Application of PEA technique to space charge measurement in cylindrical geometry HV cable systems

Space charge, as one of the major concerns for the reliability of polymeric High Voltage Direct Current (HVDC) cables, has drawn wide attention in both academia and industry. Accordingly, measurement techniques along with accurate data interpretation have been required to study space charge behaviour in insulation materials and to provide solid bases for simulation activities. In this work, a high temperature space charge measurement system for mini-cables has been developed based on the Pulsed Electro-Acoustic (PEA) method. In parallel, simulation tools for space charge accumulation, based on non-linear unipolar charge transport models, the acoustic signal formation, transmission of acoustic waves, and their detection, have been developed for PEA measurement on mini-cables to provide an alternative way for interpreting the raw experimental data rather than the traditional approaches of reconstructing space charge information by signal processing and calibration. The simulation uses 2-D simulation tools and includes the clamping unit of the PEA cell to provide, for the first time, a detailed comparison of the two commonly used shapes, flat and curved, of the base electrode. Benefiting from the ability of applying isothermal experimental conditions of 20 – 70 °C, the transient of ‘intrinsic’ space charge accumulation due to the field and temperature dependent conductivity has been studied by means of a novel experimental data analysis method proposed in this work. In addition, the analysis provides a way to assess conductivity models by matching the simulation results with the experimental space charge results. By applying the simulation tools, the effect of the possible cable defects of non-concentricity and a mismatch between the insulation and semicon layers could be assessed. Furthermore, the origin of the bulk space charge signal experimentally observed in a mini-cable was found to be consistent with a radius dependent conductivity which may be a consequence of incomplete degassing.