We found the issue with the transformer last night. The crews found a blown internal jumper cable that went from one of the back fuses, under the adjacent fuse holder / sleeve and it arced over blowing the HV cable. We proceeded to lower the oil to ensure that we could not see any other visual damage to the core. We were concerned because we found paper floating in the oil and that it may have came from a winding. Due to the oil being pretty black, we could not see very far into the transformer. After visually inspecting the core, we placed a temporary jumper over the blown HV Cable and performed a TTR test which passed. Therefore, we fill confident that the only issue was the lead blowing and that the core is ok, which is a really good thing. Our recommendation: ●I will start searching this morning for a replacement HV lead with insulation and a paper tube to replace the blown jumper. ●Recommend replacing the 550 gallons of oil. The oil is pretty black. ●Recommend purchasing another 100 gallons, i.e. total of 650 gallons so that we can clean up the inside of the tank as best as possible and flush with 100 gallons of oil. ●After replacing the jumper, complete a full test and take an as left oil sample to set a new baseline. ●Please ask the plant if they want us pulling any type of vacuum prior to filling and during filling. We do not normally pull vacuum on transformer rated less than 60kV when filling. After we inspected the core, we did pump the oil back into the transformer only to cover up the core to further protect the core.