Degradation Of Cable Insulation Research Articles

ЕЛЕКТРИЧНЕ ПОЛЕ НАПІВПРОВІДНОГО ЕКРАНА СИЛОВОГО КАБЕЛЮ З ПОЛІМЕРНОЮ ІЗОЛЯЦІЄЮ З УРАХУВАННЯМ СТРУКТУРНИХ НЕОДНОРІДНОСТЕЙ

The non-homogeneous electric field distributions in the cross-linked polyethylene insulation of 330 kV power cable with multi-conductor core depending on the electrical conductivity and thickness of the semiconducting screen as well as at different frequency of applied voltage (from 50 to 104 Hz) are studied by finite-element analysis. As shown, taking into account the periodic structure of core surface, the electrical conductivity of the material of internal semiconducting screen should exceed 10–6 S/m and its thickness should be at least 1.5 mm in order to obtain the uniform electric field in the basic insulation of the cable at the boundary with the screen. The paper also considers the effect of various types of the surface and volumetric defects in the semiconducting screen on the electric field inhomogeneity in cable insulation. It is shown that the defects in the screen can lead to a significant increase of the electric field in the local zones of their location (up to 5 times as compared to the field without defects) and therefore contribute to the formation and development of treeing structures in the insulation. The revealed results explain the reasons for the micro- and macro-level degradation of power cable insulation and confirm the need to ensure the uniform structure, smooth surface of both semiconducting screen and core during the technological process of manufacturing the power cables with cross-linked polyethylene insulation. References 23, figures 8, table 1.

Influence of Thermal Aging on Space Charge Characteristics and Electrical Conduction Behavior of Cross-Linked Polyethylene Cable Insulation.

The aging of cable insulation presents a significant threat to the safe operation of cables, with space charge serving as a crucial factor influencing cable insulation degradation. However, the characteristics related to space charge and conduction current behavior during thermal aging remain unclear. This study focused on the thermal aging of cross-linked polyethylene (XLPE) material and utilizes a combined pulse electro-acoustic (PEA) and conduction current testing system to analyze the space charge and conduction current characteristics in the sample under varying electric fields and temperatures. The average charge density, short-circuit residual electric field, electric field distortion rate, and conduction current were studied. The findings indicate that the space charge in the samples following thermal aging is predominantly governed by the injected charge. The amorphous region of XLPE decreases, while the cross-linking degree increases after aging, thereby facilitating charge carrier migration within the sample and reducing the generation of charge carriers through thermal pyrolysis. The minimum temperature required for charge injection is reduced by thermal aging. Furthermore, modifications in conduction current, residual electric field, and average charge density indicate that thermal aging has the potential to alter the microstructure and trap characteristics of XLPE. This study provides empirical evidence to elucidate the underlying mechanism of cable insulation aging.

FMCW Reflectometry Method for Diagnosis and Prognosis of Cable Insulation Degradation

FMCW Reflectometry Method for Diagnosis and Prognosis of Cable Insulation Degradation

Locating Method for Electrical Tree Degradation in XLPE Cable Insulation Based on Broadband Impedance Spectrum.

Electrical treeing is one of the main causes of crosslinked polyethylene (XLPE) cable failure. The current methods for locating electrical trees are mainly based on the partial discharge (PD) signal. However, PD signals are easily attenuated in the long cable and the PD test voltage may cause damage to the insulation. This work proposes an improved broadband impedance spectrum (BIS) method to locate electrical trees in XLPE cable. A mathematical model of a long cable containing local electrical tree degradation is established. The Gaussian signal is chosen as the simulated incident signal to reduce the spectral leakage. The location spectrum is obtained by multiplying the frequency domain function of the single-ended reflection coefficient and the Gaussian pulse. It has been found that the location spectrum of the local capacitance change can be characterized as a typical double-peak waveform and the spectrum of the local conductance change can be regarded as a typical single-peak waveform. Electrical tree experiments at different temperatures were carried out to initiate different types of electrical trees. A vector network analyzer (VNA) was used to test the high frequency capacitance characteristics in the treeing process. The location spectra of the 20 m long cable containing different types of electrical trees was calculated by the improved location algorithm. The results show that the location error of local electrical tree degradation is less than 3%. The capacitance of the sliced sample decreases with treeing time. The effect of the bush-pine tree on capacitance parameters is greater than that of the branch-pine tree. A typical double-peak is found in the bush-pine tree location spectrum and a single-peak is found in the branch-pine tree spectrum.

