Based on photo-stimulated discharge (PSD) theory about release of charges from the mono- and multi-levels of traps, two methods on the energy distribution of trapped charges (charge distribution)—step scanning method and combined calibration–scanning method are proposed. The typical polymeric dielectric, low-density polyethylene (LDPE), is selected to be investigated in this paper. The linear relation between PSD current and time at selected wavelengths in LDPE samples indicates that the effective interaction cross sections between the photons and trapped charges depend on the photon energy. It is indicated that both of the largest ratios in charge distributions based on the two methods are around the energy level of 4.97 eV for LDPE. Compared with the step scanning method, the experimental time is extremely shortened and thermal release of trapped charges is decreased in the combined calibration–scanning method, and charge distribution based on combined calibration–scanning method is more reliable.

High voltage direct current (HVDC) cable accessories are usually the weak link in the cable system for the electric field distribution is often extremely uneven, and it has a range of adverse effects in design and manufacture. In this paper, Ethylene-propylene-diene terpolymer (EPDM) was developed for ±525 kV HVDC cable accessories, and the conductivity, XLPE/EPDM interface charge behavior and breakdown strength were tested respectively. The electric field distribution of cable accessories was calculated by COMSOL analysis software. It is shown that the conductivity of EPDM is nonlinear with electric field, and it is possible to achieve the uniformity of electric field distribution by using a composite material with nonlinear conductive properties. Furthermore, the interfacial charge characteristics of XLPE/EPDM depend on the conductivity of EPDM. As a consequence, the research will make a potential application for ±525 kV HVDC cable accessories.

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The distribution of trap levels in the LLDPE (linear low density polyethylene) is studied by means of photo-stimulated discharge (PSD). And two scanning manners, continuous scanning and step scanning, are discussed. It is pointed out that there may be some problems because of the incomplete detrapping of the trapping charges for the continuous scanning. The principle and the experimental process for the step scanning were presented. It is found that there is linear relationship between the logarithm of PSD current produced under the irradiation of the monochromatic light and the time. It accords with the theoretical analysis and demonstrates that the retrapping effect of the carriers can be neglected. The charges were in the levels of 4.36-6.22 eV, and the majority of which were trapped in the energy levels around 4.97 eV in LLDPE.

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