Endurance estimation of epoxy resin systems under synergistic environment of high relative humidity and electrical stress

The authors report on an experimental investigation undertaken to quantify the effect of high humidity on the performance of epoxy resin insulations. The data obtained from the investigations have been used to calculate a new index called the cumulative characteristic factor, which uniquely grades the material and insulation systems exposed to permanently elevated humidity conditions. To assess the long-term behavior, endurance tests were also conducted with a newly designed environment cycle representing tropical climatic conditions. Based on the accelerated test data, a system model has been developed for predicting the insulation life. To validate the laboratory aging tests, a 11 kV current transformer was installed in a highly humid region in Madras City, India. The place of installation was a switchgear plant for air conditioning purposes. From the experimental investigations and model developed it was concluded that in the presence of permanently elevated humidity conditions bisphenol epoxy resins are susceptible to severe surface degradation within 2 to 3 years of installation