DC Tree Research Articles

Polarity Effect on Electric Tree Inception in HVDC Cable Insulation

This paper studies the influence of polarity of the applied voltage on DC tree inception in XLPE by analyzing dissipated and stored electric energies. Space charge dynamics, electric field distributions and resulting energies in the vicinity of the needle tip in the needle-plane type sample are calculated using the bipolar charge transport model. Simulations are performed for three modes of DC tree initiation: by applying continuous DC voltage, by grounding pre-stressed sample and by polarity reversal. In case of continuous DC, the estimated dissipated energy is found to be higher for positive applied voltage compared to negative that results in a lower inception voltage level. It is observed that the energy dissipation is controlled by the amount of mobile charge carriers and their mobility. In the case of trees due to grounding of pre-charged sample, differences in the electrostatic energy stored in the vicinity of the needle electrode right before grounding leads to tree inception at lower magnitude for the negative polarity compared to positive. Here, the amount of stored energy is dependent on the electric field controlled by the distribution of space charge in the material and the mobility of charge carriers plays a major role by defining locations of space charge clouds. When trees are initiated by polarity reversal, both stored energy before reversal and dissipated energy after, are in favor of tree inception during negative-to-positive transition than for the opposite. The obtained energy densities are compared with the threshold values required for tree inception by electromechanically induced cracks in the material. It is concluded that stored and/or dissipated energy are determining the polarity effect in DC tree inception.

Growth and partial discharge characteristics of DC electrical trees in cross-linked polyethylene

In the present study, the growth and partial discharges properties of DC electrical trees in cross-linked polyethylene (XLPE) were investigated under positive and negative DC voltages. The obtained results revealed that the voltage rise exerted significant influences on growth properties of DC trees. In either polarity, DC electrical trees grew rapidly during rising voltage and maintained the rapid growth for a certain time after DC voltage reached a constant value. The growth rate of positive DC electrical trees was quite faster than that of negative ones, and channel breakdown mainly occurred in positive ones. Although the growth rates of positive and negative DC electrical trees were different, their shapes were similar (both were branch-like electrical trees with one or two main branches). Self-healing usually occurred in negative DC electrical trees, thus electrical tree branches gradually disappeared under the application of steady DC voltage or when the voltage was off. Partial discharges mainly occurred at the beginning of electrical tree growth; however, the amount of partial discharge was relatively small, and it gradually reduced even during the rapid growth of electrical trees, consequently, almost no partial discharges were found when the voltage was constant. Finally, a discharge-avalanche theory was proposed to explain the growth properties of DC electrical trees.

Different microscopic features of AC and DC electrical trees in insulating polymer

The understanding of electrical trees plays a crucial role in the development of reliable high voltage power systems. The AC electrical trees are different from DC ones, however, there are only limited studies dealing with this topic. For the first time, the differences between AC and DC trees are carefully compared in a systematic way. A photomultiplier tube (PMT) was used to collect the electroluminescence (EL) during the AC and DC tree initiation. Optical microscope, CCD camera and scanning electron microscope (SEM) were employed to image the micro-morphology during the electrical tree initiation and propagation. Ageing characteristics were acquired by the confocal Raman spectrometer and infrared thermal imaging. The experimental results indicated that the initiation and propagation of AC and DC trees are totally different. Based on these, the growth mechanisms of AC and DC trees have been proposed, which provides a better understanding of the electrical treeing processes.

Electrical tree initiation in XLPE cable insulation under constant DC, grounded DC, and at elevated temperature

Electrical tree initiation behavior was investigated in XLPE insulation samples made from a HVDC cable by applying constant DC and grounded DC voltages under different temperatures. No trees appeared when a constant DC voltage of −60 kV was applied for 12 days at room temperature or 20 days at 90°C, and the tree initiation time was estimated to be more than 200 days by Bayes statistical processing of zero-failure data. In the grounded DC voltage test, −15 kV prestress could trigger a tree, and under −35 kV, the tree initiation rate reached 100 %. With the increase of DC prestress, both the initiation rate and tree length increased. Grounded DC trees were branch trees with dark-colored channels, and a similar material damage could be caused by the same DC prestress. The connection of a grounding resistor could reduce the damage of sample caused by the grounded DC voltage, and the tree initiation rate declined with the increase of the resistance. According to the experimental results, the tree initiation time under constant DC voltage was not significantly shortened at 90°C, whereas tree initiation under grounded DC voltage was promoted by the increase of temperature. From 25 to 70°C, the tree branches became longer and denser, and extended in more varied directions. The observations are explained by the injection, transfer, trapping and detrapping of space charges, and charge detrapping is believed to be the cause of material damage.

