Accurately determining the threshold electric field at which charge injection from the electrodes starts is important for reliable operation of dielectric materials as the presence of charge in the material can lead to electric field enhancement, resulting in degradation and early failures of the material. The two methods in charge measurements that have been commonly used to find out the threshold field have been compared to the proposed method, which overcomes the drawbacks of the two methods. Such method offers (i) high sensitivity as the effect of capacitive charge has been eliminated and (ii) contributions from both mobile and slow charges; hence, providing a more accurate value for the threshold electric field. Based on the proposed method, it has been found that the threshold field for low density polyethylene is around 8 kV/mm, which is lower than the reported value obtained from the other methods.

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Abstract-Polyethylene has been one of the widely studied polymeric insulation materials. One of the major issues related to polymeric materials is the easy formation of space charge which may cause electric field enhancement. In this paper, a numerical simulation based on a bipolar charge injection/transport model is used to obtain characteristics of space charge in polyethylene under the combined AC and DC high voltage at room temperature. The bipolar charge injection/transport model, which is widely used in HVDC space charge simulation, is applied in this combined condition. The overall applied voltage, consisted of root mean square (RMS) values of 50 Hz AC voltage and DC voltage, is kept the same, while the DC component has been varied from 0 to 1. The simulated charge distributions present notable differences when DC offset is added compared with pure AC conditions. Besides, these differences become more significant when the offset ratio is increased. The total positive and negative charge amounts are calculated respectively by integrating the charge in the material, and a curve of net charge amount changing along with time is obtained.

In this study, an improved trapping/detrapping model was used to simulate the charge dynamics in cross-linked polyethylene peelings from different-year aged cables. Injection barrier of trapping parameters was estimated by the model fitted to experimental data for each type of sample. Moreover, dc breakdown tests were operated on those samples. It has been found that the dc breakdown strength of inner-layer samples is the lowest in cable sections with thicker insulation layer taken from high-voltage ac (HVAC) 220 kV service condition, whereas for the cable with thinner insulation from HVAC 110 kV, middle-layer samples have worst breakdown performance. This might be explained by the space charge issues under long-term HVAC condition. More importantly, a clear relationship between estimated model parameters, including injection barrier, trap depth and trap density, with the dc breakdown strength in each layer has been reported in this study. High Voltage

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