MARCH 2008

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Precision and consistency in Track Clearance, Part I - Distance measurement precision

Innovation Marketplace in Lisbon: civil engineering solutions and 3D modelling

T&D Europe 2008: Power Line Inspection Tools in Amsterdam, The Netherlands

Line Inspection

Precision and consistency in Track Clearance

Part I - Distance measurement precision

Estimating precision on a known infrastructure

Quality evaluation of the of the distance measurements made by the Power Line Maintenance Inspection System involves estimation of precision and consistency (i.e. result repeatability from different observation points).
The method used to evaluate the PLMI solution is based on the analysis of a single laser sweep of the LiDAR data. In this manner, measurement errors are comparable to the LiDAR sensor error since only one sensor is used and the speed of the aircraft is negligible compared to the speed of the laser ray. The manufacturer indicates that for solid objects with smooth surfaces larger than the laser beam diameter the error is approximately 0.02 m for a distance of 50 m.

Experiments in controlled environments or measurements of structures with known dimensions such as the highway shown above reveal a standard deviation of approximately 0.03 m. The results with greatest interest, however, are those obtained during flight. In flight conditions, the diameter of the laser beam is greater than the diameter of the power line conductors and greater than the vegetation surfaces which are rough, ductile and in constant motion. The sensor precision was studied for aerial and ground ranging applications on a 400 kV line with two conductors per phase and a 500 kV line with four conductors per phase, respectively. The results were similar in both applications and the latter was chosen to illustrate the detection limitations (see figure on the right).

Conductor a, within a laser beam cone, is the best detection condition. Conductor b is only in part within the laser beam cone, resulting in less incident energy. In addition, the laser beam cone that encompasses part of conductor b also encompasses part of conductor c. Therefore, two different incident energies generate two different echos which will produce a single object located between b and c. Conductor c is within two different laser beam cones, albeit with small incident energy, increasing the measurement error. Conductor d is not detected since it is in the shadow cone of conductor a. This phenomenon also occurs with ground cables that obstruct the line of sight of the sensor to the more distant conductors.

Based on the data collected the 500 kV line has 4 conductors arranged in a square shape with side length of 47.3 cm (conductor centre to centre) with a standard deviation of 3.5 cm. The design value of 48 cm is within 1 standard deviation from the measured value. Though conductor detection limitations can be overcome with signal processing algorithms, distance measurements to vegetation include other sources of imprecision: leaves sway in the wind changing distance and incidence angle, pollen and humidity can be present on the surfaces of the leaves and translucent leaves refract the incident energy onto other leaves which repeat the refraction process. All these effects increase the measurement error and hinder a precision assessment model.

Considering the detection limitations of conductors and vegetation it is estimated that the precision is less than or equal to 10 cm which is more than adequate for track clearance maintenance inspections. Albatroz Engineering's innovative solution features real time inspection results at a significantly reduced cost with slightly greater precision than the technical sensor limits. This approach eliminates the weeks or months waiting for results while vegetation continues to grow.

Pormenores de deteccao numa fase de quatro condutores