Research of grounded DC electrical tree growth properties in XLPE

Grounded DC tree is a serious threat for the safe operation of XLPE cable. In this paper, growth properties of grounded DC electrical trees in XLPE under ±40kV DC voltage were investigated and anal...

DC electrical tree growth in epoxy resin and the influence of the size of inceptive AC trees

Electrical tree propagation is a precursor to dielectric failure in high voltage polymeric insulation. Tree growth has been widely studied under AC conditions, but its behavior under DC is not well understood. The aim of this work was to examine the importance of polarity and initiating defect size on DC tree propagation. Electrical tree propagation in epoxy resin under constant DC voltages of +60 kV and −60 kV was measured in samples with classical needle-plane electrodes, but with small initial trees (< 100 μm) incepted under lower AC voltages before the DC tests. Experimental results showed strong polarity dependence. In either polarity, the length of the initial AC tree had a major influence on the inception of the subsequent DC tree. The effect was attributed to the defect being influential if it is larger than the space charge injection region. As a consequence, there is a critical defect size that will accelerate failure of DC insulation dependent on space charge injection behavior of the polymer/electrode system in question — a critical finding for high voltage asset management.

Chemical Composition Analysis, Sensory, and Feasibility Study of Tree Peony Seed.

Eight wild species in Sect. Moutan DC (tree peony) of the genus Paeonia grown in natural habitats and 1 cultivated specie were investigated to analyze their fatty acid and bioactive phenolic compound profiles. For fatty acid composition, P. ludlowii contained the lowest α-linolenic acid (27.68%) and P. jishanensis contained the highest (51.96 %) content of the 9 species. For phenolic compounds, P. qiui contained the highest resveratrol (2.12 mg/g), P. delavayi contained the highest β-gentiobiosylpaeoniflorin (26.23 mg/g), and P. ostii contained the highest paeoniflorin (23.66 mg/g). P. ostii was selected to perform a feasibility study because of its relatively high level of α-linolenic acid 46.53%, low in ω-6 to ω-3 ratio of 1:2, and high level of the preferred bioactive phenolic compounds l including paeoniflorin and resveratrol. Physical pressing and refining process were conducted to obtain P. ostii seed oil. It exhibited bland sensory attributes described as slight grassy, very slight nutty, no painty or fishy aroma and slight grassy, slight nutty flavor with a very slight throat catch. Tocol results reported high level in tree peony seed oil 223.5 ± 13.65 mg/100 g with γ-tocopherol 70.1 ± 2.14 mg/100 g, and γ-tocotrienol 149.6 ± 15.83 mg/g. Because of the high total tocol, γ-tocopherol and γ-tocotrienol levels, and tree peony seed oil exhibited better oxidation stability than flaxseed oil even with similar α-linolenic acid levels. In addition, high levels of γ-tocopherol and γ-tocotrienol can introduce therapeutic effects such as antiinflammation and antioxidation. Therefore, this study showed that tree peony seed oil has a great potential to be used in edible oil, nutraceutical supplement, and other health care products.

