The impact of wind turbines on fixed radio links
Börje Asp, Gunnar Eriksson, Peter Holm
Information and Aeronautical Systems
FOI, Swedish Defence Research Agency
Introduction
• Strong demands for renewable energy
• Conflict of interest unavoidable
•
For example, fixed radio links (Military and civilian)
• Strong influence on frequency
allocation/use
• No reliable models available
to predict effects on links
from wind turbines
•
Most link operators
apply widely different “exclusion
zones” around their link paths
Characteristics of fixed microwave links
• Elevated antennas (line-of-sight propagation)
• Directional antennas (large directivity, narrow beams)
• Designed for very high availability
•
Permitted outages: In the order of seconds per month
•
Obtained by large fading margins + diversity arrangements
•
Outages usually caused by fading under anomalous atmospheric
conditions (e.g. ducting conditions)
Ongoing studies and measurements
• We study how scattering from wind turbines may affect
fixed radio links under natural tropospheric fading
• Measurements suggest that the strength of the scattering
from turbines is only slightly affected by the fading
•
Hence, the signal to interference ratio reduces drastically during
natural fading
•
This finding is not well known
Measurements – 8 GHz path over water (21 km)
View from one of the antennas in the direction to the other antenna
Turbine
Antenna location
Measurements – 8 GHz path over water (21 km)
(vp: green, hp: blue)
Fading event 2012-01-20
Start at 20:27 (LT)
Zoom 1 (hp)
Measurements – 8 GHz path over water (21 km)
Zoom 2
Zoom 3
Fading event 2012-01-20
Measurements – 2 GHz path (60 km)
Zoom
Fading event 2011-04-15
Start at 03:45 (LT)
Modeling of the scattered field
• Model assumptions
•
•
Turbines located between the link terminals, relatively close to the direct
path between the antennas. This implies
•
Effects dominated by forward scattering
•
Very different from the radar back-scattering case
Turbine dimensions are large
•
Neither the transmitter, nor the receiver, is in the far-field region of the turbine
• Under those assumptions, the following should be adequate
•
A wind turbine is described by it’s two-dimensional projection on an
aperture plane
•
Scattering is described by Fresnel-diffraction theory in the aperture plane
Modeling of scattered field
• Calculation of direct field (under normal and anomalous tropospheric
conditions)
• Calculation of turbine field by means of apertures
• Calculation of total received field by adding turbine field using
Babinet’s principle
Direct
field
Turbine
field
Babinet’s principle:
Total field = Direct field
– Aperture (turbine) field
Simulation at 2 GHz - PE model, no turbines
Normal troposphere
Modified
index
Duct 1
Modified
index
Duct 3
Modified
index
Simulation at 2 GHz - PE model, one turbine
Normal troposphere
Duct 1
Reciver height: 206-222 m above sea level
Distance transmitter - turbine: 38.8 km
Horizontal offset : 25 m
Turbine diameter: 100 m
Duct 3
--- Field with no turbine
--- Min field with turbine
--- Max turbine field
Simulation at 2 GHz - PE model, one turbine
Duct 3
--- Field with no turbine
--- Field with turbine
--- Turbine field
Simulation at 2 GHz - PE model, one turbine (2)
Normal troposphere
Duct 1
Reciver height: 214 m above sea level
Distance transmitter - turbine: 38.8 km
Horizontal offset : 25 m
Turbine diameter: 100 m
Turbine rotation: 0.2 Hz (typical for a large turbine)
Duct 3
--- Field with no turbine
--- Field with turbine
--- Turbine field
Simulation at 2 GHz - PE model, one turbine (3)
Duct 3
--- Field with no turbine
--- Field with turbine
--- Turbine field
Simulation at 2 GHz - PE model, one turbine(4)
Normal troposphere
Reciver height: 219 m above sea level
Distance transmitter - turbine: 38.8 km
Horizontal offset : 25 m
Turbine diameter: 100 m
Turbine rotation: 0.2 Hz
Duct 1
Duct 3
--- Field with no turbine
--- Field with turbine
--- Turbine field
Simulation at 2 GHz - PE model, one turbine(5)
Duct 3
--- Field with no turbine
--- Field with turbine
--- Turbine field
Conclusion
• Measurements and simulations suggest that the strength of
the turbine field is only slightly affected of natural fading
•
Hence, the signal to interference ratio reduces drastically during natural fading
•
Fading margin is decreased by about 10-15 dB for the investigated case
• As turbine fields are only slightly affected of natural fading
•
Simpler (less complex) propagation models can be used
•
If the strength of the turbine field is known, the fading margin can be estimated
• Simulations have also shown a remarkable slow decrease in
the turbine field for an increasing horizontal offset
•
Although, as the horizontal offset increases, the turbine field decreases faster
for higher than for lower radio frequencies
Further work
• Target: Outage statistics as a function of distance
from direct path to turbine
• Actions
•
Impact of the static fields from the turbine towers (ongoing)
•
Impact from large wind farms (more analysis, ongoing)
•
Turbine size (more analysis, ongoing)
•
Path length and distance to turbine from direct path (more analysis,
ongoing)
•
Simplified tool (first version running)
Simulation at 2 GHz - Two turbines (3)
Simple free space model
Reciver height: 219 m above sea level
Distance transmitter - turbines: 38.8 km
Horizontal offset : 50 and 100 m
Turbine diameter: 100 m
Turbine rotation: 0.2 and 0.21 Hz
Backup slides
Simulation at 2 GHz - Two turbines
Normal troposphere
Reciver height: 214 m above sea level
Distance transmitter - turbines: 38.8 km
Horizontal offset : 50 and 100 m
Turbine diameter: 100 m
Turbine rotation: 0.2 and 0.21 Hz
Duct 1
Duct 3
--- Field with no turbine
--- Field with turbines
--- Turbine field
Simulation at 2 GHz - Two turbines (2)
Normal troposphere
Reciver height: 219 m above sea level
Distance transmitter - turbines: 38.8 km
Horizontal offset : 50 and 100 m
Turbine diameter: 100 m
Turbine rotation: 0.2 and 0.21 Hz
Duct 1
Duct 3
--- Field with no turbine
--- Field with turbines
--- Turbine field
Modeling of the scattered field (3)
•
Model assumptions:
•
•
The turbines are located between the link terminals and
relatively close to the direct path between the antennas.
This implies
•
Effects dominated by forward scattering
•
Very different from the radar back-scattering case
Turbine dimensions are large
•
Neither the transmitter, nor the receiver, is in the far-field
region of the turbine
Modeling of the scattered field (4)
•
Under those assumptions, the
following should be adequate
•
A wind turbine is described by it’s twodimensional projection on an aperture
plane
•
Scattering is described by Fresneldiffraction theory in the aperture plane
•
The aperture field equals the impinging
field in the aperture plane
•
Field at Rx is computed by integration
over the aperture field
Tx
Rx
Aperture plane
Simulated scattered field
Normal atmosphere
•
Maximal turbine field:
•
•
Duct 1
-20±5 dB in all three case
Regions where scattered field is
larger than direct field:
Duct 3
•
Simulations of fading events support
observations from measurements:
•
Turbine field only slightly affected by
natural fading