Water Tree Length Research Articles

Study on water tree resistance and self‐healing properties of multi‐monomer melt‐grafted polyethylene

AbstractIn order to suppress the damage of cable insulation caused by water tree structure formed by micro‐droplets at insulation defects in low density polyethylene (LDPE) materials under AC electric field. In this paper, the siloxane group is grafted onto LDPE by melt grafting method, and different content of comonomer styrene (St) is added to improve the grafting rate of siloxane group grafted polyethylene. The effect of St on the microscopic characteristics of LDPE grafted KH570 is studied. In order to verify the accuracy and timeliness of water tree self‐healing of grafted polymer insulation materials, the microstructure characteristics of LDPE ungrafted and grafted polymer samples are analyzed by Fourier transform infrared spectroscopy, gel content test, and differential scanning calorimeter. Second, the AC dielectric properties of LDPE ungrafted and grafted polymer samples are studied by AC breakdown experiment and space charge test. Finally, the water tree length, micropore defects, and self‐healing of the samples are observed by optical microscopy and polarization and depolarization current method, and the changes of chemical composition in the water tree area before and after aging are analyzed by infrared spectroscopy. The principle and mechanism of repairing water tree defects with grafted monomers are explained. This paper not only combines the previous research results of self‐healing materials, but also provides a new method for the preparation process of LDPE grafted polymer, which has a good research prospect in practical application.

Study on the water‐tree ageing characteristics of polyethylene/organic montmorillonite and crosslinked polyethylene/organic montmorillonite nanocomposites

AbstractThe dispersion state of nanoparticles affects the properties of composites, and the crosslinking process of polymers affects the dispersion state of nanoparticles. In order to investigate the effects of crosslinking behaviour on the water‐tree ageing characteristics of nanocomposite dielectrics at the molecular level, polyethylene/organic‐montmorillonite (PE/organic montmorillonite (OMMT)) and crosslinked polyethylene/organic‐montmorillonite (XLPE/OMMT) nanocomposites were prepared by the melt blending method, after which the accelerated water‐tree ageing experiments were conducted using a needle electrode. Fourier transform infrared spectroscopy, differential scanning calorimetry, successive self‐nucleation annealing and scanning electron microscope (SEM) tests were conducted to characterise the properties of the samples before and after water‐tree ageing. The results indicate that the initiation probability and the length of water trees in XLPE/OMMT are less than those in PE/OMMT. The carbonyl index in the water‐tree area of the two nanocomposites increases, indicating that the water‐tree ageing is the result of electrochemical degradation. The crystallisability of the water‐tree area degrades, decreasing the crystallinity and the lamellar thickness. The growth of water trees destroys molecular chains and crystal structures, which results in local cracks. There are many holes for the water tree growth in the PE/OMMT specimen. However, the crosslinked meshwork not only restricts the movement of macromolecular chains but also increases the stability of the lamellar barrier effect of OMMT. The combination of the two effects significantly improves the water‐tree resistance of the XLPE/OMMT nanocomposite.

Water-Tree Characteristics and Its Mechanical Mechanism of Crosslinked Polyethylene Grafted with Polar-Group Molecules.

In order to restrain electric-stress impacts of water micro-droplets in insulation defects under alternating current (AC) electric fields in crosslinked polyethylene (XLPE) material, the present study represents chemical graft modifications of introducing chloroacetic acid allyl ester (CAAE) and maleic anhydride (MAH) individually as two specific polar-group molecules into XLPE material with peroxide melting approach. The accelerated water-tree aging experiments are implemented by means of a water-blade electrode to measure the improved water resistance and the affording mechanism of the graft-modified XLPE material in reference to benchmark XLPE. Melting–crystallization process, dynamic viscoelasticity and stress-strain characteristics are tested utilizing differential scanning calorimeter (DSC), dynamic thermomechanical analyzer (DMA) and electronic tension machine, respectively. Water-tree morphology is observed for various aging times to evaluate dimension characteristics in water-tree developing processes. Monte Carlo molecular simulations are performed to calculate free-energy, thermodynamic phase diagram, interaction parameter and mixing energy of binary mixing systems consisting of CAAE or MAH and water molecules to evaluate their thermodynamic miscibility. Water-tree experiments indicate that water-tree resistance to XLPE can be significantly improved by grafting CAAE or MAH, as indicated by reducing the characteristic length of water-trees from 120 to 80 μm. Heterogeneous nucleation centers of polyethylene crystallization are rendered by the grafted polar-group molecules to ameliorate crystalline microstructures, as manifested by crystallinity increment from 33.5 to 36.2, which favors improving water-tree resistance and mechanical performances. The highly hydrophilic nature of CAAE can evidently inhibit water molecules from aggregating into water micro-droplets in amorphous regions between crystal lamellae, thus acquiring a significant promotion in water-tree resistance of CAAE-modified XLPE. In contrast, the grafted MAH molecules can enhance van der Waals forces between polyethylene molecular chains in amorphous regions much greater than the grafted CAAE and simultaneously act as more efficient crystallization nucleation centers to ameliorate crystalline microstructures of XLPE, resulting in a greater improvement (relaxation peak magnitude increases by >10%) of mechanical toughness in amorphous phase, which primarily accounts for water-tree resistance promotion.

