Electrical Tree Initiation Research Articles

Electrical Tree Initiation Prediction in Thermally Aged DC XLPE for Cable Insulation

Electrical Tree Initiation Prediction in Thermally Aged DC XLPE for Cable Insulation

Effect of Space Charge on Electrical Tree Initiation in XLPE/Voltage Stabilizer Composites Under AC Voltage

Effect of Space Charge on Electrical Tree Initiation in XLPE/Voltage Stabilizer Composites Under AC Voltage

Characterization of Electrical Tree Initiation and Growth in XLPE Under Harmonic Waveforms

Characterization of Electrical Tree Initiation and Growth in XLPE Under Harmonic Waveforms

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.

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 Initiation and Growth in LDPE Under Negative HVDC Superimposed With AC Ripples

Understanding the role of power quality in the aging of HVdc cable systems is critical to the reliable connection of offshore renewable energy sources, and hence global ambitions to reduce carbon emissions. This work investigates electrical treeing with a needle-plane geometry in low-density polyethylene (LDPE) under a high negative dc voltage superimposed with ac ripples (−60 kV dc ± 7 kV ac). Tree initiation showed behavior similar to that widely reported under pure power frequencies. Subsequent tree growth, however, was observed to be confined in a smaller area with limited length and width than seen under pure ac voltages, even after long periods of voltage application. Traditional 2-D imaging showed overlapped tree channels which developed to cover the whole area within the tree outline. A distinguishing tree shape resulted, which we have named a “slim bouquet” shape. The volume rendering from X-ray computed tomography (XCT) showed the structure had a 3-D fractal dimension greater than 2, considerably larger than its 2-D representation. PD signals during the growth had wing-like phase-resolved partial discharge (PRPD) clusters and signal phase concentrations between 10°–45° appeared after hours of growth. There was a comb-like appearance in maximum partial discharge (PD) magnitude variation, which is typical in pure high negative dc fields. Consideration is given to both space charge distribution controlled by high dc fields and continuous degradation by ac fields to explain the slim bouquet tree formation.

Retraction Note: Characteristics of electric tree initiation in glass fiber reinforced epoxy resin under power frequency voltage

Retraction Note: Characteristics of electric tree initiation in glass fiber reinforced epoxy resin under power frequency voltage

Electrical Tree Characteristics of Epoxy Resin Under Lightning Impulse at Cryogenic Temperature

As the key cryogenic insulating material for high-temperature superconductivity equipment, epoxy resin works in the harsh environment of strong electric field and extremely cryogenic temperature, and its operational reliability faces severe challenges. In this article, aiming at bisphenol F epoxy resin for cryogenic insulation of super-conducting power equipment, researches on electrical trees in epoxy resin under lightning impulse voltage at room temperature and cryogenic temperature were carried out. The effects of temperature, impulse voltage amplitude, and polarity on the initiation characteristics, growth morphology, growth rate, and expansion coefficient of electrical trees are discussed. The results show that the electrical tree initiation under impulse voltage in a cryogenic environment also has a polarity effect. The voltage is higher than that under negative impulse voltage, the shape of electrical trees is a typical branch, the color is darker, the tree channel is wider, and the branches next to the channel are simpler. There are fewer branches, and many darker spherical or ellipsoidal cracks appear in the electrical branch channel.

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 Characteristics of Bisphenol A Epoxy Resin/Maleopimaric Anhydride Curing System.

Epoxy resin insulation materials are mainly derived from petrochemical materials which have the disadvantages of resource consumption and environmental pollution. In order to cure bisphenol A epoxy resin, a maleopimaric anhydride (MPA) curing agent was prepared from rosin, a renewable resource, and blended with a petroleum-based curing agent (methylhexahy-drophthalic anhydride). The influence of maleopimaric anhydride content on the initiation and growth characteristics of electrical trees was studied and analyzed in this paper using molecular dynamics simulation (MD) and electrical tree tests at an 18-kilovolt power frequency voltage. When the MPA content used was ≤10%, the free volume percentage of the curing system increased with MPA content, and thus the initiation voltage became lower; when the MPA content was ≥20%, the hydrogenated phenanthrene ring structure content increased significantly with increasing MPA content, and the rigidity of the curing system increased significantly; thus, the initiation voltage gradually increased. MPA4 had an 11.11% higher initiation voltage than the petroleum-based control group. The effect of the polar rigid structure within the curing system significantly inhibited the growth rate and length of electrical trees as MPA content increased. Electrical trees developed into light-colored, thin, and narrow dendritic structures when the MPA content reached 40%. The results show that curing epoxy resin with the rosin-based curing agent maleopimaric anhydride (MPA), in place of a petroleum-based curing agent, can produce environmentally friendly resins with excellent electrical tree resistance and potential application prospects.

