Water Trees In Polyethylene Research Articles

Influence of an external electric field on the rupture force of decane, 4-decanone, and dicumyl peroxide molecules: Computational insight

To clarify the degradation of the electrical insulation ability of polymeric materials at the molecular level, the effects of an external electric field on the rupture of molecules were evaluated using quantum chemical calculations. Two molecular models were evaluated with the aim of elucidating the physicochemical background of realistic insulation deterioration factors, namely, the effect of oxidation and impurities. The first model was used to evaluate the effect of oxidative degradation of alkane molecules on the C–C bond; maximum rupture force of the C–C bonds of decane and 4-decanone were compared, and it was found to decrease by approximately 13.8 %. It was also clarified that the maximum rupture force changes depending on the applied direction of the external electric field. The second model was used to examine the effect of an external electric field on the decomposition of dicumyl peroxide, which is a cross-linking agent for cross-linked polyethylene. In the decomposition process of dicumyl peroxide, the rupture of cumyl alcohol was found to be strongly influenced by an external electric field. Because the decomposition of cumyl alcohol involves dehydration, it is likely to induce the initiation and propagation of water trees in polyethylene. Therefore, it is important to remove the cumyl alcohol before applying a high voltage.

Water trees in polyethylene insulated power cables: Approach to water trees initiation mechanism

Water trees occurring in polyethylene (PE) insulated power cables at defects exist on the inner or outer semiconductor layers or in PE insulation medium. Water trees start with the existence of water source and penetration under the action of high electric fields. The inevitable micro air voids (1 μm radius) in PE medium with an internal pressure 4450 Torr, and 2.3 dielectric constant, will lead to an electric field (E) of 1.08 MV/cm which can be called tree inception field (Ei). For a protrusion defect of 200 μm tip radius and 2.5 mm insulation thickness, the minimum required breakdown field in PE is 0.417 MV/cm at 2.5 per unit (pu) over voltages. In this paper a new breakdown equation has been introduced related to the injected energy (U), electric field (E) and air void radius (r) with the so called breakdown index (n) under two cases: First case, for n<25, the air void radius increases till 5 μm diameter (water bouquet value). But for the second case, n≥ 25, the air void will crack to many branches in free space and these assume 25 cracks at n=25. The crack length (Lcr) differs according to the mechanical strength (Pm) of PE insulation. According to the new introduced equations Lcr equal 197 μm, 248 μm, and 332 μm for Pm12 N/mm2 low density polyethylene (LDPE), 15 N/mm2 high density polyethylene (HDPE), and 20 N/mm2 cross-linked polyethylene (XLPE) respectively. The water will penetrate these cracks and the progress of water trees starts.

The influence of temperature on water treeing in polyethylene

The influence of temperature on water treeing in polyethylene (PE) is investigated in this paper. Low-density polyethylene (LDPE) and peroxide cross-linked polyethylene (XLPE) are chosen as the test materials. The liquid needle electrode method is used for water tree development at 20, 40, 60 and 80 °C, and a metallographic microscope is used to observe water tree morphology. The sizes and initiation rate of water trees are measured. The results obtained from the water treeing experiments indicate that water treeing is significantly influenced by temperature. The initiation rates of water trees decrease first, and then increase as temperature rises. The sizes of water trees also exhibit an increasing trend with ascending temperature. The experiments conducted on LDPE and XLPE demonstrate similar results. However, XLPE is found to have better anti water treeing performance than LDPE. Mechanical tensile test results show that the mechanical properties of the two materials, such as elastic modulus, tensile strength, tensile breaking stress and tensile yield stress, drop significantly as temperature increases. The results of the crystallinity determination and crystal morphology observation show that the crystal blocks of XLPE are smaller and denser compared with those of LDPE, which are lager and relatively sparser. Based on the analysis, XLPE is found to have better anti water treeing performance than LDPE because of cross-linking limitation to the formation of large crystal blocks and the enhancement of the mechanical properties of the material. Moreover, the failure of the mechanical properties of the materials, pore expansion and the accelerated movement of moisture and ions are the reasons for the acceleration of water treeing at high temperatures.

Voltage Zero-crossing as a Factor Inducing Water Trees

We have been examining the effects of superposition of a high-frequency voltage to various voltages such as dc, low-frequency (0.1 to 5 Hz), and power-frequency voltages on the development of water trees in polyethylene. We have made clear that the number of voltage zero-crossings is a decisive factor in the length of water trees. In the present research, the water tree shapes grown under the various superposed voltages were carefully observed. As a result, the water tree tends to become a hand-like shape if the frequency of the lower-frequency component is between 0 and 0.5 Hz, while it becomes spherical if the frequency is higher than 1.0 Hz. This result is explained by assuming that the water tree shape is governed by the number of consecutive voltage zero-crossings. By combining the results reported in our former papers, it can be concluded that the voltage zero- crossing is a decisive factor for the formation of water trees. Frequent mechanical oscillation at the tree tip due to the Maxwell stress should play a significant role.

