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The space charge dynamic behavior and accumulation are important contributions to the formation of electrical trees, as recognized by crosslinked polyethylene (XLPE) cable insulation under alternating current electric stress. In this paper, 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.

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.

The converter transformer is the most important piece of equipment in ultrahigh voltage direct current (UHVDC) transmission systems. The rectifying valve side of the converter transformer is dealing with composite voltages, containing both ac and dc components. Therefore, the characteristics of the partial discharge (PD) in oil–paper insulation of such rectifying valves are different from those of rectifying valves subjected to pure ac or dc voltage. In this study, theories of hydrodynamic drift-diffusion and bipolar charge transfer were used to build the needle-plate electrode model with a paperboard depression, and experiments were used to verify the feasibility of the simulation method. The electric field and net charge distribution that can reflect the PD were analyzed. Besides, the effects of instantaneous voltage, the proportion of ac–dc components, and depression parameters on the development characteristics of PD were also discussed. Simulation results show that applied voltage, interface charge density, and model parameters can all affect the electric field strength, which in turn influences the development of PD in the depression. This research reveals the process and influencing factors of PD during the electrical aging process, which not only helps to optimize the oil–paper insulation at the rectifying valve side of the converter transformer but also provides a basis for further explaining the PD mechanism.

Due to the complicated structure composed of different kinds of materials, space charges tend to accumulate within cable accessories, which easily leads to the electrical breakdown. To obtain insights into the space charge accumulation in cable accessories, accelerated aging was performed for 10 kV crosslinked polyethylene (XLPE) insulated cable samples with stress cones, under an AC voltage of 26.1 kV and temperatures of 103 °C and 114 °C. The space charge distribution in the cable insulation was tested at different aging stages. The results show that space charge accumulates in the insulation covered by the stress cones, and the charge density and coaxial distribution region increase during aging. Together with the results for the charge measurement on peeled cable samples, it is concluded that charge accumulation is related to the semiconductor used in the stress cones. It is difficult for the space charge generated in the cable insulation to escape through the silicone rubber-based (SR-based) semiconductor in the stress cones, as the ability of this material to conduct charges is lower than that of a cable-shielding ethylene–vinyl acetate-based (EVA-based) semiconductor; therefore, the space charge accumulates near the outer part of the insulation. This research provides insights into the important function of semiconductor on charge accumulation in cable accessories, which will aid in research on optimizing and prolonging the lifetime of cable accessories.

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.

High‐voltage electronic equipment is widely used in power systems, which normally operate under the pulse voltage condition. However, compared to DC or AC stress, the electrical insulation suffers a more severe ageing process under a pulse voltage with steep edges and so may cause electrical equipments to fail early. The charge behaviour in polyimide under the pulse voltage with different edge times was investigated in this study. The experimental results show that charges vibrate at the pulse edge, while the amplitude of charge vibration increases with decreasing edge times. Furthermore, the detected vibration signal is strong within the nanosecond edge times, while being negligible in the microsecond range. Notably, the amplitude of charge vibration at the falling edge is larger than that at the rising edge in the presence of the column‐plate electrode. It is believed that the resultant force acting on the charge increases drastically at the pulse edges, leading to the charge movement and the generation of ultrasonic waves, with the resultant force becoming larger when the edge times decrease. Charge vibration behaviour is harmful to the microstructure of insulation materials, and so it may be concluded that the rise and fall times of the pulse voltage play an important role in insulation failure.

