Date of Award

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical and Computer Engineering

Committee Chair

Ravi Gorur

Committee Member

Shangbing Ai

Committee Member

Timothy Boykin

Committee Member

Maria Pour

Committee Member

Yuri Shtessel

Subject(s)

Electric insulators and insulation--Maintenance and repair., Electric fields--Measurement.

Abstract

The application of composite insulators in high voltage transmission and distribution has progressively increased. In recent years, composite insulators have captured a large share of the market at certain voltages due to advantages of lightweight, low cost, superior contamination performance and low maintenance. These composite insulators have a fiberglass rod housed inside a polymeric sheath. The housing material is generally made of organic materials. The material nature of composite insulators makes them easily prone to deterioration due to moisture, heat, pollution and also electric discharges. The absence of intermediate metallic structures results in highly non-uniform voltage distribution along the composite insulators. The polymeric housing material and the highly non-linear voltage distribution make the composite insulators vulnerable for the development of defects. It is important to replace the defective insulators before they fail to ensure the reliability in power delivery. Several techniques have been attempted for composite insulator condition assessment. Electric field measurement technique is a recommended method for the detection of defects as it can directly measure the enhancement in electric field due to the presence of the defect. The measurements of electric field in samples with deliberately planted superficial and internal defects are presented. Samples evaluated include full-scale composite insulators and cylindrical rods of the housing material. The goal of this work is to ascertain the viability of electric field as an indicator of impending failure of composite insulators that can develop such defects during manufacture and/or service. Commercial packages utilize the inherent axial symmetric nature of the insulator to efficiently calculate the electric field. But in the presence of defects or dry band arcs the axial symmetry of the insulator is generally lost. Hence, the electric field is to be calculated for the complete 3-dimensional insulator which can be computationally intensive. A 2-dimensional stretched grid finite difference method for the calculation of electric field in composite insulators for up to 1000 kV is presented. This method was initially used to evaluate healthy composite insulators of single and double strings with and without corona and grading rings. Further, the electric field computed with defect was compared with the actual measurements. It has been demonstrated that the proposed method can yield fairly accurate results when compared to other methods, but in a significantly reduced time taken by the latter. The performance of insulating material for tracking and erosion is an important aspect that needs to be investigated while designing the transmission lines. Surface resistance can be used as a potential candidate to understand various discharge activities occurring on the insulation material. But in the field conditions, it is not easy to obtain the actual surface resistance due to weak source or lack in the advanced measurement techniques. Hence prediction of insulator failure based on surface resistance is a challenging task. Two second order sliding mode observers based on Super-Twisting and Twisting algorithms to improve the accuracy of the measured surface resistance under weak power source situation is proposed. The efficacy of the proposed surface resistance estimation algorithms’ was verified using experimental results and simulation studies. Further the estimated surface resistance was used to predict the failure of the insulating material. Electric Power Research Institute (EPRI) has reported that the short-term, momentary interruptions, which last 5 minutes or less had a stronger impact (around 67%) on the total cost of interruptions than the less frequent sustained interruptions. One of the principal causes of momentary interruptions is flashover of outdoor insulators. Hence, a model to calculate the flashover voltage of polluted outdoor insulator in the presence of multiple dryband discharges is presented. Based on the power criterion for flashover, a generalized equation is proposed that can calculated the flashover voltage for serially burning multiple discharges on the surface of the polluted insulator. The model is further used to calculate the flashover voltage of standard insulator.

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