Electrical Tree Inception Research Articles

Mitigation of electrical treeing at high temperature in nano‐SiO2 doped epoxy resin

AbstractIn this work, electrical tree inception and ageing experiments were conducted to investigate the initiation, growth characteristics and structural characteristics of electrical trees of nano‐SiO2 doped epoxy resin at different temperatures. Electrical tree inception and ageing experiments were conducted using power frequency voltage of 15 kV at 20, 40, 60, 80 and 100°C. Electrical breakdown properties of epoxy resin were investigated by applying AC and DC voltages at 20, 40, 80 and 120°C. There are two major observations based on the experimental results. With the increase of temperature, the morphological structure of the electrical trees changes from dendritic to plexiform, the tree inception voltage decreases and the growth rate of the electrical trees increases obviously. Either under DC or AC voltage, the breakdown strength of the nano‐SiO2 doped epoxy resin decreases as the temperature increases. Finally, based on the space charge and the trap level measurement, the mechanism of the electrical tree propagation and breakdown in nano‐SiO2 doped epoxy resin was analysed.

Inhibition of electrical trees degradation of crosslinked polyethylene at high temperatures by electron‐buffering voltage stabilizers

AbstractThe effect of adding electron‐buffering voltage stabilizer (1 wt% m‐aminobenzoic acid) on the electrical tree‐induced degradation of cross‐linked polyethylene (XLPE) was studied at high temperatures. The electrical tree inception, growth, and partial discharge characteristics were simultaneously analyzed at 30, 50, and 70°C, respectively. The results showed that the growth rate of electrical tree and partial discharge activity as well as the discharge repetition rate, significantly increased with the temperature for pure XLPE and XLPE stabilizer blend. However, the XLPE stabilizer blend exhibited higher tree inception voltage, lower tree growth rate, and partial discharge activity than pure XLPE. The addition of the voltage stabilizer reduced the carbon layer thickness in the tree channel and offered higher resistance to the electrical tree growth. Surface potential decay demonstrated that the voltage stabilizer addition decreased the trap energy level, but increased the shallow trap density of the XLPE. Quantum chemical calculations showed that the voltage stabilizer enhanced the ability to buffer the high‐energy electrons in the XLPE. A high‐energy electron buffering mechanism was also proposed to explain the effect of voltage stabilizer on the electrical treeing and partial discharge activity in the XLPE at high temperatures.

Characteristics Analysis of Water Needle Induced Electrical Trees in XLPE

In this paper, electrical trees initiated from water and metal needle are investigated, and the inception and morphological characteristics of water needle electrical trees are analyzed. The existing literature shows that electrical trees can be initiated from the metal needle or transformed from the water trees. But in this study, it is found that the electrical tree can be directly induced by water needle in addition to the above cases. Moreover, the comparative results also indicate that the electrical trees initiated from water needle and metal needle have different inception features and morphological characteristics. To obtain the water needle electrical trees initiation and development mechanism, the water needle tips are observed before and after the sample degas process. Besides, the electric field distribution of the metal needle tip, the water needle tip before and after pretreatment, and of tree branch tip during the electrical tree development are calculated. The observations and calculations results show that before the degas pretreatment, a micro-gas cavity is formed at the water needle tip due to the tension of the water surface. The cavity at the needle tip may cause and support partial discharge (PD), causing accumulative damage and material defects. Then the density of defects becomes sufficiently large, and a tree branch can be formed. After the branch formation, the water flows into the tree branch, and the micro-gas remains at the branch tip, which may facilitate the PD, and trees propagation, resulting in the diversified morphological characteristics of the water needle electrical trees.

