Test for Oxygen Inhibitor
In previous sections, the need to keep the transformer dry and O2 free was emphasized. Moisture is destructive to cellulose, and even more so in the presence of oxygen. Some publications state that each time you double the moisture (ppm), you halve the life of the transformer.
As was discussed, acids are formed that attack the insulation and metals that form more acids, causing a viscous cycle. Oxygen inhibitor is a key to extending the life of transformers. The inhibitor currently used is ditertiary butyl paracresol (DBPC). This works similar to a sacrificial anode in grounding circuits. The oxygen attacks the inhibitor instead of the cellulose insulation. As this occurs and the transformer ages, the inhibitor is used up and needs to be replaced. Replacement of the inhibitor also generally requires treatment of the oil . The ideal amount of DBPC is 0.3% by total weight of the oil (shown on the transformer nameplate) .
Test the inhibitor content with the DGA every 3 to 5 years. If the inhibitor is 0.08%, the transformer is considered uninhibited, and the oxygen freely attacks the cellulose. If the inhibitor falls to 0.1%, the transformer should be re-inhibited. For example, if your transformer tested 0.1%, you need to go to 0.3% by adding 0.2% of the total weight of the transformer oil. The nameplate gives the weight of oil— for example, 5,000 pounds—so 5,000 pounds x 0.002 = 10 pounds of DBPC needs to be added. Adding a little too much DBPC does not hurt the oil. Dissolve 10 pounds of DBPC in the transformer oil that you have heated to the same temperature as the oil inside the transformer. It may take experimenting some to get the right amount of oil to dissolve the DBPC. Mix the oil and inhibitor in a clean container until all the DBPC is dissolved. Add this mixture to the transformer, using the method given in the transformer’s instruction manual for adding oil.
Do not attempt this unless you have had experience. Contact an experienced contractor or experienced Reclamation personnel if you need help.
In either case, do not neglect this important maintenance function; it is critical to transformer insulation to have the proper amount of oxygen inhibitor.