Electrical Tree Characteristics Research Articles

Electrical tree characteristics of XLPE under repetitive pulse voltage in low temperature

Cross-linked polyethylene (XLPE) may be operated under repetitive pulse voltage and low temperature environment. In recent years, many influencing factors to the treeing process, which is a main reason for the aging of XLPE have been researched. However, the tree behavior under repetitive pulse voltage and low temperature in XLPE is still not clear. In this paper, the tree behavior under repetitive pulse voltage and low temperature was investigated and analyzed. A positive pulse voltage with the amplitude of 12 kV and the frequency of 300 and 400 Hz was applied on the needle-plate electrodes. The experimental temperature was set to 30, -30, -90 and -196 °C. Four typical morphologies of the electrical trees are observed, including bush, branch, branch-pine and stagnated tree. It is revealed that low temperature has an obvious effect on tree structures and it can reduce the tree growth rate and extend the time to breakdown. It is also revealed that the tree inception probability becomes lower with the decrease of temperature. With the increase of pulse frequency, the tree growth rate, the fractal dimension and the number of tree channels increases and the tree inception probability becomes higher, but the morphologies of electrical trees stay the same.

Electrical tree propagating characteristics of polyethylene/nano-montmorillonite composites

Electrical tree occurring in polymer insulation limits the applications and reduces the service lifetime of power equipment. To improve electrical tree resistance in polymer insulation, polyethylene (PE) nanocomposites with nano-montmorillonite (MMT)- layered fillers are compounded and investigated in terms of electrical tree characteristics. By an organization process, intercalation and coupling agents are used to modify the inorganic-layered MMT nanofillers in sequence. Polyethylene/nanomontmorillonite (PE/MMT) composites with different MMT contents are prepared by melting intercalation. Fourier transform infrared spectrophotometry is used to determine the chemical characteristics of the MMT in different surface-modification stages. The crystalline morphologies and MMT distributions in the composites are characterized by polarizing microscopy and scanning electron microscopy. The electrical tree propagating characteristics and corresponding conduction properties of the PE and PE/MMT composites are investigated. Results show that the electrical tree resistance of PE/MMT composites significantly improves. The electrical tree length decreases and the fractal dimension increases in PE/MMT composites compared with pure PE. After performing electrical tree initiation, conductive current slightly increases in composites but decreases in PE. This result suggests that the characteristics and conductive mechanism of electrical tree differ between PE and PE/MMT.

Electrical tree characteristics in silicone rubber under repetitive pulse voltage

Electrical tree in silicone rubber (SiR) is a threat to the power system. Bedsides ac and dc voltage, there are pulse voltage in power system and the electrical tree behavior in SiR under pulse voltage has not been reported. In this paper, electrical treeing process in SiR under pulse voltage was investigated and analyzed. The voltage was applied to a needle-plate electrode to initiate electrical trees. The frequency of pulse voltage was 100, 200 and 300 Hz and the amplitude ranges from 8 to 14 kV. Aspects such as the patterns of electrical tree, tree length, accumulated damage, fractal dimension and tree breakdown characteristics were studied. Results indicate that the pulse amplitude plays the leading role in electrical tree initiation, propagation and breakdown processes. With the increase of pulse amplitude, the tree structure changes and the breakdown possibility increases obviously. However, the tree length is not sensitive to pulse frequency and higher pulse frequency promotes the increase of tree density. It also has been found that tree structure is closely related to pulse polarity and the inception voltage is lower with positive pulse than that with negative.

A novel approach towards the determination of the time to breakdown of electrical machine insulating materials

This paper presents new approach to using Partial Discharge (PD) data for the prediction of the Remaining Useful Life (RUL) of dielectric materials undergoing breakdown. The method presented uses a thermodynamic macro-model in conjunction with an artificial neural network to associate the features in the PD data detected during breakdown to the electrical tree characteristics. The method is presented using electrical tree simulation data from a new dielectric breakdown simulation model. The simulation model is confirmed using experimental data.

