The provisions in this clause are a major departure from the previous Cb1 and AS/NZS 1170.2 in the definition of wind regions. It acknowledges that most wind damage in Australia and New Zealand to the overhead line networks occur during severe thunderstorms and provides a more reasonable interpretation of wind regions based performance of overhead line networks over a number of years.
Figure B 1 Wind Regions for Australian Design Wind Gust Types
Figure B1 shows a zoning map to determine which storm type should be considered in design for wind. On the mainland, the regions on this map are delineated by a boundary 200 kilometres from the smoothed coastline. This contrasts to the multiple narrow 50km wide zones in AS/NZS 1170.2 for the near coastal areas. Wind velocities are selected from AS/NZS 1170.2 as appropriate to the security level selected for the relevant location and wind zone required in the standard Figure B1.
This leaves some latitude to the designer to select the V50 value for a selected Security Level as required in Section 6 of the standard.
For example in Zone 1 for Australia where cyclonic events occur AS/NZS 1170.2 provides for wind zones C, D and B. Recent experience suggests that these arbitrary 50km zones are not relevant to severe Category 4 and Category 5 cyclonic events as the storm damage paths have been observed to extend 100km inland over a width of some 20km. In these cases it would be appropriate to select a V50 value from region C
For the remainder of the non cyclonic regions within Zone 1 such as New South Wales coastal area only one V50 value is provided but a higher return period value may be adopted in some local areas where regular storm damage occurs.
For example in the coastal area immediately north of Sydney or the south east Queensland regions it might be prudent to adopt a V100 value or a higher security level as appropriate, in view of the relatively high frequency of severe thunderstorms.
It should be noted that the selection of the regional wind speed is relevant to the line’s location, and care needs to be exercised where standard designs are applied to multiple sites. Where an overhead line is of significant length, variations in wind loading may be required as the line passes through differing wind exposure situations.
For example a line emanating from a coastal substation in a cyclonic region passing inland over a coastal range to an inland supply point could pass through three significant design wind climates that should be incorporated in the line design.
B2 New Zealand
Apart from the probability in some areas of turbulent effects near large mountains the majority of New Zealand is within Region A7 of AS/NZS 1170.2. Some caution needs to be applied to locations on hills in close proximity to sea coasts.
B3 Synoptic wind regions
In Clause B3 reference is made to wind direction multipliers Md as provided in Table 3.2 of AS/NZS 1170.2 being taken as 1.0 to provide for multiple changes in direction of the route of overhead lines. In some cases it could be argued that where a line route is in a predominate direction for its entire route and the line design is unique for that line only, that consideration be given for a lower value direction multiplier. However line designs once created usually have repeat applications on other line projects which could have multidirectional characteristics and extreme caution is required if reduced values of Md are used.
Cyclonic wind amplification factors Fc and Fd provided in AS/NZS 1170 Clause 3.4 are to be taken as 1.0 for all overhead lines, based on performance of overhead lines in cyclonic areas over time. These factors are provided in AS/NZS 1170.2 to apply additional security due to some uncertainty with wind velocities in the light of the recent incidence of several major Category 5 events. Performance of major transmission lines in these regions over the last 50 years has been very good, despite some structure failures occurring. Distribution line network failures in such extreme events occur regardless of magnitude of wind velocities primarily as a result of airborne vegetation and building debris. Hence the value of 1.0 has been applied for all lines in these areas.
B4. Downdraft wind regions (Australia Zone II and Zone III and New Zealand Zones Region A 7 )
B4.1 Downdraft Winds
The standard provides for all structures to be designed for V50 3 second gust regional wind speeds as defined
in AS/NZS 1170.2. Higher or lower security levels of line design are then adjusted from this value using Security Multipliers from Section 6.
Downdraft winds are the predominate wind that governs the design of overhead lines in Australia with the exception of cyclonic coastal regions. The wind velocities provided in AS/NZS 1170.2 include this type of event.
The important aspect that is different is the span reduction factor when compared to that applicable to the larger scale synoptic wind gust events. Downdraft wind gusts are relatively narrow and when they strike the ground observations of vegetation damage suggests a burst swath varying from 100m up to 1000m in width being common occurrences during more severe thunderstorms and hence the wind can envelop one or more spans simultaneously.
The standard provides a Span Reduction Factor (SRF) to be applied as provided in Figure B 6 Terrain -Height Multiplier Mz,cat for the common range of structure in open terrain and heights < 50m is 1.0
B4.2 Tornadoes
The standard identifies that these events do occur in some parts of the country but that they are relatively rare random events and of low intensity < F2 strength when compared to those in United States of America and Argentina. Unless a line has a very high security requirement it is recommended that no special loadings be generally considered.
B5. Wind Pressures
For distribution overhead lines a simplified approach to wind loadings can be applied particularly as most lines are located in Category 3 or Category 4 exposure and significant shielding from vegetation and structures occurs.
The basic regional wind pressure (pb) as selected from Table E1 below for the relevant wind region from AS1170.2 and limit-state being considered and ps and pu represent the corresponding basic pressures for the serviceability and strength limit-states, respectively.TABLE E1