Tree Initiation Voltage Research Articles

Degradation mechanism for epoxy resins under combined electric, thermal and compressive stresses

Epoxy resins are widely used in equipment such as ultra-high voltage dry-type bushings, which are subjected to severe electric, thermal, and compressive stresses. Some physical mechanisms have already been proposed to explain the degradations generated by these different stresses, however their effects have not yet been quantified.. The degradation characteristics of epoxy resin under combined stresses of 8 kV/mm, 40 °C-120 °C, and 0–60 MPa were investigated in experiments. The results showed that the degradation characteristics turned significantly with the increase of compressive stress. With the increase in compressive stress, initially the partial discharge initiation voltage, tree initiation voltage and time to breakdown increased, and fractal dimension decreased. While the compressive stress exceeded the turning point, the characteristics were reversed. It can be suggested that opposing mechanisms exist. The free volume and phase field theories dominate at lower and higher stresses, respectively. A novel degradation model of the epoxy resin was proposed based on the theories above. When the compressive stress was low, the reduction of free volume played a dominant role in slowing down the degradation. When the compressive stress was high, the partial energy density concentration accelerated the degradation and played a dominant role. The molecular dynamics and finite element simulation of degradation process stress was carried out and proved consistent with experiments. It confirmed the proposed degradation model reliable and valid.

Electrical tree degradation of MgO/epoxy resin composites at different voltage frequencies

AbstractElectrical tree degradation is one of the main causes of insulation failure in high‐frequency transformers. Electrical tree degradation is studied on pure epoxy resin (EP) and MgO/EP composites at frequencies ranging from 50 Hz to 130 kHz. The results show that the tree initiation voltage of EP decreases, while the growth rate and the expansion coefficient increase with frequency. Moreover, the bubble phenomenon at high frequencies in EP composites is discussed. Combined with trap distribution characteristics within the material, the intrinsic mechanism of epoxy composites to inhibit the growth of the electrical tree at different frequencies is discussed. It can be concluded that more deep traps and blocking effect are introduced by doping nano‐MgO into EP bulks, which can improve the electrical tree resistance performance of EP composites in a wide frequency range.

Improvement of the electrical treeing resistance of polymers through the self-arrangement effect of the voltage stabilizer caused by dielectrophoresis under non-uniform electric field

Improving the electrical treeing resistance of polymer is of great significance for insulation reliability. To achieve this, a solution-based method is employed to introduce a voltage stabilizer with high electron affinity into the silicone rubber (SiR). Experimental findings demonstrate the voltage stabilizer's ability to improve the tree initiation voltage and provide evidence of their migration within the SiR. Additionally, by combining a finite element method, it has been confirmed that voltage stabilizers can migrate through dielectrophoresis in non-uniform electric fields and adaptively reposition themselves in response to changes in electric field distribution during the growth of the electrical tree.

Enhancing Inhibition of Electrical Treeing in XLPE Cables Based on the Targeted Movement of Rejuvenation Fluid Induced by Electric Field

In this study, the inhibition effect of electrical treeing is investigated in a cross-linked polyethylene (XLPE) cable under the targeted movement of rejuvenation fluid induced by an electric field. First, the fluid concentration around the defects is characterized by Fourier transform infrared spectroscopy (FTIR) under different voltages, application durations, and application temperatures. These results show that with increasing applied voltage, application duration and temperature, the fluid concentration around the defects increases nonlinearly. Next, the electrical treeing features of samples with and without electric field induction show that the tree initiation voltage increases and the tree accumulated damage decreases for the samples with electric field induction. By analyzing the changes in the molecular chain structure of the XLPE under different influencing factors, it is found that with applied voltage and duration increase and application temperature decreases, the methyl groups increase and the methylene groups decrease, the diffusion channel of fluid is expanded. Finally, it is proposed that the dielectrophoretic (DEP) force, changes in molecular chain structure, and molecular thermal motion determine the behaviors of the targeted movement of fluid in insulation. The fluid concentration around the defects increases after the targeted movement of fluid, and the electrical treeing inhibition effect is enhanced.

