Effect of Pre-stressed Thermal Ageing and Electric Field during Measurement on Space Charge Distribution of HVDC Insulating Materials

Transmission cables are inevitably exposed to thermal stress due to ohmic loss during cable systems operation, and severe thermal ageing may deteriorate electrical insulation performance and shorten the service life of cable. Special attention should be paid to HVDC cables as the electric field under DC voltage is highly temperature dependent so that it can have a greater adverse effect in terms of electrical insulation. Thus, it is important to investigate the effect of thermal ageing on electrical properties of an insulating material. Therefore, in this paper, space charge measurement based on the Pulsed Electro-acoustic (PEA) method was performed to evaluate the effect of thermal ageing on the space charge distribution of insulating materials. The space charge measurement is an experimental method mainly used to evaluate insulation degradation and performance of HVDC insulating materials. Polypropylene (PP), a representative eco-friendly material for HVDC cable insulation, was selected as experimental sample, and all samples were prepared in the form of sheets. Each sample was thermally pre-stressed for up to 1000 hours and then it was also applied electric field up to 30 kV/mm during measurement for 3 hours. In order to compare the difference in experimental results depending on the thermal pre-stress endurance time, the unaged PP was also included in test samples. The experimental results show that thermal ageing causes a heterogeneous charge distribution, especially near the cathode. And it could be confirmed that homogeneous charge distribution was obtained when a high electric field was continuously maintained during measurement.