Assessment of insulation condition in polymeric underground cables using polarization and depolarization current analysis

• Main Author: SUHAILA BINTI SULAIMAN
• Format: text::Thesis
• Language: English
• Published: 2023
• Subjects: Polymeric underground cables; Polarization and depolarization

A major component in the electrical distribution network is the underground cables. As such, it is considered to be a major asset to a power utility company. Gradual deterioration in a cable insulation material, whether due to continuous applications of high voltages or ageing mechanisms, may lead to eventual cable failure and subsequent electricity disruption. In the modern day, where almost every sector of a country’s economy is dependent on electricity, the consumers have high expectations for reliable and continuous supply of electricity from the power utility company. This demand has seen the growth in the need for power utility companies to explore methods to closely monitor and assess the health of cable insulation, through condition-based monitoring (CBM) exercise. Non-destructive techniques are preferred to destructive ones, in the shift to life management of highly expensive assets of the power utility companies. Many non-destructive insulation monitoring techniques have been developed over time namely tangent delta (tan δ), return voltage measurement (RVM), dielectric spectroscopy (DS) and polarization and depolarization current (PDC). PDC is a non- destructive insulation monitoring method that has been widely applied to assess the health of insulation for high voltage equipment such as power transformer, synchronous machines and surge arresters. This study proposes to expand on the recent interest in PDC’s assessment of XLPE cable insulation degradation. It attempts to investigate the PDC behaviour in a number of cable insulation conditions subjected to varying degradation conditions by performing experimental and simulation studies on ideal, artificial degraded and field degraded XLPE cables. Using a combined approach of cable insulation modeling based on the well-established Debye equivalent circuit for insulation model and optimizing its circuit elements parameters with the results from the baseline PDC measurements, highly accurate PDC simulations were obtained in MATLAB. Comparing the PDC measurements and simulations for four new, non- degraded cables, under controlled conditions, showed that the trendlines for PDC are almost overlapping, indicating negligible conduction current within the insulation material for all the baseline cables. The resulting PDC simulations for new, non-degraded cables were then validated against PDC measurement results from four experimental artificially degraded cables setups and 39 in-service, field degraded cables PDC measurements, which saw matching patterns between the measurement and simulation results. The resulting PDC patterns were then categorized by conductivity values, based on works done by established researchers, to establish five varying levels of cable degradation, ranging from new, non-degraded to moderately degraded to severely degraded cable insulation conditions. Comparisons made against results of dielectric spectroscopy measurements, a widely utilized technique for cable monitoring, further validates the high accuracy of the PDC modeling and simulation. It proposes a sound and reliable categorization of CBM levels for power utility companies which is based on levels of conductivity values, instead of generalized terms in the existing CBM schedule. and propose a more refined CBM priority index for the power utility companies. It proposes at producing a sound and reliable categorization of CBM levels for power utility companies, based on levels of conductivity values, instead of generalized terms in the existing CBM schedule of a certain power utility company. From the study, a priority index CBM table based on conductivity values, developed from past research papers, has been proposed as part of the revised CBM table in the power utility company. Comparison between the proposed and existing CBM tables saw a 76.92% match on PDC categorization levels from 39 field cables tested, i.e. nine cables as outliers. However, a fully matching categorization levels based on conductivity values, were found when validated against two other non-destructive techniques i.e. dielectric spectroscopy and tangent delta. Thus, by analyzing the trendlines of PDC behaviour in various cable conditions and comparing against established values of conductivity levels within the insulation material, a more refined CBM priority index can be proposed to facilitate cable’s continuous maintenance by the power utility company’s personnel.