Interpretation of dielectric loss data on service aged polyethylene based power cable systems using VLF test methods (original) (raw)
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JICABLE11, Versailles, France, 2011
Cable System Management requires an assessment of the health of the cables system. It is increasingly common for the assessment of aged cable systems to be made through the application of diagnostics measurements. A recent study has shown that VLF Tan Delta is perhaps the most commonly deployed cable system diagnostic. The practical use of this technique has been supported by the international standards IEEE400-2002 and IEEE400.2 (latest version). A key part of these standards is the guidance provided to a user that is detailed in the "Figures of Merit". These enable users to make practical improvements to the cable system. To aid these decisions a series of criteria have been developed. The benefit of the criteria described here is that the process is rational, reproducible and transparent. The outcomes are supported by a probabilistic assessment of service performance.
During the last decade, Very Low Frequency (VLF) testing for extruded distribution cables has gained interest among the North American utilities. The increasing interest is evidenced by recent research publications and discussions inside the expert community in which standards are being proposed and continuously discussed. While there is a general consensus as to the meaning of insulation dielectric properties, many open issues still remain for discussion in order to produce a more accurate evaluation. Consequently, this paper will discuss a number of the practical issues that arise when making these measurements at VLF on field aged and non-aged cables, particularly Tan δ measurements. The discussion is based on data from laboratory experiments and field testing.
Characterization of Ageing for MV Power Cables Using Low Frequency Tan δ Diagnostic Measurements
IEEE Transactions on Dielectrics and Electrical Insulation, 2009
This paper describes Very Low Frequency (VLF) Tan δ experiments performed on field-aged and non-aged distribution Medium Voltage (MV) cable samples. The fieldaged samples constitute a uniform set of Cross-linked Polyethylene (XLPE) of 15 kV unjacketed cables removed from the same service area having experienced similar operating and ageing conditions. The non-aged samples are a diverse set of Crosslinked Polyethylene (XLPE) and Water Tree Retardant Cross-linked Polyethylene (WTRXLPE) cables of 15 kV and 25 kV and Ethylene Propylene Rubber (EPR) cable of 25 kV. The experiments are designed to contribute in understanding, time, voltage and discharge time dependence of Tan δ diagnostic measurements at VLF of 0.1 Hz. Results help in clarifying issues that arise when characterizing MV cable insulation by Tan δ diagnostic measurements. The issues include time-on-test, voltage level as a diagnostic tool, diagnostic features, and reproducibility and repeatability of the measurements. The paper shows that higher insulation losses, non-linearity, hyteresis, and variation in voltage and time of Tan δ diagnostic measurements at VLF are indicators that can be used to properly characterize the insulation and enhance the diagnosis.
On-line cable diagnostic posibilities in an artificial aging environment
Renewable energy & power quality journal, 2011
Since more than a decade the development of reliable and accurate diagnostic tools for power cables is in the centre of interest of scientists and power utilities. With the developments in our power systems, like the increasing feed-in of renewable energy sources, several issues could be additionally generated, such as extreme concurrence factors, short-and longduration variations, voltage imbalances, waveform distortions, voltage fluctuations and power frequency variations. These impacts affect the aging rapidity of the electrical equipment and finally of the entire the electrical distribution network. Using appropriate diagnostic methods, it is possible to determine the electrical equipment's condition. The significance of the results depends on the quality of the measurement system and of the data interpretation. Based on these results, a reliable maintenance and investment strategy could be made, and the reliability of the grid could be improved. With this goal, an accelerated aging project for MV PILC (paper insulated lead covered) cables was started. A specially designed aging system for the accelerated aging of MV cables has been developed to point out the most relevant aging parameters, their limits, their development and to upgrade the accuracy of the MV cable diagnostics in this way. In this paper, the diagnostic possibilities, mainly the dielectric loss factor measurement and its dependence on environmental influences are presented.
22nd International Conference and Exhibition on Electricity Distribution (CIRED 2013), 2013
The paper focuses on the new possibilities of managing a key element of the power supply system: the power cable network. The possibilities are created by the technical condition assessment of power cables based on partial discharge diagnosis at damping (self-extinguishing) AC voltage. Both cable manufacturers and their users pursue to obtain a high technical reliability of the power network. Modern IT and measurement systems make it possible to acquire data enabling a better assessment of the line technical condition. Undoubtedly, among the new data sources there is also diagnostics based on partial discharge (PD) measurement (PD inception voltage, PD extinguishing voltage, the PD apparent charge value for different voltage levels, PD intensity, PD distribution as a function of cable length). Providing a unique data set which describes an insulation technical condition of particular line elements, such a diagnostics creates an opportunity to change the current method of power cable network management to a new more effective one, both technically and economically. Measurements of partial discharges in MV cable network were carried out in one of the distribution companies in Poland in the years 2005-2010. They were intended to develop an optimal method for the use of partial discharges diagnosis in the assessment of technical condition of power cables. The main area of research covered MV cable lines of a total length of 590 km.. The studies on power cables provided a group of measurements for Paper Insulated Lead Covered power cables (PILC). From all the measurements there were selected 18 cases where a cable failure was recorded and the measurements of partial discharges were carried out both before the failure and immediately after the repair of the failure. Basing on the carried research work and analysis it has been found that: Basic parameters indicating the risk of the failure are: reduced PD inception voltage and the occurrence of increased PD intensity in a power cable. The study shows that, the lower the PD inception voltage, the higher the percentage of cable sections of an increased PD intensity. Consequently, a greater number of cable sections which can be included to the group of increased risk of failure. No correlation between the increase of PD value and increasing probability of the failure occurrence has been found..