On-Line Partial Discharge Localization in Power Cables Based on Electromagnetic Time Reversal Theory - Numerical Validation

This paper presents the numerical validation of a new on-line location method of partial discharge (PD) in cables of Medium Voltage (MV) power networks based on the electromagnetic time reversal (EMTR) theory. Because PD events are a symptom of cable insulation degradation, PDs localization is a key topic for fault prevention on power networks, increasing their reliability and resilience and to guaranteeing electricity security. A description of the proposed EMTR PD location method is given and its effectiveness and accuracy are analyzed in lines with both homogeneous and inhomogeneous power cables. The accuracy of PD location methods is strongly affected by the distortion of the PD pulses during their propagation on power lines, caused mainly by the skin effect. For this reason, the accuracy of the proposed location method is analyzed using a lossy model of power networks that is able to reproduce PD signals distortion. The model used, realized using the transmission line matrix (TLM) method, is also presented here and theoretically validated.

Detecting XLPE cable insulation damage based on distributed optical fiber temperature sensing

To ensure the safe operation of power systems and reduce the occurrence of major accidents such as cable insulation failures and fires, research on the insulation characteristics and insulation diagnosis of XLPE cables has been a hot topic. Here, we present a new method of detecting faults in XLPE cable insulation based on optical fiber temperature sensors. First, a model of cable insulation degradation is established. Through the establishment of different degrees of insulation degradation, the temperature rise characteristics of the cable surface and the relationship between the temperature rise and the degree of insulation failure are obtained. To verify the effectiveness of the method, an insulation degradation experiment was carried out on XLPE cable based on distributed optical fiber temperature measurement technology. Based on three sets of successful experiments, we demonstrate that insulation faults can be located accurately. The temperature rise law of the cable insulation fault point was obtained, and the experimental results were consistent with simulation analysis. The proposed method feasibly and effectively identifies the type of cable insulation fault and diagnosis the fault degree based on optical fiber temperature sensing technology for XLPE cable insulation fault detection.

A Novel Dampness Diagnosis Method for Distribution Power Cables Based on Time-Frequency Domain Conversion

Dampness invasion of power cable joints may lead to cable insulation degradation gradually. Once the cable joint is severely damp, it rapidly leads to breakdown failure for the power cable insulation system and finally results in catastrophic cable accidents. To avoid power outages, it is of great significance to diagnose cable joints with severe dampness defects. Existing methods have certain limitations. In this article, a novel time-frequency domain conversion method for determining cable joints with severe dampness defects is proposed. For this method, the cable joint is first located by applying the conventional frequency domain reflectometry (FDR) technology. Then, the time domain waveform at the cable joint is obtained based on the proposed time-frequency domain conversion algorithm. Finally, the cable joint with severe dampness defect can be distinguished by judging the polarities of the time domain spectrum at the cable joint. To verify the effectiveness of the proposed method, simulations and experiments are performed with comparisons with the conventional FDR method. Simulation and field detected results prove that the proposed method can effectively distinguish cable joints with severe dampness defects.

Dielectric Spectroscopy of Low Voltage Nuclear Power Cables Under Simultaneous Thermal and Mechanical Stresses

The insulation integrity of power and instrumentation & control cables plays a crucial role in the safe and reliable operation of nuclear power plants. Different stresses affect the insulating materials of these cables, which significantly contribute the structural changes results in the polymeric insulation. Consequently, the polymer material loses its mechanical and electrical properties causing cable insulation degradation. In this work, samples of low voltage nuclear power cable have been studied with accelerated aging tests. Combined simultaneous thermal and mechanical stresses have been applied to all samples under study. While the samples being placed in an air circulating oven at 120 °C temperature, they are coiled on a cylinder with diameter of 15 cm. The dielectric properties, capacitance and dissipation factor were measured by frequency swapping from 100 mHz to 1 kHz using a commercial dielectric response analyzer. Also, the shore D hardness was measured to investigate the changes in the mechanical property of the test specimens. Over the whole frequency range, the capacitance showed a downward trend with aging. In addition, the tanδ increased with aging for frequencies higher 10 Hz. A correlation between tanδ at 100 Hz and the insulation hardness was obtained.

