wind turbine scattering at hf · 2014-01-14 · dr jen jao dr william stevens dr scott coutts 19...
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Dr Jen JaoDr William Stevens
Dr Scott Coutts
19 September 2013
Wind Turbine Scattering at HFMIT LL Quick-look Outbrief
This work was sponsored by OSD OUSD/AT&L under Air Force Contract No. FA8721-05-C-0002. Opinions, interpretations, recommendations and conclusions are those of the authors and are not necessarily endorsed by
the United States Government.
Sponsor: Michael Aimone, OSD OUSD/AT&L
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Turbine Study- 2Outbrief 19 Sept 13
• Introduction – Bottom line up front
• Modeling approach
• Summary
• Backup and Additional Information
Outline
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Turbine Study- 3Outbrief 19 Sept 13
• Wind Turbine model developed– Based on the Numerical Electromagnetics Code (NEC)
version 4.2 method of moments solution of the electric field integral equation for thin wires
• Model used with radar parameters to generate Doppler signatures of wind turbine modulated clutter
• Model used to estimate changes to standoff requirements based on wind direction
• A great deal of effort was expended attempting to validate all modeling efforts – Limited runs were performed
Bottom Line Up Front
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Turbine Study- 4Outbrief 19 Sept 13
Wind-Farm Clutter Interference
Pr
PcGround wave
Tx
Rx
Clutter patch
Sky wave
D
Pi Pi
Ps
Single turbines Effective number of turbinesTurbine-to-incidentclutter level
focusing
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Turbine Study- 5Outbrief 19 Sept 13
• ROTHR1 and Lincoln Laboratory simulations show the same dependence on Turbine Aspect Angle (i.e., wind direction)– Up to 20 dB less interference for broadside aspect vs end-on aspect
Standoff Range vs Turbine Aspect Angle
0 45 90 135 180-25
-20
-15
-10
-5
0
Aspect Angle
dB R
elat
ive
to p
eak
Interference vs Aspect Angle forROTHR and Lincoln Models
ROTHR POLincoln
N NW/SE E/W SW/NE SWind Direction
N NW/SE E/W SW/NE SWind Direction
1, “Comprehensive Modeling Analysis for Stand-Off Requirements of Wind Turbines from ROTHR Systems,” RPO-TR-WF-0712-001, ROTHR Program Office, June 2012
0 45 90 135 1806
8
10
12
14
16
18
20
Aspect Angle
Stan
d O
ff
Approximate Range of Standoff Values for 20 km Minimum
0 45 90 135 1805
10
15
20
25
30
Aspect Angle
Stan
d O
ff
Approximate Range of Standoff Values for 30 km Minimum
N NW/SE E/W SW/NE SWind Direction
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Turbine Study- 6Outbrief 19 Sept 13
Single Wind Turbine Clutter Interference
Pr
PcGround wave
Tx
Rx
Clutter patch
Sky wave
D
Pi
Ps ~ |Es|2
Pi ~ |Ei|2
Turbine RCSmodulation spectrum
Three main ingredients:1 Turbine scattering2. Antenna response3. Propagation
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Turbine Study- 7Outbrief 19 Sept 13
• Introduction
• Modeling approach– Propagation modeling– Antenna– Turbine modeling – Radar system effects– Future modeling work
• Summary
• Backup and Additional Information
Outline
Three main ingredients:1 Turbine scattering2. Antenna response3. Propagation
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Turbine Study-819 Sept 2013
HF Propagation Near the Ground
• Propagation of radio waves near the ground is comprised of a sum of– Direct wave or space wave (often referred to as line-of-sight or LOS) – Reflected wave (generates multipath propagation when combined with direct
wave) – Surface-attached wave (often referred to as ground wave)– Plus other terms (induction field and secondary effects)