Health Monitoring of Power Cable via Joint Time-Frequency Domain Reflectometry
IEEE Transactions on Instrumentation and Measurement, 2011
Utilities are experiencing premature failures of power cables. In order to prevent electrical outages and to save on repair expenses, a nondestructive and nonintrusive condition assessment technique is highly desirable to evaluate the cable status and to predict the remaining life of a cable. In this paper, the capability of joint time-frequency domain reflectometry (JTFDR) as such a condition assessment technique is studied. The health status of three popular insulations in power system cables-cross-linked polyethylene, ethylene propylene rubber, and silicone rubber-is monitored using the JTFDR in a thermal accelerated aging test. The experimental results show that the JTFDR can successfully monitor the aging process of all three insulations. Then, the results from the JTFDR are compared with the results from the elongation at break (EAB); the results show that the JTFDR technique is comparable with the EAB and has a great potential as a nondestructive and nonintrusive condition assessment technique. Index Terms-Accelerated aging test, cross-linked polyethylene (XLPE) cable, ethylene propylene rubber (EPR) cable, joint time-frequency domain reflectometry (JTFDR), prognostics, silicone rubber (SIR) cable. I. INTRODUCTION T HE INTEGRITY of the wiring in the electric power system is vital to its safe operation. However, the power cables are subjected to various stresses, including electrical, mechanical, chemical, and particularly thermal stresses [1]. Those stresses can accelerate the aging process of cable insulations in a normal service condition. The deregulated electric power utilities are facing the technical challenges of detecting and locating the faulty cables fabricated and installed up to a half century ago and are also experiencing premature failures much earlier than the life expectancy originally estimated. Some cables have to be replaced after 10-15 years, which is only a fraction of the designed lifetime of 60 years [2]-[5].
Electrical Engineering
This paper discusses the effect of thermal stress on the electrical properties, conduction and polarization, of low-voltage CSPE/XLPE-based power cables used in a nuclear power plant. The cables were subjected to an accelerated thermal stress at 120°C for an equivalent service period of 20, 40, 60 and 80 years. The capacitance, tanδ, resistivity at different frequencies and extended voltage response were the methods used for the investigation. A significant variation in the values of tanδ at low frequency, i.e., at 100 Hz, was observed. For the extended voltage response measurement, the decay voltage slope increased, showing the increase in the conduction particles inside the insulation material. While the decrease in return voltage slope showed that the slow polarization processes occurring at very low frequencies were decreased with aging. A strong correlation between the tanδ and decay voltage slope was also observed, which increased as the insulation aging increased. The results...
Research of Service Condition Impacts on Cable Polymer Insulation
2020
During last decades the intensive increase of using of polymer insulation materials in high voltage electric equipment such as cables, apparatus, surge arresters, transformers, and insulators takes place. With all wide well-known advantages the challenge of polymer insulation material deterioration and conditions of defects formation under influence of different service factors in full-scale are poorly studied. Water trees formation and its growing are concerned with humidity impact on the reduction of polymer insulation electric strength under non-uniform electric field. Water diffuses in polymer structure under electric field intensity and concentrates in volumes with high local heterogeneity, where water trees have a tendency to grow. Despite unanimous agreement that the combination of water and electric field in polymer insulation affects its dielectric properties, there is no clear understanding of the water-tree aging process. Besides proposed deterioration physical models or registered service failure statistics have not been useful for calculation of insulation degradation level or remaining life. In paper the results of laboratory research of polymer material dielectric losses over high frequency range are represented. The dielectric losses measured by using of non-destructive dielectric response measurement techniques showed a good correlation with polymer insulation deterioration. On the basis of obtained results the mathematical model and rapid method of cable remaining life assessment is offered. Thermal deterioration of polymer cable insulation (especially polyethylene insulation) under service condition cannot be ruled out. Within the framework of the project the investigations of thermal deterioration of polyethylene insulation with using of method of X-ray structure analysis are carried out. During thermal deterioration investigations the correlations between dielectric losses and temperature load at different ageing time were defined.