Electrical treeing behaviours in cross‐linked polyethylene cables after thermal ageing

AbstractLong‐term thermal ageing might have impacts on the physicochemical properties of cross‐linked polyethene (XLPE) cables, thereby influencing their electrical behaviours, such as electrical treeing initiation and growth. The main objective of this work is to report the effects of thermal ageing on the electrical treeing behaviours (initiation and growth) in XLPE cable insulation. For this purpose, the electrical treeing initiation and growth test were performed in accelerated thermal samples. In addition, Differential Scanning Calorimetry and Fourier Transform Infrared Spectroscopy were utilised to investigate corresponding changes in the microstructures of materials. In addition, a correlation was established between electrical treeing behaviours and physicochemical properties during the thermal ageing process. Test results indicate that the thermal ageing process should be divided into three phases, and the re‐crystallisation reaction is considered the primary reason for the increase of average electrical tree initiation voltage (AETIV). Furthermore, the thermal ageing index Xc is correlated to AETIV and the propagation rate of the electrical tree. As equivalent relationships have been established between the relative decrease value w of indexes AETIV, the propagating rate and Xc, this method can be employed to predict the changing pattern of electrical treeing initiation voltage and the propagating rate in future research.

Improved electrical treeing properties of silicone rubber at high temperatures by grafting aromatic hydrocarbon voltage stabilizer

Since the electrical tree initiation voltage of the silicone rubber decreases significantly at high temperatures, it is significant to improve the electrical treeing resistance of silicone rubber at high temperatures for its safe and stable operation as an insulating material. In this work, a voltage stabilizer named the 2-Allyl-4,6-dibenzoyl resorcinol was grafted onto the silicone rubber. Then, the electrical treeing properties of the silicone rubber grafted with a voltage stabilizer were measured at different temperatures. The results showed that the electrical tree initiation voltage increased by 14.5%, 42.5%, and 46.0% than that of unmodified silicone rubber at 30 °C, 50 °C, and 70 °C, respectively. Besides, the propagation structure of the electrical tree was prone to more branches. The space charge distribution exhibited that the silicone rubber grafted with the voltage stabilizer has fewer space charge injections. The trap density result demonstrated that the grafting of the voltage stabilizer significantly reduced the density of shallow traps inside the silicone rubber and increased the energy level and density of deep traps. The improved performance at high temperatures is due to the grafted rigid structure-benzene rings in the silicone rubber, reducing the degree of segment relaxation of the material at high temperatures.

Electrical Tree Evolution of BN Sheet/Epoxy Resin Composites at High Voltage Frequencies

Electrical tree aging problem under high-frequency voltages seriously affects the reliability of packaging insulation for power electronics modules. In this article, boron nitride (BN) sheet/epoxy resin (ER) composites with low doping concentration are prepared and characterized. Varied voltage frequencies (50 Hz–130 kHz) are selected, and electrical tree characteristics of composites at different voltage frequencies are studied. Results show that as the frequency increases from 50 Hz to 130 kHz, electrical tree initiation voltage of pure ER decreases by 39.8%. However, as the frequency increases, tree channels first become denser, and then, the tree branches decrease. Moreover, electrical tree shape presents sturdy single-branch-like tree at 130 kHz. By introducing BN sheet, the tree initiation voltage increases, especially at higher frequencies. The tree scale of BN sheet/ER is smaller than that of pure ER at all frequencies. Combining the dielectric properties and tree results of different materials, the influence of frequency on tree characteristics of ER is discussed, and the role of BN sheet in the bulk is revealed.

Electrical tree evolution of two-dimensional BN sheet composite epoxy resin with different particle size

The packaging insulation of power electronic devices works in high-frequency, high-voltage, and high-temperature environments, which suffers from electrical tree aging and seriously restricts the reliable operation of power electronic devices. In this paper, WZ breakdown model is used and the tree growing and fractal dimension characteristics with different sheets size are simulated. In addition, electrical tree initiation and growing characteristics of BN/epoxy resin composites are observed and studied under high-frequency voltage. Results show that with the increasing sheet size, tree initiation voltage increases first and then decreases. From the simulation and experimental results, after adding two-dimensional sheet, electrical tree tends to be more bifurcated, and as the sheet size increases, the growing rate decreases, the fractal dimension of tree increase correspondingly. It is believed that by introducing BN sheet into the matrix, the barrier effects and the forming deep trap both make electrical tree difficult to initiate and grow. The above researches could provide a solution to the regulation of electrical tree aging performance in the insulation layer of power electronic devices.