Role of number of consecutive voltage zero-crossings in propagation of water trees in polyethylene

Effects of superposition of a low-frequency (0.1 to 50 Hz) voltage and a high-frequency 2 kHz voltage on the growth of water trees in polyethylene are examined. It has become clear that both the number of total voltage zero-crossings during the whole voltage application period and the number of consecutive voltage zero-crossings in the instant of the polarity reversal of the low-frequency voltage play important roles in the length and shape of water trees. Namely, the water tree length becomes longer as the number of total zero-crossings increases. Furthermore, among the voltages with the same number of total zero-crossings, the length becomes even longer with a prolonged shape in the direction of electric field as the number of consecutive zero-crossings increases. Effects of superposition of a high-frequency voltage onto a dc voltage are also examined. In this case, the dc voltage seems not to have any influence to the tree growth. By assuming the presence of space charge around the water-tree tip, the effective zero-crossings become only sensitive to the high-frequency component. Therefore, this result also indicates that the number of voltage reversals at the tree tip plays a crucial role in the growth of water trees.

Numerical calculation of electric fields, losses and interfacial charge densities in water-treed dielectric composite structure

Results of the computer modeling and an analysis of electrical processes in a water-treed polyethylene (XLPE) are presented. The water-treed region is considered as a binary composite structure in frequency domain. Electric fields, energy losses and interfacial charge densities are calculated and are examined using a simplified layered model and 3D model of a real water-treed XLPE insulation. It is shown that the interfacial charges in the bulk of the composite structure build up mostly due to differences in conductive properties of the constituent phases, and the distributions of the electric fields and losses are determined also by the morphology of the water-treed region.

Dynamic behavior of interconnected channels in water-treed polyethylene subjected to high voltage

While increasing a 500 Hz voltage from 300 V to 2000 V, the charging and ac loss currents of a water-treed XLPE are observed to increase more with voltage than those of a non-degraded XLPE. The fundamental and the third harmonic of the ac loss current have been numerically analyzed based on a model of water-treed insulation. In the model, a nonlinear voltage dependence of the conductance of a water-filled channel region is assumed. Magnitudes and phase angles of the fundamental and 3rd harmonic components of the ac loss current calculated from the model are compared with the observed ones. It was found that the length of water-filled channel region of a water tree grows with increasing voltage, and that this length approaches that of the water-treed region estimated by the pulsed electroacoustic (PEA) method.

Influence of frequency on water tree growth in various test cells

It is well known that water treeing in polyethylene (PE) is sensitive to frequency, but there is not yet a reliable analytic model describing this empirical observation. We show from existing published data that the length L of water trees grown in the laboratory under one given ionic solution varies linearly when plotted as log L vs. log N/sub 1/ where N is the number of field cycles. This relation is nearly independent of the applied field, suggesting a fatigue-like process. Ion concentration has only a slight influence on watertree length, especially after long aging. It was observed also that field aging and thermal aging reduce the watertree length. Some authors have suggested that there is an upper frequency limit above which water treeing would be independent of frequency. We have observed that the relationship log L vs. log N deviates from its linear behavior between 30 and 445 kHz. More work is needed to define more precisely the upper frequency limit. This means that accelerated aging for watertree degradation could be done at high frequency and results could then be translated reliably to power frequency. Finally, the fact that all experimental data obtained with one given ion obey the same relation regardless of types of the test cells indicates that results obtained with one given cell can be compared directly to those obtained with any other cell. Practical considerations also are discussed briefly.

Estimation of conductivity and permittivity of water trees in PE from space charge distribution measurements

Using an improved pulsed electroacoustic system which is capable of measuring space charge distribution at minimum intervals of 25 /spl mu/s, the charge distribution inside water-treed polyethylene (PE) was examined. From the results, the frequency dependencies of conductivity and permittivity were examined for water trees grown in PE. The conductivity of the water-treed region is found to be >10/sup 10//spl times/ higher than that of non-treed regions, while the permittivity increase is only slight.

ポリエチレン中の水トリーの導電率と誘電率

ポリエチレン中の水トリーの導電率と誘電率

水トリー内の連結チャンネルの一定課電下におけるダイナミズム

水トリー内の連結チャンネルの一定課電下におけるダイナミズム

Dynamism of interconnected channels in water-treed polyethylene

Loss current through non-penetrated water treed polyethylene increases with exposure time at 1000 V, 500 Hz. The increment is discussed by taking the capacitance of the water treed sheet sample to be the Maxwell-Wagner type capacitor composed of two layers: the capacitance both for the non-treed region and the unfilled channel region is connected in series to the parallel combination of the capacitance and the conductance for the water-filled channel region. By assuming a non-ohmic dependence of the conductance, the fundamental and 3rd harmonic components of the loss current have been calculated numerically, based on the equivalent circuit. The numerical results give a good agreement with the observed changes of the magnitudes of fundamental and 3rd harmonic components I/sub 1/ and I/sub 3/, and their phase angles /spl theta//sub 1/ and /spl theta//sub 3/, respectively, for the exposure time. It has been concluded that the water-filled channel region in the water treed region grows with exposure time. It is also concluded that the conductivity of the water-filled channel region increases slightly with its growth.

