In the last edition of Tech Corner, we discussed what we do when we have bad D1816 data for in-service transformer oil. This week, we are going to 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. Oil with D1816 results that do not mean 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, < 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 the electrical equipment being filled. It is also important to consider that, especially when filling equipment of these voltage classes in the field, the results for dielectric breakdown voltage will decline somewhat when the oil is put into a new piece of equipment. If testing the oil leaving an oil processor in the field, filling electrical equipment < 230 kV, higher values 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, < 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, if testing the oil leaving an oil processor in the field, filling electrical equipment ≥ 230 kV, higher values 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 installed oil are higher for higher voltage class equipment. This makes sense. Contamination that affects the serviceability of the oil as a dielectric fluid is of greater concern at higher voltages. Removing such contamination more thoroughly results in a higher dielectric breakdown voltage value for the tested oil.
Once the equipment has been energized, the oil is considered to be in-service and the criteria that we discussed in this column on October 30, 2009 for in-service transformer oil should be used to determine whether the dielectric breakdown voltage values continue to be acceptable. For new installations, however, it is frequently a very good idea to perform the D1816 determination either 24 hours or 7 days after energizing, and then perform it again 30 or 60 days after energizing. The timing of this new installation testing can be selected to be consistent with the other new installation testing, particularly the DGA testing we discussed in this column on May 1, 2009.
Another consideration for transformer owners is the fact that dielectric breakdown voltage measurements are sensitive to the temperature of the oil at the time it is tested. The proper procedure is to allow a properly collected and representative oil sample to come to room temperature (20 to 30 degrees C). Oil at elevated temperatures will yield a higher value.
Next week, we will discuss the standard method from IEC 60156, which uses VDE electrodes similar to ASTM D1816.