Abstract Summary
In the framework of the "Smart Tower" project, a joint effort between the VUB and Belgian grid operator Elia to investigate the feasibility of SHM for transmission towers. A transmission tower is instrumented with several types of sensors, including accelerometers, strain gauges, acoustic emission sensors and PZTs. In order to evaluate the approach, the tower was damaged in a controlled manner during the monitoring campaign to simulate common failure modes of this type of structure. Scenario’s included the removal of a single bolt, bending a cross brace, grinding a cross brace and ultimately the removal of a cross brace., and after a specified period, the structure was fixed. In this contribution, the focus is on the four tri-axial acceleration sensors installed on each face of the high voltage steel lattice tower. The main objective is to perform vibration-based anomaly detection. The proposed approach is based on operational modal analysis (OMA), where the modal parameters are first estimated using the poly-reference Least Squares Complex Frequency-domain (p-LSCF) algorithm. Each sensor is processed individually, which allows differentiating local and global modes between the different faces of the structure. Then, the identified modal parameters are tracked over time in a time window of 10-minute intervals. Each tracked modal parameter is first filtered by discarding data out of the mainstream of the mode. Then, the retained modes are normalized for varying environmental conditions. The resulting compensated modal frequencies from the four sensors are used to evaluate the dynamic behavior of the structure. Results show that a large number of modes can be detected. We also notice the existence of local and global modes, as local modes can have slightly different frequency value from 1 sensor to another. This allows the estimation of the symmetry of the structure, which can be used as a non continuous structure health evaluation. Moreover, the proposed methodology was able to detect nearly all the damage events. To some level the nature of a damaging event was even identifiable as the bolt removal failure modes only affected local modes at the high frequency part of the spectrum, and it is only detectable in the sensors installed in the same face where the damage was introduced. Meanwhile, the removal of brace affected global modes and thusly was detectable in the acceleration reading of all different four faces of the transmission tower.
