The space charge features of cable insulation peelings are investigated under ac stress as a function of electrical aging imposed previously on cable sections. The investigated samples are 230-μm-thick peelings of crosslinked polyethylene cable insulation. The space charge profiles are resolved into 32 symmetric phase angles under 50 Hz ac stress. The space charge density in the phase angle of maximum applied field is the largest, as expected, but does not change visibly as a function of aging time. A method that involves averaging space charge in all phases is proposed. The all-phase averaged space charge, which appears in essence as stationary charge, exhibits an obvious increasing trend with stressing time. The aging characteristics of space charge formed under ac stress are investigated considering the all-phase averaged profiles, and the mechanical properties and degree of crosslinking are characterized as complementary methods. The change in the accumulation of space charge under ac stress follows opposite trends unlike the degree of crosslinking, which means that space charge under ac stress has a close relationship with defect concentration in cable insulation. This research contributes to the understanding of space charge features under ac conditions and the understanding of the ac aging principle of cable insulation.

When high‐voltage direct current (HVDC) cables are subjected to a direct current(DC)‐superimposed pulsed electric stress, the pulsed voltage applied facilitates electrical tree generation and breakdown; however, the mechanism involved remains unclear. To study the deterioration mechanism of cable insulation under a pulsed electric field, an experiment on the dynamic characteristics of space charge under a DC‐superimposed pulsed electric field was conducted. For this experiment, a pulsed time trigger control circuit was developed to accurately measure the change law of the space charge in cross‐linked polyethylene samples at the rising and falling edges of the pulsed electric field. The experimental results showed an unusual change law: the space charge density increases with falling voltage amplitude and decreases with rising voltage amplitude. The sudden change in the electric field breaks the balance of the forces acting on the space charge, leading to injection, extraction and migration. The energy released during these dynamic processes may destroy the microstructure of the insulation material and contributes to the growth of the electrical tree. This study provides an insight on the initiation mechanism of electrical tree in HVDC cables and proposes a strategy to suppress electrical tree initiation.

When a single-phase-to-ground fault occurs in a power system, a secondary arc will normally be generated at the fault position after the fault phase has been interrupted. A detailed investigation of multiple-reignition features of secondary arcs on extra-high-voltage/ultra-high-voltage transmission lines is, therefore, very important. In this paper, an experimental setup for generating a secondary arc across a transmission-line insulator string is presented, and both the discharge waveforms and images are synchronously recorded. According to the properties of the discharge waveforms, the discharge process is divided into two stages and each stage is analyzed. The evolution of the volt-ampere characteristic and the resistance-current curve of the secondary arc is found to exhibit hysteresis during the discharge process. Multiple-reignition images of the secondary arcs are converted into binary images using image threshold processing. The arc length and breakdown voltage are discussed in association with the discharge images. Morphological changes in the secondary arc during a single discharge process are analyzed in terms of self-magnetic compression theory, and the extinction mechanism of the arc discharge is discussed on the basis of two theoretical approaches.

A pulsed electro-acoustic equipment system was devised to measure the space charge of a power cable under 50 Hz AC stress and a high temperature gradient by adopting a new phase-matching method. Space charge profiles were recorded on a 10 kV cable with cross-linked polyethylene insulation. The AC period was resolved in 32 phase angles, while the cable underwent electrical-thermal aging for 16 days. Two types of space charge were discovered, termed periodic charge and stationary charge according to whether they change with AC phases. The periodic charge was not determined to develop over time, while the stationary charge increased gradually as the cable aged. A method called the all-phase average was employed to clearly recognize the stationary charge. An interesting phenomenon was discovered upon attempting to regenerate space charge in the cable after the charge was eliminated. The stationary charge regeneration speed was found to depend strongly on temperature and electrical field. This paper provides a theoretical interpretation for the generation of periodic charge and stationary charge, as well as the dependence of charge regeneration on temperature and electrical field. A new model is raised to account for the generation of stationary charge, explaining how residual charge in one sine period undergoes different charge migration processes around a tip defect between negative and positive periods.

Cross-linked polyethylene (XLPE) cables are widely used in power systems due to their excellent electrical and mechanical properties. Space charge is an important factor in cable aging and breakdown. At present, XLPE cables in the power system are mainly AC XLPE cables, so it is necessary to study the space charge characteristics under AC voltage. In this paper, an electro-thermal aging platform for XLPE cable was built and a 10 kV cable was aged. XLPE slices were obtained from the aging cable and AC space charge experiments were carried out on the XLPE slices at different stages of aging. The trap energy levels were calculated and the effects of different electric fields on the AC space charge characteristics were compared. The experimental results show that the trap energy level increases with the aging process and the non-averaged electric field is a necessary condition for charge migration.