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Turbine Study- 9Outbrief 19 Sept 13
Propagation Zones for Antennas and Scatterers on the Ground
Boundary LocationsPropagation Zones
OTHR
GRWAVE Handles 3 Propagation Zones: Space Wave (Direct), Sommerfeld, and Diffraction
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Turbine Study- 10Outbrief 19 Sept 13
Space Plus Surface Wave Propagation LossPropagation loss estimated with Norton’s approximation over flat ground surface
Antenna height above groundTx: 150 m, Rx: 10 m
-40
-60
-80
-100
-120
-140
Prop
agat
ion
Loss
(dB
)
Propagation at 14.3 MHzIn free spaceOver sea waterOver lossy ground (r = 30, = 0.01 S/m)
Legend:Space wave + surface wave Space wave only
1 2 5 10 20 30Ground Range (km)
Antenna height above groundTx: 10 m, Rx: 10 m
1 2 5 10 20 30Ground Range (km)
Propagationloss factor
MIT LL Implementation of GRWAVE solution allows separability of the space and ground wave terms
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Turbine Study- 11Outbrief 19 Sept 13
Ground Wave Propagation Loss Using GRWAVE
Propagation loss estimated with GRWAVE over spherical earth surface
-40
-60
-80
-100
-120
-140
Prop
agat
ion
Loss
(dB
)
1 2 5 10 20 50 100Ground Range (km)
Propagation at 14.3 MHz
In free spaceIsotropic Tx and Rx antenna
Over sea waterSmall dipolar Tx
Over lossy ground(r = 30, = 0.01 S/m)Small dipolar Tx
Antenna height above groundTx: 150 m, Rx: 10 m
Propagationloss factor
NEC will be used to compute scattered level at a given range and GRWAVE is used to shift solutions in range
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Turbine Study- 12Outbrief 19 Sept 13
NEC Computation of Ground-Wave vs Range
NEC matches ground wave calculations (GRWAVE) out to about 25 km (using monopoles with 1 radials)
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Turbine Study- 13Outbrief 19 Sept 13
-60 -55 -50 -45 -40 -35 -30 -25 -20 -150
20
40
60
80
100
120
140
160
180
200
One-way gain (dB)
Hei
ght (
m)
Multipath Propagation Factor (Hant=3 m)
H PolV Pol
Tower: 96.76m HeightBlades: 50 m LengthGround: r = 13, = 0.005 S/mAntenna ht: 3 meters
Multipath Propagation Calculation vs Height with Turbine at 10 km Range
Propagation Factor
Propagation Factor ranges from -19 dB at the top of the blade to less than -27 dB at bottom of blade – weighs the higher portions of the turbine more heavily than the lower portions
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Turbine Study-1419 Sept 2013
• Tools producing consistent results for this application– GRWAVE, NEC, MIT-LL GRWAVE, MIT-LL MPATH– Ground wave and space wave contributions understood– NEC valid in Sommerfeld region
Benefit to using NEC is that multiple effects can be computed simultaneously
• Future propagation modeling likely to be performed using VTRPE– Parabolic Equation (PE) propagation software is a full field approximation for
site-specific scenarios– Have VTRPE examples later in this brief
Propagation Tools Thoroughly Tested and Understood
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Turbine Study-1519 Sept 2013
• Introduction – Bottom line up front
• Modeling approach– Propagation modeling– Antenna– Turbine modeling – Radar system effects– Future modeling work
• Summary
• Backup and Additional Information
Outline
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Turbine Study-1619 Sept 2013
• ROTHR Uses a TWERP antenna that provides front-to-back isolation– Uses 100’ ground screen
• This quick-look study only computing front-lobe main-beam response– Monopole antenna used with up to 64 120-foot radials
What Receive Antenna to Use?
