Influence of moisture on partial discharge characteristics of oil impregnated pressboard under non-uniform field

Summary

Oil impregnated pressboard (OIP) is used abundantly as insulation barriers in transformers. However, it is hygroscopic in nature and ingress of moisture can deteriorate insulation property of OIP. During service, the influence of moisture on Partial Discharge (PD) characteristics of OIP is an important factor to be considered in insulation diagnostics and hence to confirm the reliability of transformer. In this work, estimation of moisture content in OIP is carried out using Frequency Domain Spectroscopy (FDS) and Recovery Voltage Measurement (RVM). The vacuum-drying and heating methods were employed to control the different level of moisture in OIP ie, less than 2%, 2% - 5% and more than 5% of moisture content. A test cell with needle plane electrode arrangement used to generate partial discharges in OIP under AC voltages. The needle electrode having tip diameter of 220 µm and plane electrode having diameter of 90 mm were used in the study. Investigations were carried out to evaluate the Partial Discharge Inception Voltage (PDIV) and partial discharge characteristic of OIP with different level of moisture content under non-uniform fields. The PD measurements were carried out according to IEC 60270. Pressboard samples having thickness 4 mm were used, to compare and validate the PD characteristics in OIP with different levels of moisture. The results of study indicate that increased moisture content in pressboard reduces PDIV significantly, ie, PDIV measured are 15.34kV for 2.6% of moisture and 12.21kV for 3.7% of moisture respectively (~20% reduction in PDIV). Pulse characteristics of PD such as pulse duration and rise time are observed in the range of 160 ns – 650 ns and 30 ns – 110 ns respectively for 2.6% of moisture. Similarly, the recorded pulse magnitude is in the range of 420 µV - 910 µV. The dominant frequency of PD measured is found to be in the range of 1.4 MHz – 7.6 MHz. Furthermore, the analysis is made to recognize the pattern of PD in OIP with different level of moisture. Analysis of this pattern gives the information about phase of occurrence, PD magnitude in pC and number of pulses.

Additional informations

Authors

N.R. BURJUPATI, P.M. NIRGUDE, S. THIRUMURTHY

Keywords

Oil Impregnated Pressboard, Moisture, FDS, RVM, Partial discharges, PDIV