contamination affects the serviceability of the oil as a dielectric fluid. This is of greater concern at higher voltages. Thoroughly removing contamination produces a higher dielectric breakdown voltage value.