The converter transformer is a kernel equipment in ultra-high voltage direct current(UHVDC), and some partial discharge(PD) occurs under wedge-shaped electrode of the converter transformer. For researching PD mechanism of oilpaper insulation at wedged electrode, a model based on the fluid dynamics theory and the bipolar charge transport theory was built. Microscopic images including PD and charge distribution obtained by this simulation were analyzed. Besides, the influence of parameters such as voltage amplitude and charge mobility on PD characteristics were discussed. Through the simulation, we can get the conclusion that the partial discharge process under DC stress at the wedged electrode can be broadly classified into the following parts: ionization and polarization in oil, the surface discharge at the oil-paper interface, the deep discharge in pressboard, and surface flashover. During the discharge process, the voltage plays a facilitating role in the discharge. And the effect of charge mobility especially of pressboard on discharge is twofold, the flashover voltage reaches the maximum when the electron and hole mobility is 10-13 m2V−1s−1, when the longitudinal trend of discharge is the strongest. The longitudinal trend inhibits the development of transverse discharge and increases the flashover voltage. The results of this paper have theoretical value for studying the partial discharge mechanism of converter transformer.

Stepped Controlled Shunt Reactors (SCSRs) have advantages of flexible control and reliable compensation and are likely to become widely used in Ultra-High-Voltage/Extra-High-Voltage (UHV/EHV) transmission systems. Very fast transients that occur in such systems will generate a Very Fast Transient Overvoltage (VFTO) in the SCSR. This may cause damage to the SCSR and lead to system losses. In this paper, the electric field strength and magnetic induction intensity are taken as the basic physical variables when these transients occur. Maxwell’s equations are used to separate the constraint sources, such as the conduction current, magnetization current, and polarization current, from the field source. Then, an all-current Partial Element Equivalent Circuit (PEEC) model for these component elements that considers the surface charge is established. Fast and accurate calculations of the inductance parameters, which involve multiple integrals, are performed using geometrical mean distances and a complex component segmentation method. The equivalent structures and circuit equations of the SCSR windings are presented, and the voltage distribution of the SCSR windings under VFTOs of different frequencies is obtained. The results show that, the voltage waveform in the SCSR windings more rapidly attenuates as the VFTO frequency increases. The peak value of the oscillating voltage in the high-voltage winding of the SCSR is higher than that in the low-voltage winding.

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.

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.

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.

The dynamic behavior of space charges is one of the potential factors that cause electrical aging. In this paper, a bipolar space charge transport model was established under an AC electric field in cross-linked polyethylene. The effects of physical parameters on the dynamic behavior of space charges in an AC electric field were explored by changing the injection barrier heights, mobilities, trapping coefficients, and detrapping barrier heights of positive and negative charges. The effects of voltage conditions on space charge behavior were explored by changing the amplitude and frequency. The effects of temperature were studied by setting a temperature gradient similar to that of real cable operation. The asymmetry of the physical parameters could seriously aggravate the accumulation of space charges in the AC field. A set of physical parameters based on these results was identified. Both the increase in voltage amplitude and decrease in frequency enhanced the amount of and penetration into the insulation of the accumulated space charge. The dynamic behaviors of the space charges were more active with increased voltage amplitude and frequency. The temperature gradient in the cable insulation implies that the degree of accumulation, penetration into material, and activity of dynamic behaviors of the space charges at the warmest electrode are greater than those at the coldest. Compared with the DC field, the dynamic behavior of space charges in the AC electric field is more active and releases more energy. This research provides a theoretical basis for inhibiting the accumulation of space charges in an AC electric field.

In order to study the characteristics and development process of partial discharge at the wedge-shaped oil gaps in oil-paper insulation system of converter transformers, in this paper, a simulation under DC voltage based on space charge models such as hydrodynamic drift-diffusion model and bipolar charge transport model was built. By analyzing the simulation results and characteristics of discharge, the discharge process of wedge-shaped oil gaps was divided into three stages.

