As a non-destructive diagnostic technique,return voltage measurements based on dielectric response theory have been introduced for condition assessment of transformer insulation,especially for the evaluation of water content in transformer pressboard.

One of the methods currently being investigated as a possible non-intrusive diagnostic tool for condition monitoring of power transformer and cable is the recovery voltage measurement, which will be improving the ability of detecting the content of water concentration and the ageing process in the insulation system and may thus be an indicator of insulation quality and its ageing status.