Tower Foot Monitoring in Wind Power Plants

The tubular pole of a wind power plant is subject to substantial bending loads. In order to register the stress on that safety-relevant part in service, a proven method is the recording of Rainflow collectives. For that purpose the tower foot bending is recorded and directly classified by a MATCH-II or MATCH-II-4 Recorder by means of two sensors (SG bridges) usually arranged at right angles with each other. This approach poses the problem that the main load axis depends on the wind direction. This would suggest to establish appropriate load collectives not only for downwind loads but also for other load directions. However, the installation of additional sensors is mostly discarded owing to the pertaining effort for long-term testing. Therefore, SWIFT has adopted a different approach with its MATCH-II line. Instead of individually measuring each direction, the MATCH-II box calculates additional strain signals for any selectable intermediate angles from the two typical strain gauges arranged at right angles. The resulting strain is determined online at the same high sampling rate as the SG signals and is classified in a separate Rainflow matrix. That way, up to 8 additional directions can be recorded by a MATCH-II-ADV at 2kHz sampling rate. After all, this corresponds to an 18° angle resolution at uniform distribution over the full circle. Off course, the angle resolution can also be formulated non-uniform, e.g. to increase the downwind angle resolution. Apart from the MATCH-II-ADV, two full bridges of strain gauges are sufficient at the tower foot owing to the two integrated SG amplifiers. The powerful software ensures the provision of meaningful strain data without additional hardware, also for load directions different from downwind. Thus, the MATCH-II-ADV is the perfect instrument for stress monitoring at tower footings, and, moreover, it is available at an attractive price. Customised versions of this software are available as software upgrade for all MICRO-II and MATCH-II versions as well.