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Investigations of ZnO microvaristor for stress control on polymeric outdoor insulators

Abd Rahman, Rahisham 2012. Investigations of ZnO microvaristor for stress control on polymeric outdoor insulators. PhD Thesis, Cardiff University.
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Abstract

This thesis is concerned with the investigation of the efficacy of Zinc Oxide microvaristor compound for stress control on polymeric outdoor insulators. The preliminary work has involved a comprehensive literature survey, followed by extensive computational modelling and simulation studies as well as laboratory works covering experimental investigations and fabrication of insulator prototypes. The literature survey reviewed stress-induced degradations as the cause of ageing and insulation failures, the determination of electric field distributions, considerations for outdoor insulator modelling, and field-optimisation techniques for achieving stress relief. An 11 kV polymeric insulator has been modelled and simulated under dry-clean and wet-polluted surface conditions in order to obtain electric field distribution along the insulator creepage path. The critical high field regions on polymeric surfaces were identified. In addition, clean fog solid layer tests were carried out to experimentally examine dry band formation and electric discharges. Experimental investigations confirmed the results previously achieved from theoretical simulations. A non-linear pollution model has been developed for simulating polluted outdoor insulators. The field-dependent conductivity was derived from layer conductance measurements in a non-standard low voltage test. The proposed model was used to simulate insulators under fog and light rain conditions which respectively represent a uniform and non-uniform wetting action in practice. It was demonstrated that the nonlinear pollution model yields a more detailed and realistic field distribution compared with results obtained with models using constant/linear conductivity. Short-length microvaristor coating, having a cone-shaped structure, was introduced at both insulator ends for controlling high field, particularly near the high voltage and ground terminals. The performance of field grading was evaluated through a number of simulation scenarios. The introduction of microvaristor material with an appropriate switching characteristic has led to a substantial improvement in the electric field and heat distributions along the insulator profile. The prototype of an 11kV insulator with microvaristor grading material was fabricated in-house for preliminary tests. Lightning impulse (1.2/50 μs) flashover tests were carried out using the ‘up and down’ method, and the flashover voltage was estimated by the 50% probability breakdown, U50. The results of the lightning impulse test have indicated a considerable increase in the flashover voltage up to 21% when using microvaristor-graded insulator. Favourable field distributions obtained in the simulation study have indicated a strong correlation with the experimental results.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Uncontrolled Keywords: polymeric outdoor insulators ; ZnO microvaristors ; stress control ; non-linear grading material ; electric field distributions ; finite element analysis.
Date of First Compliant Deposit: 30 March 2016
Last Modified: 19 Mar 2016 23:06
URI: http://orca-mwe.cf.ac.uk/id/eprint/38993

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