Minimum Insulation Gap Formulae for 120MVA 132/33kV Delta-Y Transformer in Oil Tank

I am studying the design of 120MVA,132/33 KV  delta-Y transformer. I am a bit confused about the distance between the conductor & earthed part & between conductors for maintaining min insulation gap inside the oil-filled tank.   I would like to know the two formulaes  for  measuring    1. the min distance between the covered conductor ( r1=bare conductor rad. & r2= rad. with insulation ) & the earthed part inside the oil-filled tank.2. the min distance between two covered conductors inside the oil-filled tank.        Concerned are cordially requested to answer.

Hi MohammadCan you explain which formulas you are talking about and which parts you are confused about?1. The mininum distance between conductors or between conductors and earth is based on the dielectric strength of the oil. The oil will be capable of withstanding say 150kv down to a distance of maybe  50mm, depending on the oil. So really the conductors could be bare and and kept a minimum distance apart of 50mm. In practice they would be kept further apart than the minimum value allowed based on the oil dielectric strength.2.  The same applies here,  the oil would have a dielectric strength value given as between phase to phase and phase to earth. The conductors inside the transformer will not be bare, but they could be as the covering that they have will not be rated to 132kv. As you can imagine the insulation required would too much. They will most likely be rated much lower than 132kv.

Alan GallagherThanks for your valuable comments, of course, the dielectric strength of the oil is vital for insulation distance design..  I am saying about active part of the transformer.  From my study, I came to know that inside the oil-filled tank, 6 kV/mm (limited stress value decreases, maybe 2kv/mm, as the barrier gap increases). Insulation is designed based on this &  650 KVp BIL is considered for 132KV voltage class. Different Weidmann formulas are used for different barrier gap for finding min distance/gap keeping the stress value within the limit. Therefore, I am seeking the Weidmann formulae for calculating distance between two covered/insulated conductors inside the oil-filled tank.