XLPE Power Cables Research Articles

A new approach for the failure prediction in XLPE power cables using acoustic emission technique

Acoustic Emission (AE) is a Non destructive testing technique which has the unique potential for evaluating degradation and possibly failure prediction of XLPE cables as well. The authors have demonstrated its capability towards failure prediction on XLPE cables. In this study, five sets of fifty number of identical XLPE cables were taken up for the investigation. These tests were carried out on both damaged and undamaged conditions. The acoustic emission data acquired during their loading cycles is analyzed and a lucid empirical relation is developed to predict their failure performance. Using this approach one could appreciate that impending failure is significant even at 50–60% of maximum expected operating voltage (MEOV) with a reasonable error margin. In fact, till date there is no lucid method spelt out in the open literature for the failure prediction of XLPE Cables. Moreover, this methodology can also be applied to predict in real time the failure of similar XLPE cables made of other material systems.

The Fault Analysis and On-Line Monitoring Technique for XLPE Power Cables

XLPE power cable is widely used in grid system for its series of merits, but the fault become more and more frequent with increase of the use of cable and longer run time. In this paper, the reasons of the fault are introduced and the on-line monitoring technique of XLPE power cables is presented.

Development of a Portable XLPE Cable Insulation Detection Device

In this article, a portable XLPE cable insulation detection device is introduced. Such a device utilizes electromagnetic coupling, UHF electromagnetic wave and acoustic emission to detect partial discharge signals in power cables. By analyzing the partial discharge signals and cable temperatures, the insulation status of XLPE power cables is judged.

ELECTRIC FIELD EFFECT IN THE FORMATION OF WATER TREEING IN MV POWER CABLES

In this paper a MV (medium voltage) of 11 KV XLPE (cross linked polyethylene) power cable is implemented in COMSOL MULTIPHYSICS. This software simulates the results based on FEM (Finite Element Method) in order to know the stress effects of electric field in the formation of water trees. Water voids form when there is mechanical damage while laying directly or buried in ducts and water penetrates through the cable sheath. So the impurities fill the cable and result in treeing which develops slowly. The results show the field effect is more at the corners of the ellipse shaped water void which direct radial outwards from the cable axis. This maximum value of electric field causes the formation of channels between the micro cavities which further leads to breakdown.

Fractal features-based pattern recognition of partial discharge in XLPE power cables using extension method

Partial discharge (PD) may cause insulation deteriorations in power facilities and may demonstrate impact on the reliability. Therefore PD detection with pattern recognition is seen as an effective approach in the high-voltage (HV) insulation diagnostics of power systems. Based on an extension method, a PD recognition system for HV power cables is proposed in this study. A PD detector is used to measure raw three-dimension PD patterns of cross-linked polyethylene (XLPE) power cables using an L sensor, according to which two fractal features (the fractal dimension and the lacunarity) and the mean discharge are extracted as critical PD features that form the cluster domains of defect types. The matter-element models of the PD defect types are then built according to the PD features derived from practical experimental results. The PD defect type can be directly identified by the correlation degrees between a tested pattern and the matter-element models. The recognition ability is investigated on 160 sets of PD patterns of XLPE power cables and compared with a multilayer neural network and K-means method. The results show that a high accuracy together with a high tolerance in the presence of noise interference is reached by use of the extension method.

Study on Partial Discharge Detection of 10kV XLPE Power Cable

Partial discharge is a main factor which causes cross-linked polyethylene (XLPE) cable damaged. In order to research the Partial Discharge detecting methods of 10kV XLPE power cable, the electrostatic field is firstly simulated in order to analysis the electric filed at the distortion of the cable. According to the simulation mode, portable thermal infrared imager and ultraviolet imaging instrument is utilized to detect partial discharge of power cable terminals. The experimental results demonstrate that the use of ultraviolet imaging instrument to detect the location of defects is efficient and can observe corona photons; and thermal infrared imager can detect the temperature rising caused by Partial discharge. And this result also verified the simulation result that electric field concentration resulted in partial discharge. Test results also show that in short period of time partial discharge changes not much. This is mainly because the energy caused by PD does not accumulate so much in short time. That is contributing to the discovery of potential defects. DOI: http://dx.doi.org/10.11591/telkomnika.v10i7.1577 Full Text: PDF

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.