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Turbine Study-1719 Sept 2013
TWERP and Monopole Elevation Gain Patterns Comparison
16 MHz
Monopole Height = 15.5 ftTWERP Separation = 13.8 ft
Medium Ground (=0.005 S/m, =13)
-15
-10
-5
0
5
0
10
20
30
40
50
6070
8090100110
120130
140
150
160
170
180Result Provided by Henry Thomas, MIT LL
Directivity close at the 5-degree elevation direction of the main clutter signal
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Turbine Study-1819 Sept 2013
• Introduction – Bottom line up front
• Modeling approach– Propagation modeling– Antenna– Turbine modeling
Model development and monostatic testingBistatic RCSModel results
– Radar system effects– Future modeling work
• Summary
• Backup and Additional Information
Outline
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Turbine Study-1919 Sept 2013
Model Development Progression
Long WireMonostaticRCS
Long WireBistatic RCS
5-Wire TurbineModel
Full MeshTurbineModel
• Ground types– Free space (no ground)– Perfect Electric Conductor (PEC) Ground– Real ground ( e.g., average ground with r= 13, = 0.005 S/m)
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Turbine Study-2019 Sept 2013
NEC Monostatic RCS Calculations for Long Wire
• NEC Broadside Calculations – Free space: 7.46 dB
( Add 26 dB for =20 m: 33.46 dBsm)– PEC Ground: 19.5 dB (45.5 dBsm)– Average ground (2.5 degrees from
broadside): 15 dB (41 dBsm)
NEC Spot Check
Avg. Ground 13/.0055.221 Wire (104 m)
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Turbine Study-2119 Sept 2013
NEC Bistatic RCS Calculations for Long Wire • NEC Bistatic RCS Calculations
85º incidence • RCS at 0º scattered angle (Add 26 dB to
plot values (=20m))– Free Space: 4 + 26 = 30 dBsm– PEC: 15.5 + 26 = 41.5 dBsm– Real Ground: = 0 m2 (-125 dB) null at 0º
-2 + 26 = 24 dBsm up 1º29 dBsm up 2º
Avg. Ground 13/.005
5.221 Wire (104 m)
Free Space PEC
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Turbine Study-2219 Sept 2013
Scattered Power Ratio Calculations for Long Wire“Back of the Envelope”
• RCS is defined at a ratio of incident and scattered power density:
• At 25 km, 4R2 = 99 dB and power ratios for the long wire are – Monostatic free space: 33.5 dBsm – 99 = -65.5 dB– Monostatic PEC Ground: 45.5 dBsm – 99 = -53.4 dB– Monstatic average ground (2.5 degrees from
broadside): 41 dBsm – 99 = -58– Bistatic free space: 30 dBsm – 99 = -69– Bistatic PEC = 41.5 dBsm – 99 = -57.5– Bistatic average ground = 24 dBsm (up 1º) -99 = -75
5.221 Wire (104 m)
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Turbine Study-2319 Sept 2013
Simple 5-Wire Model – Monostatic RCS
0 5 10 15 20 250
50
100
150
200
250
300
Blade rotation (deg)
Sqr
t (R
CS
) (m
)
0306090
Validation of 5-wire NEC turbine model
MIT LL NEC 4.2IEEE OCEANS 2012 Conference reference
65-meter tower, 42-meter blades, PEC Ground
Teague, Barrick
Near exact agreement between Barrick NEC 2 model and Lincoln NEC 4.2 Model
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Turbine Study-2419 Sept 2013
Simple 5-Wire Model – Doppler Spectra
-15 -10 -5 0 5 10 15-30
-20
-10
0
10
20
30
40
50
Harmonic number
Rad
ar c
ross
-sec
tion
(dB
sm)
0306090
Both RCS and Doppler spectra agreement with Barrick for monostatic PEC case
5-wire tower
Teague, Barrick Lincoln
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Turbine Study-2519 Sept 2013
Evolve Wire Model for Current Study
0 5 10 15 20 2510
15
20
25
30
35
40
45
50
55
Blade rotation (deg)
RC
S (d
Bsm
)
0306090
65-meter tower height
42-meter blade length
100-meter tower height
50-meter blade length
0 5 10 15 20 2510
15
20
25
30
35
40
45
50
55
Blade rotation (deg)
RC
S (d
Bsm
)
0306090
RCS increases by several dB for larger turbine for this PEC example
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Turbine Study-2619 Sept 2013
Monostatic RCS at 85 and 90-Degree Incidence Angles
0 20 40 60 80 100 12020
25
30
35
40
45
50
55
60
Blade rotation (deg)
Rad
ar c
ross
-sec
tion
(dB
sm)
Monostatic RCS i,s = 85 deg
PEC 04590135180
0 20 40 60 80 100 12020
25
30
35
40
45
50
55
60
Blade rotation (deg)
Rad
ar c
ross
-sec
tion
(dB
sm)
Monostatic RCS i,s = 90 deg
PEC 04590135180
RCS strongly depended on incidence angle for PEC case (dropped by almost 10 dB)
90º Incidence 85º Incidence
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Turbine Study-2719 Sept 2013
Monostatic and Bistatic RCS
0 20 40 60 80 100 12020
25
30
35
40
45
50
55
60
Blade rotation (deg)
Rad
ar c
ross
-sec
tion
(dB
sm)
Monostatic RCS i,s = 85 deg
PEC 04590135180
0 20 40 60 80 100 12020
25
30
35
40
45
50
55
60
Blade rotation (deg)R
adar
cro
ss-s
ectio
n (d
Bsm
)
Forward-scatter RCS i = 85 deg, s = 90
PEC 04590135180
Oblique incidence combined with forward-scattering produces even greater RCS reduction for PEC Case
85º Incidence Monstatic 85º Incidence, 90º Scattered Angle
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Turbine Study-2819 Sept 2013
PEC Real ground r=30, s=.01
i=85 deg, s=89.5
0 20 40 60 80 100 12010
15
20
25
30
35
40
45
50
55
Blade rotation (deg)R
CS
(dB
sm)
04590135
0 20 40 60 80 100 12010
15
20
25
30
35
40
45
50
55
Blade rotation (deg)
RC
S (d
Bsm
)
04590135
Bistatic RCS with Real Ground
Real ground further reduces effective RCS and flattens out spatial variation – may be due to multipath propagation weighting the higher portions of the turbine more heavily and attenuating the blade-column interaction.