Influence of DBD treated nano‐Al2O3 on thermal and electrical insulation properties of epoxy nanocomposites at high temperatures

AbstractThis paper presents the effect of addition of dielectric barrier discharge (DBD) plasma‐treated nano‐Al2O3 particles on electrical treeing characteristics of epoxy resin (EP) at 30, 60, and 90°C, respectively. The electrical treeing and simultaneous partial discharge measurements, surface potential decay, scanning electron microscope, Fourier transform infrared spectrograms and differential scanning calorimetry are adopted to analyze the influence of plasma‐treated nano‐Al2O3 particles on the different insulation properties of EP and its composites. The results show that the high temperature adversely affects the electrical tree initiation, growth rate, and discharge activities of the EP nanocomposites. The DBD‐treated nano‐Al2O3 particles increase the electrical tree initiation voltage of the EP nanocomposites compared to the nanocomposite with the untreated nano‐Al2O3 particles. In contrast, the electrical tree growth rate decreases in the EP nanocomposite with the DBD treated nano‐Al2O3 particles at high temperatures. It is found that DBD treated nanoparticles affect the trap density and energy level of the composites by generating new chemical bonds, and then change their electrical properties. A charge carrier transport model is proposed to explain the obtained experimental results.

Space charge evolution and its effects on electrical tree degradation in silicone rubber at varied temperature under polarity reversal voltage

Electrical tree is one of the main reasons for cable failure. In this paper, the space charge evolution and electrical tree characteristics under polarity reversal voltage with varied temperature in silicone rubber (SIR) are carried out. Results show that as temperature rises from 30 °C to 120 °C, electrical tree initiation voltage declines by 17.8%. Meanwhile, the trees become denser, and the tree length is larger correspondingly. In order to ascertain the charge transportation and its influence on tree initiation process under polarity reversal voltage, we build a bipolar carrier transportation model with needle-plate electrode. Space charge transportation characteristics and corresponding electric field are simulated. Moreover, the evolution model of electrical tree under polarity reversal voltage is discussed. The presence of homo space charge leads electric field to become larger during the voltage reversal process. The segment relaxation of SIR and more, deeper space charge injection make tree initiate more easily at higher temperature.

Grafted UV absorber as voltage stabilizer against electrical degradation and breakdown in cross-linked polyethylene for high voltage cable insulation

Voltage stabilizers are widely used to improve the resistance to electrical treeing degradation and breakdown of cross-linked polyethylene (XLPE) insulating materials. However, the effects of voltage stabilizers under high temperatures are not fully investigated by now, and the long-term effect of voltage stabilizers is still a concern due to its poor compatibility with polymer. In this work, the UV absorber 4-allyloxy-2-hydroxybenzophenone (AOHBP) which contains an unsaturated graftable vinyl group was demonstrated as a high-efficiency voltage stabilizer and compared with an ungraftable UV absorber as reference. Characterization by Fourier transform infrared (FTIR) spectra and differential scanning calorimetry (DSC) indicated that the AOHBP could be grafted onto the macromolecules of XLPE through radical addition reaction on the vinyl during peroxide cross-linking process. The test results of electrical tree initiation, electrical tree propagation and breakdown strength indicated that the grafted voltage stabilizer AOHBP exhibited higher efficiency than the ungrafted UV absorber and could comprehensively improve the electrical tree initiation voltage, inhibit the electrical tree propagation and improve the breakdown strength of XLPE at different temperatures. Through the electrical aging test with the longest aging time of up to 6000h, the AOHBP was confirmed to still maintain the initial effect on improving the breakdown strength after aging, which indicated that the migration and precipitation of AOHBP was restrained by the grafting reaction. The mechanism of AOHBP improving the electrical properties by scavenging high-energy electrons was analyzed by theoretical calculation based on density functional theory (DFT). It was proposed that the graftable AOHBP is a promising voltage stabilizer.

Charge transport dynamics and the effects on electrical tree degradation under DC voltages in thermally aged silicone rubber

Electrical tree is one of the major issues for insulation failure in high voltage power cables. In this paper, we investigate electrical tree degradation under DC voltages in thermally aged silicone rubber (SIR). Charge transport dynamics are revealed by means of space charge measurement and simulation to give a better insight into the electrical tree initiation and evolution. The results show that a thermal oxygen reaction enhances the SIR cross-linked networking and thus gives rise to a higher tree initiation voltage in the early stage of aging. Subsequently intensified thermal degradation and cracking reaction accelerate SIR cross-linked network breakdown and generate micro-cracks. The increased deep traps and significantly decreased charge carrier mobility after thermal aging result in greater homo charge accumulating near the needle tip. The mechanism of DC electrical tree initiation in SIR is clarified based on the trap theory. It is argued that more trapped charges in the low-density region near the tip in the late thermally aged stage would decrease the tree initiation voltage, while the resultant homo charges would in turn reduce the electric field around the tip. Therefore, after thermal aging, the reduction of tree initiation voltage under DC voltage is less than that under AC voltage.

