Initiation Of Water Trees Research Articles

Enhancement in Electrical Properties of Impact Polypropylene Copolymer for Cable Insulation Induced by Rapid Cooling

Impact polypropylene copolymer (IPC) is a promising eco-friendly power cable insulating material with great potential to replace the thermosetting cross-linked polyethylene (XLPE). This article compares the breakdown strength and water tree resistance of IPC and isotactic polypropylene (iPP) after different cooling rates. Unexpectedly, an abnormal enhancement of IPC’s insulating property is discovered. The breakdown strength and water tree resistance of IPC are both increased after rapid cooling, which is opposite to the electrical property deterioration generally found on iPP and XLPE with similar treatment. To reveal the mechanism of abnormal enhancement of IPC, the crystalline structure, phase morphology, and trap distribution are characterized. The results indicate that rapid cooling induces an abnormal increase in the crystallinity of IPC due to the formation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> crystals, which in turn increases the deep trap level and density and leads to enhanced breakdown strength. Moreover, the formation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> crystal can also enhance the inter-lamellar coupling and result in higher tensile yield strength, further, decreasing the water tree initiation probability. These results reveal that rapid cooling can significantly regulate the performance of IPC, which should be given serious consideration for the application of IPC in cable insulation.

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.

A New Approach to the Analysis of Water Treeing Using Feature Extraction of Vented Type Water Tree Images

In this study, vented type water trees were initiated and grown in laboratory environment. A smart test platform was used to accelerate the initiation and growth of vented type water trees. 6 kV/4 kHz voltage was applied to the specimens to initiate and grow water trees. Mel-frequency cepstral coefficients of the vented type water tree images are obtained after 2 h and 10 h of aging respectively. The insignificant regions in the vented type water tree images were removed by using morphological filtering method before MFCC feature extraction. Finally, the statistical values of these features were analyzed. Scatter plots of the standard deviations and mean values of the cepstral coefficients were plotted. As expected, it has been observed that the points in the scatter plot are clustered in a certain area. MFCC is a popular and frequently used feature extraction method in speech recognition, however there are some studies which employs MFCC as a successful feature extraction method in image processing applications. This study provides a new approach to the analysis of vented water treeing using image processing techniques. The other new approach is using MFCC as a feature extraction method in microscopic water tree images.

Axial Water Ingress MV XLPE Cable Designs with Watertight Barrier

Humidity can enter the insulation system when the outer sheath of a cable becomes damaged. The relative humidity can then increase above a critical value (70 % RH), facilitating initiation and growth of water trees. The purpose of this experimental and numerical work is to determine how fast water vapour will diffuse axially in a soaked cable with a damaged outer sheath.Before drying (evacuation) relative humidity and temperature sensors were placed within the outer sheath at different axial positions. After the dryingprocess a hole was cut at the cable end facilitating water ingress. Numerical calculations of axial water diffusion were performed using Comsol.The numerical calculations show that the axial water vapour diffusion in the cable is slow and dependent of the air gap close to the swelling tape. After 430 days the measured humidity at 0.5 m had increased by about 75 % (initial 20%), and the sensor at 1 m had increased by 40%. Numerical calculations show a slower increase in the relative humidity. The actual axial liquid water penetration extensions, is yet not determined. The numerical calculations show that this is an important factor, as the calculations seem to be in more agreement with measurements when adjusting the position of the water front.

Enhancement of Water Tree initiation due to residual and applied Mechanical Strain on XLPE Cables

This paper present results from laboratory water tree experiments performed on samples of commercial available 6/10 kV XLPE insulated cables. The main purpose of the investigation was to examine possible water tree enhancement, due to mechanical tension of the insulationDuring water tree ageing at 50 Hz AC voltage of 14 kV (E_max = 5.2 kV/mm) the cable samples were mechanically stretched at 1%, using both static and dynamic mechanical load varying at 0.1 Hz. In addition, thermal treatment indicated a total residual longitudinal mechanical strains of 6%, frozen in during th emanufacturing process.Both the density and growth rate of vented and bow-tie water trees was found to increase with increasing applied mechanical tension. During non-strained thermal treatment the axial length of cables was reduced by about 6%. The low density of vented trees originating from te outer insulation screen supports the assumption that frozen in compressive stresses at the outer insulation surface may balance the effect of applied strain.

