Observation of Interfacial Charge Formation Process by Space Charge Microscopy

In high-voltage insulation systems, the space charge that accumulates in the insulation is a problem, as the distortion of the local electric field may lead to a reduction in insulation performance. Generally, insulation design assumes that space charge is not existent. To evaluate insulation performance, it is important to know the difference between the actual electric field distribution and the designed electric field. Space charge measurement is one of the solutions. Space charge measurements of insulators are usually made in one dimension using a film-like sample. However, practical insulating systems have complex structures, so we believe that space-charge measurements with two - or three -dimensional resolution will be required in the future. Therefore, we developed a prototype space-charge microscope based on the pulsed electro-acoustic method. An insulating system was mounted on a polymer substrate, and the signal was acquired by scanning a focused ultrasonic transducer. In this study, we observed a transition in the space charge distribution across the insulating interface between different insulating materials. The electrostatic stress signal generated by the action of the pulse voltage was received by the transducer. Immediately after the bias voltage is applied, signals were seen at the parallel plate electrodes, and almost no signal was observed at the interface between the different insulation materials. At the interface, it was observed that the charge gradually spreads out from the triple junction as if it were growing out of the junction. This is thought to represent the process of transition from the electric field distribution determined by the dielectric constant distribution to the electric field distribution determined by the conductivity.