Effect of DC Prestressing on Periodic Grounded DC Tree in Cross-Linked Polyethylene at Different Temperatures

Periodic grounded dc trees in cross-linked polyethylene under various dc prestressing times are investigated in the temperature range of 20 °C–80 °C. Space charge behaviors in the samples during dc prestressing are simulated based on the bipolar charge transport model. The results reveal that the dc prestressing time has different effects on the tree growth at different temperatures, which is because that the space charge behavior in the sample during dc prestressing is closely related to the dc prestressing time and the temperature, and it has both promotion effect and impedance effect on the tree growth. The moderate promotion effect and impedance effect under 160 s dc prestressing at 20 °C, and the strongest promotion effect and the weakest impedance effect under 240 s dc prestressing at 40 °C result in the largest tree lengths, widths, and accumulated damages under each corresponding condition. At 60 °C and 80 °C, the lengths, widths, and accumulated damages are the largest under 240 s dc prestressing, smaller under 80 s dc prestressing, and the smallest under 160 s dc prestressing, and the reason is believed to be the growth rates reversal during the tree growth. Also it is found that the dc prestressing time has significant effects on the tree shape at 60 °C and 80 °C, which is believed to be related to the wider charge diffusion range under longer dc prestressing time. The electrical tree characteristics are discussed in detail combined with the space charge behaviors.

Effect of temperature on space charge detrapping and periodic grounded DC tree in cross-linked polyethylene

In a high voltage direct current (HVDC) cross-linked polyethylene (XLPE) cable system, the temperature of the insulation varies with the load condition, which significantly influences the electrical performance of the XLPE. In this study, space charge detrapping and periodic grounded DC tree in XLPE are investigated and analyzed at various electric fields in a broad temperature range of 20–80 °C. Results reveal that with the increase in temperature, the apparent trap-controlled mobility of space charge becomes larger while its decay rate becomes smaller in the initial phases of depolarization. These findings indicate that charge detrapping becomes easier at higher temperature. The electrical tree initiation ratios, lengths, and widths all increase as temperature increases, except for the negative tree width under 16 kV at 80 °C. An increment in length and width of positive trees is observed at 80 °C. Electrical trees become thicker at higher temperature, and branches into the direction perpendicular to the applied electric field are observed at temperatures above 60 °C. Furthermore, the negative trees are easier to initiate and are thicker in shape than positive trees, and sometimes have an advantage over positive trees in length and width in the temperature range of 20–60 °C.

Breaking More Composition Symmetries Using Search Heuristics

The pruning power of partial symmetry breaking depends on the given subset of symmetries to break as well as the interactions among symmetry breaking constraints. In the context of Partial Symmetry Breaking During Search (ParSBDS), the search order determines the set of symmetry breaking constraints to add and thus also makes an impact on node and solution pruning. In this paper, we give the first formal characterization of the pruning behavior of ParSBDS and its improved variants. Introducing the notion of Dominance-Completeness (DC-ness), we show that ParSBDS and variants eliminate the symmetry group of the given subset of symmetries if the resultant search tree is DC, and give an example scenario. Unfortunately, building a DC tree is not always possible. We propose two search heuristics with the aim of having more nodes dominated and thus also pruned during search. Extensive experimentation demonstrates how the proposed heuristics and their combination can drastically reduce the solution set size, search space and runtime when compared against the state-of-the-art static and dynamic symmetry breaking methods.

Lack of photosynthetic or stomatal regulation after 9 years of elevated [CO2] and 4 years of soil warming in two conifer species at the alpine treeline

Alpine treelines are temperature-limited vegetation boundaries. Understanding the effects of elevated [CO2 ] and warming on CO2 and H2 O gas exchange may help predict responses of treelines to global change. We measured needle gas exchange of Larix decidua Mill. and Pinus mugo ssp. uncinata DC trees after 9 years of free air CO2 enrichment (575 µmol mol(-1) ) and 4 years of soil warming (+4 °C) and analysed δ(13) C and δ(18) O values of needles and tree rings. Tree needles under elevated [CO2 ] showed neither nitrogen limitation nor end-product inhibition, and no down-regulation of maximal photosynthetic rate (Amax ) was found. Both tree species showed increased net photosynthetic rates (An ) under elevated [CO2 ] (L. decidua: +39%; P. mugo: +35%). Stomatal conductance (gH2O ) was insensitive to changes in [CO2 ], thus transpiration rates remained unchanged and intrinsic water-use efficiency (iWUE) increased due to higher An . Soil warming affected neither An nor gH2O . Unresponsiveness of gH2O to [CO2 ] and warming was confirmed by δ(18) O needle and tree ring values. Consequently, under sufficient water supply, elevated [CO2 ] induced sustained enhancement in An and lead to increased C inputs into this ecosystem, while soil warming hardly affected gas exchange of L. decidua and P. mugo at the alpine treeline.