Under lightning and operating overvoltage, cable transmission lines are more prone to occur insulation degradation and breakdown resulting in power interruption. To study the mechanism of insulation breakdown under pulse voltage caused by overvoltage, an experiment about the dynamic characteristic of space charge at rising and falling edges in cross-linked polyethylene under DC-superimposed pulsed voltage was conducted. The experiment result shows that the change of space charge density at the rising and falling edges is opposite to the change of voltage amplitude: the space charge density decreases when the voltage amplitude increases, and increases when the voltage amplitude decreases. An analysis method from the perspective of forces acting on the space charge is proposed to explain this unusual phenomenon. The suddenly changed electric field at the rising and falling edges breaks the balance of force acting on the space charge, resulting injection and extraction of charge. The dynamic behavior of space charge is accompanied by the release of energy, which will destroy the microstructure of insulation material and result breakdown eventually. This study is of great significant to help reveal the essence of insulation failure and improve the performance of insulation materials.

International audience; When HVDC cables are subjected to a 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 pulse 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.

The dynamic behavior of the space charge in the crosslinked polyethylene insulation is one of the important reasons for the initiation of the aging of electrical branches in ac cables. In this paper, a bipolar charge model is established to numerically simulate the dynamic behavior of space charge in each AC cycle and the accumulation of space charge under long-term stress. Space charge under AC stress can be divided into two types: periodic charge and aperiodic charge. Space charge density and electric field are influenced by each other. It is found that the periodic charge at the electrode interface changes with the polarity and intensity of the electric field, and the accumulation rate is relatively slow. The aperiodic charge has a multi-period accumulation effect, which causes a certain degree of distortion of the electric field near the two electrodes. Compared with periodic charge, aperiodic charge has a more significant effect on cable insulation performance.

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.

Converter transformers are the core device in ultrahigh voltage direct current (UHVDC) transmission systems, and the oil-paper insulation at its rectifying valve side deals with the composite AC-DC voltage. A detailed investigation of the partial discharge (PD) mechanism in the oil-paper insulation under the composite AC-DC voltage is, therefore, very important. The oilpaper insulated needle plate electrode simulation model for the electric field and charge distribution can be established using hydrodynamic drift-diffusion theory and bipolar charge carrier theory, and the simulation results can reflect the morphological changes of PD. The development characteristics of PD under composite AC-DC voltage are researched. In addition, the effects of simulation time, the AC-DC component proportion, and model parameters on PD are also explored. Results indicate that differences in applied voltage, interface charge, and geometry structure can cause changes in the electric field strength, which consequently influences the PD process in the simulation model. Oil-immersed paperboard is a type of liquid-immersed dielectric (LID). When the LID is arranged in perpendicular, only the positive streamers have narrow channels oriented toward the LID. When the LID is arranged in parallel, the PDs are in the form of creeping discharges and the negative streamers are more dispersed. The development characteristics of PD and its influencing factors reported here may provide theoretical and simulation support to explain the physical mechanism of PD at the rectifying valve side of the converter transformer.

The application of hybrid reactive power compensation (HRPC) in ultra-high-voltage substations has a nonnegligible impact on the characteristics of transient ground potential rise (TGPR) in the gas-insulated switchgear (GIS) enclosure. In this paper, according to the principle of series compensation and stepped controlled shunt reactors under a very fast transient overvoltage, a simplified model of an HRPC device is established. Based on the generation mechanism of TGPR and the theory of transmission lines, the equivalent simulation circuit of GIS in a substation is designed under the occurrence of TGPR. Simulation experiments comparing the characteristics of TGPR with and without HRPC are conducted to explore the factors that influence TGPR. This paper proposes specific methods of reducing TGPR, and provides a theoretical basis for the construction of GIS protection devices for ultra-high-voltage substations.

Safety use of hydrogen leads to the development of highly sensitive materials for hydrogen sensing. Herein, Pd(II)@alkyne-PVA/d-Ti3C2Tx composite was prepared by spin-coating Pd(II)@alkyne-PVA solution onto the filter membrane supported with delaminated Ti3C2Tx (d-Ti3C2Tx). The properties of the composite were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results demonstrate that the composite shows high sensitivity and stability for hydrogen sensing.

