Design of a ground-deployed sensor for detecting the sag of Overhead Transmission Lines by measurement of the magnetic field

Abstract

The main constraint on the capacity of an overhead transmission line (OTL) is the sag in the wires caused by thermal expansion. A sensor network that monitors the sag in real time could allow operators to run transmission lines above their designed capacity. Most commercial sensor systems deploy the sensors on the wires but having a human come into contact with the wires during deployment or maintenance is expensive and dangerous. This thesis explores the possibility of deploying a sensor at ground level under a span of an OTL. A sensor is developed that consists of induction coils. By measuring the voltage induced in the coils, and with prior knowledge of the geometries of the span, the sensor can deduce the line sag. The mathematical strategy employed by the sensor was developed through simulating the magnetic field while accounting for a number of affects, such as: the shape of the hanging wires, the possible unbalance of the transmission and the induced eddy currents in the conductive ground.The accuracy in the developed strategy depends on the span geometry and increases with sag, typically falling below 0.5m before sag reaches the critical value. A prototype sensor was designed and built that utilizes the developed method. The manufacturing of the induction coils was found to be the largest source of measurement error.The sensor was tested 11 times under 11 spans of 6 different transmission lines. In 6 of the tests the sensor correctly predicted the height of the wires to within the accuracy predicted by the developed method. The reason for the other 5 tests failing is the subject of further research.The developed method and sensor design, if refined further, could be used to monitor the sag of OTLs, providing greater operational safety and could allow increased capacity. Keywords: sag monitoring, magnetic field sensing, overhead transmission line, magnetostatics