Pearl-chain-type Trees in Silicone Gel Under AC Voltage

The tree growth has been usually classified to three stages; inception, propagation and runaway stage under AV voltage. When the two-dimensional (2-D) specimen of silicone rubber was used, the propagation stage was divided to two stages: straight and isotropic growth stages. 2-D specimen is useful to observe AC trees with many branches since the number of the branches of AC tree is usually larger than DC and impulse tree. When the silicone rubber is fabricated, two types of silicone gel is mixed up; one includes catalytic agent. If the mixed ratio is changed from 1:1, silicone rubber has fluidity or viscosity. In theses cases, breakdown or partial discharge phenomena also change. Under certain conditions of gel, tree has been developing with bubble-like and string-like pattern alternatively. This tree is called pearl-chain-like tree. At first the tree developed like bubble and then a projection like cone was generated on the surface of the bubble. The tip of the cone expanded and formed a new bubble. This process was repeated. If two tips of the cone are formed on the surface of the bubble, the branching occurs. The shape of tree is like pearl-chain. The pattern of the tree is usually string (or straight) channel and branching. However In the pearl-chain like tree, string channel has consisted of two channels; bubble and string. Branching has occurred at bubbles. These data is important to investigate the mechanism of tree.

Effect of land management and Prosopis juliflora (Sw.) DC trees on soil microbial community and enzymatic activities in intensive silvopastoral systems of Colombia

Livestock production in Latin America has replaced tropical dry forests with conventional monocultures pastures (CP) that have degraded soils. As an alternative to CP, intensive silvopastoral systems (ISS) have been developed with multi-canopied vegetation that mimics native forest (F). The litter inputs and year-round presence of the tree rhizosphere in ISS, contribute to the formation of “fertile islands,” which is expected to impact biological activity and crop productivity. This study, investigated the impact of the conversion of CP to ISS, as well as the effect of canopy of Prosopis juliflora trees in a chronosequence of ISS (3–15years) on soil microbial communities and the physicochemical properties of soil. FAME (fatty acid methyl ester) profiles indicated that soil microbial community structure and composition shifted depending on land management systems. CP promoted the dominance of G− bacteria, while ISS chronosequence and F favored actinomycetes and fungal biomass (total and arbuscular mycorrhizal fungi). In addition, soil microbial community (FAME profiles) of ISS chronosequence and F were more similar than with CP. An increase in the Cy/pre FAME ratio in CP suggested that the microbial community was under higher stress. The advantage of including trees in pasture systems was reinforced by the observation that FAME biomarkers, enzymatic activities and nutrient status were significantly higher beneath the canopy of P. juliflora. The results indicate that ISS are viable alternatives for improving soil quality and metabolic function, which is reflected in the significant increase in microbial biomass, FAME biomarkers and enzyme activities compared with CP.

Environmental modulation of the seasonal cambial activity in Prosopis flexuosa DC trees from the Monte woodlands of Argentina

The rhythm of cambial activity and its relationship with precipitation, temperature and phenology was analyzed in Prosopis flexuosa trees from the xerophytic woodlands of western Argentina. Periodic tree growth measurements made in a P. flexuosa-dominated stand between 2005 and 2008 showed that earlywood tree-ring development starts in the early spring (October), when young vegetative buds begin expansion and completes its formation at the end of November when fruits are almost ripe. The development of the tree-ring latewood is completed in early January (the middle of summer). Redundancy Analysis suggests that cumulative daily rainfall 4–6 days prior to core sampling significantly influences cambial growth, while temperatures 0–3 days preceding core sampling show the most significant effect on growth. An Artificial Neural Network analysis suggests a positive, monotonic effect of rainfall on cambial growth, whereas temperature shows a modal effect on growth with a peak at 18–20 °C. The results provide important new information about the eco-physiology of P. flexuosa trees in the Monte region of Argentina, and indicate that this species responds rapidly to changes in rainfall and temperatures during the growing season.