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Turbine Study- 29Outbrief 19 Sept 13
Monostatic RCS and Spectra for 5-Wire and Full Mesh Model
-15 -10 -5 0 5 10 15-20
-10
0
10
20
30
40
50
Rad
ar c
ross
-sec
tion
(dB
sm)
Harmonic number
Wire mesh tower5-wire turbine
0 20 40 60 80 100 12010
20
30
40
50
60
Rad
ar c
ross
-sec
tion
(dB
sm)
Blade rotation (deg)
Wire mesh tower5-wire turbine
Simple 5-wire model captures much of the spectral behavior for monostatic PEC case
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Turbine Study-3019 Sept 2013
• Introduction – Bottom line up front
• Modeling approach– Propagation modeling– Antenna– Turbine modeling
Model development and monostatic testingBistatic RCSModel results
- Principal results for one turbine, one element
– Radar system effects– Future modeling work
• Summary
• Backup and additional information
Outline
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Turbine Study- 31Outbrief 19 Sept 13
RCS Modeling/Simulation Block Diagram
Thinkmate 64-processor AMD Opteron series 6200 Computer
MATLAB-based NEC driver and post processor
Input parameters
Tower• Upper diameter• Lower diameter• Height• Number of faces
Turbine• Nacelle angle• Blade wire length• Blade wire diameter• Number of rotor positions• Rotation direction
Radar System and Environment
• Frequency• Incident, scattering geometry angles
• Incident wave polarization• Underlying surface electrical parameters
Desired Output• Radar cross section• Electric field
Read tower, nacelle, hub, blade descriptors
Calculate wire mesh positions, orientations and sizes
Read radar system / environment / output parameters
NEC input file generation
System Call to NEC executable program
Pre-processor/initiator .m
Post-processor .mSpecify NEC output parameter of interest (RCS, electric field, current)
Parse NEC .out file
Parallelize turbine blade angle
computations by dividing among64 processors
Numerical Electromagnetics
Code NEC-4.2Numerical
ElectromagneticsCode NEC-4.2
Numerical Electromagnetics
Code NEC-4.2
Numerical Electromagnetics
Code NEC-4.2
1
64
NEC executable program
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Turbine Study- 32Outbrief 19 Sept 13
• “All in One” or “One-Step” approach where direct path and scattered paths are measured in a single NEC run– Required modifications to NEC 4.2 source code which was obtained
from Lawrence Livermore National Laboratory• Precision of NEC output files was increased
• “Two-Step” approach where the scattered field is measured separately from the direct path with “surface wave mode” feature of NEC turned on– Measured field at antenna location converted to received voltage
using the antenna-effective height – Direct and scattered path ratio formed and Doppler spectra
computed
Two Modes of Operation
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Turbine Study-3319 Sept 2013
One-Step and Two-Step ExamplesSingle Turbine, Single Receiver Element
14 MHz, 25 km Range5 degree grazing, forward scatter
ROTHR Study Figure 5
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
Harmonic numberPo
wer
rela
tive
to in
cide
nt p
lane
wav
e (d
Bc)
04590135180
Lincoln Calculation Using ROTHR Model
Two-step result shows turbine scatter only
One-step result shows Direct and Scattered signal in a single run
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Turbine Study-3419 Sept 2013
Turbine Model Comparison
Lincoln Dense Mesh Model
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
0
45
90
135
180
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)-30 -20 -10 0 10 20 30
-160
-140
-120
-100
-80
-60
-40
-20
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
Lincoln One-Step Calculation Using ROTHR Model
14 MHz, 25 km Range5 degree grazing, forward scatter
ROTHR and Lincoln turbine models predict very similar scattering spectra
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Turbine Study-3519 Sept 2013
Turbine Model Comparison (cont.)