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.

RETRACTED ARTICLE: Characteristics of electric tree initiation in glass fiber reinforced epoxy resin under power frequency voltage

RETRACTED ARTICLE: Characteristics of electric tree initiation in glass fiber reinforced epoxy resin under power frequency voltage

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.

Research on the discharge characteristics of XLPE insulated DC cables

This paper focuses on the discharge of cross-linked polyethylene (XLPE) insulated DC cables, which has become a concern of power utilities when carrying out a voltage withstand test or an outage maintenance. Based on the electrical tree initiation test of XLPE block samples under grounded DC voltages, the main cause and key factors for damages of DC insulation resulted by grounding discharge were investigated, and the allowable voltage level to start the accelerated discharge operation was estimated. By the laboratory tests of two XLPE cable samples discharged under different conditions, the basic discharge rules were observed, the equivalent circuit was set up, and the equipment parameters were determined. In addition, a method to estimate the discharge time for cable lines with different lengths was proposed. Finally, the on-site measured data of decay voltage and leakage current were presented for a 200 kV DC XLPE cable line which had been discharged steppedly through a resistance grounding device after the DC voltage withstand test, and the experimental results were found to be in good agreement with the theoretical data.

Effects of bipolar repetitive square wave voltage parameters on electrical tree characteristics of epoxy resin

This work investigates the electrical tree growth of epoxy resins. Repetitive bipolar square wave voltage is applied to the epoxy resins and different parameters rise time, frequency, and duty cycle are studied. The frequency range of repetitive square voltage ranges from 50 Hz to 20 kHz, the rise time ranges from 70 ns to 620 ns, and the duty cycle ranges from 5 to 90%. The results reveal that the initiation of the electrical tree is more rapid under the influence of high voltage frequency having a shorter rise time. Moreover, the electrical tree growth is faster under the impact of a shorter rise time with a duty cycle greater than 10%. The rise time mainly affects the initiation process of electrical trees. In comparison, the primary influence on the electrical tree initiation and its growth is attributed to the impact of the frequency applied.

Detection of Electrical Tree Formation in XLPE Insulation through Applying Disturbed DC Waveforms

Initiation and growth of electrical trees can be linked to the development of partial discharge (PD) activity within an insulation system. We present in this paper changes in PD characteristics that appear during the electrical tree growth in a crosslinked polyethylene (XLPE) based insulation at different stages of its degradation. A DC voltage with superimposed high frequency component is applied to the material specimens for the detection and characterization of PD traces observed during the degradation. The method is flexible and offers a possibility to vary DC voltage level independently of the superimposed disturbance, its frequency and rise time, including rapid voltage steps. It also enables to create a phase-locked phase-resolved PD pattern (PRPD) facilitating various analyses of the characteristics. It is postulated that measurements of the changes in PD characteristics found under such excitations can be used for performing diagnostics and prognostics of material condition. This approach also implies new possibilities to evaluate different material concepts.

Effects of Harmonic Component on Electrical Tree in EPDM for HVDC Cable Accessories Insulation

In this paper, the effects of harmonic superimposed DC voltage on electrical tree growth characteristics in ethylene propylene diene monomer (EPDM) for HVDC cable accessories insulation are studied. Harmonic component with a frequency from 50 to 450 Hz is superimposed on DC voltage to analyze the mechanism of harmonic amplitude, order and voltage polarity on electrical tree growth. The results show that the electrical treeing process is promoted with an increase in harmonic amplitude and order, which brings a longer tree length and more accumulated damage to the insulation. Especially when the amplitude of harmonic component exceeds that of DC voltage, the electrical treeing processes are significantly deteriorated, resulting from the large amount of energy released by the recombination of hetero charges. The growth of electrical tree has an obvious polarity effect which tends to be branch-like tree under positive voltage and bush-like tree under negative voltage. The polarity effect may be caused by different propagation processes of electron avalanche under different voltage polarities. As harmonic order increases, the main channel of positive electrical tree is broadened, while the negative tree becomes denser with a lot of bifurcation. These results indicate that harmonic component accelerates the initiation and propagation of electrical tree by affecting the charge transport behavior under the superimposed electric field.

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.