The insulation condition of Voltage Transformers can be assessed by the Polarisation Depolarisation Current (PDC) technique.

PDC results of oil-paper type Voltage Transformers include:

• Moisture-in-paper
• Insulation Resistance and Polarisation Index
• Frequency scan of Capacitance
• Frequency scan of Dielectric Dissipation Factor (DDF) or tan δ
• Recovery Voltage Polarisation Spectrum

Some PDC results of a leaky, new 110 kV Voltage Transformer are presented in the charts below: