Water Tree Degradation Research Articles

Study on the Acceleration Factors and Dielectric Breakdown Process of Water-tree Degradation in 6.6kV E-T Type XLPE Power Cables

Study on the Acceleration Factors and Dielectric Breakdown Process of Water-tree Degradation in 6.6kV E-T Type XLPE Power Cables

Study on Artificial Water Tree Degradation Method for XLPE Power Cables (Part 2)

Study on Artificial Water Tree Degradation Method for XLPE Power Cables (Part 2)

Modeling of Water Trees based on Investigation of Water Tree Degradation of 6.6 kV XLPE Cable Subjected to Accelerated Water-treeing Test and Calculation of DC Leakage Current and AC Superposition Current and Estimation of AC Breakdown Voltage by Transient Electric Field Analysis

Modeling of Water Trees based on Investigation of Water Tree Degradation of 6.6 kV XLPE Cable Subjected to Accelerated Water-treeing Test and Calculation of DC Leakage Current and AC Superposition Current and Estimation of AC Breakdown Voltage by Transient Electric Field Analysis

ＣＶケーブル（3kV〜22kV）未橋絡水トリー診断方法

ＣＶケーブル（3kV〜22kV）未橋絡水トリー診断方法

Assessment of charge behavior in water tree‐degraded cables under pulse voltages

In this article, water tree degradation was detected and was located by applying the method entitled ‘Charge Radar method’. We studied the reduction of charge depending on the number of pulse voltages and the time interval between each pulse (5 µs or 1 s). It was revealed that the charge amount remained stable at 5 µs of the time interval and decayed significantly at 1 s of the time interval. This tendency suggested that the number of pulses did not provide a significant change to the reduction of charge amount. We investigated the space charge behavior of a thin crosslinked polyethylene (XLPE) film sample using the Pulse Electro‐Acoustic (PEA) method. The result suggested that the shallowly trapped charge was released shortly after the film sample had been short‐circuited, and the residual charge would stay for a long time depending on the depth of the trapping layer. It was also revealed that the residual charge was not disturbed by a short pulse voltage. It was suggested that the reduction in charge amount depended on the short‐circuiting time after polarization. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Fundamental Study on Water-tree Degradation Diagnosis for Dry-cured and Extruded Three-layer (E-E Type) 6.6 kV XLPE Cables with Penetrated Water Tree

Fundamental Study on Water-tree Degradation Diagnosis for Dry-cured and Extruded Three-layer (E-E Type) 6.6 kV XLPE Cables with Penetrated Water Tree

Evaluation of relationship between residual charge signal and AC breakdown strength of water-tree degraded 22 to 77 kV classes XLPE cables removed from service using pulsed voltages

The authors have been developing a diagnostic method for water-tree degraded cross-linked polyethylene (XLPE) cables, in which pulsed voltages are used rather than the DC and AC voltages used in the conventional residual charge method. In this study, we first applied the proposed method to the diagnosis of water-tree degraded XLPE cables in classes from 22 to 77 kV, and successfully detected current signals related to water-tree degradation due to residual charges in the XLPE cables removed from service. Next, we investigated the relationship between the residual charge signal and the AC breakdown strength, and our results show that the correlation between the residual charge magnitude and the AC breakdown strength was weak, but the removed cables with a longer duration of the current signal related to water-tree degradation had a lower AC breakdown strength. Also, the ratio of the duration to the rise time of the applied pulsed voltage was strongly correlated with the AC breakdown strength. Therefore, we determined that this ratio can serve as a water-tree degradation index. Thirdly, we demonstrated the reason for this strong correlation using a simplified water tree model. Finally, to obtain greater sensitivity in current signal detection, we developed positive and negative pulse generators and confirmed that an increase in the applied voltage would increase the current signal detection sensitivity.

CVケーブルの設備形態ごとの劣化様相に関する調査

As XLPE cable laid in duct is mainly degraded by water-tree, water-tree growth and its affection to breakdown voltage of aged XLPE cable has been investigated. For XLPE cables laid under dry condition (substation, tunnel, etc.) or with water barrier, the growth rate of water-tree is considered to be lower than those laid in duct because of little water invading into the insulating layer. However, few reports related to this issue have been published and the degradation of aged XLPE cable laid under dry condition or with water barrier is not revealed. From these view-points, we investigated the breakdown voltage and the condition of water-tree degradation on removed 33kV XLPE cables laid under dry condition and with water barrier.

Investigation of Water Tree Degradation in Dry‐Cured and Extruded Three‐Layer (E‐E Type) 6.6‐kV Removed XLPE Cables

SUMMARYDry‐cured and extruded three‐layer (E‐E type) 6.6‐kV cross‐linked polyethylene (XLPE) cables were introduced into electric power systems more than 30 years ago, but they do not experience failures because of water tree degradation. Also, the degradation index of water treeing for these cables has not been established. Therefore, investigating results of residual breakdown voltage and water tree degradation of these cables will help us plan for cable replacement and determine water tree degradation diagnosis scheduling, and will be fundamental data for cable lifetime evaluation. In this study, the authors measured the ac breakdown voltages of dry‐cured and E‐E type 6.6‐kV XLPE cables removed after 18 to 25 years of operation and observed the water trees in their XLPE insulation. As a result, it was observed that breakdown voltages were larger than the maximum operating voltage (6.9 kV) and the ac voltage for the dielectric withstanding test (10.3 kV). Water trees were mainly bow‐tie water trees and their maximum length was approximately 1 mm. Although the number of measured cables was limited, the lifetime of this type of cable was estimated to be approximately 40 years, even experiencing water immersion.

乾式架橋・三層同時押出（E-E方式）の6.6kV撤去CVケーブルの水トリー劣化調査

Dry-cured and extruded three-layer (E-E type) 6.6kV cross-linked polyethylene (XLPE) cables were introduced into electric power systems more than 30 years ago, but they do not experience failures because of water tree degradation. Also, the degradation index of water treeing for these cables has not been established. Therefore, investigating results of residual breakdown voltage and water tree degradation of these cables will help us plan for cable replacement and determine water tree degradation diagnosis scheduling, and will be fundamental data for cable lifetime evaluation. In this study, the authors measured the AC breakdown voltages of dry-cured and E-E type 6.6kV XLPE cables removed after 18-25 years of operation and observed the water trees in their XLPE insulation. As a result, it was observed that breakdown voltages were larger than the maximum operating voltage (6.9kV) and the AC voltage for the dielectric withstanding test (10.3kV). Water trees were mainly bow-tie water trees and their maximum length was approximately 1mm. Although the number of measured cables was limited, the lifetime of this type of cable was estimated to be approximately 40 years, even experiencing water immersion.

直流漏れ電流測定による水トリー劣化位置標定

In general, the degradation span of 6.6kV XLPE cable with plural connections is determined by DC leakage current measurement. This method needs to disconnect and reconnect the shielding layer and the outer semi-conducting layer at the cable connecting part, and hence it takes a lot of time and cost. We investigated a new method to locate the water tree degradation by DC leakage current. In 6.6kV XLPE cables the pulse-like leakage current called “kick” due to water tree degradation is often observed. Consequently, we detected the currents at the DC applying terminal and another terminal, and transformed them to the signals like a sine-wave having the phase difference exactly proportional to the time difference of arrival (TDOA) by using ceramic filters at 455kHz. The position of the water tree is calculated from TDOA obtained by the phase difference. In the experiments to locate the water tree using 6.6kV XLPE cable samples up to 85m long, the mean squared error was 2.2m. From the results, it is possible to locate the water tree in 6.6kV XLPE cable 2km long with an error of about 20m by optimizing the center frequency of the ceramic filter.

Measurement of residual charge using pulse voltages for water tree degraded xLPE cables diagnosis

The residual charge method is one of the main degradation diagnostic methods for water tree degradation of XLPE cables. In this method, a DC voltage is applied to a water tree-degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages since the method can be applied to the XLPE cables that are connected to gas insulated switchgears. If a diagnostic method using pulse voltages for water tree-degraded XLPE cables is developed, the improvement of the detection sensibility for signals related to water trees is expected because of the averaging effect of signals by the repeated application of pulse voltages in a time that is shorter than conventional method. The diagnostic method with pulse voltages enables us to use more compact and lightweight high voltage devices. In this study, we applied pulse voltages with a pulse width of about 1 ms to an artificial water tree-degraded XLPE cable and a removed XLPE cable with a class of 6.6 kV, and successfully detected current signals related to water tree degradation.

Measurement of residual charge using pulse voltages for water tree degraded xLPE cables diagnosis

The residual charge method is one of the main degradation diagnostic methods for water tree degradation of XLPE cables. In this method, a DC voltage is applied to a water tree-degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages since the method can be applied to the XLPE cables that are connected to gas insulated switchgears. If a diagnostic method using pulse voltages for water tree-degraded XLPE cables is developed, the improvement of the detection sensibility for signals related to water trees is expected because of the averaging effect of signals by the repeated application of pulse voltages in a time that is shorter than conventional method. The diagnostic method with pulse voltages enables us to use more compact and lightweight high voltage devices. In this study, we applied pulse voltages with a pulse width of about 1 ms to an artificial water tree-degraded XLPE cable and a removed XLPE cable with a class of 6.6 kV, and successfully detected current signals related to water tree degradation.