Water Tree Propagation in a Wide Temperature Range: Insight into the Role of Mechanical Behaviors of Crosslinked Polyethylene (XLPE) Material.

To understand the propagation characteristics of water trees at a wide temperature range, this paper presents the effect of mechanical behaviors on the sizes of water trees. An accelerated water tree aging experiment was performed at −15 °C, 0 °C, 20 °C, 40 °C, 60 °C, and 80 °C for crosslinked polyethylene (XLPE) specimens, respectively. Depending on the micro observations of water tree slices, water tree length is not always increasing with the increase in temperature. From 0 °C to 60 °C, water tree length shows a trend from decline to rise. Above 60 °C, water tree length continues to reduce. Dynamic mechanical analysis (DMA) shows that the glass transition temperature of the new XLPE specimen is about −5 °C, and the α-relaxation is significant at about 60 °C. With the increase in temperature, the XLPE material presents different deformation. Meanwhile, according to the result of the yield strength of XLPE at different temperatures, with the increase in temperature, the yield strength decreases from 120 MPa to 75 MPa, which can promote the water tree propagation. According to the early stage in the water tree propagation, a water tree model was constructed with water tree branches like a string of pearls to calculate electric field force. According to the results of electric field force at different expansion conditions, with the increase in temperature, due to expansion of the water tree branches, the electric field force at water tree tips drops, which can suppress the water tree propagation. Regardless of high temperature or low temperature, the water tree propagation is closely related to the mechanical behaviors of the material. With the increase in temperature, the increased deformation will suppress the water tree propagation, whereas the decreased yield strength will promote water tree propagation. For this reason, at different temperatures, the promotion or suppression in water tree propagation is determined by who plays a dominant role.

Dynamic Thermomechanical Analysis on Water Tree Resistance of Crosslinked Polyethylene

The water tree resistance of crosslinked polyethylene (XLPE) initiated by ultraviolet (UV) irradiation technique is investigated through a water blade electrode method, and the effects of the mechanism of UV irradiation crosslinking on inhibiting water tree growth are revealed with dynamic thermomechanical analysis (DMA). The accelerated water tree aging experiment shows that UV irradiation crosslinking inhibits the growth rate of water trees, and the water tree length and width is reduced with the increase of the crosslinking degree of XLPE. The DMA result demonstrates that the molecular activity of the amorphous phase in XLPE as represented by polyethylene β-relaxation is gradually intensified with the increase of the crosslinking reaction. Combined with the fatigue mechanism of water tree growth in semi-crystalline polymers, it is suggested that the UV irradiation crosslinking reaction can significantly improve the anti-water-tree performance of linear low-density polyethylene (LLDPE). The crosslinking bond in the amorphous phase of UV-photoinitiated crosslinking polyethylene can produce a large number of cross-connected polymer chains, by which the length of fiber is obviously increased, leading to an reduced force from the micro-water beads onto the crack tip and thus decreasing the rate of the material being destroyed by micro-water beads.

Research on the discharge characteristics for water tree in crosslinked polyethylene cable based on plasma-chemical model

Water tree is the predominant defect in high-voltage crosslinked polyethylene cables. The microscopic mechanism in the discharge process is not fully understood; hence, a drawback is created towards an effective method to evaluate the insulation status. In order to investigate the growth of water tree, a plasma-chemical model is developed. The dynamic characteristics of the discharge process including voltage waveform, current waveform, electron density, electric potential, and electric field intensity are analyzed. Our results show that the distorted electric field is the predominant contributing factor of electron avalanche formation, which inevitably leads to the formation of pulse current. In addition, it is found that characteristic parameters such as the pulse width and pulse number have a great relevance to the length of water tree. Accordingly, the growth of water tree can be divided into the initial stage, development stage, and pre-breakdown stage, which provides a reference for evaluating the deteriorated stages of crosslinked polyethylene cables.

