3. Electronic Theses and Dissertations (ETDs) - All submissions

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    Developing a reliability model for an 88 kV XLPE cable termination
    (2019) Chimunda, Samantha
    Whereas there are life models for cable systems, the focus has been on cables and not on accessories. Literature shows that cable accessories are the problematic area in cable systems. There is therefore a need to develop cable accessory reliability models which can be used for informed asset management decision. This research work has conceived a reliability model for cable terminations using a case study municipal substation which has been experiencing failure of 88 kV XLPE power cable terminations. In the literature, there is an electrothermal model developed for cable systems. In the present work the existing model was adopted and extended. This study involved identifying the modes of degradation by forensic investigation of the failed terminations. For each mode of degradation, a corresponding mathematical model of the stress life factor was developed. Using the Weakest Link Principle, the product of reliabilities due to each degradation mechanism computes the overall reliability of the termination system. The results indicate that electrothermal stress hardly reduces the reliability of the termination from the time of installation to 40 years in service. However, thermomechanical stress causes a reliability of less than 10% after 18 years and galvanic degradation reduces the termination reliability to less than 10% after 24 years in service. Each new cable termination design should have a corresponding reliability model. The latter entails conducting laboratory aging tests which give the parameters required in developing the stress life factors. For existing terminations, root-cause analysis of failure can lead to determination of the stress factor parameters. The model developed in the present research work can be used in real-time asset management decisions by processing the real time operating conditions.
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    Time-evolution of partial discharge characteristics of XLPE MV cable termination defects
    (2018) Haikali, Elizabeth NN
    Power cable terminations and joints experience high electrical stress due to the abrupt change in geometry of the cable; hence the insulation at these points is more prone to partial discharges and has been reported as the main root cause of power cable system failures. Electrical failure of the insulation is known to occur due to a phenomenon of Partial Discharges (PD). Modern practice (especially in higher voltage installations) entails installation of PD sensors at strategic locations during installation of electrical equipment such as cable joints and terminations. This enables continuous monitoring of PD events in the plant, and this is termed on-line PD diagnosis. However, with limited knowledge to interpret the meaning of certain PD changes during the service period, this practice remains limited. It is therefore the interest of the study to understand the time evolution behaviour of PD characteristics in order to discern the insulation condition or deteriorating stages. The present study is on XLPE power cables, focusing on PD in artificial defects in the cable termination insulation that in most cases arise from poor workmanship. The power cables were subjected to accelerated ageing to emulate their ageing under service conditions. PD measurements were then conducted at periodic time intervals and characterized PD in terms of PD Inception Voltage (PDIV), maximum apparent PD magnitude (Qmax), Pulse Repetition Rate (PRR) and Phase-Resolved-PartialDischarge-Pattern (PRPDP). The findings are that, Qmax, PRR and PDIV did not show any time-evolution trends unique to a defect, the general trends observed were that of a fairly constant PDIV with several fluctuations of a 5 kV band. Qmax showed a decreasing trend over ageing time. The PRR decreased overall, with a pick up increase near the end of the tests. Qmax and PRR were noted significantly fluctuative between 23% and 57% of the total ageing period, distinct characteristics were that, the tram line had the largest PRR which is expected since it is a flat cavity, and iii the PRPDP appeared more skewed than other defects. The semicon feather had a PRPDP that seemed like a combination of a void discharge and corona discharge. The ring cut PRPDP was similar to that of the tram line except that it was not skewed. Furthermore, a capacitance PD model was constructed in Matlab R Simulink R to emulate experimental observed PD behaviour and therefore confirm the theory explaining the observed time-dependency of PD phenomena. Simulated void discharge PRPDP which corresponded with experimentally measured PRPDP were obtained for the unaged, moderately aged and severely aged cavity defect. The corona-surface discharge effect observed in the semicon PRPDP was also successfully emulated. The study outcomes suggest that PD characteristics evolve over time, and that the behaviour of the observed trend is unique at different stages during ageing. The time evolution characteristics of PD are The PRPDP signatures did not change with time of ageing despite the variations in Qmax and PRR. This means that, defect signatures obtained prior ageing or in-service operation of the cables can still serve as a good reference of identifying the nature of the defect at different ageing stages except in the event of PD evanescence. From the simulations, it was derived that the PD region surface conductivity as well as the geometry of the defect are the main contributing factors to the unique signatures observed at different stages and per defect.
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    Voltage uprating of existing high voltage substations when transient voltage stress and available withstand strength are coordinated
    (2017) Schutte, Peet
    Servitude availability in space-constrained built-up areas within the Johannesburg or Central Load Network (CLN) poses every-day challenges for power system engineers. Strengthening the backbone 88/275 kV transmission system within the CLN becomes even more difficult when multi-circuit transmission lines are required for increased power transfer capabilities. When uprating is considered to increase the power transfer capability, the withstand levels of existing external insulation demands an optimisation to find a new stress versus strength balance that allows reliable operation of substations at higher voltages. The research includes primarily an investigative simulation study to evaluate the current Eskom available design clearances in terms of their withstand capability when subjected to over-voltage transients. Two voltage range classes were evaluated and the results are discussed. For voltage range 1, it was found that the over-voltage stress was low enough to allow for a higher nominal operating voltage while maintaining the existing clearances. For voltage range 2, existing clearances are also found to be conservative and smaller safety margins will most likely be acceptable. From a transient analysis evaluation, voltage uprating is considered as a very attractive option to increase the power transfer capability of existing substations. Current Eskom clearances for 88 kV and 275 kV are expected to perform well during transients generated in uprated systems. Electrode grading to improve the field gradients in the substation will require attention to increase gap factors. Additional surge arresters are considered to be a cost effective solution to control over-voltages throughout the whole uprated substation. The physical modification of substations to replace strung conductors with tubular conductors, ensuring sufficient outage time to refurbish and rebuild with new equipment will be the most challenging part of uprating existing substations.
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    PD characteristics of typical MV XLPE power cable termination defects at VLF and power frequency test voltages
    (2016-10-11) Fynes-Clinton, Daniel
    It has been reported in literature that newly installed cable accessories can pass typical commissioning tests and yet show signi cant PD activity leading to premature failure in the long term. This research report details the partial discharge (PD) characterisation of three arti cial defects in MV cable terminations, at both power and very low frequency (VLF). The defects included a semi-conductor feather, a tram line in the cross-linked polyethylene (XLPE) insulation and an XLPE ring cut. The experimental methodology involved subjecting the terminations to a standard overvoltage withstand test, after which the PD activity was monitored, at 0.1 Hz and at 50 Hz excitation frequencies. All the defects passed the overvoltage test, despite some showing signi cant discharges. The defects gave distinct phase-resolved PD patterns that are e ectively invariant of excitation frequency. The PD magnitudes at power frequency were typically bigger than at VLF, while the PD inception voltages tended to be lower at VLF. The PD magnitudes of the semi-conductor feather defect surface discharges were found to increase with increasing applied voltage, while no increase was observed for the tram line and ring cut void discharges. The results show that common installation defects in MV cable terminations can be suitably characterised using VLF PD measurements
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