Temperature Effect on Electrical Treeing and Partial Discharge Characteristics of Silicone Rubber‐Based Nanocomposites

This study investigated electrical treeing and its associated phase‐resolved partial discharge (PD) activities in room‐temperature, vulcanized silicone rubber/organomontmorillonite nanocomposite sample materials over a range of temperatures in order to assess the effect of temperature on different filler concentrations under AC voltage. The samples were prepared with three levels of nanofiller content: 0% by weight (wt), 1% by wt, and 3% by wt. The electrical treeing and PD activities of these samples were investigated at temperatures of 20°C, 40°C, and 60°C. The results show that the characteristics of the electrical tree changed with increasing temperature. The tree inception times decreased at 20°C due to space charge dynamics, and the tree growth time increased at 40°C due to the increase in the number of cross‐link network structures caused by the vulcanization process. At 60°C, more enhanced and reinforced properties of the silicone rubber‐based nanocomposite samples occurred. This led to an increase in electrical tree inception time and electrical tree growth time. However, the PD characteristics, particularly the mean phase angle of occurrence of the positive and negative discharge distributions, were insensitive to variations in temperature. This reflects an enhanced stability in the nanocomposite electrical properties compared with the base polymer.

Influence of montmorillonite on electrical treeing and breakdown characteristics of low-density polyethylene

Electrical treeing is an electrical aging phenomenon that severely threatens the operation safety of power equipment, in which polymer is used as primary insulation material. A type of low-density polyethylene-montmorillonite (LDPE/MMT) nanocomposite was prepared using melt method to study the impact of MMT nanoparticles on the growth of the electrical tree and on the breakdown properties of the LDPE material. The distribution characteristic of space charges of the specimens under DC voltage of –50 kV/mm and short-circuit conditions was measured using pulsed electro-acoustic method. The electrical tree aging characteristics of specimens under AC voltage of power frequency were studied using needle-plate electrode structure test platform. The starting voltage of initiation, the triggering rate, and the growth rate of the electrical trees, as well as the breakdown performance of the specimens under AC voltage of power frequency were measured. The inhibitory mechanism of MMT nano-particles on the growth of electrical trees in nano-composites, and the mechanism behind the improvement of the breakdown strength of the LDPE/MMT specimens were illustrated. Experimental results showed that, the addition of MMT can effectively reduce the amount of space charges injected from electrodes and the accumulation amount in the center of samples. Moreover, space charges dissipated more slowly during short-circuit conditions. The speed and the scale of the growth of the electrical tree declined as the initiation conditions of electrical trees growing in LDPE/MMT nanocomposites increased; meanwhile, the breakdown strength increased substantially. Here are the reasons: MMT can reduce the mobility of carriers in LDPE material, thereby contributing to the improvement of the distribution of space charges. In the meanwhile, MMT changed the development path of electrical trees, and extended the discharge distance. Consequently, MMT postponed the occurrence of penetrable breakdown in the LDPE/MMT specimen, and ultimately improved the breakdown strength of the specimen.

Imaging and analysis techniques for electrical trees using X-ray computed tomography

Electrical treeing is one of the main mechanisms of degradation in polymeric high voltage insulation, a precursor of power equipment failure. Electrical trees have been previously imaged mostly using two-dimensional imaging techniques; thereby loosing valuable information. Here we review the techniques that have been previously used and present the novel application of X-ray computed tomography (XCT) for electrical tree imaging. This non-destructive technique is able to reveal electrical trees, providing a threedimensional (3-D) view and therefore, a more complete representation of the phenomenon can be achieved. Moreover, taking virtual slices through the replica so created brings the possibility of internal exploration of the electrical tree, without the destruction of the specimen. Here, laboratory created electrical trees have been scanned using XCT with phase contrast enhancement, and 3-D virtual replicas created through which the trees are analyzed. Serial Block-Face scanning electron microscopy (SBFSEM) is shown to be a successful complementary technique. Computed tomography enables quantification of electrical tree characteristics that previously were not available. Characteristics such as the diameter and tortuosity of tree channels, as well as the overall tree volume can be calculated. Through the cross-section analysis, the progression of the number of tree channels and the area covered by them can be investigated. Using this approach it is expected that a better understanding of electrical treeing phenomenon will be developed.

Imaging and analysis techniques for electrical trees using X-ray computed tomography

Electrical treeing is one of the main mechanisms of degradation in polymeric high voltage insulation, a precursor of power equipment failure. Electrical trees have been previously imaged mostly using two-dimensional imaging techniques; thereby loosing valuable information. Here we review the techniques that have been previously used and present the novel application of X-ray computed tomography (XCT) for electrical tree imaging. This non-destructive technique is able to reveal electrical trees, providing a threedimensional (3-D) view and therefore, a more complete representation of the phenomenon can be achieved. Moreover, taking virtual slices through the replica so created brings the possibility of internal exploration of the electrical tree, without the destruction of the specimen. Here, laboratory created electrical trees have been scanned using XCT with phase contrast enhancement, and 3-D virtual replicas created through which the trees are analyzed. Serial Block-Face scanning electron microscopy (SBFSEM) is shown to be a successful complementary technique. Computed tomography enables quantification of electrical tree characteristics that previously were not available. Characteristics such as the diameter and tortuosity of tree channels, as well as the overall tree volume can be calculated. Through the cross-section analysis, the progression of the number of tree channels and the area covered by them can be investigated. Using this approach it is expected that a better understanding of electrical treeing phenomenon will be developed.

