All transformer manufacturers provide required insulation depending on the voltage level so they do not require any additional increase in insulation.
If it is an old and used transformer affected by moisture effect, then take it out heat it in a heating chamber at appropriate temperature and then re insulate it with insulating varnish and again heat it (if required)depending on the insulating varnish used (as directed by the manufacturer).Check the insulation with suitable megger before re use.
electrical engineering
Re: electrical engineering
