Wake Effects from Wind Turbines - aeolus.bz · Wake effects from wind turbines Update 3 Brian Wareing.tech Potential problems for the Turbines nIt has been known for many years that

Wake Effects from Wind Turbines

Brian WareingBrian Wareing.Tech Ltd

Chester, UK

Secretary WG28 ‘Meteorology for overhead lines’

Wake effects from wind turbines Update 2

Brian Wareing.tech Wind power

n Over 70GW installed capacity of wind power world-wide

n Europe is by far the leading player with 48GW installed and further developments going in at 14GW/yr.

n Major players in Europe are Germany, Spain, Denmark and the UK

n 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.


Brian Wareing.tech

Potential problems for the Turbines

n 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


Brian Wareing.tech

Potential Problems for Overhead Lines

n 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

n 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.

n Problems can arise from two areas:– More wind in the low speed region (vibration)– Turbulence affecting bundled conductors (1-3Hz)


Brian Wareing.tech UK concernsn 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)

0

100

200

300

400

500

600

700

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

MW


Brian Wareing.tech

The Basic problem 1 Vibration

n Overhead lines (OHL) use vibration dampers on their conductors in normal ‘classical’ wind flow.

n Most of the problems due to vibration, however, occur in relatively light winds (<5m/s).

n So a tower line that was present before a wind turbine existed would already have damping suitable for the wind pattern.

n Wind turbines commonly don’t start turning until wind speeds >4m/s so current low wind speeds not affected.

n 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.

n This second area can bring about a major reduction in wind speedwhen the OHL is downwind of the turbine.

n This means that for a higher percentage of the time, the OHL conductors will be in an air flow of <5m/s.

n This may require further damping for the conductors to survive.


Brian Wareing.tech

The Basic problem 2Turbulence

n Turbulence will occur at the boundary between the two air flows.

n 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).

n 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.

n This could cause conductor damage at the spacer clamp.

n The investigation will look to see where and over what areas these two effects could be significant for the OHL.


Brian Wareing.tech Possibility of damagen Overhead lines (structures and spans) can have a

1-3Hz resonance. n 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.

n Due to the possibility of continuous excitation caused by the wind turbine wake, fatigue may occur at clamping points

n So extra strengthening/protection/damping may be required on the OHL structure and components.


Brian Wareing.tech Jean-Louis conclusions

n If wind turbine blades > 33% of conductor span then 0.3-5Hz oscillations possible.

n 1.5MW turbine has 80m bladesn Peak-to-peak amplitudes of 15cm possiblen Fretting/fatigue damage possiblen Could lead to major problems in bundled

conductors (sub-span oscillation)n OHLs could suffer damage up to 10x rotor

diameter (800m) downwind


Brian Wareing.tech Wake from a single turbine

nxD

Wind spectra

D


Brian Wareing.tech

Wind spectra downstream

2.5D

5D

10D

D ~ 80m


Brian Wareing.tech Turbulence area

Long distance away- No problem

Turbulent areas

Danger areaFor low frequencyturbulence


Brian Wareing.tech

Horizontal turbulence intensity at hub height


Brian Wareing.tech Vertical turbulence

So the turbulence can be way above the OHL!


Brian Wareing.tech

Effect on conductor near clamps (Sefag)

The more time the line spends in winds at<4m/sthe shorter its lifetime!


Brian Wareing.tech 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


Brian Wareing.tech My conclusions!n 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.

n Further information is required, however, on the level of disturbances and how these affect the OHL conductors and fittings.

n 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.


Brian Wareing.tech Scottish Power proposaln 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

n Monitor an existing line both at current spacer dampers and the earthwire as well as the nearby wind turbine operation and meteorological parameters.

n Determine the wind profile of a particular site of concern to Scottish Power so that results from the wind tunnel tests can be applied.

n Evaluation of the increased vibration and other oscillatory problems identified from the above data and determine their likely effect on OHL conductor security.


Brian Wareing.tech Areas to be investigated

3 x Rotor diameter D

Earth wire (for vibration)

Conductor/clamp (for low frequency effect)

German recommendations


Brian Wareing.tech Required output

n Recommend changes to negate the effects of the turbulence that can be applied to: -

– The OHL support structure.– The span(s) of conductor(s).

n Together with recommended clearance distances that should be applied between any Wind Turbine and: -

– An OHL support structure.– The span(s) of conductor(s).


Brian Wareing.tech Current status

n Costs involved in this project have been determined.

n Scottish Power considering whether to go ahead on their own or in conjunction with other UK utilities.


Brian Wareing.tech


Brian Wareing.tech

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Wake Effects from Wind Turbines - aeolus.bz · Wake effects from wind turbines Update 3 Brian Wareing.tech Potential problems for the Turbines nIt has been known for many years that