By the way there are heaters or dessicants on the iso-phase bus enclosures, ensure they are working. You may find water accumulation on several sections and check for the low point drain valve maybe they are plugged.
Perform dc withstand test but consult the manufacturer first for recommendation. Ensure the CT and PT are isolated, GCB open and the bus isolated from GSU. NETA MTS Table 10.17 gives you the maximum test voltages.
Insulation resist