Inception and Propagation of Electrical Trees in the Presence of Space Charge in HVAC Extruded Cables

This paper presents the space charge impact on the inception and propagation of electrical trees in cross-linked polyethylene (XLPE) insulation via simulations and experimentation. A 3D finite element analysis (FEA)-based modeling is proposed to simulate electrical trees via a needle embedded on the XLPE insulation. The proposed FEA model demonstrates the influence of the space charge magnitude and polarity on the initiation of partial discharges (PD). Then the critical parameters at the tip of an 8 µm. tree are to be examined. Experimentation is necessary to verify the simulation results acquired and study the mechanism of electrical trees at the inception and propagation stage. Therefore, in the experiment, a needle with a curvature radius of 5 µm. is embedded in the XLPE as a method of simulating defects, such as protrusions, voids, or cracks found in cable insulation. The power source supplies several high AC voltages on the 20 kV (rated voltage) cable under test for observing the inception and propagation behavior of the electrical trees. Phase-resolved partial discharge (PRPD) patterns are extracted at the inception and propagation stages of the electrical tree to determine the severity of the PDs occurring. A relationship is obtained for the PD magnitude with respect to the applied AC voltage while considering the positive and negative polarities of the PDs. This relationship then allows for the acquisition of the maximum electric field-tree length characteristics. The goal of the simulation of electrical trees is to provide the reader with a better understanding about the effect of space charge magnitude and polarity to comprehend the behavior of the treeing inception and propagation mechanisms. The injection and extraction of space charges is a complex phenomenon that requires further visualization through simulations. Therefore, the results of both the simulation and experiment are compared with the aim of establishing a clear relationship between space charge distribution and tree initiation.

Digital image processing for physical basis analysis of electrical failure forecasting in XLPE power cables based on field simulation using finite‐element method

In this research, digital image processing method (DIPM) is used as an innovative approach to predict precisely the shape of electrical tree (ET) in cross-linked polyethylene (XLPE) power cables in the presence of air voids based on the field calculation using finite-element method (FEM). With the help of DIPM, two case studies are held to detect the accurate parameters of either the first initiated major branch or the tips of the major branches of ET. A hyperbolic needle-to-plane simulation model is proposed to illustrate the ET inception and propagation stages. The non-uniform electric fields thatare accompanied with the electrical treeing phenomenon are calculated using FEM as one of the most effective numerical methods to deal with non-uniform shapes. The predicted shapes of ET initiation and growth are provided in an innovative manner with the implemented hybrid connection between FEM and DIPM for the two proposed case studies. Direction branching approach and deviation angle branching approach are provided in this work to predict the shape and the direction of ET branched voids. The validity of the proposed model is assessed with the help of available previous experimental and simulation data.

An Improved Physical-Stochastic Model for Simulating Electrical Tree Propagation in Solid Polymeric Dielectrics.

The dielectric breakdown of solid polymeric materials is due to the inception and propagation of electrical trees inside them. The remaining useful life of the solid dielectrics could be determined using propagation simulations correlated with non-intrusive measurements such as partial discharges (PD). This paper presents a brief review of the different models for simulating electrical tree propagation in solid dielectrics. A novel improved physical-stochastic model is proposed, which allows quantitatively and qualitatively analyzing the electrical tree propagation process in polymeric dielectrics. Simulation results exhibit good agreement with measurements presented in the literature. It is concluded that the model allows adequately predicting the tree propagation behavior and additional experimental analyses are required in order to improve the model accuracy.

Electrical Detection of Degradation in Specimens of HVDC Cable Insulation

One of the challenges in laboratory investigation of degradation and ageing of HVDC cable insulation is related to securing, or in other words, imitating the real service environment of the material specimens. So far, the published data refer to experiments conducted in thermo-oxidative conditions, which is not the case during normal cable operation. In service, the cable insulation is protected by a metallic barrier that blocks the transfer of any substances in and out of the construction. By-products from the cross-linking reactions cannot diffuse out and any foreign substances are blocked from entering the insulation. Thus, in order to generate results that are valid, these conditions must be replicated in laboratory experiments. This contribution presents a methodology elaborated for performing ageing experiments in a hermetically sealed environment. Degradation of the material is evaluated through changes in the electrical tree inception voltage and test object capacitance over time. Securing the environmental isolation is accomplished with an isolation system consisting of a glass enclosure with attached metallic electrodes. Indium is used to create a glass-to-metal seal between the glass and the electrodes. The electrode geometry is of needle–plane type and the needle injection process is semi-automated to ensure specimen repeatability.

