Effect of thermal and humidity ageing on space charge accumulation in epoxy resin

Insulation systems are subjected to environmental stresses such as temperature and moisture. These atmospheric stresses cause ageing of polymeric dielectric materials through change in material properties. Under application of DC field, ageing might affect the accumulation of space charge within the dielectric material. As we are aware, space charge accumulation in a material has an important role to play in determining subsequent behaviour of the dielectric and its expected life. In this work, space charge measurement is carried out using the Pulsed Electro Acoustic (PEA) technique. Epoxy resin is chosen as the dielectric material on account of its prevalence in the high voltage industry. The specimens are subjected to thermal and and/or ambient (humidity) aging. For thermal aging, the samples are subjected to elevated temperatures for given time periods. Humidity ageing is carried out under environmental conditions with high moisture content. Space charge measurements are performed before and after aging. Also FTIR analysis is carried out to estimate the change in material properties due to heat/humidity treatment. A quantitative estimation of charge accumulation is done on epoxy samples aged under different conditions. Ageing markers like trap depth, trap controlled mobility are estimated and compared with unaged material. Charge trapping and transport mechanisms are investigated under different ageing conditions. Further, breakdown tests are also performed on the unaged/ aged specimens with an aim to correlate the charge accumulation and charge transport mechanisms to the breakdown strength of the material.