Condition assessment of XLPE insulated cables based on polarization/depolarization current method

The water tree is one of the key factors leading to the insulation property deterioration in cross-linked polyethylene (XLPE) cables. An accelerated ageing experiment has been performed to produce water trees with the purpose of ageing the full-size XLPE cable under the high-frequency electric field. Based on Polarization and Depolarization Current (PDC) method, the diagnostic tests are employed to evaluate the ageing extent of the cable subjected to water tree at different ageing time. Results show that the average length of water trees is proportional to the ageing time, and thus the experiment is effective to accelerate the growth of water trees. Furthermore, such various diagnostic indicators as the I∗t∼lnt curves, the loss factor at different frequency, have their rules to assess the ageing condition of the cable with the duration of ageing time. Therefore, information obtained from PDC current can be used for determining the ageing extent of the cables.

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.

Study on Inhibition of Water Tree in XLPE Insulation Material

The formation of water tree in XLPE insulation material was inhibited by adding EAA(Ethylene Acrylic Acid),DBS(dibenzylidene sorbitol) into cross-linked polyethylene-based materials. The mixture was crosslinked through mixing and tabletting the blend at 160 degrees. The effect of additive content on the length of the water tree was investigated. The water tree cultivation experiment showed that the length of water tree decreased with increasing of the content of EAA and DBS in the modified compound.Its full property experiments results showed that the property of the insulation material fully meet the related requirements for the use of anti-watertreeing XLPE cable.

Performance of water tight cables produced in Saudi Arabia under accelerated aging

This paper reports on the long term performance of water tight cables produced in Saudi Arabia subjected to accelerated aging. In this context, two types of water tight constructions were designed and implemented under the same production conditions. The first type was a completely water tight cable while the other was partially water tight. In addition, normal cross linked polyethylene (XLPE) insulated cables with medium density polyethylene (MDPE) and polyvinyle chloride (PVC) jackets were also included in this testing program. After an accelerated aging lasting more than 500 days, the retained dielectric strength and the water tree lengths were measured. The results show that the fully water tight cable outperforms the partially water tight cable followed by the normal XLPE cable. MDPE sheathed cable exhibited better performance than PVC sheathed one. The production cost of water tight construction was evaluated for different cable designs. The analysis shows that the water tight construction is economical and such cables should be preferred in areas where water treeing degradation is of concern.

EFFECT OF NANOPARTICLES ON WATER TREEING CHARACTERISTICS IN XLPE INDUSTRIAL I NSULATING MATERIALS

Water treeing is still now one of the main causes of cable deterioration and so, it is widely known that water treeing is the responsible for the reduction of the service life of polymeric cables. This paper has been studied the effects of adding various amounts of nanoparticles as ZnO, and Al2O3 on water treeing characteristics in XLPE industrial materials. This research has focused on studying development of the nano-composite materials performance with water tree growth resistance superior to the unfilled matrix, and has stressed particularly the effect of filler volume fraction on the water tree length and via various frequencies. Finally, this research aims to present a systematic and comparative study about the effect of adding nanoparticles to change dielectric insulation properties, and to understand the role effect of these nanoparticles in the industrial insulation materials which can make significant improvement in water tree resistance, life-time of insulation and decreasing the water tree length in the industrial insulation materials.

Study of Artificial Water Tree Degradation Method for XLPE Power Cables

In order to propose a method for producing water trees that measure a few millimeters in length in 20kV class to 60kV class XLPE cables, we discussed defect types and thermal conditions. First, surface roughness, needle protrusions, and line cracks were formed on the XLPE insulation surface of 6.6kV cables, and it was found that the water tree grew to a few millimeters in length from the needle protrusions and the line cracks. Second, water trees were generated in XLPE sheet and cable samples at room temperature and under a heat cycle and continuous elevated temperature conditions. Here, the longest water trees grew at room temperature. From these results, we selected needle protrusions and line cracks as defects and set room temperature as an artificial water tree degradation method. This method was applied with needle protrusions to a 22kV class XLPE cable, and we found that water tree length reached a few millimeters and AC breakdown voltage decreased to approximately 35% of the initial value.

