Effect of introduced charge on frequency domain dielectric response of oil-paper insulation

Frequency Domain Spectroscopy (FDS) technique is widely used to perform diagnosis of oil-paper insulation. Results reported in this paper suggest that it is imperative to obtain charge free insulation for FDS measurement in order to obtain reliable insulation diagnosis result. It is practically difficult to ensure such condition for practical transformer as it often contains trapped charge. This means the results of FDS must be studied along with the effect of charge for better interpretation of the physical condition of oil-paper insulation under test. This paper presents a comparative study of FDS data obtained from a charged and uncharged oil-paper insulation sample. FDS measurement of uncharged insulation sample is performed and tanδ profile is measured first. Thereafter, different amount of charge is introduced in the insulation sample by applying high dc voltage across the sample for different time intervals followed by FDS measurement of the charged sample. It is observed that presence of charge affects the frequency domain dielectric response of the sample non-linearly. Being a linear model, this non-linear behavior of oil-paper insulation in presence of charge cannot be addressed by popular insulation model like Conventional Debye Model (CDM).