Impact of High-Frequency Voltage Distortion Emitted by Large Photovoltaic Power Plant on Medium Voltage Cable Systems

Switching processes of voltage source converters can emit significant high-frequency distortions of frequencies between 5 to 20 kHz. Furthermore, the distortions can partly propagate from the place of their origin into a medium voltage system via capacitive coupling of power transformers. The origin, parameters, and propagation process of distortions were clarified by a numerical simulation of a 1-MW photovoltaic power plant connected to the medium voltage distribution grid. The determined results provided input data for the following accelerated aging test setup. The main idea behind the accelerated aging tests was to confirm the effect of the high-frequency distortions on the degradation rate of cable insulation systems. The tests were performed on cable samples, with terminations mounted on each end, which were split into three groups. The first sample group was stressed by increased power frequency voltage and by maximum operating temperature. The second group was stressed similarly, but with superposed high-frequency distortions present. The last group was not stressed to provide reference values for the other groups. Partial discharge, dissipation factor, and breakdown voltage measurements were carried out on both aged groups. The results indicate that cable insulation degradation progresses faster under power frequency voltage with superposed high-frequency distortions.

Electrical Detection of Degradation in Specimens of HVDC Cable Insulation

One of the challenges in laboratory investigation of degradation and ageing of HVDC cable insulation is related to securing, or in other words, imitating the real service environment of the material specimens. So far, the published data refer to experiments conducted in thermo-oxidative conditions, which is not the case during normal cable operation. In service, the cable insulation is protected by a metallic barrier that blocks the transfer of any substances in and out of the construction. By-products from the cross-linking reactions cannot diffuse out and any foreign substances are blocked from entering the insulation. Thus, in order to generate results that are valid, these conditions must be replicated in laboratory experiments. This contribution presents a methodology elaborated for performing ageing experiments in a hermetically sealed environment. Degradation of the material is evaluated through changes in the electrical tree inception voltage and test object capacitance over time. Securing the environmental isolation is accomplished with an isolation system consisting of a glass enclosure with attached metallic electrodes. Indium is used to create a glass-to-metal seal between the glass and the electrodes. The electrode geometry is of needle–plane type and the needle injection process is semi-automated to ensure specimen repeatability.

Novel Waveforms, Models, Algorithms for Cable Health Monitoring

Cables in power generation and delivery are under high thermal stress cycles. Such high temperature can lead to cable insulation degradation, which will reduce the projected lifetime. Existing methods mainly focus on cable fault detection or insulation degradation mechanism. There is no existing tools for diagnosing the insulation degradation level and predicting the remaining useful life of the cable. The goal of my Ph.D. research is to develop reflectometry and data based approaches to monitor the health status of cables. The research will be conducted in three steps: (1) development of reflectometry based method to monitor the cable insulation degradation; (2) feature extraction and cable insulation degradation dynamic modeling based on the accelerated aging test data; (3) development of risksenstive particle filtering based fault diagnosis and prognosis algorithms for cable degradation; and (4) verification and validation the proposed solution with new experiment data and comparison with existing approaches.

Electrical detection of degradation in specimens of HVDC cable insulation

One of the challenges in laboratory investigation of degradation and ageing of HVDC cable insulation is related to securing, or in other words, imitating the real service environment of the material specimens. So far, the published data refer to experiments conducted in thermo-oxidative conditions, which is not the case during normal cable operation. In reality the cable insulation is protected by a metallic barrier that blocks the transfer of any substances in and out of the construction. By-products from the cross-linking reactions cannot diffuse out and any foreign substances, such as oxygen and water, are blocked from entering into the insulation. Thus, in order to generate results that are valid, these conditions must be replicated in laboratory experiments.This contribution presents a methodology elaborated for performing ageing experiments in a hermetically sealed environment. Degradation of the material is evaluated through measurements of changes in the electrical tree inception voltage and test object capacitance over time. Securing the environmental isolation is primarily accomplished with an isolation system consisting of a glass enclosure with attached metallic electrodes. Indium is used to create a glass-to-metal seal between the glass and the electrodes. The electrode geometry is of needle-plane type and the needle injection process is semi-automated to secure a large degree of repeatability in specimen preparation.

Propagation coefficient spectrum based locating method for cable insulation degradation

To solve the complexity of the conventional time-domain reflectometry method for locating minor cable ageing degradation, a frequency domain-based method for locating cable ageing degradation based on propagation coefficient spectrum is proposed in this study. First, the change pattern of cable input impedance spectrum in frequency-domain reflectometry is analysed to reveal the relationship between input impedance spectrum and propagation coefficient, and thus finds out the working mechanism of ageing degradation on propagation coefficient spectrum. Then, in the experiment, a vector network analyser is used to measure propagation coefficient spectrum and the inverse discrete Fourier transform is applied to locate cable ageing degradation. Finally, kurtosis analysis is performed on locating curves of different bandwidth and the upper limit of frequency of the testing signal is decided. Within a certain limit, the method proposed in this study can locate degradation with higher accuracy and resolution than several time/frequency-domain based methods currently adopted.

