The condition of CT Main Insulation system can be assessed by the Polarisation Depolarisation Current (PDC) technique, no matter whether the Capacitance tap of main insulation is brought out or not.

PDC results of oil-paper type Current 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 good, new 72 kV CT are presented in the charts below: