Very-Low-Frequency Testing of Cables

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Table of Contents

Introduction
Review of Evidence
Discussion of Findings
References

Introduction

When a hypothetical utility has to test its cables, the choice has to be made on the basis of several values that require organizations to check in with the quality of cables and their life cycles. Even though many utilities tend to overlook the importance of cable-testing activities, it may be safe to say that the inability to test cables in time might result in costly emergency repairs and damage given to adjacent cables. From the commercial point of view, this could lead to decreased consumption revenue and multiple malcontent customers. This is why many organizations embrace very-low-frequency (VLF) testing of cables and ensure that bad cables are recurrently weeded out and replaced by new ones.

In the future, if the IEEE releases a new standard, the VLF is going to become a unified procedure aimed to reduce the occurrence of cable failures and develop an ethic of proper workmanship. As a decent splice checker, the VLF could also be utilized to test substation cables, newly-installed equipment, and vital feeder runs. The current paper aims to review the key elements of the VLF cable testing and outline the potential areas of development that could be addressed by the experts in the field to improve the quality of cables and protect organizations from unwanted power outages and collateral damage.

Review of Evidence

Power Frequency Test Voltage and Medium Voltage Power Cable

As it was suggested by the authors of the Reference [1], high potential tests may only be survived by medium-voltage power cables under the condition where there are heat shrink terminations. Also, it could lead to partial discharges and premature cable failures, but not in all cases because the insulation does not degrade in 100% of testing results. The experiments conducted by the researchers showed, on the other hand, that the majority of typical cable defects could be easily detected with the help of the VLF. Those defects included (but were not limited to) tramline, ring-cut, and semicon feather. There was no substantial difference between the VLF and power frequencies, which also hints at the fact that tests completed under the VLF conditions could disclose more cable issues than any other testing option. Power frequencies proved to cause larger discharges and cause more damage to cables than the common VLF voltage [2]. To conclude, the authors of the article suggested that the power frequency test voltages, and the VLF technique could be used together to recognize unknown cable defects and inform the experts.

Low Frequency Tests for Distribution Cables

Throughout the last twenty years, XLPE cables were significantly improved in terms of their water-tree resistance and the overall production quality. A lot of cables also depend on their working environment, as water tress may not be found in some of the XLPE cables that are at least ten years old. Accordingly, the application of VLF testing would not cause any insulation degradation in those cables. As it was suggested by the authors of the article from Reference [3], the current improvements in OW test equipment also contribute to a greater understanding of how cables could be protected with the help of VLF testing. According to the findings, the VLFs could be used to determine the locations of sources of PD signals and then perform advanced analysis of cables in the area. The experts would be able to determine the possible cable defects and assess their severity (in specific insulation locations) without having to gain physical access to cables.

On the other hand, the value of the VLF testing is also supported by the ability to run monitoring and diagnostic check-ups on distribution cables. Despite the fact that there are limitations associated with any type of testing, insulation resistance and discharge evaluation are inferior to the VLF, especially knowing that dielectric loss cannot be used to discover insulation defects [4]. When a cable undergoes the VLF testing, paper insulation (or its oil counterpart), terminations, and joints will be assessed to the fullest, leaving no room for unwanted or unrecognized defects. The existing PD detection may also be deemed as inferior to the VLF testing because of greater accuracy of the latter and higher allowable level for distribution cables. Therefore, the authors of the two articles above agreed on the idea that the VLF is the most straightforward technique to identify defected cables and accessories.

Partial Discharge in Cable Testing

The results of tests performed on polyethylene and epoxy resin proved the need for additional measurements in the areas of low frequencies and partial discharge. Within the range from 0.1 Hz to 50 Hz, the samples as mentioned above were tested with the help of VLFs and custom-designed low-noise low-frequency generators [5]. The research shows that unwanted pulses from the test components could be effectively removed by the discharge detection system. The findings presented by the authors of Reference [5] are of rather high value because they proved the existence of discharges during the utilization of VLFs.

On the other hand, partial discharge measurement, as one of the best diagnostic methods for cable and cable accessories performance, was critically evaluated by the authors of Reference [6]. According to their comprehensive testing, the VLF testing turned out to be the most advantageous technique because it allowed for proper assessment of cables and accessories that had already been installed in their final form. This also hints at the fact that the quality of installation could also be reviewed, even if secondarily. High voltage cable accessories testing, though, might be a severe disadvantage because it could lead to inconclusive results caused by discharges in the case of standalone accessory testing.

Tan δ Diagnostic Measurements

The importance of diagnostic measurements linked to Tan δ values cannot be underestimated because there is a possible correlation between the breakdown strength of VLFs and XLPE cables. According to the evidence presented by the authors of Reference [7], 40-year-old cables from the same service area could be easily tested for defects with the help of VLFs, meaning that aging conditions do not cause any delays or complex deployments in the process of cable assessment. The test program offered by the researchers included Tan δ capacities at different voltages and a to-breakdown test performed with the help of the VLF technique.

The Tan δ measurements can be regarded as a relatively new diagnostic feature that significantly benefits from the utilization of VLF testing. Particular voltage levels may degrade only those cables that had already been damaged, helping cable experts gain more insight into which cables have to be replaced and when. The authors of the article from Reference [8] also introduced a new method of assessing cable quality called Performance Ranking. This innovative technique allows for a thorough evaluation of cable diagnostics effectiveness and provides cable experts with quantitative data on breakdown performance. The cable population that was used for the study also created premises for future research in the area where even more insight into the breakdown performance could be gained.

Correlation between Tan δ Values and Frequencies

The authors of Reference [9] were able to find a correlation between Tan δ values and frequencies, as there was identified a higher level of sensitivity across the lower frequencies. They also performed ANOVA to define the relationship between the variation of Tan δ values and the values of discharge times. The results of another important research in the area (see Reference [10]) showed that the discharge time is a significant variable, while the magnitude is practically inconsequential. These findings create the room for the conclusion that different insulation losses and variation in Tan δ diagnostics and voltage cannot be associated with the VLF testing measurements. This means that the VLF form of cable testing can be used to enhance the results of cable assessment and properly typify cable insulation. The most important finding from both articles is that Tan δ is an inextricable diagnostic tool that may provide cable experts with rich insights into the health of cables and their potential life cycle.

Discussion of Findings

Based on the existing findings, it may be concluded that the VLF does not cause any premature failures as in DC voltage testing, which also makes it perfect for insulation. The insulation is not going to be degraded because of the VLF testing, making this testing method one of the most beneficial out of all that are currently available to the experts in the field. Even though testing might cause splice defects or water tress, it only relates to the cables that have already been damaged before the VLF testing. Eventually, if repeated testing shows that the cable does not hold standard voltage, the cable should be replaced in order to prevent an unwanted outage in the nearest future. Therefore, the utilization of VLF allows specialists to run proper installation and reparation of new cables and find faults rather quickly. As cables regularly experience AC voltage throughout their functioning periods, it would make perfect sense to run the VLF from time to time to see if cables still perform at a decent level.

Even though each cable is factory-tested at higher levels than usual field test voltages, it does not allow for a hypothesis that the VLF is not as valuable as it is described in the literature on the subject. As the existing evidence shows, the VLF testing is not destructive, unless there is a defect somewhere in a joint, splice, or cable. Given the fact that the effectiveness of a cable should be tested based on its ability to carry AC voltage, there is no doubt that the VLF should become the standard for cable-testing specialists. As a stress test, the VLF testing is a perfect opportunity to go beyond mere diagnostics and gain insight into existing defects. Therefore, the VLF is a strong testing method that may serve as a preventive instrument intended to maintain the quality of cables, joints, and splices at the highest level possible.

References

D. Fynes-Clinton and C. Nyamupangedengu, “Partial discharge characterization of cross-linked polyethylene medium voltage power cable termination defects at very low frequency (0.1 Hz) and power frequency test voltages”, IEEE Electrical Insulation Magazine, vol. 32, no. 4, pp. 15-23, 2016. Web.
B. Oyegoke, P. Hyvonen, M. Aro, Ning Gao and M. Danikas, “Selectivity of damped ac (DAC) and VLF voltages in after-laying tests of extruded MV cable systems”, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 10, no. 5, pp. 874-882, 2003. Web.
C. Su and C. Li, “Using very-low-frequency and oscillating-wave tests to improve the reliability of distribution cables”, IEEE Electrical Insulation Magazine, vol. 29, no. 1, pp. 38-45, 2013. Web.
Z. Hou, H. Li, S. Chen, B. Li, Y. Lu and S. Ji, “Development of a novel 20 kV 0.1 Hz very low frequency cosine-rectangular voltage generator for multi-functional insulation testing of MV power cables”, IET Generation, Transmission & Distribution, vol. 12, no. 1, pp. 1-8, 2018. Web.
R. Miller and I. Black, “Partial Discharge Measurements over the Frequency Range 0.1 Hz to 50 Hz”, IEEE Transactions on Electrical Insulation, vol. -12, no. 3, pp. 224-233, 1977. Available: 10.1109/tei.1977.298026.
A. Eigner and K. Rethmeier, “An overview on the current status of partial discharge measurements on AC high voltage cable accessories”, IEEE Electrical Insulation Magazine, vol. 32, no. 2, pp. 48-55, 2016. Web.
J. Hernandez-Mejia, J. Perkel, R. Harley, N. Hampton and R. Hartlein, “Correlation between tan δ diagnostic measurements and breakdown performance at VLF for MV XLPE cables”, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 16, no. 1, pp. 162-170, 2009. Web.
Y. Liu, Y. Su, L. Wang and Y. Xiao, “Ageing condition assessment of DC cable XLPE insulation by Tan δ measurement at 0.1 Hz voltage”, 2016 IEEE International Conference on Dielectrics, 2016. Web.
J. Hernandez-Mejia, R. Harley, N. Hampton and R. Hartlein, “Characterization of Ageing for MV Power Cables Using Low Frequency Tan δ Diagnostic Measurements”, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 16, no. 3, pp. 862-870, 2009. Web.
S. Morsalin, B. Phung and M. Danikas, “Influence of partial discharge on dissipation factor measurement at very low frequency”, Condition Monitoring and Diagnosis, pp. 1-5, 2018. Web.

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