Conference Agenda

Electricity distribution is essential to modern society, with medium voltage (MV) power cables playing a critical role in delivering power from transformer stations to end users. However, these cables often operate under adverse conditions, that can compromise their performance and longevity. One of the primary stresses for MV cable insulation is water treeing, a process that occurs when polymeric materials are exposed to high humidity and electrical stress. Water treeing is characterized by the formation of microchannels within the insulating material, which can eventually lead to electrical treeing and material breakdown. For these reasons, it is essential to understand the mechanisms of water treeing formation in order to identify preventive strategies to optimize cable efficiency and durability.

This study investigates water treeing within the EPDM (Ethylene Propylene Diene Monomer) insulation, widely used in medium voltage cables through a water blade electrode test cell, which generates a localized high electric field able to accelerate the initiation and propagation of water trees. This method enables the reproduction accelerated aging conditions in the lab, subjecting samples to electrical stress, to observe long-term effects in a shorter timeframe. The goal is to establish a testing methodology which can compare different insulating materials in flat sample form, identifying the best candidates for the standard Accelerated Water Tree Test (AWTT) a time-intensive and costly test typically conducted on actual cables

To assess change in material properties due to aging, the study applied various diagnostic techniques, including dielectric spectroscopy, DC conductivity measurement, and FTIR spectroscopy. The results showed an increase in dielectric loss factor (tanδ) and insulator capacitance, which are attributable to the increase in complex permittivity of the material. The rise of ε’’, the imaginary part of the complex permittivity, indicates increasing energy dissipation with the presence of relaxation peaks associated with dipolar polarization, associated with water absorption. Conductivity analysis further supports the observed deterioration in insulating properties and the increase in dielectric losses within the material. FTIR analysis revealed new peaks associated with hydroxyl groups related to water absorption. These structural changes demonstrate how moisture absorption degrades dielectric properties, suggesting that these diagnostic techniques could be effective for ongoing insulation monitoring, In conclusion, the proposed method can be seen as a valuable tool for the preliminary investigation of insulation systems' resistance to water treeing. It may assist in distinguishing between different polymeric compound formulations, helping to select materials more likely to succeed in the AWTT, and contributing to the improvement of cable design and durability.

One important milestone for a wider acceptance of DFR (Dielectric Frequency Response) in condition assessment of capacitance graded bushings was the publication of IEEE C57.12.200 in 2022. Since then, additional experience has been collected.

Baseline testing using DFR for new bushing installations to allow for more accurate condition assessment during service is a well-established method which is encouraged by the authors. It is however important to understand the influence of fringing effects caused by the different electrical context when testing is carried on installed bushings, compared to the separate routine testing in the manufacturer’s facility required by the bushing standards.

Some examples of bushing degradation and methods for result analysis are outlined in IEEE C57.12.200 however additional are available for both oil impregnated as well as dry concepts. The additional findings are based on extensive practical use of DFR. The most important conclusion is that testing at 1400 V provides more reliable results when accessing the condition of bushings, compared to testing at lower voltages.