Delocalized aromatic molecules with matched electron‐donating and electron‐withdrawing groups enhancing insulating performance of polyethylene blends

AbstractSeven delocalized aromatic molecules with electron‐donating and electron‐withdrawing groups are applied as voltage stabilizers to improve the insulation properties of polyethylene blends. Voltage stabilizers 1 wt% are added into the blends (90 wt% low‐density polyethylene and 10 wt% high density polyethylene) by diffusion loading method. Electrical measurements including electrical treeing, space charge distribution, and direct current conductivity are conducted to disclose their effects. The results show that the co‐existence of matched electron‐donating and electron‐withdrawing groups in the molecules is favorable for the insulation properties. A 50% increase of tree initiation voltage is achieved with the addition of 3‐aminobenzoic acid, which is also able to inhibit the space charge and decrease the conductivity at lower temperature. Besides, the grafting sample of the optimal molecule is successfully prepared and displayed enhanced electrical properties. Finally, the common internal mechanism of the delocalized molecules on different electrical properties is revealed.

Effect of silicone grease on electrical tree degradation of silicon rubber under AC electric field

Silicone grease (SG) is usually applied on the interface between the insulation of cable and cable joint to enhance the sealing of interface. However, in long-term operating process, SG will gradually diffuse into silicone rubber (SiR) matrix and cause the electrical performance degradation of SiR, which can even result in the failure of cable joints. The commercial HTV SiR with functional silica filler was used in this study, and the electrical tree characteristics SiR samples with different SG coating times were investigated. The crosslinking density and trap distribution were analyzed to explain the transformation of electrical tree with different coating times. It was found that the electrical tree initiation voltage dropped from 13.35 to 10.46 kV and the fractal dimension increased from 1.25 to 1.52, while the electrical tree length showed an almost constant trend. The morphology of the electrical trees gradually changed from branch-type to bush-type. The decline of crosslinking density from 1.92 × 10−4 to 1.76 × 10−4 mol/g is attributed to the breaking of crosslinking structure between silica and SiR matrix, which can lead to the expanding free volume and decreased deep trap density. More high-energy electrons can be generated, which aggravates the degradation of molecular chains and further results in the decreased electrical tree initiation voltage. In addition, the denser morphology of electrical tree is considered to be caused by the more growth direction for electrical tree provided by the expanding free volume.

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.

Enhancement of insulating properties of polyethylene blends by delocalization type voltage stabilizers

This paper reports on the effects of four voltage stabilizers, m-aminophenylboric acid, 2-methoxy-5-pyridineboric acid, m-aminobenzoic acid and 4-dimethylaminobenzoic acid, on the insulation properties of polyethylene blends. 1 wt% voltage stabilizers were added into low density polyethylene (LDPE) blend containing 10 wt% high density polyethylene (HDPE) by a solution method. Physicochemical and electrical properties of the blends were analyzed by infrared spectroscopy, thermo-gravimetric analysis, differential scanning calorimetry, electric tree initiation tests, space charge and surface potential decay measurements. The results showed that the addition of the voltage stabilizers had a little effect on the thermal stability and melting properties of the LDPE/HDPE blend. When the voltage stabilizers were included into the blends, the tree initiation voltage (TIV) increased, in particular the blend with m-aminobenzoic acid showed the highest increase of 41% compared to the reference LDPE/HDPE. It was found that space charge accumulations of the blends were apparently suppressed by adding voltage stabilizers. Moreover, the blend with the m-aminobenzoic acid had the least space charge accumulation. The surface potential decay rate of the blends was significantly increased after adding voltage stabilizers. Quantum chemical calculations illustrated that the HOMO-LUMO energy gap was closely related to the TIV of the blends, whereas the m-aminobenzoic acid has the lowest energy gap level and the best effect of restraining the electrical tree initiation. The trap level calculation showed that the trap depth in the blends decreased with the addition of the voltage stabilizers, which was beneficial to suppress the accumulation of space charge in the materials.

Clarification of the optimum silica nanofiller amount for electrical treeing resistance

This paper aims to clarify the optimum amount of fumed silica (SiO2) nanofiller in resisting the initiation and propagation of electrical treeing in silicone rubber (SiR). Unlike other works, SiR/SiO2 nanocomposites containing seven different weight percentages of SiO2 nanofiller were prepared for this purpose. To achieve the objective, the electrical tree characteristics of the SiR/SiO2 nanocomposites were investigated by comparing the tree initiation voltage, tree breakdown time, tree propagation length and tree growth rate with its equivalent unfilled SiR. Moreover, the structural and morphological analyses were conducted on the SiR/SiO2 nanocomposite samples. The results showed that the SiR, when added with an appropriate amount of SiO2 nanofiller, could result in an improved electrical tree resistance. It implies that the 5 wt% of silica is the optimum amount to achieve the optimal electrical tree resistance such that above 5 wt%, the tree resistance performance has been abruptly reduced, subjected to the agglomeration issue.

