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 line 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 swing-angles (θ-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 “θ-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 long-term meteorological data. With considerably varying wind speeds in the six zones, the maximum and intermediate values of swing angle θ for the line conductors vary over these zones, resulting in higher values of θ 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 θ but smaller values of C, while lower wind zones require lower values of θ but higher values of C.
Table 1. Swing Angle vs. Clearance Combinations and Other Parameters for Indian Transmission Lines+7*1.57mmSt
30
76.0
45
112
61.0
+7*2.59mmSt
30
76.0
45
132
61.0
+7*3.00mmSt
30
45
137
122.0
18
230
+7*3.18mmSt
30
45
183.0
167.5
155
205
+7*3.53mmSt
186.0
215
The θ-C combinations, adopted for tower design, determine the horizontal and vertical separation of conductors, and thus the basic tower configuration. Expectations arose about the impact of the various wind speeds on θ and C, and a scientific review was conducted on the different transmission lines in each of the six wind zones.
Higher Wind Speed/Wind Zone Increase Conductor Swing Angles
The values of basic wind speed (that is, peak gust velocity averaged over an interval of 3 sec) have been specified for the six wind zones of India. These values are converted into equivalent values of mean wind speed, Wm (mean wind value averaged over a 1-minute period), by applying suitable conversion factors to ensure conservative results. The values of Wm specified for wind zones 1 through 6 are found to be 110, 130, 147, 157, 166 and 183 km/hour, respectively.
Based on the results of a test project, the conductor swing angles θ, corresponding to the six values of Wm, can be determined for lines of different voltages using the respective conductor diameter, conductor weight and a conservative value of 1.2 for vertical-to-horizontal span ratio. These “Wm-θ” relationships for the five lines under consideration are shown in Fig. 1, from which the following parameters can be determined:
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The values for mean wind speed, which correspond to the currently used swing angles for the five transmission lines. (These values are shown in Table 1, column 10).
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The maximum values for swing angles, which correspond to the Wm specified for the six wind zones. These values, shown in Fig. 2, confirm that there are large variations ranging from 197% to 231% in the values of swing angles across the six wind zones; the swing angle θ varying in proportion to zone wind speed for transmission lines of different voltages.
Reduced Conductor Clearances
It is well known that C varies with the wind speed due to the latter's effect on deionization of the intervening space. The relationship between clearance and wind speed has been well documented but the results vary considerably due to a number of complex factors. However, the pattern of variation C with mean wind speed, Wm, which has given satisfactory performance over a long period for different transmission lines in India, provides a dependable guide.
Using Wm, shown in column 10 of Table 1, the “Wm-C” relationship for the five typical Indian transmission lines is shown in Fig. 3. Note that these relationships are nearly straight-line expressions.
Using these relationships, the requirement of C is evaluated for the wind speeds in each zone. These values, presented in Fig. 4, confirm considerable variation (ranging from 116% to 51% in the values of C across the six wind zones) but in reverse order (lower values of C for higher wind zones).
Opposing Variations
There are wide variations in the values of θ and C across the six zones, but as these variations are in opposite directions, it is necessary to estimate their net impact on the configuration of conductors on the tower that, in turn, can affect the dimensions and, therefore, the weight/cost of the towers.
The θ-C combinations mainly affect the horizontal spacing, H, of the conductors. An increase of H will increase the width and, correspondingly, the height (for the same shield angle of the ground wires) of the towers, resulting in heavier, costlier structures. The variation in horizontal spacing of conductors provides a fair yardstick to determine cost savings that can be achieved by adopting different sets of θ-C combinations for some or all wind zones.
The other component of conductor configuration (vertical spacing) is mainly affected by the conductor clearance with 0° swing. As this corresponds to a no-wind condition, it does not generally change because of a variation of wind speeds and, therefore, remains unaffected across the six wind zones.
The conductor's H for different values of θ°C combinations for the six wind zones have been determined using well-established methodology, as shown in Fig. 5.
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For the two 66-kV lines, the variation in the horizontal spacing of conductors is within 7% of the presently used values.
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For 400-kV line, this variation is even less (within 3%).
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However, for 132-kV and 220-kV lines, there is considerable negative variation (reduction) in the horizontal spacing of conductors with reference to the presently used values, ranging from -9% to -19% for 132-kV lines and between -3% to -17% for 220-kV lines.
These observations suggest that although the presently used θ-C values could continue to be used for 66-kV and 400-kV lines across all six wind zones without sacrificing any economic benefit, there would be some techno-economic benefit if one or two additional sets of θ-C combinations were introduced for 132-kV and 220-kV lines, especially in lower wind-speed zones.
Summary
The introduction of multiple θ-C combinations on transmission lines of the same voltage would result in different tower designs, which would increase design, construction and maintenance costs. Proper line design criteria should result in appropriately designed lines that will result in transmission line projects built to meet India's quickly developing power infrastructure needs.
Although the configuration of towers used to support the transmission lines for all voltages will remain the same across the six wind zones, the weight/cost of these towers has increased because of higher wind pressures experienced in higher speed wind zones.
Dr. V.N. Rikh received a BE degree in 1953, an ME degree in 1977 and a PhD in engineering and technology in 1986. Rikh retired from the U.P. State Electricity Board, one of India's largest public sector power undertakings, after a 36-year career. He continues to work as an advisor and consultant to several public sector undertakings and the Power Grid Corporation of India. He has published more than 100 technical papers and presented numerous papers at national and international conferences. He has received several honors in India and has worked on the CIGR… Study Committee 36 for six years.
vnrikh@vsnl.net
