Electrical Tree Inception Research Articles

Development of a method for evaluation of the life curve of the HV and EHV cable polymeric insulation: The first results and fields of application

A test procedure and an experimental setup are presented that enable derivation of the “life curve,” i.e., the electrical tree induction period vs. applied voltage dependence for the low-density crosslinked polyethylene insulation. The developed equipment allows testing samples of materials both made in a laboratory and cut out from commercially manufactured cables at room temperature and at long-term permissible and overload temperatures. The fact of the electrical tree inception is established by light microscopy directly during simultaneous high-voltage test of ten samples. The optical system and the applied micromanipulation technique provide spatial resolution of approximately 1.5 μm, high-precision delivery of the microelectrode to the area of interest inside the sample, a permanent and reliable contact between the microelectrode and the material, and preservation of the residual mechanical stress field. The obtained results can be applied to development of new high-voltage cables with polymer insulation. The derived “life curve” enabled—using the corresponding mathematical model—calculation of the electric insulation thickness, which reaches 19–20 mm for the 220-kV cables.

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

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

Electrical treeing behavior of DC and thermally aged polyethylenes utilizing wire-plane electrode geometries

This paper presents electrical treeing behavior in low density polyethylene (LDPE) and cross-linked polyethylene (XLPE) after exposure to thermal and DC electro-thermal ageing. Both the ageing and the treeing tests were performed by means of two different types of test objects with wire-plane electrode geometry. One type of the tested objects contained only wire electrode of 10 μm diameter, whereas in the other type the wire electrode was attached through a semiconducting tab. The ageing was performed at 80°C with and without 10 kV DC voltage of both polarities connected to the wire and lasted up to 800 hours. The AC electrical treeing tests were applied afterwards for detecting changes of material properties after the ageing. The results showed that the electrical tree inception voltage consistently decreased with increasing time of thermal exposure, whereas the applied DC electric stress had a negligible effect on the observed behavior. Similar effects were found in both the tested materials (LDPE and XLPE) though the object type also influenced the results. For the objects with semiconducting tab, a higher level of the scale parameter was registered because of shielding effect of the tab on the electric field strength at the wire electrode. It also yielded less number of trees growing in parallel at the electrode. The dominant effect of thermal stress on the ageing of LDPE was elucidated by using various analytical techniques, like differential scanning calorimetry, infrared spectroscopy, inductively coupled plasma optical emission spectrometry and oxidation induction time, and it is believed to mainly affect antioxidant content in the test objects.

Electrical treeing behavior of DC and thermally aged polyethylenes utilizing wire-plane electrode geometries

This paper presents electrical treeing behavior in low density polyethylene (LDPE) and cross-linked polyethylene (XLPE) after exposure to thermal and DC electro-thermal ageing. Both the ageing and the treeing tests were performed by means of two different types of test objects with wire-plane electrode geometry. One type of the tested objects contained only wire electrode of 10 μm diameter, whereas in the other type the wire electrode was attached through a semiconducting tab. The ageing was performed at 80°C with and without 10 kV DC voltage of both polarities connected to the wire and lasted up to 800 hours. The AC electrical treeing tests were applied afterwards for detecting changes of material properties after the ageing. The results showed that the electrical tree inception voltage consistently decreased with increasing time of thermal exposure, whereas the applied DC electric stress had a negligible effect on the observed behavior. Similar effects were found in both the tested materials (LDPE and XLPE) though the object type also influenced the results. For the objects with semiconducting tab, a higher level of the scale parameter was registered because of shielding effect of the tab on the electric field strength at the wire electrode. It also yielded less number of trees growing in parallel at the electrode. The dominant effect of thermal stress on the ageing of LDPE was elucidated by using various analytical techniques, like differential scanning calorimetry, infrared spectroscopy, inductively coupled plasma optical emission spectrometry and oxidation induction time, and it is believed to mainly affect antioxidant content in the test objects.