Lincoln Dense Mesh Model
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
04590135180
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
14 MHz, 25 km Range5 degree grazing, forward scatter
Simple 5-wire model captures much of the spectral behavior but scattering is slightly weaker
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
04590135180
Lincoln 5-Wire Model
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Presentation Name - 36Author Initials MM/DD/YY
Frequency Dependence of Turbine Spectra 5-wire model, 97m height, 100m blade diameter
5 MHz 8 MHz
-30 -20 -10 0 10 20 30-160
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-20
0
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic numberPo
wer
rela
tive
to in
cide
nt p
lane
wav
e (d
Bc)
04590135180
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
11 MHz
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
14 MHz
• 15 km separation
• Average ground(13, .005)
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Presentation Name - 37Author Initials MM/DD/YY
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
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-20
0
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
Frequency Dependence of Turbine Spectra 5-wire model, 97m height, 100m blade diameter
17 MHz 20 MHz
23 MHz 26 MHz
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic numberPo
wer
rela
tive
to in
cide
nt p
lane
wav
e (d
Bc)
04590135180
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
• 15 km separation
• Average ground(13, .005)
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Turbine Study-3819 Sept 2013
5-Wire Model Spectra for Three GE Turbines
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
GE 1.85-87
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
GE 2.85-103
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
Pow
er re
lativ
e to
inci
dent
pla
ne w
ave
(dB
c)
04590135180
GE 2.5-120
5-Wire model, 15 km receiver-turbine distance, average ground (=13, =0.005)
TurbineModel
Hub Height Blade Diameter
GE 1.85-87 85 m 87 m
GE 2.85-103 97 m 103 m
GE 2.5-120 139 m 120 m
F=14 MHz
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Turbine Study-3919 Sept 2013
• Introduction – Bottom line up front
• Modeling approach– Propagation modeling– Antenna– Turbine modeling – Radar system effects– Future modeling work
• Summary
• Backup and Additional Information
Outline
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Turbine Study-4019 Sept 2013
Wind-Farm Clutter Interference
Pr
PcGround wave
Tx
Rx
Clutter patch
Sky wave
D
Pi Pi
Ps
Single turbines Effective number of turbinesTurbine-to-incidentclutter level
focusing
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Turbine Study- 41Outbrief 19 Sept 13
Adjustment Factor for Wind-Farm Interference
• GRWAVE propagation loss over wet ground- r = 30- = 0.01 s/m- Receiving antenna: 10 m- Transmitting antenna: 150 m
• Ratio of Tx/Rx beam width
• Near field array factor- Single turbine- Wind farm
(5 x 5, 1-km spacing)
0 20 40 60 80 100Ground Distance (km)
60
40
20
0
-20
-40
Loss
(dB
-10
7 dB
)
Used to scale single-turbine, single-element result to entire wind-farm
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Turbine Study- 42Outbrief 19 Sept 13
Standoff Distance Determination
Adjust single-element single-turbine spectra to achieve standoff distance estimates
+ Standoff
Adjustment Factor (5x5 Farm)
Single-Element Single-Turbine Spectra
-30 -20 -10 0 10 20 30-160
-140
-120
-100
-80
-60
-40
-20
0
Harmonic number
04590135180
Pow
er (d
Bc)
Lincoln Dense Mesh Model
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Turbine Study-4319 Sept 2013
Error Analysis
• Standard practice for reporting expected RCS measurement accuracy is to tabulate all error sources– Worst case condition is to assume all errors combine at their maximum value
Can be shown as error bars on plots – Root-Sum-Squaring (RMS) the errors results in a reduced expected error
Unlikely that all errors will combine in the same direction• Modeling error sources for this problem are 1) Propagation, 2) Turbine
Modeling, and 3) Antenna modeling
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Turbine Study- 44Outbrief 19 Sept 13
• Introduction – Bottom line up front
• Modeling approach– Propagation modeling– Antenna– Turbine modeling – Radar system effects– Future modeling work