Interface Damages of Electrical Insulation in Factory Joints of High Voltage Submarine Cables

As a key accessory of high-voltage (HV) insulated submarine cable, the factory joints of the cross-linked polyethylene (XLPE) represent an unpredictable uncertainty in cable-connecting fabrications by means of the extruded molding joint (EMJ) technique. The electrical breakdown pathways formed at the interfaces between recovery insulation and cable body under alternative current 500 kV voltages are specifically investigated by microstructure characterizations in combination with the electric field and fractal simulations. Dielectric-defected cracks in tens of micrometers in insulation interfaces are identified as the strings of voids, which dominate insulation damages. The abnormal arrangements of XLPE lamellae from scanning electron microscopy (SEM) imply that the structural micro-cracks will be formed under interface stresses. Electrical-tree inception is expedited to a faster propagation due to the poor dielectric property of interface region, manifesting as 30% lower of tree inception voltage. The longer free-paths for accelerating charge carriers in the cracks of interface region will stimulate partial discharging from needle electrodes. The carbonized discharging micro-channels arising in interface region illustrate that the partial discharging will be triggered by the electrical-trees growing preferentially along the defect cracks and could finally develop into insulation damages. The mechanism of forming cracks in the fusion processes between the molten XLPE of cable body and the molten cross-linkable PE of recovery insulation is elucidated, according to which the crack-caused degradation of insulation performance is expected to be alleviated.

Effects of Temperature Gradient on Electrical Tree Growth and Partial Discharge in Silicone Rubber Under AC Voltage

The Joule heat produced by the current in conductor decreases in the radial direction, producing a temperature gradient in the insulation. This existing temperature gradient can affect the electrical tree phenomenon in insulation materials because of the temperature dependence of insulation material, charge migration and tree growth. Different temperature gradients were obtained by controlling the temperature of high voltage (HV) and ground electrode (GD) sides. The electrical tree was recorded by a microscope, while the partial discharge (PD) was observed simultaneously. For better understanding the effects of the temperature gradient on electrical tree inception, the trap distribution in silicone rubber (SIR) with different temperature was analyzed by isothermal discharge current (IDC) method. With the fixed temperature in HV or GD side, it was found that the electrical tree inception voltage decreased linearly with increasing temperature in either side. The IDC results indicate that the increasing temperature contributes to the de-trapping of charges and lowers the inception voltage. Besides, the electrical tree length and accumulated damage gradually increase with the temperature gradient. The observed PD amplitude and quantity also increase with the temperature gradient, which facilitates the growth of electrical trees. These results indicate that the tree growth is promoted with an increase in the temperature gradient of the cable insulation, posing a threat to the safety of the cable system.

Autonomous Self-Healing of Electrical Degradation in Dielectric Polymers Using In Situ Electroluminescence

Summary Dielectric polymers are ubiquitous as electrical insulation in electrical power systems and electronic devices. Electrical treeing is typically regarded as an irreversible degradation process of dielectric polymers subjected to high electric stresses, which significantly limits their lifespan and endangers the reliability of electronics and electrical power systems. Here, we report self-healing polymers consisting of microcapsules that are capable of completely restoring the dielectric properties as a direct response to electrical tree degradation. We design the microcapsules with a relatively higher dielectric constant than polymer matrix to guide the propagation of electrical trees. The electroluminescence generated in situ during electrical treeing of polymers triggers crosslinking of the healing agents released from the capsules to repair the electrically damaged areas under ambient conditions. Notably, the breakdown strength of the mended region is largely enhanced upon healing, which was initiated at different locations. The integration of autonomous healing capability into dielectric materials opens new avenues to smart polymers for electronics and energy applications.