Comparison of Propensity toward Water Treeing in XLPE Insulated Power Cables Produced in Arab Gulf Countries

Electric power utilities and industrial units in Saudi Arabia and other gulf countries are extensively using XLPE cables produced in the Arabian Gulf region. Due to prevailing high temperatures and high sulphate and chloride contents in the soil coupled with high sub surface water table in some areas, these cables have suffered serious breakdowns in a relatively short time span after their installation. This investigation was undertaken to compare the resistance to water treeing of 15 kV, XLPE insulated cables produced by four regional cable manufacturers. In the first stage, aging was carried out using AEIC protocol under high electric stress and temperature cycling in the presence of deionized water. After 120 days of such aging, the retained dielectric strength of these cables was determined by high voltage time test. The results showed poor performance of one of the cables while the three others exhibited close values. In the second stage, the cables were aged in the presence of aqueous ionic solution of CuSO4 and elevated electric stress for aging periods that lasted in the range of 500 to 1500 hours. Water tree number and lengths were measured and were analyzed using Weibull and log-normal statistical distributions and were found to fit better on log-normal model. Furthermore, water tree density which varied as a function of aging period obeyed a log-linear relation. These data also show that for a fixed aging period, water tree density in these cables as evaluated in the second stage of aging, comparatively varies significantly and leads to a better method of discrimination between different cables in order to select a cable that will serve best in wet local environmental conditions.

Effect of nanoparticle surface treatment on morphology, electrical and water treeing behavior of LLDPE composites

This paper reports on the role of silica nanoparticle surface treatment in affecting the morphology, charging current, frequency dependence of dielectric parameters, breakdown electric field and water treeing behaviors of linear low density polyethylene (LLDPE). Octyl-trimethoxysilane was used as a silane coupling agent for the surface treatment of the silica nanoparticles. The composite sample with silane-treated nanoparticles shows improved dielectric properties when compared with the sample with unsurface-treated nanoparticles. However, although both types of silica nanoparticles show apparent water tree resistance in LLDPE, the sample with unsurface-treated nanoparticles shows shorter water tree length.

Effect of silane-grafting on water tree resistance of XLPE cable insulation

Water treeing is one of the main deterioration phenomena observed in the polymeric insulation of extruded crosslinked polyethylene (XLPE) cables, which can affect the service life of power cables. In this work, we investigated the effect of grafting of a silane (vinyl trimethoxysilane, VTMS) on the resistance of XLPE to water treeing. A series of water-treeing tests, the mechanical and dielectric measurements indicated that the silane-grafting could significantly improve the water tree resistance of the conventional XLPE cable insulation with little influences on its dielectric properties, e.g., the dielectric breakdown strength, dielectric constant and loss tangent, and its mechanical performance. It was found that there exists an optimum value of VTMS concentration (about 0.6 phr) corresponding to the minimum water tree length. The water tree resistance mechanism of silane-grafted XLPE was proposed on the basis of the process of silane hydrolysis and crosslinking. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Life-Cycle-Management of CV Cables based on Degradation Diagnosis -Usability Assessment in Small-scale Application Considering Variation of Evaluated Cost-

Life-cycle management (LCM) based on degradation diagnosis, which is called condi- tion-based maintenance (CBM) can be useful for preventing an unexpected failure and extending service life of electric power apparatuses, resulting in reduced life-cycle cost (LCC). In our previous study, we formulated numerical model of CBM and applied the model to calculate the LCC of 6.6 kV XLPE cables, for which relatively rich data on degradation are available. Assuming that cables with longer water-tree length as a nondestructively measurable replacement criterion are replaced after the diagnosis in CBM, we evaluated the economic advantage of CBM against time-based maintenance (TBM) where all the ca- bles are replaced with a certain fixed interval. To carry out reliable CBM, accurate data are necessary as well as a well-established diagnostic method. However, power apparatuses are used under various condi- tions and furthermore failure probability would be different even with the same condition, and the data available for CBM are limited. Therefore, it is important to carry out robust CBM even with the insuffi- cient and/or inaccurate data. In this study, by using the data which is intentionally formulated to be dif- ferent from the master data, we examined the influence of accuracy of the data on CBM and evaluated its reliability and robustness. The main results are as follows. In TBM, if the replacement of 6.6 kV XLPE cables is carried out with the apparent optimal interval which is calculated by using the incorrect or in- complete data, the LCC can be much larger than the optimum LCC with the accurate date. On the other hand in CBM, the LCC is smaller than that of the optimum TBM, and does not change so much with the diagnosis parameters, i.e. replacement criterion and diagnosis interval. The results suggest that CBM can contribute to reliable life-cycle management even if the correct and complete data are not available to de- termine the diagnosis parameters.

