Hypothesis of Interfacial Charge Transportation in Oil-Paper Insulation for Transformer Moisture Estimation

Accumulated space charge on the interface between oil and cellulosic paper has a significant impact on the dielectric excitation and relaxation behavior of transformer insulation system. Conventional differential time-domain dielectric spectroscopy (DTDS) function of polarization and depolarization current (PDC) measurement treats the whole oil-cellulosic paper insulation as a linear dielectric medium. This approach may not be able to separate the interfacial charge transportation from the relaxation process of dipoles. In this presented work, the effect of interfacial charge transportation on dielectric relaxation of oil-cellulosic paper insulation is investigated. A distributed parameter DTDS model of detrapping current caused by the interfacial charge transportation is established from the PDC measurement. Subsequently the characteristics parameters of the primary component of the distributed parameter DTDS model are extracted to estimate the moisture concentration of cellulosic insulation. The proposed method is verified using measurement results obtained at different stages of ageing experiment performed on prototype transformers.