Statistical Analysis of Electrical Tree Inception Voltage, Breakdown Voltage and Tree Breakdown Time Data of Unsaturated Polyester Resin

This paper presents a statistical approach to analyze electrical tree inception voltage, electrical tree breakdown voltage and tree breakdown time of unsaturated polyester resin subjected to AC voltage. The aim of this work was to show that Weibull and lognormal distribution may not be the most suitable distributions for analysis of electrical treeing data. In this paper, an investigation of statistical distributions of electrical tree inception voltage, electrical tree breakdown voltage and breakdown time data was performed on 108 leaf-like specimen samples. Revelations from the test results showed that Johnson SB distribution is the best fit for electrical tree inception voltage and tree breakdown time data while electrical tree breakdown voltage data is best suited with Wakeby distribution. The fitting step was performed by means of Anderson-Darling (AD) Goodness-of-fit test (GOF). Based on the fitting results of tree inception voltage, tree breakdown time and tree breakdown voltage data, Johnson SB and Wakeby exhibit the lowest error value respectively compared to Weibull and lognormal.

N-tridecane as a model system for polyethylene: comparison of pre-breakdown phenomena in liquid and solid phase stressed by a fast transient

Pre-breakdown phenomena in pure n-tridecane in liquid and solid state have been investigated experimentally in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Pre-inception currents in liquid and solid state were recorded and compared to finite element calculations to find plausible high-field conduction models for n-tridecane and cyclohexane. n-tridecane was chosen as a model system for polyethylene based on the known similarities in energy bands for polyethylene and alkanes. The results obtained in pure n-tridecane were compared to similar experiments on cyclohexane and polymer systems. High-field conduction in liquid n-tridecane was found to have a similar field dependence as previously reported for cross linked polyethylene (XLPE). Conduction currents and inception probabilities were found to be almost independent of polarity in frozen and liquid n-tridecane, which is also in line with results obtained for XLPE. Conduction currents were observed at lower voltages in n-tridecane than in cyclohexane, reflecting the lower space charge limited field (SCLF) in n-tridecane. Inception of streamers in liquid cyclohexane occurs at lower voltages than in liquid n-tridecane, and a stronger polarity dependence for all phenomena was observed in cyclohexane compared with n-tridecane. A possible explanation for the differences between n-tridecane and cyclohexane is given based on the field needed for electron avalanches and the SCLF in the two liquids. Inception voltages, light emission and charge injection were found to be similar in liquid and solid n-tridecane. This indicates that the same processes are responsible for inception and propagation of electrical trees in solids and streamers in liquids when the material is stressed by a fast transient. The similarity between the measured properties in solid and liquid phases leads to the conclusion that electrical treeing mainly takes place in the amorphous regions of the solid phase.

Effects of N,N-dimethylaniline and trichloroethene on prebreakdown phenomena in liquid and solid N-tridecane

Prebreakdown phenomena in 0.1 M N,N-dimethylaniline (DMA) and 0.1 M trichloroethene (TCE) in n-tridecane have been investigated experimentally in liquid and solid phases in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Results have been compared with results obtained in neat n-tridecane and in cyclohexane with the same additives. Adding TCE to n-tridecane increases the p re-inception current and decreases the inception voltage for negative and positive polarity. The increase in pre-inception current is probably caused by trap assisted conduction, and may be responsible for the decrease in inception voltage. TCE had no effect on propagation of streamers in n-tridecane. The main effect of DMA in n-tridecane is to enhance the propagation of streamers and electrical trees with positive polarity. This is in line with what has been reported for cyclohexane, and is explained by the low ionization potential of DMA. Charge injection for negative streamers is reduced by the addition of DMA in n-tridecane, while the emitted light is increased. This indicates that some of the energy otherwise used for propagation is emitted as light. A low electronic excitation energy for DMA at 3.8 eV supports this hypothesis. The additives have the same effect on prebreakdown phenomena in solid and liquid phases with positive polarity, and thus the same mechanism is suggested to be responsible for electrical treeing and streamer inception and propagation. The scatter is generally found to increase when going from liquid to solid phase, which is explained by the inhomogeneity in the solid phase. The transition to solid phase with negative polarity typically results in an even larger scatter than for positive polarity. The effect of additives seems to be secondary to the morphology at the point electrode with negative polarity.

Understanding Electrical Treeing Phenomena in XLPE Cable Insulation Adopting UHF Technique

Understanding Electrical Treeing Phenomena in XLPE Cable Insulation Adopting UHF TechniqueA major cause for failure of underground cables is due to formation of electrical trees in the cable insulation. A variety of tree structure can form from a defect site in cable insulationvizbush-type trees, tree-like trees, fibrillar type trees, intrinsic type, depending on the applied voltage. Weibull studies indicate that a higher applied voltage enhances the rate of tree propagation thereby reducing the life of cable insulation. Measurements of injected current during tree propagation indicates that the rise time and fall time of the signal is of few nano seconds. In the present study, an attempt has been made to identify the partial discharges caused due to inception and propagation of electrical trees adopting UHF technique. It is realized that UHF signal generated during tree growth have signal bandwidth in the range of 0.5-2.0 GHz. The formation of streamer type discharge and Townsend type discharges during tree inception and propagation alters the shape of the tree formed. The UHF signal generated due to partial discharges formed during tree growth were analyzed adopting Ternary plot, which can allow one to classify the shape of tree structure formed.