Annual variation and influence of climate on the ring width and wood hydrosystem of Prosopis flexuosa DC trees using image analysis

We developed chronologies based on the width of tree rings, total area of vessels, and the number of vessels per tree ring of the Prosopis flexuosa wood samples from the xerophytic woodlands of central Argentina. We evaluated the influence of climate on these tree-ring characteristics considering the period from 1940 to 2004 (65 years). The width of the rings, the number of vessels, and the total area of vessels were positively influenced by regional precipitation corresponding to the seasonalized November to December period, which reflects the importance of the water availability in the initial stage of the formation of the wood. The width of the rings and the total area of vessels were negatively influenced by temperature during the same period, while the number of vessels was not significantly correlated with temperature. The high temperatures in spring increase evapotranspiration, which reduces water availability to plants and results in an inverse growth response. This study was the first to develop chronologies based on anatomical characters of wood from the arid and semiarid regions of South America and with significant applications in ecological and climatic studies.

Effect of Antioxidants on DC Tree and Grounded DC Tree in XLPE

To study the effects of antioxidants on the initiation of the DC tree and the grounded DC tree, experiments were conducted using XLPE specimens containing phenolic and sulfur type antioxidants. Experimental results showed that sulfur type antioxidants in XLPE have the effect of increasing inception voltages of both the DC tree and the grounded DC tree. Based on results of those experiments, the mechanism of increase in the inception voltage of the DC tree and the grounded DC tree by antioxidants was examined along with the mechanism of polarity effects on those trees. Results showed a promotional effect of charge injection from a needle electrode by antioxidants, which are responsible for the increased inception voltages of the DC tree. Charge trapping by antioxidants explains the increase of inception voltages of the grounded DC tree.

DC electrical treeing phenomena and space charge

Influence of space charge on dc tree propagation is studied using many insulating polymer materials under divergent fields. The inception voltage and the propagation of dc treeing are affected strongly by space charge. Space charge formation in polymers was observed directly by using thermally stimulated current (TSC) techniques under divergent field. These results of the space charge effect on dc treeing phenomena are consistent with those obtained by the TSC method. From these results, the origin of space charge in polymers and the suppression techniques for space charge formation were discussed.

Comments, with reply, on "Fractal character of DC trees in polymethylmethacrylate" by M. Fujii et al

In the above-named work by M. Fujii et al. (ibid., vol.26, no.6, pp.1159-1162, Dec. 1991), the differing fractal descriptions of DC trees generated in polymethylmethacrylate (PMMA) are attributed to the local electric fields generated by space charges. The commenter argues that, while a fractal dimension approach to DC tree characterization is certainly useful, a knowledge of the value of the exponent would be much more informative from the point of view of understanding the physical mechanism involved in the stochastic development of trees. This is also important because it is eta which controls the dimension of the tree structure. In their reply, Fujii et al. points out that, since the mechanisms of the streamers in solids are not understood well at present, it is difficult to discuss the difference of the patterns of the negative and the positive point electrodes. Therefore, the difference of the patterns for both polarities cannot be discussed using only the field dependence parameter eta .< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fractal character of DC trees in polymethylmethacrylate

The fractal dimensions of DC trees in polymethylmethacrylate have been investigated using the self-correlation function. The fractal-established range is proportional to the tree length for both positive and negative polarities of the applied voltage. For positive polarity, the fractal dimension increases with increasing inception voltage and saturates at 1.5 to 1.6. For negative polarity, the fractal dimension is nearly constant, approximately=1.5, as a function of voltage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of additives on DC treeing breakdown in polyethylene under divergent fields

The effect of additives on DC treeing breakdown phenomena and space-charge formation in polymeric materials is studied under divergent fields. Experimental results are presented to show that: (1) The DC treeing breakdown phenomena are considerably affected by the space charge. (2) The presence of acetophenon in low-density polyethylene (LDPE) enhances the amount of accumulated space charge, while blending with ionomer or polyamide suppresses it. (3) Blending ionomer or polyamide into LDPE improves the DC tree resistance. >