• Site specific propagation modeling using VTRPE
• Measurements to refine models
• Summary • Backup and Additional Information
Outline
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Turbine Study- 45Outbrief 19 Sept 13
VTRPE* vs GRWAVE Test Case(Variable Terrain Radio Parabolic Equation)
GRWAVE, ht =10m
VTRPE, ht =10m
Hei
ght (
m)
0 5 10 15 20 25 30 35 40 45 500
50
100
150
200
250
300
-120
-115
-110
-105
-100
-95
-90
-85
-80
-75
-70
Range (km)
Hei
ght (
m)
0 5 10 15 20 25 30 35 40 45 500
50
100
150
200
250
300
-120
-115
-110
-105
-100
-95
-90
-85
-80
-75
-70
-120 -115 -110 -105 -100 -95 -900
50
100
150
200
250
300
Propagation Loss (dB)
Heig
ht (m
)
GRWAVEVTRPE
10 MHz, V-pol r=13, =0.005
Propagation Loss vs Height at Range of 15 km
* Ryan, Frank J., User's Guide for the VTRPE Computer Model, NOSC, San Diego, CA, October 1991
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Turbine Study- 46Outbrief 19 Sept 13
Range (km)
Heigh
t (m)
0 20 40 60 80 100 120 140 160 180 2000
50
100
150
200
250
300
350
400
450
500
-60
-50
-40
-30
-20
-10
0
10
VTRPE Runs for Flat Earth and Variable Terrain
Flat Earth
Variable TerrainRange (km)
Heigh
t (m)
50 100 150 200 250 3000
50
100
150
200
250
300
350
400
450
-60
-50
-40
-30
-20
-10
0
10
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Turbine Study- 47Outbrief 19 Sept 13
VTRPE: Flat Spherical Earth vs Variable Terrain
0 20 40 60 80 100 120 140 160 180 200-60
-50
-40
-30
-20
-10
0
Range (km)
Prop
agat
ion
Fact
or (d
B)
Hei
ght (
m)
0 20 40 60 80 100 120 140 160 180 2000
50
100
150
200
250
300
350
400
450
500
-60
-50
-40
-30
-20
-10
0
10
h=100m, Flat Earthh=100m, Variable Terrainh=200m, Flat Earthh=200m, Variable Terrain
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Turbine Study- 48Outbrief 19 Sept 13
• Objective:– Calibrated EM field measurement of HF propagation loss,
wind turbine scattering cross section and modulation spectrum
– Validate models of wind-turbine scattering, ground wave propagation, and spectral modulation on both Tx or Rx signal
• Various measurement approaches– Radar measurement by leverage ROTHR transmitter and or
receiver– Radio transmission measurement with dedicated transmitter
and receiver equipment
Wind Turbine Scattering Measurements to Verify Models
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Turbine Study- 49Outbrief 19 Sept 13
2
3 4
4
4
1
~15 km
Notional Testing ScenarioUse Helicopter to Measure Propagation from LOS to Ground
DSTO-TR-0654Altitude
Prop
agat
ion
Line of Sight
Ground Wave
Helicopter measures propagation factor from Line-of-Sight (LOS) altitudes down to the ground. LOS portion of flight removes calibration uncertainties related to ground-based monopole antennas mounted over imperfect ground planes.
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Turbine Study- 50Outbrief 19 Sept 13
Recent LL Test Using a Helicopter-Borne TransmitterAugust 2013
Signal
Propeller Harmonics
Low-power Transmitter in HelicopterReceiver on Ground
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Turbine Study- 51Outbrief 19 Sept 13
Turbine Experiment withHelicopter-Borne Transmitter
Altitude
DistanceWind turbine
Tx
Over sea measurement
Altitude
DistanceWind turbine
Tx
Over land measurement
Azimuth
Rx
Rx
Rx
Azimuth
Rx
Rx
Rx
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Turbine Study- 52Outbrief 19 Sept 13
Turbine Experiment withHelicopter-Borne Receiver
Altitude
Distance
AzimuthTx
Wind turbineRx
Over sea measurement
Altitude
Distance
AzimuthTx
Wind turbineRx
Over land measurement
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Turbine Study- 53Outbrief 19 Sept 13
Turbine Experiment With Surface Vehicle-Based Receiver
Tx
Wind turbine Rx
Over sea measurement
Tx
Wind turbine
Rx
Over land measurement
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Turbine Study- 54Outbrief 19 Sept 13
• Wind Turbine model developed– Matlab – NEC 4.2 based exploits parallel processing to
compute 64 rotation angles simultaneously
• Model used with radar parameters to generate Doppler signatures of wind turbine modulated clutter
• A great deal of effort was expended to validate modeling– Propagation, RCS, antennas, and NEC models evaluated and
understood• Multiple approaches used including hand calculations
– Limited “final” runs were performed
