To test insulation with a megger, disconnect all “H” high- and “X” low-side cables including secondary-side X0-bonding jumper. At 1kV, test three times—high-side to ground, then low-side to ground, and high-side to low-side. Roughly one megohm minimum on each.
I doubt it’s an insulation problem. Carefully check primary current on each high-side phase with the secondary cables disconnected and meter on a low range. Offhand, it should be less than amp on all three phases. If the drytype seems noisy or overly warm with no secondary load, there may be a shorted turn in the transformer. A shorted turn won’t automatically fail an insulation-resistance test.