Analysis of electrical tree propagation in XLPE power cable insulation

Electrical treeing is one of the major breakdown mechanisms for solid dielectrics subjected to high electrical stress. In this paper, the characteristics of electrical tree growth in XLPE samples have been investigated. XLPE samples are obtained from a commercial XLPE power cable, in which electrical trees have been grown from pin to plane in the frequency range of 4000–10,000 Hz, voltage range of 4–10 kV, and the distances between electrodes of 1 and 2 mm. Images of trees and their growing processes were taken by a CCD camera. The fractal dimensions of electric trees were obtained by using a simple box-counting technique. The results show that the tree growth rate and fractal dimension was bigger when the frequency or voltage was higher, or the distance between electrodes was smaller. Contrary to our expectation, it has been found that when the distance between electrodes changed from 1 to 2 mm, the required voltage of the similar electrical trees decreased only 1or 2 kV. In order to evaluate the difficulties of electrical tree propagation in different conditions, a simple energy threshold analysis method has been proposed. The threshold energy, which presents the minimum energy that a charge carrier in the well at the top of the tree should have to make the tree grow, has been computed considering the length of electrical tree, the fractal dimension, and the growth time. The computed results indicate that when one of the three parameters of voltage, frequency, and local electric field increase, the trends of energy threshold can be split into 3 regions.

Effects of interfacial pressure on tracking failure between XLPE and silicon rubber

XLPE power cables are increasingly used in power distribution system for their excellent mechanical and electrical performance. However, interface between XLPE and silicon rubber in cable joint remains one of the weakest part of the cable system, because aging and relaxation of silicon rubber may reduce the interfacial pressure and then cause interfacial discharge and tracking failure. Therefore, this paper investigates the effects of interfacial pressure on interface discharge and tracking failure at the interface. Test samples were made by pressing a piece of XLPE and silicon rubber together under different interfacial pressures. AC voltage was applied on a pair of needle-plate electrodes at the interface with the insulation distance of 10 mm. Both the initial discharge voltage and the time to tracking failure were recorded with variation of the interfacial pressure and the applied voltage. Meanwhile, the discharge light and the carbonized configuration were also captured by using a high-speed camera. In order to quantify the light and the carbonization, the methods of image processing and fractal dimension (FD) were employed to establish the relationship between the quantitative characteristics of tracking process and the interfacial pressures. The results show that both the initial discharge voltage and the time to tracking failure increase with the increase of interfacial pressure. With increasing the interfacial pressure, the FDs of discharge light and carbonization path show the decreasing tendency, which indicates that the higher interfacial pressure can effectively delay the interfacial discharges and the tracking failure.

Effects of Reclosing for Insulation Coordination in 345kV Combined Transmission Lines

This paper describes switching surge analysis for reclosing decision in 345kV combined transmission line with XLPE power cable. Reclosing operation should be decided based on the detailed technical analysis in combined transmission line because this line includes power cable section which is week on insulation. Insulation of power cable can be breakdown at the week point in case of reclosing moment. Therefore the detailed analysis has to be carried out by considering several conditions such as length ratio of power cable section, arrestor, inserting resistance, charging rate, grounding resistance, etc.. On the other hand sheath voltage on IJ(Insulated Joint) is analyzed to check dangerous condition on cable cover. Analysis is performed by EMTP/ATP. Analysis results show that reclosing can be operated as the single line-to-ground fault occurs on overhead line in 345kV combined transmission line, if the inserting resistance is considered before the operation of main circuit breaker.