A predictive model for polyethylene cable insulation degradation in combined thermal and radiation environments

Reliable prediction of material service lifetime is important for ensuring safe operation in safety-critical systems such as nuclear facilities. Materials in these environments degrade due to several simultaneous effects, most important stressors are heat and radiation. In this work, samples of medium density polyethylene cable insulation were subjected to thermal (110 °C) and γ-radiation stress (500 Gy h−1); a combined thermal and radiation test (85 °C and 4.5 Gy h−1) was also performed. Stability of cable insulation was studied by differential scanning calorimetry and isoconversional kinetic analysis employing a non-Arrhenian temperature function was carried out. From the dependence of residual stability on the degradation dose a predictive model for the cable service life was developed based on the thermal and radiation stress tests; the model validity was verified using the combined stress test results. The model assumes a first-order exponential decay of the residual stability with degradation dose. It allows to predict the degree of insulation damage in wide range of environmental conditions as both temperature and radiation dose effects are considered and radiation dose rate effect is also taken into account.

Application of temperature field modeling in monitoring of optic-electric composite submarine cable with insulation degradation

To effectively monitor the insulation state of the optic-electric composite submarine cable, the finite element numerical model for the temperature field of a 110 kV YJQ41 × 300 mm2 buried submarine cable was constructed in COMSOL, based on its thermoelectric coupling module. The results were validated by the improved IEC 60287 standard method and the optimal size of the physical dimensions of the model was obtained. On the basis of study of the effects of ambient temperature, buried depth, soil thermal conductivity, heat transfer coefficient of seawater and ampacity on the optical fiber's temperature in normal insulation state, the optical fiber's temperature rise in different levels of global insulation degradation was investigated. The results reveal that, when the cable insulation is globally degraded, the relative temperature rise of optical fiber is proportional to dielectric loss factor. Comparatively, the ambient temperature has little influence on it. A formula for estimation of dielectric loss factor based on relative temperature rise and ampacity of cable is presented. According to the formula, a method for evaluation of the submarine cable insulation degradation based on relative temperature rise of optical fiber is proposed. By tuning the structure and electrical parameters of a part of cylinder area in the insulation layer of submarine cable, the cable with different local insulation states was modeled by the combination of ATP and COMSOL. The effects of ambient temperature, volume resistivity of insulation degradation area, ampacity of cable and the size of the degraded area on the maximum relative temperature rise of optical fiber were investigated. When the cable insulation is locally degraded, the maximum relative temperature rise of optical fiber is inversely proportional to the volume resistivity of the insulation degradation area. It is proportional to the central angle of the insulation degradation area and increases with the square of length of the insulation degradation area. If the threshold of the relative temperature rise of optical fiber is set to 1 °C, the range of optical fiber's temperature rise caused by local insulation degradation decreases exponentially with volume resistivity of the insulation degradation area and always has a spatial scale of meters for the cases with severe insulation degradation, which is similar to the spatial resolution of optical fiber distributed temperature sensing system based on BOTDR (Brillouin Optical Time Domain Reflectometer). This work can be taken as a reference to evaluation and location of insulation degradation of submarine cable based on temperature rise of optical fiber.

Thermal degradation of flame-retarded high-voltage cable sheath and insulation via TG-FTIR

In energy application system, electrical energy is economic, practical, clean and easy to control and transfer. Electrical cables, especially high-voltage cables, have been extensively and increasingly applied in industries, and however their fire safety deserves more attention. Thermal behaviours of flame-retarded high-voltage Cable Sheath and Insulation were studied using SDT Q600 thermal analyser. The cable sheath pyrolysis profiles show three peaks while the insulation has only one peak. Possible reaction pathways and activation energy evolution with various conversion rates were analysed using both model-free and model-fitting methods. The zero order model and the 6th order model are responsible in two cable sheath pyrolysis stages, respectively. The one order model plays an important role in the pyrolysis of cable insulation. Evolved gas analysis by Fourier transform infrared spectroscopy (FTIR) shows that the amount of six main components produced in the order of most to least is CO2 > CH stretching > CH bending > CH aliphatic bending > CH2 deformation > CCl stretching for the cable sheath while the relationship of C–H stretching > CH scissor vibration > CO2 > CO & CC stretching is determined for the cable insulation.

