Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.585218
Title: Uprating of overhead lines
Author: Bhattarai, Roshan
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2011
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Abstract:
This thesis establishes a rigorous procedure for overhead line uprating with guidance for calculating the increase in voltage rating that may be achieved for given configurations. It initially investigates different technical issues and conventional methods of voltage uprating of overhead transmission lines. Various issues such as clearance, insulation, pollution, transient overvoltages, surge arresters and its combinations were studied for optimised insulation coordination of voltage uprated transmission systems with reference to international standard IEC 60071 and British standard BSEN 50341. It then considers a case of existing 275kV line in 'L3' structures to analyse these issues and propose appropriate process for its possible uprating to 400kV system. In this investigation, overhead line uprating techniques used by different utilities around the world, as published in the literature were analysed. It was found that, the decision to uprate overhead lines is influenced by technical, institutional and financial issues. In this thesis, issues such as conductor air clearance, insulation electrical strength and overvoltages are investigated and taken into account to develop an appropriate methodology. For uprating overhead lines, an exemplar case study of uprating an L3, 275kV line to 400kV was used, introducing minimum structural changes to the tower. The selection of the L3 tower is made on the basis that it is not readily upratable to 400kV due to restricted air clearances it offers. This work has demonstrated that the voltage uprating of overhead transmission lines is possible with minimal modification to the existing line. In this case, the phase-to-earth clearance was found to be the critical factor which determines the level to which the voltage level of the line can be increased. Computations of overvoltages due to switching and lightning phenomena were conducted to estimate overvoltage levels and optimise the protection scheme required to minimise the required minimum electrical clearances. Employing gapless metal-oxide line surge arresters were proposed to be the most effective solution to control the overvoltages, thereby reducing the minimum phase-to-earth clearance requirements. This solution is preferred to modifying the tower structure in order to achieve the required clearance for 400kV system. The study of lightning and switching surge performances along the line under different arrester configurations was carried out so that the appropriate surge arrester configuration could be selected to maintain overvoltage levels within the targeted withstand level for the line. The extensive transient simulations performed in this work identified that, for a double circuit overhead transmission line as used on the UK system, the top-most phase conductors are prone to shielding failure lightning strikes whilst the bottom-most phase conductors are likely to be subjected to backflashover surges for the case of high tower footing resistance. The assessment of electric and magnetic field profiles of a 275kV line uprated to 400kV was computed. It showed that, the field intensities of voltage uprated lines are within the limits adopted by national and international standards and requires no additional wayleave for uprating.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.585218  DOI: Not available
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