Huson, L. Stationary Wind Turbine Infrasound Emissions, WTN Glasgow
6th International Conference on
Wind Turbine Noise
Glasgow 20-23 April 2015
Stationary wind turbine infrasound emissions and propagation loss measurements
W Les Huson, L Huson & Associates Pty Ltd, PO Box 290, Woodend, VIC 3442, Australia.
Summary
Microbarometers have been used to quantify the infrasonic emissions (0.05Hz to 20Hz) from five wind farms in Victoria, Australia. The wind farms measured include; Macarthur wind farm (140 turbines type Vestas V112 3MW); Cape Bridgewater (29 turbines type MM82 2MW); Leonards Hill (2 turbines type MM82 2MW); Mount Mercer (64 turbines type MM92 2MW), and; Waubra (128 turbines 3 types of Acciona Windpower 2MW).
Upwind indoor measurements at the Macarthur wind farm during an unplanned shutdown from full power and subsequent startup to 30% load has shown that stationary turbines subject to high winds emit infrasound pressure below 8 Hz at levels similar to the infrasound emissions at blade pass frequencies and harmonics.
The stationary V112 turbine infrasound emissions are caused primarily by blade and tower resonances excited by the wind. It is apparent from the mismatch of resonances and blade pass frequency components that Vestas have carefully designed this unit to minimise fatigue of the wind turbine.
Short range (up to 2km) measurements from the Leonards Hill wind farm have shown the determination of attenuation rate with distance to be problematic due to interference between the two turbines. A model to explain the unexpected attenuation results at Leonards Hill has demonstrated that the commonly observed amplitude modulation of blade pass tones is the result of changing phase between turbine rotor speed and changes in wind speed.
Long range measurements from two different wind farms over a distance of 80km have shown that infrasound below 6Hz has a propagation loss approximating 3dB per doubling of distance.
1 Introduction
Planning development applications prepared by acoustical consultants for wind farms around the world have until recently neglected the potential impacts of infrasound.
A number of recent studies1,2,3 have described adverse human reactions to infrasound and low frequency sound. Similar adverse reactions have been voiced by residents near other wind farms. The research work described in this paper is based on the measurement of infrasound immissions observed within a number of dwellings near Australian wind farms. Some of the dwellings used to gather infrasound data have been vacated by the owners.
The objective behind this work was to quantify and better understand the propagation and causes of infrasound from modern wind farms having a generating capacity of 2MW and above.
This work was not commissioned by any external organisation and was funded by the author. The author has worked for both the wind farm industry and residents and is not affiliated with any pro or anti-wind organisations.
2 Infrasound Instrumentation
Acoustics professionals are most familiar with the common microphone connected to a sound level meter that has some form of standardisation such as IEC61672. Unfortunately, IEC61672 is designed for the audible frequency range and fails to address infrasound measurements4.
An alternative to the microphone for infrasound measurement is a MEMS pressure transducer. This type of device can be calibrated at static pressures and is used as a reference to calibrate infrasound equipment that detect nuclear explosions at many listening stations around the world5. The nuclear test ban treaty monitoring stations are located outdoors and have wind filters implemented by sampling the air over large areas with the use of tubes connected back to the sensor. This has the potential to influence the frequency response of the system5 so this approach is not used in these studies.
The equipment (LHA-IR1) used for the monitoring results reported in this paper was developed in-house in accordance with AS/NZS 3817 using a MEMS pressure sensor. Four units were used in this study. The system response is 0.05Hz to 30Hz (-3dB) with phase variation limited to +/- 10 ̊ in the frequency range 0.3Hz to 4Hz. An accurate phase response is required to faithfully represent the infrasonic pressure waveforms from wind turbines that predominantly fall within this frequency range. Calibration was with reference to a Class 1 sound level meter having known characteristics at 20Hz and 10Hz. The MEMS sensor is a static pressure measurement device with known sensitivity. The LHA-IR1 is configured to sample every 3ms and store the time history data in 20-minute blocks. This data is then converted into WAV file format for subsequent post processing. The storage capacity is limited only by the size of the microSDCARD in each unit and allows typically, for a 16GB card, over 100 days of data to be collected without replacement. With a 64GB card it can run unattended for over a year.
The two LHA-IR1 units used in the Leonards Hill wind farm propagation measurements were calibration matched by recording data from both units inside a car near to the wind farm. This measurement setup was recognised as a useful technique to minimise wind noise in the Shirley wind farm study2.
3 Infrasound Measurements
Infrasound measurements are reported for three wind farms. The Macarthur wind farm has 140 Vestas V112 3MW wind turbines located over flat terrain. Seven measurement locations are reported around this wind farm.
The Cape Bridgewater wind farm has 29 MM82 wind turbines located on the coast in Victoria. Eighteen of the turbines are grouped together in a relatively flat terrain 1000m from a dwelling in which a start-up was observed.
The Leonards Hill wind farm has two MM82 wind turbines located on top of a small hill and measurements are reported from within dwellings located 650m and 2000m away.
At the time of taking measurements at the Leonards Hill wind farm it was thought a good idea to monitor at a remote location indoors 34km to the East in Woodend as a reference ‘zero’. Contrary to obtaining a reference zero it was found that a number of wind farms could be identified from this location. Measurements at this location have proved useful in assessing infrasound propagation from another two wind farms in the area: Waubra and Mount Mercer.