Dear Sir, Thank you very much for your reply. If there is PD, Methane should have also come and CH4/H2 should have been < 0.2 as per IEC. If there is leakage from OLTC compartment to main tank, other fault gasses potentially C2H2 should have shown. There are no gases at present building in Buchholz relay. We have analyzed chemical composition of oil. Aromatic compounds are 1.3% less than 4% min as per IEC. Is low aromatic content the cause of H2 generation? Is the gassing tendency of oil creating issue? Is this a case of stray gas (H2) generation? What to look for if transformer is opened? Regards Sreelatha