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India reassesses transmission line tower designs for different wind zones considering swing angles and clearances.
In the late 1990s, the introduction of six wind zones in India created expectations that, for optimized designs, different swing angle-clearance combinations would be used for each wind zone. However, studies showed that although the conductor swing angles vary considerably due to wind speed characteristics in each zone, their net impact on the tower configuration and cost are largely offset by a counter variation of conductor clearances. Faced with additional costs, delays in construction and other problems associated with the introduction of multiple tower configurations, India's transmission lino designers decided to use the same tower configuration for each voltage class in all wind zones.
India's Power System Background
Indian power systems are designed with 66-kV, 132-kV, 220-kV and 400-kV transmission lines. In addition, some 800-kV ac and 500-kV HVDC lines are in operation. Generally, the conductor sizes used on these lines have been standardized to help reduce construction times and spare requirements. Three sets of conductor swingangles ([theta]-C) versus conductor-to-tower clearance combinations determine the tower configuration and generally are specified for 66-kV, 132-kV and 220-kV lines. Two sets of these combinations are specified for 400-kV lines.
These "[theta]-C" combinations were adopted nearly 50 years ago on the basis of experience and wind speed data available at that time. During this period, they were used uniformly in all parts of the country, generally providing trouble-free performance.
India is geographically divided into six wind zones on the basis of longterm meteorological data. With considerably varying wind speeds in the six zones, the maximum and intermediate values of swing angle [theta] for the line conductors vary over these zones, resulting in higher values of [theta] for higher wind speeds. Similarly, the requirement of clearance, C, between the (live) conductors and the nearest (earthed) structural member of the tower varies with the wind speed because of the latter's effect on deionization of the intervening air space. This results in smaller values of C with higher wind speeds. Thus, higher wind zones require higher values of [theta] but smaller values of C, while lower wind zones...