Water tree in polymeric cables: a review

The power cable breakdown suspected due to water treeing in polymeric cables are said to be the main cause. The inconsistent and unreliable nature brings out the importance of research in water treeing detection in polymeric cables. Early detection of water tree in polymeric cables is important in order to increase the cable efficiency by reducing the timeframe of the cable failure. This paper presents a comprehensive review of water treeing detections in polymeric cables, in particular, the relation to the background of water treeing, the types of water treeing, experimental investigations on water treeing, and factors affecting the initiation and growth of water trees.

A possible water tree initiation mechanism for service-aged XLPE cables: Conversion of electrical tree to water tree

In this paper, the conversion of electrical tree to water tree is found in a cross-linked polyethylene (XLPE) cable, and a possible conversion mechanism is presented. An electrical-tree initiation experiment was performed under AC voltages in XLPE cable specimens. After that, a 390-hour accelerated water-tree aging experiment was performed for the specimens, and water trees at the tips of the electrical tree were observed in the slices by an optical microscope. Also, to further understand and confirm the conversion, similar experiments were performed, and the same conversion phenomenon was observed in XLPE block specimens. Depending on the geometric size observed in water-tree cable slices, the 2D simulation model, including an electrical tree and a water tree, was constructed for electric field simulation. According to the results of electric field simulation, the electrical trees can be directly initiated at microscopic defects in service XLPE cables under the effect of transient over voltages. After initiation of the electrical tree, the forked branches can reduce the electric field around the tips of the electrical tree. When the over voltages disappear and moisture invades, the tips of the electrical tree can become the initiation sites of water trees because of stagnant propagation of the electrical tree. For this reason, the conversion of electrical tree to water tree is possible for service-aged cables, and these water trees even can make the electric field uniform at the tips of electrical tree.

Research on the water blade electrode method for assessing water tree resistance of cross-linked polyethylene

A new water blade electrode method is presented to accelerate water tree initiation and propagation in cross-linked polyethylene (XLPE) insulation, and the validity was verified by a series of experiments. Water tree morphology and length over time were monitored using a polarized light microscope and compared to the results through the conventional water needle electrode method. The influence of electric field distribution along the water electrodes on water tree initiation and morphology was also investigated on the basis of experimental data and finite element simulation. Furthermore, the effect of voltage frequency on water tree initiation was also investigated. It was found that the water blade electrode method could greatly accelerate water tree initiation and propagation and it was more convenient to slice XLPE for observation. The high frequency voltage could more efficiently facilitate water tree initiation and propagation.

A New Water Electrode Method for Short‐Time Testing of Water Treeing and Consideration of Temperature Effects of Water Tree Initiation and Propagation

AbstractWater treeing occurs when an AC voltage is applied to an electrical insulation system such as those in power cables and power apparatus. From a practical viewpoint, research on water treeing has mostly been performed under high‐frequency AC voltage application. However, power cables and power apparatus are also used under DC voltage application owing to progress in inverter technology. Inverter drive of rotating electrical machines has spread and expanded for global warming prevention and maintenance improvement. Underwater motors are used in water. In an underwater motor driven by the voltage of an inverter, it is necessary to consider water treeing of the motor insulation and cable insulation connected with a drive board. Thus far, a water electrode method that utilizes an AC voltage of high frequency has been used in the accelerated water treeing experimental method for a long time. On the other hand, an inverter with a surge voltage generates an equivalent AC frequency by the application of a multifrequency pulse voltage similar to a pulse width modulation (PWM) control wave. Therefore, there is a possibility of mistakes in real‐machine reliability evaluation because the initiation and propagation mechanisms are different from those in a real machine, even if we can reproduce water treeing by increasing the basic frequency. This paper reports our investigation of a new water electrode method and of temperature effects on water tree initiation and propagation. We also compare experimental results between 500 Hz and 50 Hz using the new water electrode method from the viewpoint of temperature. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 184(2): 1–9, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22422

Proposal of New Water Electrode Method Used for Short-time Testing Method of Water Treeing and Consideration on Temperature Effects of Water Tree Initiation and Propagation

Proposal of New Water Electrode Method Used for Short-time Testing Method of Water Treeing and Consideration on Temperature Effects of Water Tree Initiation and Propagation

Calculation of water ingress in a HV subsea XLPE cable with a layered water barrier sheath system