Electrical treeing characteristics in XLPE power cable insulation in frequency range between 20 and 500 Hz

Electrical treeing is one of the main reasons for long term degradation of polymeric materials used in high voltage AC applications. In this paper we report on an investigation of electrical tree growth characteristics in XLPE samples from a commercial XLPE power cable. Electrical trees have been grown over a frequency range from 20 Hz to 500 Hz and images of trees were taken using CCD camera without interrupting the application of voltage. The fractal dimension of electric tree is obtained using a simple box-counting technique. Contrary to our expectation it has been found that the fractal dimension prior to the breakdown shows no significant change when frequency of the applied voltage increases. Instead, the frequency accelerates tree growth rate and reduces the time to breakdown. A new approach for investigating the frequency effect on trees has been devised. In addition to looking into the fractal analysis of tree as a whole, regions of growth are being sectioned to reveal differences in terms of growth rate, accumulated damage and fractal dimension.

On-line dynamic cable rating for underground cables based on DTS and FEM

The development of surface temperature on-line monitoring techniques for 110kV and above XLPE power cable gives a strong support to power cable load dynamic adjustment. However, the problem is how to determine the conductor temperature from the surface temperature. A technique based on distributed temperature sensing (DTS) and finite element method (FEM) is presented to estimate the soil thermal properties by matching the computed thermal fields to that obtained from measurement and predict the real-time current carrying capacity by iterative method in order to prevent overheat of insulation. Compare with the thermal circuit model, the method based on DTS and FEM is more accuracy.

The electrical trees that developed in inner and outer layer of XLPE cable insulation

Due to the limitations of specific insulation structures and the influence of technical process，the growth of electrical trees in high-voltage power equipments with polymer as main insulation materials is more complicated than those in uniform materials，especially in equipment employing semi-crystalline polymer，so it is difficult to characterize by mathematical means. The experiment reported in this paper is the first attempt to focus on the electrical treeing rules in both inner and outer insulation layers of high-voltage XLPE power cable. Through statistical experiments and the SEM observation，large difference in treeing character between the inner and outer layers of cable insulation due to the different crystalline status are found. Electrical trees initiating in the inner layer of insulation has shorter initiation time，higher growth rate and greater variety of shapes compared than in the outer layers. The initiation and propagation mechanisms are also discussed.

Influence of Ethylene Ionomers on the Electrical Properties of Crosslinked Polyethylene

AbstractSummary: Water treeing is a deterioration mechanism observed in the polymeric insulation of extruded cables, which can affect the service life of the transmission and distribution XLPE power cables. To improve the water‐tree resistance of XLPE, it was blended with sodium‐neutralized EAA‐Na ionomers which were formed by neutralization of EAA with NaOH. A series of XLPE/EAA‐Na ionomer blends were investigated for their electrical properties, such as water treeing, electrical breakdown strength, dielectric constant, and dissipation factor. The results strongly suggest that EAA‐Na ionomers can improve the water‐tree resistance of XLPE, and the XLPE/EAA‐Na blends retain excellent dielectric properties. Characterization of XLPE/EAA‐Na blends by using FTIR indicates that the neutralization reaction is effectively achieved. In addition, it can be found that XLPE/EAA‐Na blends are partially compatible from observing morphology observations made by SEM and, therefore, EAA‐Na ionomers can be well dispersed in the matrix.Water tree length of the XLPE/EAA‐0.5Na blends.magnified imageWater tree length of the XLPE/EAA‐0.5Na blends.

Derating Factor for Underground Power Cables in Pipes

The current carrying capacity (ampere capacity Ampacity) of underground power cables are calculated according to their installation and operation conditions. In this study, a derating factor is determined for underground power cable which is installed in pipe. The 154 kV XLPE power cable which is used in the underground power transmission in Turkey is considered in computations. In addition the calculations are carried out for different two high voltage underground power cables. First, the ampacity of buried cable system is calculated and then the derating factor is determined for the same cable installed in pipe according to IEC 60287 standards. The results indicate that the permissible maximum conductor temperature of the cable can increase noticeably. Therefore, the ampacity of the cable in pipe should be reduced about %10. According to the results of ampacity calculation for different cable types, a derating factor of 0.9 can be used for high voltage underground power cables which are installed in pipe.