에폭시수지의 고전압 전원주파수 변화에 따른 장시간 전기적 트리잉 열화 특성연구

Electrical tree structure is one of the most important influencing factors for electrical treeing characteristics in polymers. In this paper, we focused on the structure characteristics of electrical treeing in epoxy resins (original) insulation under different high-frequency voltages (60, 500, 1000 ㎐). Effects of voltage frequency on the ac electrical treeing phenomena in an epoxy resins were carried out in needle-plate electrode arrangement. To measure the treeing initiation and propagation, and the breakdown rate, constant AC of 10 ㎸ with three different voltage frequencies (60, 500 and 1,000 ㎐) was applied to the specimen in needle-plate electrode specimen at 30 ℃ of insulating oil bath. At 60 ㎐, the treeing initiation time was 360 min and the propagation rate was 6.85×10-4㎜/min, and the morphology was dense branch type. As the voltage frequency increased, the treeing initiation time decreased and the propagation rate increased. At 1,000 ㎐, the treeing initiation time was 0 min and the propagation rate was 7.81×10-2㎜/min, and the morphology was dense bush type.

Electrical Treeing Characteristics in Polydimethylsiloxane-Based Organic-Inorganic Hybrid Materials

Electrical treeing characteristics of hybrid materials made from titanium alkoxide Ti(OCH2CH(C2H5)C4H9)4 and alkoxysilane-terminated polydimethylsiloxane (PDMS) in different TA/PDMS ratio was investigated. The development of AC tree of sample was mounted on a glass substrate with needle electrode was observed by CCD camera and computer. The tree inception voltage increased and tree growth speed decreased with increase in TA/PDMS ratio of the hybrid. It is suggested that the variation of crosslinking density and homogeneity of dispersion of silica clusters influenced on their tree propagation characteristic. We found that the hybrid made from the alkoxysilane-terminated PDMS and TA has good electrical insulating property compared with conventional PDMS-based hybrid materials and silicone rubber.

Study on conducting characteristics of electrical trees in cross-linked polyethylene cable insulation

The conducting characteristics of two typical electrical trees in cross-linked polyethylene (XLPE) cable insulation are studied by a combination of optical microscopy observation, partial discharge measurement and con-focal Raman spectroscopy analysis. Although they are grown under similar conditions, these two trees display very different shapes. One is a typical branch-pine tree grown at 9 kV, and the other is a branch tree grown at 11 kV. The growth and the partial discharge regularities show obvious differences. The disordered graphitic carbon is condensed in the main tree channels of the branch-pine tree. From the relative intensity of the graphitic carbon G band to D band, the graphitic domain is estimated to be about 8 nm in size. The tree channel resistance per unit length is less than 10 m-1, which is sufficient to prevent the partial discharge from developing within the tree structure. The branch-pine tree shows the features of the conducting tree. The fluorescence background is observed in the channels of branch tree, which shows the existence of the products of the material degradation, but no disordered graphitic carbon is observed in these tree channels. These tree channels display obvious non-conducting characteristics, which is not sufficient to prevent the continuous effect of the partial discharges. Finally, a single channel growth model is proposed for the conducting and non-conducting trees grown in XLPE cable insulation. Based on the equivalent circuit theory, the growth mechanisms of the two trees with different conducting characteristics in XLPE cable insulation are discussed.

Structure characteristics of electrical treeing in XLPE insulation under high frequencies

Electrical tree structure is one of the most important influencing factors for electrical treeing characteristics in polymers. In this paper, we focused on the structure characteristics of electrical treeing in cross-linked polyethylene (XLPE) insulation under high-frequency voltages. The tree structure characteristics include structure distribution characteristics and structure conversion characteristics. The influences of voltage, frequency, and pin-plane spacing on tree structure characteristics were analyzed based on the experimental results. It can be concluded that tree structures regularly change with the local electric field and frequency. The electric field in a very small zone near the needle tip is an important influencing factor for the formation of bush-like trees, and the lowest frequencies for the observed pure-vine-like trees increased with voltage. For double-structure trees, the local electric field at the transition location of the two structures remained almost unchanged with voltage and pin-plane spacing, but obviously increased with frequency. In order to investigate the relations of the growth rate and fractal dimension with tree structure characteristics, a new parameter, the energy threshold W t , has been introduced and calculated for different tree structures.