Electrical detection of degradation in specimens of HVDC cable insulation

One of the challenges in laboratory investigation of degradation and ageing of HVDC cable insulation is related to securing, or in other words, imitating the real service environment of the material specimens. So far, the published data refer to experiments conducted in thermo-oxidative conditions, which is not the case during normal cable operation. In reality the cable insulation is protected by a metallic barrier that blocks the transfer of any substances in and out of the construction. By-products from the cross-linking reactions cannot diffuse out and any foreign substances, such as oxygen and water, are blocked from entering into the insulation. Thus, in order to generate results that are valid, these conditions must be replicated in laboratory experiments.This contribution presents a methodology elaborated for performing ageing experiments in a hermetically sealed environment. Degradation of the material is evaluated through measurements of changes in the electrical tree inception voltage and test object capacitance over time. Securing the environmental isolation is primarily accomplished with an isolation system consisting of a glass enclosure with attached metallic electrodes. Indium is used to create a glass-to-metal seal between the glass and the electrodes. The electrode geometry is of needle-plane type and the needle injection process is semi-automated to secure a large degree of repeatability in specimen preparation.

Modelling and simulation of PD characteristics in non-conductive electrical trees

Electric tree growth is a key ageing mechanism leading to breakdown of high voltage electrical insulation. Partial discharges (PDs) are invariably associated with electrical tree inception and propagation. In turn, the physical structure of an electrical tree influences the characteristics of partial discharge activity. Interpretation of PD patterns is therefore central to developing an understanding of the tree propagation process, and also to the use of PD patterns as an asset management tool. Our previous research indicates that the phase resolved PD (PRPD) patterns and pulse sequence analysis (PSA) patterns evolve with tree propagation. A method was proposed to estimate the point-on-wave inception and extinction voltages of PDs in tree channels within each power cycle. It was shown that the evolution of PD patterns is a consequence of changes to PD inception and extinction voltages as a tree develops. This paper provides a deterministic model of partial discharge in tree channels. Simulations of PDs in a straight non-conductive tree channel are based on experimental PD inception, extinction and residual voltages. The quantitative simulations reproduce almost all the characteristics of observed PRPD and PSA patterns. It is concluded that PD events are determined by five key parameters: tree structure, applied voltage, PD inception voltage, PD extinction voltage and PD residual voltage. Key parameters estimated by the method, and the models proposed explain PD activity in non-conductive trees. It is suggested that the PSA and PRPD patterns should be discussed together to fully understand the PD events. This model forms a platform for generating robust information for asset managers using PD measurements from high voltage equipment in service.

Effects of mechanical stretching on electrical treeing characteristics in EPDM

Effects of mechanical stretching, produced after the expansion of cable accessories, on the electrical tree growth characteristics of EPDM (Ethylene Propylene Diene Monomer) are reported in the present work. FFV (fractional free volume) and CED (cohesion energy density), which were respectively related to free path of electrons and molecular chains splitting during electrical tree propagation process, were calculated through molecular dynamics simulations under different stretching ratios. A needle-plane electrode system with the stretching ratio ranging from 0 to 30% was used to investigate the electrical treeing characteristics in the experimental study. The simulation results show that the FFV becomes larger at higher stretching ratio, which enhances the collision between the hot electron and the molecular chain during the treeing process by extending the free path of the hot electron. While CED tends to become smaller at higher stretching ratio, reducing the cohesion strength for the tree channel crack to grow. Moreover, FFV and CED are investigated during the stretching recovery process, where FFV tends to decrease and CED turns to be larger. The experimental results indicate that mechanical stretching affects the electrical tree inception and propagation characteristics. The inception time of electrical tree becomes shorter as stretching ratio increases from 0 to 30%. Tree length and accumulated damage become larger with increasing the stretching ratio. Electrical tree propagation mechanism considering chemical and physical traps, which are generated under mechanical stretching, are also discussed and analyzed.