High efficiency voltage stabilizers for XLPE cable insulation

A set of new voltage stabilizers has been synthesized, tested and has shown to suppress a degradation mechanism, i.e. electrical treeing, present in cross-linked polyethylene used for high-voltage cables. Electrical treeing is seen at very high and divergent electrical fields and has a rapid lapse from initiation to total breakdown of the insulation material. The new voltage stabilizers presented in this paper have increased the electrical tree inception field with up to 50% at such low additions as 0.4%-wt. Furthermore, the best-performing materials have also proven to increase the threshold level for tree inception, i.e. before this level no deterioration of the material is seen, up to 50%.

A versatile method to study electrical treeing in polymeric materials

Electrical treeing in high voltage cable insulation has been studied widely as it is one of the major breakdown mechanisms for solid dielectrics subjected to high electrical stresses. The most frequently experimental set-up used to study electrical treeing is utilizing needles to achieve a high divergent field to promote the initiation of electrical trees. In this paper a new type of electrode configuration is described, which will be beneficial when examining electrical treeing in terms of both the capability to follow the course of events in real time and the small amount of insulation material is needed. This new electrode configuration makes use of a thin tungsten wire as electrode to obtain the high divergent electrical stress needed to initiate electrical trees. Multiple electrical trees are at times formed at different points along the wire during the testing, this indicates that the field are homogenous along the wire. This allows for determining the tree initiation field and the rate of tree growth. It is also shown that this electrode configuration is able to distinguish the increased electrical tree inception obtained by the addition of voltage stabilizers.

Study of electrical treeing phenomena in XLPE cable samples using acoustic techniques

Electrical trees were generated experimentally in the actual 33 kV underground XLPE cable insulation material under the AC voltages. A tree like structure and a bush type of tree structure can form from the point of defect site under the AC voltages. Acoustic emission technique was adopted to identify the point of inception, propagation and termination of electrical trees. A variation in the dominant frequency content of the acoustic signal was observed as and when the tree propagates in the insulation structure. The characteristic variation in the magnitude of the acoustic emission signal with time, indicates that tree propagation as an intermittent growth process. The energy content of the acoustic signal characterizes that the energy released due to partial discharges, at every step growth of the tree structure is not the same. The partial power measurement in the present study provides an indication to the growth process of electrical trees and to the near point of failure of polymer insulation material due to electrical treeing.

Diagnostic study of electrical treeing in underground XLPE cables using acoustic emission technique

Electrical trees were generated experimentally in samples from a 33 kV underground XLPE cable material under AC voltages. The formation of tree like structure and a bush type of tree structure was observed near the defect site of the insulation structure. To understand the point of inception and propagation of electrical trees, an acoustic emission technique was adopted in the present study. A characteristic change in the frequency content of the acoustic signal is observed as and when the tree propagates in the insulation structure. The partial power measurement used in the study provides an indication of the growth of electrical trees and of the imminent failure of polymer insulation material due to electrical treeing.

Selectivity of damped ac (DAC) and VLF voltages in after-laying tests of extruded MV cable systems

The purpose of HV after-laying tests on cable systems on-site is to check the quality of installation. The test on extruded MV cable systems is usually a voltage test. However, in order to enhance the quality of after installation many researchers have proposed performance of diagnosis tests such as detection, location and identification of partial discharges (PD) and tan /spl delta/ measurements. Damped AC voltage (DAC) also called oscillating voltage waves (OVW) is used for PD measurement in after-laying tests of new cables and in diagnostic test of old cables. Continuous AC voltage of very low frequency (VLF) is used for withstand voltage tests as well as for diagnostic tests with PD and tan /spl delta/ measurements. Review on the DAC and VLF tests to detect defects during on-site after-laying tests of extruded MV cable systems is presented. Selectivity of DAC and VLF voltages in after-laying testing depends on different test parameters. PD process depends on type and frequency of the test voltage and hence, the breakdown voltage is different. The withstand voltage of XLPE cable insulation decreases linearly with increasing frequency in log scale. Experimental studies with artificial XLPE cable model indicate that detection of defects with DAC or VLF voltage can be done at a lower voltage than with DC. DAC voltage is sensitive in detecting defects that cause a breakdown due to void discharge, while VLF is sensitive in detecting defects that cause breakdown directly led by inception of electrical trees.

