A 400 KV OHTL in Mexico facing particular reliability, environmental and design problems

The present work give a description of a OHTL project carried out on years 2002÷2003 by ABB Power Technologies S.p.A. on behalf of the Mexican Utility CFE on the basis of an EPC (Engineering Procurement and Construction) Contract. This paper deals with the reliability and the safety of the line, with the resolution of special environmental problems, with the design and construction peculiarities of the long spans involved. Special environmental issues were studied and solved in terms of selection of line route (cost benefit balance of different routes were analysed and commented), of study of mixed solutions (part overhead and part underground cable) and of mitigation of impact on the environment. Biologists were key actors in the project to find environmental solutions to restore, after the construction works, the natural site conditions very rich in protected wild life and plant life. Reliability is a key issues after the catastrophic collapses of large part of high voltage networks in different parts of the world due to exceptional atmospheric conditions, see Quebec and France. For this reason it is now very sensitive the issue of the safety measures to be adopted when planning a new transmission line. Special attention is to be paid to the possibility of cascading effects, which is of particular importance in terms of reliability of the lines. This subject is analysed by Standard Code CEI IEC 60-826 which recommends in the design of towers the introduction of some special loading conditions - torsion and longitudinal residual static loads and unbalanced longitudinal - with the aim of giving a sufficient torsion and longitudinal resistance to all towers, suspension towers included. Additional recommendations are relevant to the installation of some anticascade towers, typically one each 10 normal towers in order to limit the damages to the line in case of failure of a single tower. These recommendations were considered by CFE when planning the project and were introduced in the Technical Specifications. Reliability and safety were also considered in the definition of the additional design requirements to be adopted in the design of the special crossing towers. Specific factors of safety, second order analysis, deformation limits, slope of truss members and constraint to tower tests due to physical dimensions of towers are considered. A comparison of requirements given for normal line section versus crossing section is commented and results detailed. Construction works were heavily conditioned by the reduction to the minimum of the damages to the environment. Some of the precautions and solutions adopted to reach towers in hilly and mountainous area are discussed. The choice of foundation types was again leaded by environment conditions. The line is crossing some stretch of water with the need of long spans and very high towers ( one of these is a tension tower with 45 deg line angle). Details of solutions adopted (i.e. special conductors with higher tensions) again to reduce the impact on the environment are presented and discussed.