One of our clients had a question about performing a LiDAR survey when there was significant wind, and wanted to know how this would affect the accuracy of the survey. If the purpose of the survey is to be able to model the line at max blowout position, you need to model the line plumb in the first instance.
In my view this comes down to knowing at what point we should apply a correction — or even not fly the circuit. When does the wind factor become enough to affect the quality of the conductor sag in the PLS-CADD model? These numbers vary depending on the conductor span length, the conductor weight, and the conductor diameter. The horizontal loads which will cause the conductor to deflect are the wind pressure multiplied by the conductor diameter; the conductor weight is what causes it to remain plumb.
A Worked Example
As an example, I’ve run some numbers for Araucaria 700mm² — a common 700mm² transmission conductor — with a span length of 300m:
- Up to 6 m/s — deflection is trivial: a 6° blow-out angle, causing a delta sag of 31mm.
- At 8 m/s — a blow-out angle of 10.7°, with a delta sag of 98mm. Above 6 m/s, this is the point at which we may wish to consider sag correction.
- At 10 m/s — a blow-out angle of 16.5°, with a delta sag of 232mm.
The wind speed here is, of course, the component which is perpendicular to the conductor. As a power line will have numerous changes of direction, it’s prudent to consider the worst-case wind and accept that most spans will not be so badly affected. There are exceptions to this: in certain areas where prevailing winds arise, we may wish to only consider the higher wind events where they occur perpendicular to the conductor, or determine the perpendicular and parallel components from that prevailing wind direction.
When to Suspend the Survey
At some point, the helicopter pilot may decide that the winds are causing excessive crabbing, making the required survey flight problematic — that’s outside the scope of this piece. Purely in terms of excessive angle and delta sag, corrections to the sag delta cease to be as effective for larger blow-out angles and sag changes.
“With this in mind, it is normally prudent to suspend LiDAR survey at wind speeds of 14m/s and higher.”
The Bottom Line
Below around 6 m/s, wind-induced sag error is trivial and safe to ignore. Between 6 and 14 m/s, it’s a correction problem: know your span, weight and diameter, and adjust. Above 14 m/s, the corrections themselves become unreliable, and it’s time to stand the survey down rather than trust the numbers. This is the same judgement call our survey teams apply on every LiDAR flight — knowing not just how to correct for wind, but when a correction stops being trustworthy.
As Director of Engineering and heading up research and development at NM Group, Paul Richardson is a renowned power industry expert and long-time proponent of the use of 3D modelling in improving reliability and utilization, while minimizing cost. He is a chartered civil engineer (BEng Hons, CEng, MICE), and retains a wealth of experience, including more than 20 years of power line engineering.
Paul will be answering more questions as part of the Ask an Engineer series. If you have a question about LiDAR, PLS-CADD, or any aspect of power line engineering, get in touch.