Performance Evaluation of Power Transmission Line Tower Made of Polymer Matrix Composite

Abstract

The design of power transmission lines is done to meet multiple constraints – electrical, mechanical and environmental. Thus designers are generous in deciding the margin to meet the above. But presently, with limited space for transmission lines, need for reduction in transmission line space in both horizontal i.e., Right of Way (ROW) and vertical i.e., height of tower has arisen. Several attempts are made to achieve this reduction at the same time reducing the cost. Use of composites for tower and its components is an attempt directed to decrease the space and the cost. Polymer composite materials have emerged as promising engineering materials due to their light weight and non – corrosiveness. The available literature provide few details of polymer matrix composites as alternative materials for tower but a systematic and holistic study on developing and testing of a tower with composites is yet to see the light. Thus, the present work is focused on development of a tower with composite members and test it for meeting mechanical and electrical performances and also achieve reduction in ROW and cost. The work considers two approaches, first is FE analysis and the next is physical building of tower components at different levels and the full tower to test for the performance. As a preliminary step, properties of glassepoxy material processed with pultrusion are determined to assess its suitability in tower applications. Subsequently, various tower members are fabricated with pultrusion process the details of which are provided in Table.1 The tower considered for present work is a 66 kV vertical double circuit lattice type in a line of 200m span operating at a wind speed of 47 m/s. Initially tower and its components are designed as suggested in standard IS: 802 providing all mandatory clearances from the point of electrical insulation. Cross arm which is one of the major components in tower, is modelled in FEM using dimensions determined earlier. The design of cross arm is verified with FE analysis. Subsequently, FE analysis of a portion of the tower body, tower sub assembly, followed by analysis with cross arm mounted is taken up. FE analysis of a full length tower made of composite member is envisaged as an ultimate part of the study.Analysis indicated that stress levels in members far below the permissible ones of a material. Thus design of tower and its components is verified. Table 1. Details of GE pultruded cross arm and tower members Sl. No Member Dimensions of member cross section Reinforcement Matrix 1 Solid rectangle section 20 mm x 70 mm E- Glass continuous fibres ( 70 - 75 % ) Epoxy (20-25%) Lapox L-12 Hardener K-6 2 Solid angle section 50x50x6mm 76.2 x 76.2x6.35 - do- - do- 3 Solid circular section Ø30 mm, Ø33 mm - do- - do- 4 Hollow sections 101.6x101.6x9.525 101.6x101.6x6.35 - do- - doIn order to reinforce the feasibility of tower with composite material, physical construction and testing of its components and in the end full tower is taken up. All tests are carried out at station in Central Power Research Institute (CPRI), Bangalore. Initially cross arm is constructed and loads as suggested in standard IS: 802 are applied on the cross arm. The deflection measured at the tip of cross arm is only about 44 mm also strains in members of the cross arm are found to be not vey excessive. Prototype testing is extended to a tower sub assembly without cross arm and with cross arm mounted successfully. Later a full length tower with all cross arms mounted in place is constructed and tested. The tower with composite member performed satisfactorily without any visible damage at 100 % full load suggested in standard. The maximum deflection of tower is found to be only 1.4 % of tower height and is within permissible limit of 5 %. The tower with composite member successfully withstood even 300 % full load without any visible signs of failure suggesting a Factor of safety 3.0.Tests for electrical performance of cross arm and tower with composite members are carried out. Table.2 provides the results of electrical test wherein it can be observed that the test parameters determined are higher than the suggested minimum values. Thus the cross arm and tower satisfactorily meet the electrical requirements. Table 2. Results of electrical testing Electrical Performance test Suggested minimum values in IS:2165 Experimentally determined values Cross arm with tower sub-assembly Full tower Power frequency ( kV ) 140 150 143 Impulse voltage ( kV ) 325 328 328 From the study it could be inferred that the tower with composite members satisfied both mechanical and electrical requirements. Since the tower is without insulator strings and the associated problems of their swing, the ROW for the line is less and a saving of about 17 % is achieved in ROW. The height of the proposed tower is only 15 m as against 18 m for metallic tower suggested by Indian standard IS: 5613. Thus a saving of about 18 % is achieved. Consequently on account of this lesser height and lower weight of composite members, the saving in total weight of the tower against a metallic tower is about 33 %. Thus with savings and benefits mentioned above, the proposed tower could be most suitable for earthquake prone zones and for Emergency Restoration Systems (ERS).