Space charge measurement in polymer insulated power cables using the PEA method

The Pulsed Electro-Acoustic (PEA) technique has been widely used to measure space charge distribution in solid dielectrics. A considerable amount of work has been done on the charge dynamics in plaque samples. Recently, there have been several attempts to apply the technique to polymeric cables, but it has been hampered by experimental difficulties in achieving a universal electrode system to maintain an intimate contact between the curved ground electrode, transducer and absorber.In this research, a modified system employing a flat ground electrode, transducer and acoustic absorber has been introduced. By thoroughly analysing the propagation of the acoustic pressure in the new electrode arrangement, it is concluded that the revised electrode system will give the same detection sensitivity and spatial resolution as that of the old version. The new PEA system not only achieves an intimate contact acoustically but also has advantages of easy assembly of cable sample and is suitable for different sizes of cables without any modification.The basic principle of space charge measurement in cable geometry with the new system is fully obtained. To remove the influences of the geometry factor, the acoustic propagation attenuation and dispersion through the polymer material, a compensation algorithm has been developed to improve the measurement accuracy both on charge density and position.A comprehensive program with the functions of data acquisition, deconvolution of original signal, calibration and compensation both to geometry and propagation factors has been developed in the LabViewTM environment. The method to calculate the electric stress distribution from the results of the space charge profile in cable geometries has been proposed.For practical application, space charge distribution measurements were carried out on several prototype cable samples which adopt modified XLPEs (undegassed and degassed) as insulating materials. The comparisons were conducted on both the space charge accumulation and its influence on the electric stress distribution. The results showed that the modification to the XLPE material is profitable from the point of view of space charge suppression. Moreover, the thermal treatment to the sample showed that the removal of the volatile residue had a significant influence on the space charge behaviour.