Lately we had a problem on one of our old (33 years) but reliable Desalter Transformers. The Transformer is NWL 150KVA 4.16KV/23KV (3each) on top of our crude oil desalter trap. Two of these transformers experienced failure and we found the high voltage cable was burned and cut. The oil in the entrance housing was discolored (black) and the oil in the third transformer was clear. Later on we found the secondary on the third transformer was disconnected. the floating switch and the grid were OK.
What are the possible causes for such failure?
We replaced the entrance bushings with the high their high voltage power cables for the damaged bushings.
We performed Megger test and Polarization Index (PI) test on the transformers. The PI was 1.4, 1.6 and 1.5 for these transformers. Are these PI readings acceptable to put back the system in service? What are other tests shall we perform on these transformers?