| A major challenge faced by power engineers today is to meet the growing energy demands while ensuring an optimized grid operation. A transformer is a highly reliable device, if serviced and maintained regularly. 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. Life of a transformer is essentially life of its insulation. Some of the newer insulation diagnostic techniques like Polarization & Depolarization Current (PDC) measurement and Frequency Domain Spectroscopy (FDS) are based on the measurement of dielectric response functions. PDC measurement is an on-site, off-line, noninvasive, time--domain technique effective in differentiating the aging effects of paper and oil insulation.;In this research work, two medium voltage (69kV) Current Transformers (CTs) were subjected to thermally accelerated tests up to 450,000 aging hours. The elapsed insulation life was calculated in accordance with Arrhenius reaction rate theory as per IEEE C57.91-1995. The thermal aging was interrupted at several stages to perform various diagnostic tests including PDC. In addition, the PDC measurements were correlated with FDS results by modeling the current transformer insulation structure as a linear dielectric. Oil and paper conductivity values were estimated from the PDC measurements to study the dielectric properties of the composite insulation system with aging. Various experimental factors that could affect the PDC test were explored.;Classical diagnostic tests like insulation resistance, polarization index, step voltage, and dielectric discharge were also performed at each stage of accelerated thermal aging. Oil samples were periodically sent to a test laboratory for furanic compound analysis. The test results were studied and compared with Arrhenius aging analysis. Results indicate the potential of PDC for insulation diagnostics of instrument transformers. The frequency domain parameters in the low frequency range can be obtained from PDC measurements by accurate modeling and analysis. Therefore, PDC may be an inexpensive and viable alternative to FDS. |