Compressive stress dependence of electrical tree growth characteristics in EPDM

To ensure a certain interface electrical strength resistance, the cable accessories are densely coated onto the main cable insulation. Compressive stress is generated in the normal direction of high voltage cable by the spring clamp device or the banding force after the expansion of EPDM (Ethylene Propylene Diene Monomer) stress cone. This paper presented a compressive stress experimental study considering electrical treeing propagation in EPDM by employing a needle-plane electrode system. The results indicate that compressive stress affects the electrical tree inception and propagation characteristics. The inception time of electrical tree becomes longer as compressive rate increases from 0 to 30%. Tree length and accumulated damage become smaller with increasing the compressive rate. Electrical tree structure is also depended on compressive stress, and branch tree structure becomes the main electrical tree morphology at higher compressive rate. For better understanding the effects of compressive stress on electrical tree propagation characteristics, FFV (fractional free volume) and CED (cohesion energy density) were calculated and analyzed through molecular dynamics simulations under different compressive rates. Results show that the FFV becomes smaller at higher compressive rate, which shortens the free path of the hot electron and weakens the collision with the molecular chain during the treeing process. While CED tends to be lager under higher compressive rate, which improves the cohesion strength for the crack to grow in the tree channel tips and restrains the propagation of electrical tree.

Analysis of electrical tree inception in silicone gels

This work assesses the initial and crucial part of electrical treeing degradation, the inception stage, focusing on its dependence on applied voltage waveform and frequency. Tests have been performed on needle-plane configuration samples in solids and gels. A physical model has been formulated through an adaptation of an established theory for solids in which electrical tree inception is related to damage-producing injection currents. The voltage rise time appeared to be the most important parameter influencing the tree inception in the gel, while in the solid material the frequency is more relevant. The analysis leads to the conclusion that tree inception in gels is due to a single high-energy event, in contrast to what is commonly known for solids where damage accumulation takes place. A tree inception model is proposed for the gel, in which initiation is driven by a pressure wave generated by the electric field and the space charge injected into the sample. The model fits the experimental data and may be used to predict the tree initiation for different waveforms and voltage values.

Temperature-dependent electrical tree in silicone rubber under repetitive pulse voltage

Silicone rubber (SiR), which is employed as the main insulation materials in the cable accessories, faces the challenge of high temperature during the overload running process. Pulse voltage is inevitable during the switch operation process of the converters or lighting, which aggravates the operation state of the insulation materials. To understand the electrical treeing process at the high temperature with repetitive pulse voltage considering the actual operating state, the needle-plate electrode system was stressed in the experiment. In the test, the ambient temperature (Tamb) was set to 30, 60, 90, 120 and 150 °C, while the pulse frequency was 5, 20, 100, 200 and 1000 Hz. Electrical tree shapes, tree length, fractal dimension (FD), tree inception and tree breakdown probability were studied to investigate electrical tree growth characteristics. The results indicate that the tree shapes are depended not only the temperature environment but also the pulse frequency. Higher pulse frequency and higher temperature facilitate bush tree shape to be generated. With the pulse frequency increasing, the tree length becomes longer, while FD tends to be larger. However, the tree length decreases as the FD increases with the temperature elevating at the same pulse amplitude. As more hot electrons to collide the molecular chain with pulse frequency increasing, the tree inception and breakdown probability become higher. Nevertheless, the tree inception probability tends to be larger as the mechanical property of silicone rubber drops with the temperature elevating, while breakdown probability becomes lower as the shorter tree length grows for a given pulse number.