AbstractThe main aging mechanism of electrical cables with polymeric electrical insulation is the growth of water trees. Water trees are initiated if the relative humidity (RH) in the electrical insulation is above a critical level. Delaying the water ingress into the electrical insulation system delays the water tree initiation and reduces water tree growth, thus extending the service life of the cable. For a cable without any metallic water barrier, the water ingress can be significantly delayed by the use of an outer sheath material with low water permeability. An even greater delay in the water ingress into the electrical insulation can be achieved using a layered sheath system. To explore the possibilities of a layered sheath system, calculations of water ingress into a typical cable cross section has been performed using a finite element method. The water diffusion and sorption data used in the calculation has been measured for typical cable materials. Calculations have been performed for uniform temperature conditions and for a temperature gradient due to resistive current heating. The time to reach critical humidity levels and stationary humidity levels in the insulation system has been determined for several different arrangements of the sheath system. A sheath system with an outer layer of a material with low water permeability and an inner layer of a material with a high water absorption capacity is shown to give a significant delay of the water ingress into the electrical insulation. For the sheath materials used in this study, there is an optimum distribution of thickness of each layer. The calculations also show that a temperature gradient across the insulation system of a cable in operation gives an advantageous RH profile. With a temperature gradient the equilibrium RH level in parts of the electrical insulation can be lower than the critical value for water tree initiation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Effect of Cable Restoration Fluid on Inhibiting Water Tree Initiation

Silicone fluid has been applied in cable rejuvenation for decades to “cure” existing water trees. However, whether it inhibits future water tree initiation is not known. An electrical aging test has been carried out at 5 kV/mm (127 V/mil) based on a sample which includes semicon electrodes on both sides of the insulation. Samples were tested untreated, treated prior to the test, and treated at the midpoint of the test (3500 h). The results suggest that treatment reduces water tree initiation by a factor of 25 for bow-tie and 100 for vented water trees for the samples treated prior to aging. In the case of samples treated at the mid-point of the test, both bow-tie and vented water tree density are reduced by an order of magnitude relative to the density present at the time of treatment.

Chemical crosslinking of polyethylene and its effect on water tree initiation and propagation

The water tree resistance of chemically crosslinked polyethylene and of low density polyethylene were compared in order to elucidate whether the crosslinking itself influences or not the water tree characteristics of polymeric cable insulation materials. For this purpose, water trees were grown in compression moulded disks, obtained from pellets of either thermoplastic or chemically crosslinked polyethylene. Three types of crosslinked polyethylene were evaluated: one containing only peroxide (XLA) and two others having, besides peroxide, a tree retarding additive system (XLB and XLC), and the results were compared with those obtained on their thermoplastic correspondents (A, B and C). Results were determined for both Water Tree Propagation and Water Tree Initiation. The results obtained for Water Tree Propagation indicate that there is generally a large difference between the types (A, B & C) yet only small difference between thermoplastic and crosslinked samples. However for two cases (A & B) these small differences between crosslinking are statistically significant. The complementary Water Tree Initiation results show both large and significant differences between material types and the crosslinking.

Finite element analysis of electric field distribution in water treed XLPE cable insulation (1): The influence of geometrical configuration of water electrode for accelerated water treeing test

Water trees are localized degraded zones that can be found in the polymeric insulation of service-aged underground cables, and observed as microchannels or chains of microcavities in the bulk of a solid dielectric material. Since water trees have microscale local structures, some phenomena related to them can be understood and analyzed in detail only when using numerical analysis methods such as finite element method (FEM). For the artificial accelerated growth of water trees, water electrodes in needle form or with conical structure are often utilized, and then some unexpected initiation positions and shapes of water trees have frequently been observed. This paper deals with the investigation of the influence of the geometrical configuration of water electrodes on water tree initiation and its shape, on the basis of numerical experiment results using the FEM and accelerated water treeing test data.

Development of water tree structure in polyester resin

It is commonly accepted that water trees can be initiated and developed within insulation by the simultaneous action of moisture and alternating electric field. Despite the appreciable achievements and extensive works in the field of water treeing, there is no definite mechanism explaining its initiation and development. In this work, the results of a regular optical examination of the detailed structure of water trees are reported. Various patterns of water droplets are clearly illustrated before the initiation of water trees. The role of water absorption, as a main source of the tree, is not limited to the initial stage of tree formation but it is important for the creation of channels in the later stages. This idea is highlighted via a number of photographs, describing the main stages of the building process of water trees. New evidences on the mechanisms interpreting the development of water trees are introduced. Finally, the relationship between the obtained patterns of water filled voids and the subsequent shapes of water trees is discussed

Dielectric properties and spatial distribution of polarization in polyethylene aged under ac voltage in a humid atmosphere