水トリー内の連結チャンネルの課電電圧に対するダイナミズム

水トリー内の連結チャンネルの課電電圧に対するダイナミズム

Space charge behavior under ac voltage in water-treed PE observed by the PEA method

The pulsed electroacoustic method (PEA) has been applied to observe space charge formation under ac application (7 kV/sub pk/, 50, 0.1 and 0.001 Hz) in water-treed polyethylene samples in order to understand the degradation mechanism of water trees. A system with phase-resolving capability has been developed to measure the space charge distribution at any phase angle of the applied ac voltage. By comparing the observed charge distributions with theoretically predicted ones, it has been shown that the interfacial polarization due to the difference in conductivity between the treed and the regions without trees dominate the space charge behavior. It has become clear that the PEA method can be a powerful nondestructive method for evaluating water tree length.

Silver tree

In this paper, AgNO/sub 3/ (silver nitrate) solution was used as liquid electrode to accelerate the growth of water trees in polyethylene. It was found that water trees grown in AgNO/sub 3/ solution can be colored spontaneously during their formation, their areas are brownish-yellow, and they grow fast. Scanning electron microscopy (SEM) revealed microchannels in the AgNO/sub 3/-grown water tree with diameters from 0.2 to 0.5 /spl mu/m. In fact, these microchannels are possibly filled with metallic silver reduced from silver ions, and these water trees are referred to as 'silver trees'. The structure of the silver trees is studied and analyzed in comparison with sulfide trees and blue water trees. The effects of silver ions (Ag/sup +/) in the silver tree region are investigated and a comparison is made with other ions such as K/sup +/, Na/sup +/, Mg/sup ++/, and Cu/sup ++/ in water trees. The trees from AgNO/sub 3/ solution grow much faster than trees from the other solutions. Finally, the mechanism of the formation of silver trees is suggested.

Correlation of optical and dielectric properties of water trees in polyethylene

Many experimental works report that the presence of water trees lowers the voltage needed for the initiation of dielectric breakdown in polyethylene. The influence of water trees on the dielectric breakdown is generally related to their length. Here, in addition to this spatial property, we consider the extinction of the light by the water trees due mainly to scattering of the incident light by water microcavities. This effect is described by means of the water tree absorbance. The absorbance itself is related to another main variable: the water content. We also take into account the existence of a structure in the water tree, i.e., of a heterogeneous distribution of water microcavities inside the trees. Water content and spatial water distribution allows us to define a local permittivity for each region in the tree. These elements are the basis of a model for the computation of the disturbance induced by the tree in the spatial distribution of the electric field (reduction of the field inside the tree, enhancement outside). The field enhancement effect is computed as a function of the water concentration. Measurements of the light transmitted by given water trees are performed; they allow us to propose an estimate of the water contained by different water trees. From these concentrations, different field enhancement coefficients are computed from the model. They are then compared with measurements of electrical treeing inception voltage performed with and without water trees. Theoretical values obtained from the model are found in good agreement with experimental data.

Three-dimensional optical microscopy of water trees in polyethylene

Polyethylene is used as high-voltage electrical insulation in cable, and fails when subjected to the combination of water environment and electrical field. The three-dimensional microstructure of water trees, which are precursors to dielectric breakdown, has been revealed with laser scanning confocal optical microscopy. Two-dimensional images which represented optical serial sections were recorded in digital form, and reconstructed in a computer to produce three-dimensional views of the microstructure. Both stereo pairs and computed tomographic reconstructions were made. The material was stained with a fluorescent dye and the fluorescent image was compared to the standard unstained image. The dye was found to increase the signal-to-noise ratio and improve the quality of the image significantly.

Electrode influence on the propagation of water trees in polyethylene

The influence of the nature of various solid electrodes in contact with the liquid on the propagation of water trees was studied in laboratory experiments, using the water-needle method. Several hypotheses put forward to account for the observed electrode effect are examined. It is shown that the role played by the electrode is essentially due to ions entering into solution in the liquid through electrode corrosion.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Water treeing using the water needle method: the influence of the magnitude of the electric field at the needle tip

The initial growth kinetics of water trees in polyethylene is measured for different values of the radius of curvature of the needle tip and for different applied voltages. The initial propagation rate is shown to be determined by the magnitude of the electric field at the needle tip, except at its highest values. The local electric field at the solution/polymer interface or in its vicinity plays an essential role in the growth mechanisms of water trees; it determines the initial propagation rate up to values of about 100 to 300 V/ mu m. Above these field values the water tree growth is found to be controlled by slower processes, e.g. the migration of the liquid through the polymer. These results are used in discussing some experimental conditions which are required to get reproducible results in water needle tests.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An Investigation Into the Initiation Mechanisms of Water Trees in Polyethylene from Attempts to Detect an Emission of Light

A very sensitive device has been used to try to detect the emission of light during the initial phase of the growth of water trees. No emission was detected, either before the onset of growth of the trees or during the propagation, contrary to what is observed in electrical treeing. This experiment eliminates light emission as a tool to examine initiation and propagation mechanisms in water treeing. It is suggested that there is no basic difference in the very nature of the mechanisms which govern the initiation and propagation phases of the growth of water trees, contrary to what is accepted for electrical treeing.