Fundamental Study on Diagnostic Method of Water Tree Degraded XLPE Cables using Residual Charge Detection with Pulse Voltages

One of the degradation diagnostic methods for water tree degradation of XLPE cables is the residual charge method. In this method, a DC voltage is applied to a water tree degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages. In this study, we first applied pulse voltages with a pulse width of about 1ms to an artificial water tree degraded XLPE cable, and found that the residual charge increased with the increase in the degradation degree. Second, we confirmed that residual charges were successfully detected in the removed 6.6kV XLPE cables that had water tree degradation. Finally, we investigated the relationship between the residual charge signal and the AC breakdown voltage of the removed 6.6kV XLPE cables. The results clarified that the removed cables with larger residual charge and longer duration of current signal related to water tree degradation had a lower AC breakdown voltage.

Pulse response measurement for locating water tree degradation in XLPE cables

AbstractWater treeing is a degradation mode of power cable with polymeric insulation. A water tree is composed of small droplets filled with water. As the conductivity in a water tree is very high, it leads to dielectric breakdown when it grows. Because the inside of the water tree is filled with trap sites, it is polarized with a certain distribution of relaxation time when a DC poling voltage is applied. Although its depolarization process after removing the poling voltage depends on the ambient temperature, applying a “depolarizing voltage” with the opposite polarity can accelerate the process. If a short pulse propagating through the cable is employed as a depolarization voltage, we may locate the water tree by looking at the time‐resolved pulse response. This would lead to a diagnosis method with spatial resolution. In order to retain 100‐m spatial resolution, the response should be as sharp as 1 μs. As a preliminary study, a coaxial communication cable was aged to form water trees. A DC poling voltage was applied followed by a pulse voltage with opposite polarity. The rise time of the pulse was several hundreds of microseconds. A sharp pulse current response 50 μs wide was observed, suggesting that rapid depolarization took place. No such response was seen when the cable specimen was not aged. We concluded that the technique is quite feasible. Because the response was found to be as quick as several microseconds, an experiment using a cable 405 m long with 5 m of degraded length in the middle was performed. It was shown that the degraded point was successfully located. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 180(3): 18–24, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21294

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.

A nonlinear frequency analysis based approach for power cable insulation fault detection

PurposeThe present study is concerned with the application of a nonlinear frequency analysis approach to the detection and location of water tree degradation of power cable XLPE insulation without turning off electric power.Design/methodology/approachThe use of power cable system responses to power line carrier signals are proposed to conduct the required signal analysis for damage location purpose. This technique is based on the fact that the water tree degradation in power cables can make the system behave nonlinearly. Consequently, the location of water tree degradation can be determined by detecting the position of nonlinear components in power cable systems.FindingsA novel method has been proposed for locating water tree degradation in power cable systems; numerical simulation studies have demonstrated the effectiveness of the new technique.Originality/valueThe proposed technique has the potential to be applied in practice to more effectively resolve the power cable damage location problem.

Statistical analysis of diagnostic indicators during an accelerated ageing experiment for XLPE cable specimens

An accelerated ageing experiment has been performed, in order to age XLPE cable samples in a wet environment to advanced levels of water tree degradation. During this accelerated ageing experiment, semi-periodic measurements were performed using high voltage Frequency Domain Spectroscopy (FDS). This paper presents the rigorous statistical analysis that was performed on these populations of measurements. Various diagnostic indicators will be compared and assessed with respect to their correlation with the duration of the ageing time. In addition to this, the effect of environmental variables, such as humidity, will also be analysed using linear regression techniques. It will be shown that certain diagnostic indicators perform better than others at following the expected positive relationship with the time of accelerated ageing. It will also be shown that the humidity plays an important role in the low frequency dielectric response, even in the presence of a guard terminal, and therefore will show the need of taking such effects into account when analysing data. Comments will also be made about the level of fluctuation of the dielectric response encountered within the sample set.

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.

Aiming at a more rigorous understanding in electrical insulating materials research

Professors Inuishi, Ieda and Yahagi were pioneers of research in dielectrics in Japan. Their philosophy was that in order to understand a certain property of a material or to clarify the mechanism of a phenomenon, research must be done with scientific rigor and a scholarly approach. Here, the author wishes to illustrate their philosophy through a number of examples of his own research on both inorganic and organic materials and on topics ranging from vacuum discharge, through defect centers in inorganic glasses, to the water tree degradation phenomena in insulating polymers. Finally he will make some comments upon the way in which fundamental research can be useful in an engineering context.