Evaluation of voltage endurance characteristics for new and aged XLPE cable insulation by electrical treeing test

The voltage endurance coefficient of solid insulating materials can be determined by electrical treeing tests. The measured value is influenced by the test conditions, and its effectiveness remains to be further demonstrated. In this study, samples were made from the XLPE insulation of three 110 kV AC cables, two new and one operated for 16 years. In treeing tests performed with a pin-plane electrode system, the tree initiation times were measured under both progressive and constant voltage, and voltage endurance coefficients were determined by the residual voltage method. The material properties of XLPE were measured using the FTIR, DSC and TSC methods. For the same sample, the voltage endurance coefficient measured with a 10 μm needle electrode was larger than that measured with a 5 μm electrode, indicating the influence of space charges on the electrical field around the needle tip. For the insulation samples of the two new cables, the measured voltage endurance coefficients were different, and the difference became more significant when the rise rate of the progressive voltage decreased. The superior resistance to electrical aging of the insulation with a higher voltage endurance coefficient was confirmed via material property measurements. For the insulation samples of the aged cable, along with a decrease of the progressive voltage rise rate, the tree initiation voltage increased, which was opposite to the test results obtained for the new cable insulation samples. The voltage endurance coefficient could not be measured using the residual voltage method because the trees always initiated at the rising edge of the constant voltage, except for a low value measured at the lowest constant voltage. The abnormal performance of the aged cable insulation might be the result of increased defects and impurities caused by operation aging, which enhanced the space charge effect, especially in the pin-plane electrical field of the electrical treeing test.

Use of Grafted Voltage Stabilizer to Enhance Dielectric Strength of Cross-Linked Polyethylene.

Aromatic voltage stabilizers can improve the dielectric properties of cross-linked polyethylene (XLPE); however, their poor compatibility with XLPE hinders their practical application. Improving the compatibility of aromatic voltage stabilizers with XLPE has, therefore, become a new research goal. Herein 1-(4-vinyloxy)phenylethenone (VPE) was prepared and characterized. It can be grafted onto polyethylene molecules during the cross-linking processes to promote stability of the aromatic voltage stabilizers in XLPE. Fourier transform infrared spectroscopy confirmed that VPE was successfully grafted onto XLPE, and effectively inhibited thermal migration. Thermogravimetric analysis showed that the grafted VPE/XLPE composite exhibits a better thermal stability than a VPE/PE blend composite. Evaluation of the electrical properties showed that the breakdown strength and electrical tree initiation voltage of the VPE/XLPE composite were increased by 15.5% and 39.6%, respectively, when compared to those of bare XLPE. After thermal aging, the breakdown strength and electrical tree initiation voltage of the VPE/XLPE composite were increased by 9.4% and 25.8%, respectively, in comparison to those of bare XLPE, which indicates that the grafted voltage stabilizer can effectively inhibit its migration and enhance the stability of the composite material.

DC electrical tree initiation in silicone rubber under temperature gradient

In this paper, initiation behaviors of DC electrical tree in silicone rubber were studied under different voltage polarities and temperature gradients (temperature of needle and ground electrodes ranged from 20 to 120 °C). Electrical tree initiation probability, tree shape, and tree length were obtained via the statistics. The results show that polarity effects, where the positive DC voltage is easier for trees to initiate, appear evidently under various temperature situations. As for the temperature gradient, tree initiation voltage decreases first and then returns to a larger value with the increasing needle temperature when the ground electrode temperature is fixed at 20 °C. Both the probability of multiple-branch tree and corresponding tree length increase and subsequently decrease. However, tree initiation voltage decreases alone and tree length increases with the rise of ground electrode temperature when the needle temperature is fixed at 20 °C. Homo charge injection becomes easier with increasing needle temperature following the Schottky-Richardson approximation under ramped DC voltages. In addition, electric field strength at the tip decreases when the needle temperature is higher than ground temperature, since the conductivity is sensitive to the local temperature and electric field. It is believed that the needle temperature and the temperature gradient dominate tree initiation behaviors, where the former affects the collisional ionization and partial relaxation of silicone rubber chains, and the latter decides electric field distribution in the bulk.