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• The packaging insulation material is exposed to extreme operating conditions with rapid voltage rise rate. • The voltage rise time appeared to be the most important parameter influencing the early failure of insulating materials. • A strong local vibration of the surrounding molecules to produce phonons in addition to the normal thermal vibration. • With assistance of phonons, a portion of charges is excited to high-energy states, thereby leading to insulation deterioration. Due to the extreme operating conditions of high voltage rising rate, silicone gel materials used in the encapsulation insulation of power electronic devices are prone to insulation failure. In this paper, we proposed a mechanism of insulation degradation under the ultra-fast pulsed electric field by considering the synergistic effect of molecular vibrations and charge behaviors. With the quantum dynamic simulation, it is found that the motion of space charge will induce a strong local vibration of the surrounding molecules to produce phonons in addition to the normal thermal vibration. The steeper the edge time, the higher the amplitude of vibration. With assistance of phonons, a portion of charge is excited to a high-energy state, resulting in an increased probability of induced charge trapping and detrapping. This image well microcosmically confirms the phenomenon of molecular vibration and intensified electric tree formation under ultra-fast pulsed electric fields. [ABSTRACT FROM AUTHOR]

As an important piece of equipment of ultrahigh-voltage direct current, the valve side of a converter transformer contains a large dc component prone to partial discharge (PD) under the action of a slightly uneven electric field, which can be simulated using a wedge electrode. To study the PD characteristics of oil–paper insulation under a wedge-shaped electrode, a 2-D axisymmetric simulation model was established based on the bipolar-charge transport theory and the fluid dynamics drift-diffusion theory. The microscopic discharge images at nanosecond scale under different dc voltage polarities were calculated, and the change process of the space charge and electric field intensity distribution was obtained. The effects of the voltage amplitude, polarity, and dielectric constant on PD characteristics are discussed. The simulation results show that the discharge pattern and velocity exhibit a notable polarity effect. This polarity effect is related to the adsorption of the cellulose hydroxyl group to negative charge, which leads to the amplitude of the homopolar charge accumulated in negative polarity being greater than that accumulated in positive polarity under the same applied voltage intensity so that the homopolar charge has a greater barrier to the discharge. This type of adsorption is positively correlated with the difference of dielectric constant between the paperboard and insulating oil. The results of this study are of great theoretical significance for elucidating the dc discharge mechanism under wedge electrodes. [ABSTRACT FROM AUTHOR]

The space charge dynamic behavior and accumulation are important contributions to the formation of electrical trees, as recognized by cross-linked polyethylene (XLPE) cable insulation under alternating current electric stress. In this article, in order to investigate the reasons for the charge accumulation phenomenon revealed by experiments under ac stress, a bipolar space charge transport model with symmetrical and asymmetrical parameters is built under ac stress. The charge accumulation mechanism is analyzed based on the variation of space charge density with different voltage application time and voltage phase. The effects of each major physical parameter are simulated and analyzed, respectively. The results show that there is no obvious space charge accumulation with the transport model under symmetrical physical parameters. However, asymmetrical parameters in the transport model result in different motions between positive and negative charges, which greatly enhances the charge accumulation. Each main parameter has an influence on charge accumulation in different perspectives and extents. The applicability of the simulation model with asymmetrical parameters is evaluated by comparing experiment and simulation results. This study contributes to the understanding of space charge generation mechanism under ac voltage and serves as a basis for improving the stability of XLPE electric power cables. [ABSTRACT FROM AUTHOR]

The stabilization of cross-linked polyethylene (XLPE) cables is crucial to the safe working of power systems. In order to investigate the aging characteristics of cable insulation in the process of operation, 10 kV XLPE cables were electrically-thermally aged at an AC voltage of 26.1 kV and three temperatures: 103°C, 114°C, and 135°C. Cable samples at five aging stages were cut into slices and tested to determine their mechanical, physicochemical properties and dielectric strength. The changes in these properties were analyzed in terms of aging time. The mechanical strength and oxidation induction time have the same changing trend of decrease, because the chemical bonds of the cable insulation material are fractured when the XLPE cable is aged. The AC space charge presented a trend of gradual accumulation with aging time. The breakdown field strength after AC voltage application decreases monotonically with aging time, which could be a characteristic index to evaluate the degradation degree of the cable insulation. This research contributes to the comprehension of degradation mechanism and discovery of aging assessment criteria, which is of great significance in assessments of insulation properties and fault detection on power cables. [ABSTRACT FROM AUTHOR]

To study the effect of semiconductive (SC) material on space charge accumulation in cable accessories, an accelerated aging of cable samples with cable accessories was carried out in the laboratory. The experimental results show accumulation of space charge in the cable insulation covered by a stress cone, whereas the space charge was negligible in the region wrapped by the accessory-reinforced insulation as well as regions not in contact with the accessory. With increased aging time, the space charge density gradually increased, and the axial distribution gradually extended. A comparative experiment on cable insulation slices was carried out using silicone rubber and EVA SC material as the upper electrodes to determine the cause of the space charge accumulation. The results showed that a large quantity of heteropolar charges accumulated near the silicone rubber SC upper electrode, whereas the charges near the EVA SC electrode were negligible. It was concluded that due to the weak charge conducting ability of the silicone rubber SC material used as the electrode, space charges cannot escape from the stress cone and thus accumulate on the outside of the cable insulation at the stress cone position. This paper shows that the SC material used in cable accessories plays an important role in space charge accumulation. [ABSTRACT FROM AUTHOR]

The DC voltage related partial discharge (PD) classification in solid dielectrics under a uniform electric field has been studied, but it is more complicated in liquid-solid composite insulation under a highly non-uniform electric field. Therefore, PD pulse waveforms and PD channel shapes in oil-pressboard insulation were investigated by simulating a needle-plate model with a positive DC voltage via pulse current method. The elements of PD can be classified in order of ionizing intensity strength as corona, tree-like PD, single-streamer PD, or multi-streamer PD according to the discharge channel shape. The corona pulses are sharply oscillating, whereas the other types of PD can generate unimodal pulses with no over-zero oscillation. A finite element analysis reveals the PD types are related to the charge quantity at the oil-pressboard interface, which influences the streamer evolution by distorting the space electric field. This study uncovers the physical process during PD in oil-pressboard insulation and provides basis for explaining the PD mechanism. [ABSTRACT FROM AUTHOR]

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-?m-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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]