Correlation between time and frequency domain measurements for the insulation diagnosis of current transformers

Transformer maintenance has evolved over the past few decades from an item of periodic expenditure to a strategic planning tool in the management of transmission and distribution systems. Some of the newer insulation diagnostic techniques like Polarization and Depolarization Current (PDC) are based on the measurement of dielectric response functions. PDC analysis is an onsite, offline, time-domain technique effective in differentiating the aging effects of paper and oil insulation. In this research work, current transformers (CTs) were subjected to thermally accelerated tests. Diagnostic tests were performed at various aging stages of CTs. The PDC measurements were correlated with Frequency Domain Spectroscopy (FDS) by modeling the current transformer insulation structure as a linear dielectric. The model parameters were evaluated with the use of an iterative non-linear curve fitting program.