Effect of ambient temperature on electrical treeing characteristics in silicone rubber

In this paper, electrical treeing was investigated in room temperature vulcanized (RTV) silicone rubber (SiR) over a range of ambient temperatures. An ac voltage with a frequency of 50 Hz was applied between a needle-plate electrode to initiate the electrical tree at different ambient temperatures. Both the structures and the growth characteristics were observed by using a digital microscope system. Obtained results show that the tree initiates from a single branch with a white gap. Four typical tree structures, namely branch, bush, pine branch and bush-pine mixed tree, were observed within the sample. The occurrence of tree structures changes with the increase of ambient temperature, in which branch tree takes up a great proportion at 30°C while bush tree becomes dominated as the temperature rises up to 90°C. Meanwhile, the cumulative inception probability within the same time decreases obviously with the increase of ambient temperature. The growth rate of the tree is closely related to the ambient temperature. It is suggested that the increase of vulcanization network density and elastic modulus with ambient temperature may have great influence on the treeing characteristics (including growth rate, fractal dimension and treeing proportion).

Investigations of electrical trees in the inner layer of XLPE cable insulation using computer-aided image recording monitoring

Using a computer-aided image recording monitoring system, extensive measurements have been performed in the inner layer of 66 kV cross-linked polyethylene (XLPE) cables. It has been found that there are three kinds of electrical trees in the samples, the branch-like tree, the bush-like tree and the mixed tree that is a mixture of the above two kinds. When the applied voltage frequency is less than or equal to 250 Hz, only the mixed tree appears in XLPE samples, when the frequency is greater than or equal to 500 Hz, only the dense branch-like tree develops, both of which are attributed to the coexistence of non-uniform crystallization and internal residual stress in semicrystalline XLPE cables during the process of manufacturing. Through the fractal analyses of these electrical trees, it has been found that both the propagation and structure characteristics can be described by fractal dimension directly or indirectly. It is suggested that the propagation and structural characteristics of electrical trees are closely related to the morphology and the residual stress in material at low frequency, i.e., the propagation characteristics of electrical trees depends upon not only the boundaries between big spherulites and amorphous region, but also the impurity, micropore concentration and the relative position of needle electrode tip with respect to spherulites or amorphous region in the low frequency range. However, at high frequency, it has nothing to do with the morphology of material. It is suggested that the injection and extraction process of charge from and to dielectrics via the needle electrode are more intense at high frequency than in low frequency. Thus, it can form relatively uniform dielectric weak region in front of needle electrode, which leads to similar initiation and propagation characteristics of electrical trees at high frequency.

Investigating the influence of the lubricant coating on hypodermic needles on electrical tree characteristics in epoxy resin

Many electrical treeing studies have employed the use of hypodermic needles in the point-plane geometry configuration. These hypodermic needles developed for medical usage are coated with a transparent lubricant. Limited literature exists on sample preparation techniques addressing the removal of this lubricant coating, which affects adhesion of the epoxy resin to the hypodermic needle. In this experimental study, the impact of the lubricant coating on electrical tree growth and associated partial discharge activity was investigated. The findings confirm that the lubricant coating inhibits electrical tree growth resulting in limited spread of the electrical tree while yielding the occurrence of 'rabbit-ear' partial discharge patterns. This study also reports pronounced visible gaseous activity upon electrical tree initiation in the presence of the lubricant coating. Electrical trees were developed using point-plane samples of LY/HY 5052 epoxy resin with a gap spacing of 2 ± 0.5 mm. All electrical trees were 50 Hz initiated at either 8.4 or 14.4 kV peak. After electrical tree initiation, 50 Hz composite waveforms incorporating harmonic content were applied as the excitation waveforms.