Propagation of Electrical Tree in Extended Chain Crystal Polyethylene

In the present paper the electrical tree growth and tree inception voltage on oriented highly crystallised polyethylene is reported. The electrical treeing discharges were investigated with a needle to plate electrode system under a lightning impulse voltage application.

Critical parameters for electrical tree formation in XLPE

Published data for electrical tree inception field at a defect in XLPE vs. the Laplacian field at the tip of the defect are explained on the basis of a minimum distance which the space charged limited field must extend from the defect tip into the XLPE in order to damage enough dielectric in the field direction to facilitate PD inception and tree initiation.

Aging diagnosis of insulation systems by PD measurements. Extraction of partial discharge features in electrical treeing

The charge height values associated with PD measured during electrical tree inception and growth are processed according to the Weibull distribution. Test arrangements, based on the needle-plane electrode system, consider specimens having either a cavity at the tip or an intimate contact with insulation. It is shown that the Weibull-function parameters have close relation to discharges occurring in cavity and tree channels, and that an indication of inception of electrical trees can be obtained.

Luminescence in crosslinked polyethylene at elevated temperatures

Electrical treeing is often responsible for the breakdown of insulating materials used in power apparatus such as high-voltage transformers, cables, and capacitors. Insulation, such as crosslinked polyethylene (XLPE), used in underground high-voltage cables usually operates at temperatures above ambient. This paper describes the characteristics of luminescence, emitted prior to electrical tree inception, at a crosslinked polyethylene-semiconducting material (XLPE-semicon) interface held above room temperature. Use of a sensitive light detection system showed that XLPE subjected to elevated temperatures emits luminescence even without voltage application. This light was attributed to thermoluminescence which decreased with the decrease in the concentration of the crosslinking by-products present in the polymer. The spectra of thermoluminescence were only in the visible range. On the other hand, electroluminescence occurred when the XLPE-semicon interface was held above room temperature and subjected to high electric stress. This light did not depend on the concentration of the crosslinking by-products and the spectra of electroluminescence were in the visible and the ultraviolet ranges. It is proposed that XLPE-semicon interface held at elevated temperature and subjected to long-term voltage application initially emits both thermoluminescence and electroluminescence. As the crosslinking by-products exude out of the polymer, thermoluminescence decreases with time and ultimately ceases, but electroluminescence occurs as long as the voltage is applied to the polymer. Although the intensity of electroluminescence emitted at high temperature was lower than that emitted at ambient, sufficient ultraviolet radiation was emitted. The ultraviolet radiation could photodegrade the polymer and lead to electrical tree inception.

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.

Threshold voltage for electrical tree inception in underground HV transmission cables

Experimental evidence is provided to show that irreversible aging which leads to electrical treeing in polymeric insulation such as low-density and crosslinked polyethylene used in HV transmission cables, does not start at local fields between 5 and 20 kV/mm. It is shown that a much higher stress is required for electrical tree inception and that the electroluminescence inception voltage is the threshold voltage at which the polymer starts to degrade. >

Degradation mechanism at XLPE/semicon interface subjected to high electrical stress

In HV devices such as power cables, electrical stress enhancement can occur at the interface of semiconducting shields (semicon) and polymeric insulation. In this study, points of electrical stress enhancement are simulated by embedding semicon protrusions into the polymer. XLPE with two different concentrations of crosslinking byproducts and impregnated with various gases has been used. It it shown that, prior to electrical tree inception, electroluminescence occurs at the semicon tips and visible and ultraviolet light is emitted. The characteristics and the spectra of electroluminescence at semicon protrusions embedded in XLPE are similar to those previously observed at metallic needles embedded in LDPE. The ultraviolet light, emitted at points of electrical stress enhancement, can photodegrade the polymer, cause bond scission and lead to electrical treeing. Also, the results for XLPE impregnated with different gases indicate that, as in the case of LPPE/metal interface, oxygen in the free volume of the polymer plays an important role in the deterioration of XLPE insulation subjected to high electrical stress. >