• Model used to estimate changes to standoff requirements based on wind direction– Significant reductions in standoff predicted vs wind direction
• More work is required including measurements to verify models
Summary
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Turbine Study- 55Outbrief 19 Sept 13
Backup and Additional Information
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Turbine Study-5619 Sept 2013
• Objectives:– Evaluate wind-turbine interference and clutter modulation at HF– Assess impacts of wind farm siting on ROTHR operations– Recommend wind-turbine interference approaches
• Scope of efforts: 3 staff-months study
• Tasks:
1. Review relevant literature2. Electromagnetic modeling at HF of wind turbine scattering and ground wave
propagation, develop and test computation tools. This model will be capable of rotation and arbitrary angle orientation
3. Investigate Doppler modulation signatures of wind turbines and their dependence on wind direction, explore interference mitigation approaches
4. Develop radar signal and system model, evaluate wind-turbine interference to ROTHR, assess effects of siting, geometry, and wind direction
• Deliverables: briefing of study results
Wind Farm StudyStatement of Work
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Turbine Study-5719 Sept 2013
1. Develop EM computational tools to model wind-turbine interference1. RCS at HF of wind blades as function of frequency, viewing geometry
(aspect at selected elevation angles) and rotational angles2. Time series of interference as function of wind-blade rotation3. Start with analytic solutions of long wires and supplements with accurate
numerical modeling of actual blade structures
2. Generic OTH propagation modeling1. Nominal HF sky wave propagation in one (and < a few) scenarios2. Use available tools such as NEC and GRWAVE to model attenuation ground
wave propagation and its dependence on frequency and stand-off distance
3. Implement ROTHR radar signal and system model1. Investigate wind-farm clutter modulation level and spectra2. Assess wind-farm impacts on ROTHR operations and performance such as
effects of stand-off distance, size, lay-out, and wind
• Explore wind-farm interference mitigation approaches, define processing algorithms
Study Plan
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Turbine Study- 58Outbrief 19 Sept 13
Literature Review (1 of 5)
• ROTHR Program Office, “Comprehensive Modeling Analysis for Stand-Off Requirements of Wind Turbines from Relocatable Over The Horizon Radar (ROTHR) Systems,” Executive Report II, Version 2.0, RPO-TR-WF-0712-001, June 2012.
• ROTHR Program Office, “Stand-Off Requirement of Wind Turbines from Relocatable Over The Horizon Radar (ROTHR) Systems,” Executive Report, July 2011.
• S. Rodriguez, R. Jennett, J. Bucknam, “Wind turbine Impact on High Frequency Skywave Radar (Initial Assessment),” NRL Tech Report NRL/MR/5320-11-9334, September 30, 2011. Distribution Statement C: Distribution authorized to U.S. Government agencies and their contractors.
• S. Rodriguez, B. Root, “Wind turbine Impact on high Frequency Radar (Line-of-Sight and Skywave Measurements) ,” Proc. Tri-Service Radar Symp., TSR-2011V1TP61, 1 June 2011. Distribution Statement C: Distribution authorized to U.S. Government agencies and their contractors.
ROTHR-specific
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Turbine Study- 59Outbrief 19 Sept 13
Literature Review (2 of 5)
Microwave• B. Kent, K. Hill, A. Buterbaugh, G. Zelinski, R. Hawley, L. Cravens, T. Van, C. Vogel, and T.
Coveyou,” Dynamic radar cross section and radar Doppler measurements of commercial General Electric windmill power turbines, part I: Predicted and measured radar signatures,” IEEE Antennas Propag. Mag., vol. 50, no. 2, pp. 211-219, Apr. 2008.
• A. Buterbaugh, B. Kent, K. Hill, G. Zelinski, R. Hawley, L. Cravens, T. Van, C. Vogel, and T. Coveyou,” Dynamic radar cross section and radar Doppler measurements of commercial General Electric windmill power turbines, part 2: Predicted and measured Doppler signatures,” IEEE Antennas Propag. Mag., vol. 50, no. 2, pp. 211-219, Apr. 2008.
• J. Browning, B. Wilson, J. Burns, B. Thelen, “Wind Farm Radar Interference Characterization and Mitigation (wricm): Initial Data Analysis Results,” Proc. of Tri-Service Radar Symp., TSR-2010-TA06, 12 July, 2010.
