Both mVA and MVA are correct unit but depends upon the
rating of transformer
1. mVA mean milli volt ampere, used for small rating
2. MVA means megga volt ampere used for large rating
MV.A (not 'MVA') is the symbol for megavolt ampere.and in some low volatge cases we consider VA that is (Voltage Ampere). where VA is the true vector sum of circuit power or reactive power.
So, i thik it is true. NOT 'mVA' in case of transformer.
we all know that corona is nothing but ionization of air around
the conductor and there by allowing current to pass through
ionized air.but we are not getting any short circuit in between
the respective phases though the air turned as a conductive media