Time domain spectroscopy measurements for the insulation diagnosis of a current transformer

There is a growing need for accurate but nonintrusive insulation diagnostic techniques and better maintenance tools to assay the internal condition of transformers. Classical diagnostic methods and periodic maintenance routines have been in practice for several decades. Some of the newer non-destructive diagnostic techniques are based on the measurement of dielectric response functions in time and frequency domain. Polarization and Depolarization Current (PDC) analysis is an on-site, off-line, time-domain diagnostic technique capable of differentiating the aging effects of paper and oil insulation. In this research work, a medium voltage (69 kV, 1200 A/5 A) current transformer (CT) was subjected to thermally accelerated tests. The elapsed insulation life was calculated as per IEEE C57.91-1995. Insulation diagnostic tests including PDC were performed at various aging stages of the CT. Results indicate that PDC tests can be applied to instrument transformers as well. A correlation between time and frequency domain spectroscopic measurements was also established. Measurement of furanic compounds in the oil was consistent with the paper conductivity trends observed in the PDC measurements.