HF• L. Wyatt and A. Robinson, “Wind farm impacts on HF radar current and wave measurements in
Liverpool Bay,” Proc. OCEANS 2011, Spain, IEEE, 6-9 June 2011, pp. 1-3.
Full-size Turbine Measurements
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Turbine Study- 60Outbrief 19 Sept 13
Literature Review (3 of 5)
Microwave• A. Naqvi, S. Yang and H. Ling, “Investigation of Doppler features from wind turbine
scattering,” IEEE Antennas Wireless Propagat. Lett., vol. 9, pp. 485-488, 2010.
• A. Naqvi, N. Whitelonis, H. Ling, “Doppler features from wind turbine scattering in the presence of ground,” “Progress in Electromagnetics Research Letters,” vol. 35, pp. 1-10.
• F. Kong, Y. Zhang, R. Palmer, and Y. Bai, “Wind turbine radar signature characterization by laboratory measurements,” Proc. of RADAR 2011, pp.162-166, 23-27 May, Kansas City, MO, IEEE.
• Y. Zhang, et al., “Using scaled models for wind turbine EM scattering characterization: Techniques and Experiments,” IEEE Trans. Instrum. Meas., vol. 60, no. 4, pp. 1298-1306, Nov. 2010.
Scale-model Measurements
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Turbine Study- 61Outbrief 19 Sept 13
Study
• M. Brenner, et al., “Wind farms and radar,” JASON Program Office, The MITRE Corporation, McLean, VA JSR-08-125, 2008.
• Audrey Durmanian, et al., “Wind Turbine RCS Modeling and Validation”, The Applied Computational Electromagnetics Society, April 2011, presentation at Williamsburg, VA and conference paper published online (ACES 2011 Conference – Williamsburg, VA, Volume: Topics in Radar Scattering), MIT LL.
• J.K. Jao, C. Ho, P. Jardin, M. Yamaguchi, and P. Monticciolo, “FORESTER GMTI Processing and Performance Test Results of Target Detection and Signature Data Exploitation,” 56th 2010 Tri-Service Radar Symposium, Orlando, Florida, 21 – 25 June, 2010."
Simulation
• L. Rashid and A. Brown, “RCS and Radar Propagation Near Offshore Wind Farms,” Proc. IEEE Antennas and Propagation Conf., Honolulu, HI, 2007, pp4605-4608.
• D. Jenn and C. Ton, “Wind turbine radar cross section,” Int. Journal of Antennas and Propagation, Vol. 2012, Article ID 252689, 2012.
• C. Teague and D. Barrick, “Estimation of wind turbine radar signature at 13.5 MHz,” Proc. of IEEE OCEANS Conf., Virginia Beach, VA, 2012.
Simulations and Analysis
Literature Review (4 of 5)
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Turbine Study- 62Outbrief 19 Sept 13
• N Maslin, HF Communications, A Systems Approach, Plenum Press, New York, 1987
• S. Rotheram, “Ground-Wave Propagation Part 1: Theory for Short Distances,” IEE Proceedings, October 1981
• S. Rotheram, “Ground-Wave Propagation Part 2: Theory for Medium And Long Distances and Reference Propagation Curves,” IEE Proceedings, October 1981
• Rec. ITU-R P.368-7, “Ground-wave Propagation Curves For Frequencies Between 10 Khz And 30 Mhz” The ITU Radiocommunication Assembly, 1992
• E. Miller, et al., “Radar cross section of a long wire,” IEEE Trans AP, May 1969
Propagation
Literature Review (5 of 5)
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Turbine Study- 63Outbrief 19 Sept 13
Use NEC to Determine Reference Voltage and Effective Antenna Height
25 km
85 deg inc.
1 2 3 4 5 6 7 8 9 10
x 10-6
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
E field (V/m)
Hei
ght (
m)
Lossy ground
Lossy ground
1. Excite monopole with a plane wave for clutter reference voltage
Source atTurbine Height
E field vs Height at Monopole
2. Determine effective antenna height V=hE for turbine scattered signal
Reference Voltage due to distant clutter signal and effective antenna height from scattered signal required for NEC based 2-step interference solution
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Turbine Study- 64Outbrief 19 Sept 13
Wind Turbine NEC Models
Full Tower & HubFull Tower5-Wire Tower
6673 segments 6844 segments254 segments