Lifetime Investigations on Insulating Materials for High Voltage DC Airborne Applications

The purpose of the presented study is to evaluate the lifetime of an insulating material for a high DC voltage in an airborne and an off-shore application. In order to accelerate the aging process, a thermal stress at 200°C was applied to coupons made of epoxy material (E). Some samples also undergo combined stresses. For a DC electric field of 3.8kV/mm, no dielectric breakdown was observed after 3 months of test, while for a field of 4.6kV/mm, breakdown of all coupons occurred in less than 30 hours. In order to understand aging mechanisms, several parameters were monitored during the tests. Space charge mapping, using the Pulsed Electro Acoustic (PEA) method, done at 150°C under 4.6kV/mm, showed that hetero-charges were trapped inside the material, leading to a local field enhancement in the material. For the applied field of 3.8kV/mm, the local electric field was below the critical threshold, and no breakdown was initiated. Dielectric spectroscopy was monitored during aging tests in the range of 10mHz-30Hz. A continuous increase of coupons impedance was observed during aging. The increase was even higher in the case of coupons which received the combined thermal and electrical stresses. Charge mapping with PEA showed that most of the charge relaxed after the voltage switch off. Hence it was concluded that the higher impedance increase during time with the cumulated stresses was probably not due to trapped charge inside the material. Instead, the applied DC stress had provoked an additional change of the material properties, compared to the thermal stress alone.