Surveying high-voltage transmission lines
NIWA has recently been involved in Transpower’s sixth Aerial Laser Survey (ALS) of an additional 2300 kilometres of power transmission line. Our role: to measure meteorological conditions along parts of the NZ transmission grid all the way from Manapouri in Fiordland to Kaitaia in the Far North.
Transmission lines move
Transmission lines move – and not just because of the wind.
The amount of power that can be safely transmitted through a line is limited by how hot the line gets. The temperature of a transmission line at any time depends mainly on the amount of electric current flowing through it and local weather conditions: air temperature, direct heating from the sun, and the cooling effect of wind.
As the line temperature increases, the line expands and sags. Sagging reduces the clearance between the line and trees, buildings, or the ground. Transpower controls the amount of power flow through its lines to ensure that there is not excessive sag and that the lines are operated safely.
The 2010 survey
This year our role was to support the ALS activity by measuring the ambient meteorological conditions along the more than 2300 kilometres of lines during the helicopter survey.
We provided sixteen portable meteorological stations, two helikyte vertical profiling systems and two operating teams to capture meteorological information.
The helikyte (balloon) is winched to above transmission line height and then lowered, and its on-board weather station continuously records air temperature, and wind speed and direction, against height. The data collected are used to model the meteorological conditions at or about the time the helicopter-borne Light Detection and Ranging (LiDAR) system measures the exact location (and amount of ‘sag’) of the transmission lines.
Once the field campaign is completed, the different data are used to accurately model the lines during known conditions. Line operators can then form a better picture of how each surveyed line performs at all times under all electrical load and weather conditions. Ultimately the survey helps Transpower operate the national grid more safely and efficiently.
NIWA was subcontracted by Power Systems Consultants (Wellington), and the LIDAR surveys were conducted by their project partner Opten Ltd (Russia). Meterological modelling was done by NIWA in conjunction with Atmos Works limited (Wellington).