ＣＶケーブル絶縁劣化診断技術

XLPE power cables have been in use for long time now and some of them are approaching 30 years of their designed lifetime. However, under the present severe economic situation, it is necessary to use the existing apparatus as long as possible. So, degradation diagnostic methods with high reliability are needed earnestly. This paper explains the insulation degradation mechanism of XLPE power cables and the newest diagnostic methods on each voltage class. In 3.3-77kV classes, fairly high extents of the cable accidents depend on the water-tree degradation. For 3.3-6.6kV class cables, on-line diagnostic methods with high reliability to detect bridged water-trees have been developed and put in practical use. And, for 22-77kV classes, two promising off-line diagnostic methods to detect un-bridged water-trees have begun to carry out in on-site tests.

界面拡散法によるCVケーブル半導電層界面の改良

界面拡散法によるCVケーブル半導電層界面の改良

CVケーブル活線絶縁測定における発生モードによる1Hzの信号電流検出動作

1Hz current signals can be detected when in-service insulation diagnostic measurement is used for the maintenance test of XLPE power cables. By using “transmission mode” and the water tree deteriorated XLPE cables, the author had disclosed the detection mechanism of the 1Hz current signals By using “generation mode”, this paper describes detection mechanism of 1Hz current signals of in-service insulation diagnosis of XLPE power cables. Two kinds of generation mode operation mechanism had been studied. One, non-liner voltage and current characteristics of deteriorated insulator and beat-down operation create 1Hz signal, and another create wide band frequency signals like void discharge which is not proofed sufficiently using dried deteriorated specimen.

The ac superposition method, a new hot-line diagnostic method for 6-kV XLPE power cables

Water trees are the most hazardous factor affecting the life of XLPE distribution cables and the major cause of insulation failure. It is well known that insulation failure causes stoppages in electrical service and requires extensive repair work. Up to now, deterioration of cable insulation has been diagnosed mainly by the dc leakage current method. However, application of this method for diagnostic measurements requires interruption of electrical service. Several types of hot-line diagnostic methods (including the dc component current and dc superposition methods) were developed to detect water tree deterioration. However, these methods have some shortcomings, such as being subject to effects of stray currents and the accuracy of measurements not being sufficiently high. Therefore, we have attempted to develop a new hot-line diagnostic method. We investigated whether the signals produced by superposition of voltages of various frequencies to the cover layer of energized cables were correlated with deterioration by the water treeing. As a result, we found that a deterioration signal of 1 Hz was observed when we superposed an ac voltage (commercial frequency × 2 +1 Hz) on the cover layer of cable where the water tree had occurred in the insulation. © 1999 Scripta Technica, Electr Eng Jpn, 130(2): 49–58, 2000

ESCA analysis of semiconducting layer materials in XLPE power cables

This paper describes an application of ESCA analysis to semiconducting materials in XLPE power cables in terms of the oxygen ratio (integrated intensity of oxygen spectrum/integrated intensity of carbon spectrum). Analyzed materials were carbon black, an additive mixed into semiconducting materials, and several semiconducting materials treated in various cross-linking conditions. It was found that the additive increases the oxygen ratio of semiconducting materials as well as does the cross-linking treatment. The higher the oxygen ratio of the semiconducting materials, the smaller is the average lamellar angle in the XLPE insulation at the semiconducting interface. The angle is thought to be a parameter which expresses the hyperstructure of polymer insulation and smaller angles correspond to a better state of the semiconducting interface in XLPE power cables. © 1998 Scripta Technica, Electr Eng Jpn, 126(2): 15–22, 1999