High field space charge measurements in XLPE using a novel pulsed electro-acoustic system

This thesis describes the development of a novel high-voltage, high-speed Pulsed Electro- Acoustic (PEA) instrument for real-time measurements of space charge distribution in solid insulators subjected to DC, AC or transient voltages. Very high-rate PEA measurement is accomplished with a state of the art data acquisition module and a purpose-built high-voltage FET switch pulse generator. This advanced PEA also has excellent temperature monitoring and control. Cross-linked polyethylene (XLPE) peelings taken from un-aged power cable was used in this study. The average thickness of the XLPE sheet was 150 um. A measurement at 50 Hz power frequency was performed using the new PEA system for validation purposes. The study of DC electric field criteria for the formation of space charge packets in XLPE was carried out to determine the DC onset field for charge packet formation. The average DC applied electric field ranged from 80 - 120 kV/mm. The calculation of the internal electric field showed that the packets required a field of 140 kV/mm for their initiation. The generation of charge packets is governed by the field at the anode and the balance of charge injection, transport and extraction process. Researchers in Toulouse (France) conducted electroluminescence experiments the author and researchers from Bologna (Italy) performed space charge measurements using the PEA at various DC electric field. Complementary information was obtained from these two techniques. The correspondence between the pertinent values of the DC electric field obtained from space charge and EL support the existence of degradation thresholds in this particular XLPE material. Charge packet formation, propagation and EL results have been analysed. Charge recombination appears to be the excitation process of EL. Whilst continuous EL excitation can be initiated without the involvement of charge packets, oscillatory EL has been found, which corresponds to packet formation or annihilation.