Transformers megger zero

[Electricrifying]

Dear sir's, pl advise, In a 100MVA, BHEL make Tx, found IR value between C-F , C-T is Zero, internal inspection was carried out no fault like shorting between core to frame & core to tank found, in this Tx. As pet design Frame & Tank is on same potential. After that a cable lead connected from core point to ground and found 31amp initial circulating current and after 2-3 days charging & loding circulating current measured 27Amp photographs attached and DGA test carried out found gradual increase in carbohydrates gasses including acetelene ppm 1 to 9 from no load to load 40 to 60%..... corrective advice is required for safe guard of Tx.....