密度汎関数法を使用した酸化防止剤の電気トリー抑制作用の解析

This paper describes the effect of antioxidants mixed in XLPE on electrical tree inception voltage by using the density functional theory (DFT) approach. It already has been demonstrated experimentally that among the four phenolic antioxidants, 3-methyl-6-tert-butylphenol has the highest tree inception voltage. First, it is shown DFT calculation results which are the energy levels, electric dipole moment and 3D electrostatic potential distributions of antioxidants. We could be confirmed that the electrical tree inception voltage increases with increasing the dipole moment of antioxidants, and the inhibitory effect becomes larger when the phenolic antioxidant contains sulfur. The DFT approach is very useful to consider the mechanism of the relationship between tree inception voltage and antioxidants mixed in XLPE.

AC and DC pre-stressed electrical trees in LDPE and its aluminum oxide nanocomposites

Resistance of pure low density polyethylene (LDPE) and its aluminum oxide nanocomposites (up to 3.0 wt%) to degradation by electrical treeing under AC stress and DC pre-stress is analyzed. The experiments were carried out on wire-plane electrode specimens before and after exposure to thermal and DC electro-thermal ageing at 80 °C. The obtained results showed enhanced resistance of the nanocomposites to electrical tree inception under AC stress and the tree inception voltage (TIV) increased with nanoparticles content. It has been shown that there was an improved partial discharge (PD) resistance in the nanocomposites compared to the unfilled LDPE. The results also showed that the AC TIV in the nanocomposites consistently increased with the ageing and especially the DC electro-thermally aged specimens had about 30% higher the AC TIV as compared to the unaged material. This effect is attributed to significantly reduced mobility of charge carriers in the nanocomposites. The DC pre-stressed electrical trees generated in the investigated materials were of filamentary-branch structure and the branch channels content increases with the addition of nanoparticles. The mean tree number of the DC pre-stressed electrical trees decreased in the LDPE and its nanocomposites while the mean maximum tree length increased with the ageing treatments. It is postulated that material recrystallization and a very high electric field level on the wire electrode during the DC pre-stressed electrical tree test are the main reasons for the observed effects.

Chemical component analysis of electrical treeing in polyethylene by micro-infrared spectroscopy

Conventional infrared spectroscopy is used to investigate the degradation region after electrical treeing in polymers. However, the dimension of the infrared light spot is too large compared with the tree channel, and the position of the light spot is imprecise. Consequently, the micro-infrared spectroscopy is proposed in this paper, which is aim to do chemical analysis for electrical tree area in polyethylene at a micrometer scale. A ramped ac voltage was applied for electrical tree inception in polyethylene, utilizing a wire-plane electrode structure. Slices of samples with electrical trees were then analyzed through micro-infrared spectroscopy. The characteristic group distribution within a selected treeing area was provided through micro-infrared mapping. The experiment results indicated the existence of C=O, C-OH and C-O-C groups, and the content of C-OH and C-O-C groups was more than C=O group. The appearance of C=O group was accompanied with the C-OH group, and the latter absorbance is higher, so it is inferred the C=O group tend to degrade further according to the Norrish type II regime. In addition, not all the electrical tree area had the oxygen containing groups. The material degradation during electrical treeing was discussed with and without oxygen.

Analysis of Electrical Tree Inception and Propagation in XLPE Nano-Composites

In this present work, electrical treeing analysis of the virgin and nano size filled Cross Linked Polyethylene (XLPE) material has been studied. Silicone Oxide (SiO2)nano-filler was used in the XLPE base material. Nano-fillers were added at 1% concentration by weight in the base material. Laboratory experiments were conducted as per IEC procedures at 12 kV AC voltage. Influence of filler concentration on the inception and propagation of electrical treeing were studied. Analysis of time to failure of XLPE materials at 1%nano-filler concentrations were carried out. Tensile strength were measured for understanding the mechanical properties of the nano filled XLPE material, Scanning Electron Microscope (SEM) carried out for understanding the dispersion of nano fillers in the nano filled XLPE material.