A new dielectric response model for water tree degraded XLPE insulation - part b: dielectric response interpretation

Dielectric response measurements, in both time and frequency domains, can generate valuable information about the condition of cables affected by water tree degradation. However, the interpretation of how these dielectric response measurements relate to water tree density and length is a difficult task. This difficulty in assessing in particular the longest water tree length from dielectric response measurements is a substantial limitation, as it is well known that the remaining electrical strength of a water tree degraded cable is related to the longest tree length. This paper will detail a study whose goal was to enhance the understanding of how water tree degradation (in particular long vented trees) influences the dielectric response, and therefore seeks to reduce this interpretation limitation. The paper examines the application of a finite element model for water tree degraded insulation. This application involves using the dynamic electrical behavior as predicted by the model to generate frequency domain dielectric responses. The predicted electrical behaviour of both bow-tie and vented trees and the dielectric response that such trees will produce is examined and compared to actual Frequency Domain Spectroscopy (FDS) measurements on water tree degraded cable samples. It is shown that the developed model can accurately reproduce such measurement behavior, including the non-linear response. Because of the success of the model in reproducing actual dielectric response measurements of water tree degraded cables, it has proven itself to be a useful interpretive tool for water tree detection and assessment.

A new dielectric response model for water tree degraded XLPE insulation - part a: model development with small sample verification

Water tree degradation in underground XLPE insulated cables is a growing, worldwide problem. This form of degradation is ultimately fatal for affected cables, and therefore the detection of damaging trees in power cable insulation is vital for distribution companies to avoid catastrophic failure. Dielectric response measurements, in both the time and frequency domains, can generate valuable information about the condition of the cable. However, the interpretation of how these dielectric response measurements relate to water tree density and length is a difficult task. This paper will present a new dielectric response model for water tree degraded XLPE insulation. The model is based on finite element analysis to determine the electrical behaviour of water tree degraded insulation. Preliminary simulations will verify the model development by comparing the results to small sample pulsed electro-acoustic (PEA) measurements performed by other researchers. The importance of a strong non-linearity mechanism for accurate modelling will also be elucidated.

Water treeing, AC breakdown, and dielectric loss characteristics of EVA‐OH as a function of conversion rate

AbstractFour kinds of ethylene–vinyl acetate–vinyl alcohol (EVA–OH) terpolymers containing different contents of hydroxyl groups were prepared using the transesterification reaction between a ethylene–vinyl acetate (EVA) copolymer and alcohol. Structural and thermal analyses of the EVA‐OH produced were performed with Fourier transform infrared spectroscopy and differential scanning calorimetry. Electrical properties such as water treeing, AC breakdown, and dielectric loss characteristics were investigated. It was found that both water tree length and the probability of water tree generation decreased as a function of hydroxyl group content. AC breakdown characteristics such as average breakdown strength and characteristic breakdown strength (breakdown probability = 63.2%) of EVA improved as a function of the conversion rate of EVA‐OH from acetate to hydroxyl groups. Dielectric loss increased with hydroxyl group content in the low‐frequency range. In contrast, it was observed that dielectric loss decreased with hydroxyl group content in the high‐frequency range. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 420–424, 2006

Compatibilizing Effect of SEBS for Electrical Properties of LDPE/PS Blends

AbstractSummary: In the previous study, we observed compatibilizing effects of low density polyethylene (LDPE)/polystyrene (PS) with polystyrene‐block‐poly(ethylene‐co‐butylene)‐block‐polystyrene (SEBS), a triblock copolymer. Blends consisting of 70 wt.‐% LDPE and 30 wt.‐% PS were prepared with a SEBS concentration of up to 10 wt.‐%. This study examined the electrical properties such as the electrical breakdown, water tree length, permittivity and tan δ in the blends. The possibility of using these blends as insulating material substitutes for LDPE was investigated. The electrical breakdown strength reached a maximum of 66.67 kV/mm, which is superior to 50.27 kV/mm of the LDPE used as electrical insulators for cables. In addition, the water tree length decreased with increasing SEBS concentration. The water tree lengths of the blends containing SEBS were shorter than that of the LDPE. The permittivity of the blends was 2.28–2.48 F/m, and decreased with increasing SEBS concentration with the exception of S‐0. Tan δ of the blends increased smoothly with increasing SEBS content.Breakdown strength , water tree length, permittivity and tan δ of the LDPE/PS/SEBS blends and raw materials.imageBreakdown strength , water tree length, permittivity and tan δ of the LDPE/PS/SEBS blends and raw materials.