Monitoring and Modelling the Vibrational Effects of Small (<50 kW) Wind Turbines on the Eskdalemuir IMS Station

It is known (Styles et al., 2005) that windfarms generate low frequency vibrations which propagate through the ground and have the potential to adversely affect sensitive installations, most notably seismometer arrays set up to monitor for nuclear tests. Significant work on the effects of large wind turbines has been carried out by Keele University as well as by Schofield (2002) and Fiori et al (2009). For the Eskdalemuir International Monitoring System station in Scotland, a vibration threshold was set, for wind farms within 50 km of Eskdalemuir, at frequencies around the 4 to 5 Hz region. However, with increased development, the threshold is being approached and small wind turbines (less than 50kW), even of the order of 15kW have also been restricted despite the differences in scale and modes of vibration. In order to protect Eskdalemuir a threshold limit was set for any turbine as a holding measure and a programme to try to establish whether they were really problematic has been carried out. Models for two wind turbine types from the manufacturers Proven and Gaia-Wind have been calculated and measurement programmes carried out. It has been possible to demonstrate that in most cases these small turbines do not generate significant energy in the band of concern and that the levels are low enough to be negligible. Small turbines once evaluated and monitored by Keele University and given approval by the UK Ministry of Defence, will receive clearance for deployment around the Eskdalemuir site at distances greater than 10km

Seismic monitoring and vibrational characterization of small wind turbines: A case study of the potential effects on the Eskdalemuir International Monitoring System Station in Scotland

This paper presents a case study of the ground-borne vibrations generated by small wind turbines. This is of particular importance for assessing the possible impact on the detection capabilities of the International Monitoring System seismic array at Eskdalemuir in Scotland. Measurements were gathered from a selection of small wind turbines (≤50 kW), from three different manufacturers, varying in hub height, power and construction using a range of accelerometers and seismometers fixed to the tower and buried in the adjacent ground at increasing distances. Previous studies have shown that medium and large turbines generate harmonic vibrations. Planning guidelines exist but have inadvertently led to a ban on the deployment of individual small-turbines within 50 km of the array. This study investigates whether their inclusion in these guidelines is necessary. Vibration levels on the tower are in the region of 10 -3 m/sqrt(Hz), are transferred into the ground and decay rapidly away from the turbine (10 -5 m/sqrt(Hz) in the ground at 200 m). The spectral content of the vibrations shows variations between the turbines, but each have peaks in the band of interest for Eskdalemuir. Further, sources of the peaks in the spectra are identified as either originating from the blades’ rotation or the resonant modes of the turbines. Analysis of the relationship between wind speed and seismic amplitude on the tower shows an exponential trend, individual to each wind turbine and the frequency band of interest. Similarly, analysis of the seismic amplitude between the tower and foundation shows that this can be described by a power law, but this is individual to each turbine. It is our opinion that there is little requirement to include turbines of 50 kW or less in planning guidelines outside the statutory exclusion zone of 10 km, but each turbine should be considered on an individual basis

Studies of Vibrations from Wind Turbines in the Vicinity of the Eskdalemuir (AS104) IMS Station

Styles et al (2005) describe an extensive microseismic and infrasound monitoring programme to characterise the low-frequency vibration spectra produced by wind turbines of various types, both fixed and variable speed. They demonstrated that small but significant harmonic vibrations (modal eigentones) of the towers, excited by blade passing, tower braking and wind loading while parked, can propagate many kilometres and be detected on broadband seismometers. This meant that protective measures were required to protect the IMS seismic monitoring station (EKA), located at Eskdalemuir in the Scottish Borders, UK. Over 2 GW capacity of wind turbines were planned for this region and planning restriction were imposed to control development because of potential effects on the IMS station. This work established that vibrations of concern in the 2 to 6 Hz band, while small, were critical for this monitoring. Propagation laws were derived and an aggregate vibration budget established which would not prejudice the operation of Eskdalemuir, to aid planning and permit appropriate wind farm development. Subsequently, further work has been carried out to determine if small wind turbines (<50kW) should be covered by the same restrictions as large turbines. The UK Ministry of Defence has now issued new guidelines that should allow small wind projects to be developed in the vicinity of Eskdalemuir after modeling and measurement confirms that they do not generate significant vibrations within the band of concern. The work may have relevance to other IMS sites where new windfarm developments are planned or already exist