The conduction characteristics of electrical trees in XLPE cable insulation

AbstractDepending on the morphology of the material and applied voltage frequency, three kinds of electrical trees can exist in cross‐linked polyethylene (XLPE) cable insulation, which are conducting, non‐conducting, and mixed trees with different growth mechanisms. It is suggested that when the needle is inserted into large spherulites, conducting trees will form in those spherulites; when it is inserted among spherulites, non‐conducting trees will appear along the boundaries of spherulites. Frequency will accelerate the growth of non‐conducting trees but have little influence on the initiation and growth processes of conducting trees. If the initiation process of non‐conducting trees is too difficult, they will grow into mixed trees. Finally, it is concluded that the space charge limited tiny breakdown around the tips of electrical trees is responsible for the propagation process of conducting trees; on the other hand, fast expansion occurs due to local high temperature and pressure along the boundaries, partial discharge in electrical tree paths and charge recombination, etc., which are the main reason for the growth of non‐conducting trees. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

The characteristics of electrical trees in the inner and outer layers of different voltage rating XLPE cable insulation

The statistical initiation and propagation characteristics of electrical trees in cross-linked polyethylene (XLPE) cables with different voltage ratings from 66 to 500 kV were investigated under a constant test voltage of 50 Hz/7 kV (the 66 kV rating cable is from UK, the others from China). It was found that the characteristics of electrical trees in the inner region of 66 kV cable insulation differed considerably from those in the outer region under the same test conditions; however, no significant differences appeared in the 110 kV rating cable and above. The initiation time of electrical trees in both the inner and the outer regions of the 66 kV cable is much shorter than that in higher voltage rating cables; in addition the growth rate of electrical trees in the 66 kV cable is much larger than that in the higher voltage rating cables. By using x-ray diffraction, differential scanning calorimetry and thermogravimetry methods, it was revealed that besides the extrusion process, the molecular weight of base polymer material and its distribution are the prime factors deciding the crystallization state. The crystallization state and the impurity content are responsible for the resistance to electrical trees. Furthermore, it was proposed that big spherulites will cooperate with high impurity content in enhancing the initiation and growth processes of electrical trees via the ‘synergetic effect’. Finally, dense and small spherulites, high crystallinity, high purity level of base polymer material and super-clean production processes are desirable for higher voltage rating cables.

Three‐dimensional tree simulation utilizing a three‐layer model

AbstractRecently, polymer/polymer composite insulating systems have been widely used in high‐voltage equipment and power cables. Such composite insulating systems always have an interface between two polymeric insulating materials. This interface may cause partial discharge and electrical treeing under high electric stresses. The electrical properties of this polymer/polymer interface have not yet been fully understood. It has been shown that the propagation characteristics of electrical trees in polymer/polymer composite insulating materials considerably vary depending on the combination of polymers. It has also been shown that a cavity or a thin gas layer generates near the polymer barrier molded in the base polymer, by the pressure of decomposing gas occurring in the tree channels during AC tree extension.In this study, we will investigate the three‐dimensional simulated tree utilizing a three‐layer model considering a thin gas layer on a barrier based on a DBM model with growth probability.The results show that the critical fields of air layer are related to the easiness of tree propagation in the air layer. Moreover, it is suggested that the three‐layer model is more effective than the two‐layer model when the tree propagation is discussed in detail. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(3): 1–9, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20808

Electrical treeing characteristics in XLPE power cable insulation in frequency range between 20 and 500 Hz

Electrical treeing is one of the main reasons for long term degradation of polymeric materials used in high voltage AC applications. In this paper we report on an investigation of electrical tree growth characteristics in XLPE samples from a commercial XLPE power cable. Electrical trees have been grown over a frequency range from 20 Hz to 500 Hz and images of trees were taken using CCD camera without interrupting the application of voltage. The fractal dimension of electric tree is obtained using a simple box-counting technique. Contrary to our expectation it has been found that the fractal dimension prior to the breakdown shows no significant change when frequency of the applied voltage increases. Instead, the frequency accelerates tree growth rate and reduces the time to breakdown. A new approach for investigating the frequency effect on trees has been devised. In addition to looking into the fractal analysis of tree as a whole, regions of growth are being sectioned to reveal differences in terms of growth rate, accumulated damage and fractal dimension.

Dynamics model for electrical tree propagation in cross-linked polyethylene cable insulation under high frequency voltage

In this paper we investigated the structures and propagation characteristics of electrical trees observed in the high volatage cross-linked polyethylene (XLPE) cable insulation samples under high volatage of 10kV at frequency of 1000—2000Hz. Based on the specific tree growth mechanism and the fractal nature of tree structures, a dynamic model has been developed to qualitatively predict the electrical tree growth characteristics in XLPE cable insulation when subjected to an electrical stress under high frequency voltage, and then the tree growth rate equation and life formula of tree growth to breakdown are derived.We performed electrical tree growth tests in XLPE cable insulation samples at high frequency and compared the results, which showed that the proposed model was in good agreement with the expreriment.So,it seems that the proposeed model can be applied to the qualitative analysis of aging law of electrical tree in XLPE cable insulation.