Wake Effects from Wind Turbines Brian Wareing Brian Wareing.Tech Ltd Chester, UK Secretary WG28 Meteorology for overhead lines
Wind power Over 70GW installed capacity of wind power world-wide Europe is by far the leading player with 48GW installed and further developments going in at 14GW/yr. Major players in Europe are Germany, Spain, Denmark and the UK As with much new technology, the rate of introduction can far exceed the recognition of potential problems and the development of standards to deal with the problems. Wake effects from wind turbines Update 2
Potential problems for Brian Wareing.tech the Turbines It has been known for many years that wind reduction and turbulence effects mean that turbines should not be installed within 2xrotor diameter of other turbines across the prevailing wind direction and 8xrotor diameter along the prevailing wind direction, otherwise economic power loss and possible blade problems can occur Wake effects from wind turbines Update 3
Potential Problems for Brian Wareing.tech Overhead Lines It has been suggested in a study over 8 years ago [Jean-Louis] that excitation of vibrations in a power line is possible due to wake eddies from a wind turbine Germany has placed recommendations for the distance of OHLs from wind turbines in their National Normative Annexe (NNA) in the CENELEC European standard EN50341-3-4. Problems can arise from two areas: More wind in the low speed region (vibration) Turbulence affecting bundled conductors (1-3Hz) Wake effects from wind turbines Update 4
UK concerns Including planned projects, the overall wind power capacity of the UK is currently set to be 17.4GW from over 500 wind farms. Annual Installation Rate (BWEA) 700 600 500 400 MW 300 200 100 0 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Wake effects from wind turbines Update 5
The Basic problem 1 Brian Wareing.tech Vibration Overhead lines (OHL) use vibration dampers on their conductors in normal classical wind flow. Most of the problems due to vibration, however, occur in relatively light winds (<5m/s). So a tower line that was present before a wind turbine existed would already have damping suitable for the wind pattern. Wind turbines commonly don t start turning until wind speeds >4m/s so current low wind speeds not affected. With a wind turbine nearby there are two basic wind patterns the relatively undisturbed flow around but away from the turbine blades and the disturbed flow (wave eddies) which passes through the turbine blade envelope. This second area can bring about a major reduction in wind speed when the OHL is downwind of the turbine. This means that for a higher percentage of the time, the OHL conductors will be in an air flow of <5m/s. This may require further damping for the conductors to survive. Wake effects from wind turbines Update 6
The Basic problem 2 Brian Wareing.tech Turbulence Turbulence will occur at the boundary between the two air flows. This will be at a frequency similar to the rotational speed of the turbine blades (0.5 to 1Hz) but over a relatively short length of conductor (~50-100m). This could mean that, for a line with bundled conductors using spacers, one section (such as a sub-span) could be in a major turbulent flow whilst the next section may not be. This could cause conductor damage at the spacer clamp. The investigation will look to see where and over what areas these two effects could be significant for the OHL. Wake effects from wind turbines Update 7
Possibility of damage Overhead lines (structures and spans) can have a 1-3Hz resonance. Therefore, the turbulence of the new wind pattern created by the rotating blades may disturb any structure/span of OHL in the vicinity of wind turbines. Due to the possibility of continuous excitation caused by the wind turbine wake, fatigue may occur at clamping points So extra strengthening/protection/damping may be required on the OHL structure and components. Wake effects from wind turbines Update 8
Jean-Louis conclusions If wind turbine blades > 33% of conductor span then 0.3-5Hz oscillations possible. 1.5MW turbine has 80m blades Peak-to-peak amplitudes of 15cm possible Fretting/fatigue damage possible Could lead to major problems in bundled conductors (sub-span oscillation) OHLs could suffer damage up to 10x rotor diameter (800m) downwind Wake effects from wind turbines Update 9
Wake from a single turbine D nxd Wind spectra Wake effects from wind turbines Update 10
Wind spectra Brian Wareing.tech downstream 2.5D 5D D ~ 80m 10D Wake effects from wind turbines Update 11
Turbulence area Turbulent areas Danger area For low frequency turbulence Long distance away -No problem Wake effects from wind turbines Update 12
Horizontal turbulence Brian Wareing.tech intensity at hub height Wake effects from wind turbines Update 13
Vertical turbulence So the turbulence can be way above the OHL! Wake effects from wind turbines Update 14
Effect on conductor Brian Wareing.tech near clamps (Sefag) The more time the line spends in winds at<4m/s the shorter its lifetime! Wake effects from wind turbines Update 15
BSEN50341-3-4 (2001) 5.4.5 DE.2 Clearance to wind energy converters Between the rotor blade tip of the wind energy converter in the most unfavourable position and the closest conductor of the overhead line in still air the following clearances shall be obeyed: 1. more than or equal to three times the rotor diameter if the conductors are not damped against wind induced vibrations 2. more than one times rotor diameter if the conductors are damped against wind induced vibrations 3. If it is ensured that the overhead transmission line is outside the wake of the wind energy converter and the distance between the rotor blade tip in the most unfavourable position and the closest conductor is more than one times rotor diameter, it is possible to avoid the damping of the conductors Wake effects from wind turbines Update 16
My conclusions! That model and field measurements on wind turbine wave eddy and wind speed reduction effects have indicated that there is a significant risk to the earthwires and upper phase conductors of tower lines situated within 5 rotor diameters of the nearest point of a wind turbine. Further information is required, however, on the level of disturbances and how these affect the OHL conductors and fittings. It is therefore necessary that further studies be set up to more closely define the effect and thereby to identify the means of nullifying any increased oscillatory problems by using specific damper or configuration methods. Wake effects from wind turbines Update 17
Scottish Power proposal Use a wind tunnel to investigate the wave eddy and wind speed reduction phenomena over a range of wind speeds and turbine modes of operation and orientation Monitor an existing line both at current spacer dampers and the earthwire as well as the nearby wind turbine operation and meteorological parameters. Determine the wind profile of a particular site of concern to Scottish Power so that results from the wind tunnel tests can be applied. Evaluation of the increased vibration and other oscillatory problems identified from the above data and determine their likely effect on OHL conductor security. Wake effects from wind turbines Update 18
Areas to be investigated Earth wire (for vibration) Conductor/clamp (for low frequency effect) 3 x Rotor diameter D German recommendations Wake effects from wind turbines Update 19
Required output Recommend changes to negate the effects of the turbulence that can be applied to: - The OHL support structure. The span(s) of conductor(s). Together with recommended clearance distances that should be applied between any Wind Turbine and: - An OHL support structure. The span(s) of conductor(s). Wake effects from wind turbines Update 20
Current status Costs involved in this project have been determined. Scottish Power considering whether to go ahead on their own or in conjunction with other UK utilities. Wake effects from wind turbines Update 21
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