Insulating paper is a traditional insulation material used for transformer insulation. Transformer development is not only limited to small sizes and large capacities, but also limited to insulation life as insulating paper cannot withstand high temperatures. Therefore, recent studies have focused on improving the performance of insulation paper and discovering better insulation materials. In this study, two types of polymeric materials, polycarbonate (PC) and polyester film (PET), were chosen for comparative analysis. In order to test whether these two materials could be used in oil-immersed transformers, the PC and PET were placed in transformer oil for thermal aging at 110°C and 130°C, respectively. The thermal cracking processes and fragmentation mechanisms of the PC, PET, and insulation paper were analyzed using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and atomic force microscopy (AFM), as well as their degrees of polymerization and surface morphologies. According to the test results, the initial thermal decomposition temperature of PC and PET are higher than insulation paper and exhibited a better thermal resistance. PC and PET have the potential to substitute insulation paper for large capacity power transformer.

To study the effect of semi-conducting(SC) material on space charge accumulation in cable accessory, cable electrical-thermal accelerated aging with cable accessories was carried out in laboratory. The experiment results show that a large amount of space charge accumulated in the cable insulation area covered by the stress cone, while there was no space charge in other areas. To explain the reason, the comparative experiments of Cross-linked polyethylene(XLPE) slices of cable insulation were carried out using silicone rubber SC material and ethylene-vinyl acetate copolymer(EVA) SC material as upper electrode respectively. The results show that a small amount of heteropolar charge accumulated near the EVA SC upper electrode, while a large amount of heteropolar charge accumulated near the silicone rubber SC upper electrode. It is concluded that due to the low conductivity and high contact barrier between XLPE and silicone rubber SC material, the ability of conducting sparce charge is weak. Space charge cannot escape from the stress cone SC electrode and accumulate on the outside of the cable insulation at stress cone. The research work shows that the SC material of cable accessory plays a key role in the space charge accumulation. It is necessary to carry out the next work and study how to select materials to suppress the space charge accumulation at cable stress cone. The research work has important theoretical guiding significance for the improvement of manufacturing technology and voltage level of HVDC cable.

Hybrid reactive power compensation (HRPC) combines step-controlled shunt reactors and series compensation, and will be employed in ultra-high-voltage (UHV) power systems. The single-phase auto-reclosure characteristics of secondary arcs in systems with HRPC require further investigation. In this paper, both the arc-recalling voltage and subsidiary variations in arc current are investigated with and without HRPC. The frequency components of the secondary arc current and variations in arcing time are analyzed for various influential factors, such as the neutral reactor, arc resistance, fault location, degrees of compensation of HRPC, and the length of the transmission line. The non-dominated sorting genetic algorithm II (NSGA-II) and support vector machine regression are combined to create a multi-variable dependent forecasting algorithm to predict the characteristics of the secondary arc in UHV systems with HRPC. This paper provides a theoretical reference for optimizing the parameters of HRPC, and for developing adaptive auto-reclosure schemes and protection equipment.

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.

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.

The shortcomings of oil-immersed transformer solid insulation like low high-temperature resistance of the insulation paper and the uneven distribution of electric field caused by the large difference of dielectric constants between insulation paper and transformer oil, restricted the development of transformer with smaller size and larger capacity. In view of this situation and the transformers demand for the dielectric physicochemical and mechanical property of insulation material, polycarbonate, polyester film and polyphenylene sulfide with high high-temperature resistance were chosen for the comparison analysis in this paper We did the 300-day thermal aging experiment on four samples in transformer oil under different aging temperatures of 90℃, 110℃ and 130℃, then analyzed the changes of their dielectric properties. The experiment results are as follows; the permittivity and dielectric dissipation factor of the three polymer materials are apparently smaller than those of the paper both before and after 300-day aging; the volume resistivity and partial discharge inception voltage of the three materials are higher than those of the paper both before and after aging; and the breakdown field strength of polycarbonate is higher than that of the paper both before and after aging, while that of polyester film and polyphenylene sulfide are slightly lower than that of the paper.