Advancement of TDR Technique for Locating Power Cable Insulation Degradation

This paper discusses the advancement of time domain reflectometry (TDR) technique for locating degradation in power cable insulation with and without jointing. TDR technique is commonly used for localizing processes in various fields. It applies the concept of radar where a signal propagates through a medium will be reflected when there is an impedance mismatch. However, the application of TDR technique in power cable is still limited and its potential to pinpoint the exact location of degradation in power cables is to be ascertained. This study conducts experiments to investigate the potential of this technique in pinpointing the location of degradation in power cable with and without jointing. Experiments are conducted on un-degraded cable without any joint and followed by cable with jointing up to 2 cable joints. For experiments with jointing, the un-degraded cable is sectionalized into 2 parts, and one of the sections is replaced by a degraded cable. Experiments are repeated by sectionalizing the un-degraded cable into 3 parts, and each section is replaced with a degraded cable up to 2 sections. TDR results from all experiments are compiled and then analyzed. The results obtained from this study have proven that TDR technique is capable of identifying the degradation along a cable and at the same time pinpointing the exact location of degraded cable section.

Assessment of HDPE aged under DC voltage combined with AC harmonic stresses of various frequencies

One of the challenges on the increasing reliance on isolated renewable generation sources is the transmission of power from these sources to centers of power demand. One possible approach is the use of high voltage direct current (HVDC) transmission. The power electronic converters are key components in HVDC transmission system. The converters produce the intended DC voltage for transmission but there may also be AC harmonics superimposed. The superimposed harmonics on the HVDC may have synergistic effects and may lead to further degradation in the cable insulation. Previous research has shown that partial discharge was the main cause of degradation in polymeric insulation under AC stress. However, few publications have demonstrated the effect of combined stress on cable insulation degradation. Additionally, the most popular cable insulation material, cross-linked polyethylene (XLPE), cannot be recycled. Alternative materials which can be recycled have been proposed and one such solution could be thermoplastic materials. In this study, HDPE was investigated as a reference material for thermoplastics and their potential use as insulation in HVDC cables. In this paper the effect of frequency on HDPE degradation under superimposed stresses was studied using the following approaches; equivalent phase resolved partial discharge (PRPD) plots, fourier transform infrared spectroscopy — attenuated total reflection (FTIR-ATR) and dielectric spectroscopy (DS) measurements were carried out. The results show that during aging and with a frequency increase, the voltage of PD events increased which in turn created more polar molecule groups on the surface. The amount of polar molecule groups was found to affect e' and tano, with both increasing when more polar molecules were created. The results show that applying a higher AC frequency enhances polymer degradation.

Improvement in Cable Defects Assessment using Time Domain Reflectometry Technique

Background/Objectives: This study examines the potential of time domain reflectometry (TDR) technique in locating fault accurately along a cable due to cable insulation degradation and degradation at cable joint. Methods/Statistical Analysis: This study involves experimental verification of TDR technique in cable defect localization and identification. The experiment is divided into two parts. Part 1 investigates the ability of TDR to detect cable defect due to cable insulation degradation while part 2 investigates the effect of defect due to degraded cable joint on the TDR results. TDR results are analyzed to identify the reflections due to insulation and jointing degradations. Findings: A degraded cable can be identified by evaluating the velocity of propagation where signal propagates slower in a degraded cable, which causes delay in the reflections. A cable joint can be detected by TDR through a pair of reflections, which consists of a positive peak followed by a negative peak. A degraded joint can be recognized by comparing the amplitudes of the positive and negative peaks reflected where degraded joint gives higher positive and negative peaks. Application/Improvements: Improves the cable defects assessment by indicating the degradation of cable insulation and jointing, and pinpointing the degradation location through the time domain reflectometry technique.

Mechanical response of high density polyethylene to gamma radiation from a Cobalt-60 irradiator

The response of High Density Polyethylene (HDPE) to gamma irradiation is important for a variety of applications, from the durability of hip replacements after gamma sterilization to the degradation of power cable insulation that guides the licensing and regulation of nuclear power plants. HDPE samples are irradiated with up to 58.8 kGy from a Cobalt-60 gamma irradiator, and mechanical properties are examined using models assuming exponential behavior. Increasing the radiation dose led to increases to the ultimate strength and the Rockwell hardness with a corresponding reduction in the maximum elongation at ultimate strength, supporting the hypothesis that the samples increased their strength and brittleness.