This paper reports the results of an investigation of the dielectric properties, growth of water tree density and spatial distribution of polarization in samples of non-stabilized LDPE. Samples were initially sand-blasted to accelerate initiation of water trees, and subsequently aged in salt solution for approximately three weeks under two regimes of temperature (room temperature and 65/spl deg/C). Aging both with and without the presence of electric field stress was studied. The capacitance and dielectric loss of each sample were measured using a HV capacitance bridge, and compared to theoretical models over the frequency range 10/sup -5/ to 10/sup 4/ Hz. The results indicate a real movement of charges/ions in the polymer bulk which causes a deterioration of the electrical insulation properties even in the absence of AC fields. The presence of a field further degrades the insulator. Samples were studied using the LIMM (laser-intensity-modulation method) to determine the distribution of polarization, assuming mean polarization is zero and space charge is absent. In the absence of the external field there is a presence of ionic charges of opposite polarity, which increases with increasing aging time and is highest at room temperature. The length of the twenty longest vented water trees was measured after various aging times, and it was found that trees grown at room temperature are longer than those grown at 65/spl deg/C. Also, the tree density was higher in specimens tested at room temperature.

Electrical, chemical and mechanical processes in water treeing

Water treeing is a complex phenomenon involving several processes with many synergistic effects. Although a huge number of papers on the subject have been published over the last 25 years, there is no comprehensive theory able to describe the often contradictory experimental results. However, there are some tendencies that are always observed, whatever the experimental conditions. A critical review of some electrical, chemical and mechanical processes is made. In fact, water treeing is likely to be due to a mixture of all these processes. When existing data is examined carefully, it appears that very few theories can explain the results. It is concluded that voltage and frequency are major aging factors, and the role of oxidation may be less important than sometime was suggested. The energy of ion reduction is pointed out as a factor that may possibly influence water tree initiation and growth. The solubility parameter of additives and solutions is another factor that deserves some attention. Finally, the polymer morphology appears to affect water treeing greatly through modifications of its mechanical properties. Before a detailed model could be proposed, several studies remain to be done and thorough investigations devoted to the initiation phase and to synergistic effects are required. The development of a reliable and significant accelerated aging test simulating field conditions is also suggested.

Water treeing and dielectric loss of WTR-XLPE cable insulation

Abstract: In the investigation, the authors have tested three commercially available water-tree retardant XLPE (WTR) materials and compared some of their properties with conventional XLPE insulation. They have focused on measurements of the initiation and growth of water trees and dielectric loss at power frequency (50 Hz). Water- tree tests were performed on Rogowski-type test objects kept at 20°C and energised at 10 kV/mm for up to nine weeks of aging. Measurements of the dissipation factor tan 6 were performed on 12 kV cable samples in the temperature range 20- 130C, up to a maximum voltage of 96 kV,,, . The results show that water trees are also readily initi- ated in WTR materials, and that their growth rates are approximately as for conventional XLPE. In all the materials tested, introduction of mechanical strain strongly enhanced the initiation and growth of water trees. The WTR-cable samples examined showed different temperature- and voltage-dependent dissipation factors. Typic- ally, the dissipation factor of degassed cable samples at 20°C was about two times higher than that of conventional XLPE, and at 90°C it was six times higher. This difference was found to increase with increasing voltage. Prior to the degassing of the by-products from the curing process, some of the WTR cables showed high dissipation factors even at 20C, and their values were found to increase nearly proportionally with applied voltage.

Mechanical aspects of water treeing

The initiation and growth of water trees in a clear polyester resin are correlated with the mechanical properties of the material. The mechanical properties of the resin were varied systematically by the addition of controlled amounts of a plasticizing polyester. The polymer was in contact with 0.1-M NaCl solution and a 50-Hz AC voltage was applied to each specimen. It was found that the initiation time for water trees decreased (and the growth rate increased) with the plasticizer content. A correlation has been made between the initiation time and the tensile strength, both of which decreased due to plasticization, based on the assumption that initiation occurs when the osmotic pressure in a cavity at the water needle tip exceeds the mechanical strength of the material.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance of Ethylene-Propylene Rubber Insulation in Medium and High Voltage Power Cable

A new class of EPR insulations having significantly improved electrical loss properties is described. These elastomeric insulations are compared in a number of key properties vs. XLPE which are believed significant in certain failure modes of power cables. Laboratory studies have shown that this new class of EPR's is extremely resistant to the initiation and growth of water and electrical trees. Data from the AEIC Accelerated Water/Electrochemical Tree Test are presented for high molecular weight thermoplastic PE, XLPE and EPR. In this test EPR shows a significant improvement in performance vs. PE and XLPE which is particularly evident in the high temperature (high current load) test mode.