what is the preventive maintenance procedures of toroidal transformer?

Answer 1

Normally none. Toroidal TXFMRs are usually a single current coil without an iron core. They fit around cables or busbars and are sized to provide a secondary current generally 1 or 5 amps for protection and/or control.

It is rather a strange question as there is nothing on a toroidal transformer to maintain

Source: Electrical engineer (ret.) with 45 years experience