2014 IEEE PES Transmission & Distribution
Conference & Exposition
Chicago, April 14-17
Methods for Improving Ground Resistance/Transient
Ground Impedance of Transmission Structures
Dr. Emanuel Petrache
Kinectrics Inc., Canada
CEATI Project: T113700 #3227
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Background
Previous Work: CEATI Report T093700-3227
“Methods for Improving Ground Resistance of
Transmission Structures – Phase I”
 Reviews methods for improving the ground resistance of
transmission line structures
 A simplified equation, based on shape and fill factors, is
proposed to calculate the resistance of any electrode.
 The approach is used to both visualize and calculate the
effectiveness of possible electrode installations.
 The report offers an illustrated guide for of possible
ground electrode installations for difficult soil: single-pole,
H-frame, four-leg steel lattice, and guyed towers.
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Single-Pole Structures: Possible Ground
Electrode Installations for Difficult Soil
Base Case
Two Wires, 180º,
length L, depth d
Short Rod Nearby,
distance s
from edge,
length L
Four Wires, 90º
length L, depth d
Long Rod Nearby,
length below
foundation L
Four Wires, 90° to 10 m then
Bent 45° along ROW,
length L, depth d
Single Horizontal Wire, length
L, buried at depth d
Resistance of Cylindrical Foundation Electrode with
Radial Counterpoise, comparing ChizWhiz Model (Trench
area) with Reference Calculations. Scale result by
Observed Resistivity / 1000 m
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H-frame Structures: Possible Ground Electrode
Installations for Difficult Soil
Calculated Resistance of One, Two, Three or Four Counterpoise in
line with Two-Pole Tower Leg with  = 1000 m. Scale result by
(Observed Resistivity / 1000 m)
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Guyed Structures: Possible Ground Electrode
Installations for Difficult Soil
Resistance of Guyed Tower, comparing Counterpoise
Connections to Central Pad or Four Guy Anchors. Scale
result by resistivity/1000 m.
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Scope
?
2. High frequency
response of
transmission line
structure footings.
1. Low-frequency
resistance Rf
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Scope
• compared a reference low frequency
approach with two different high frequency
models for the ground resistance/transient
ground impedance of transmission
structures.
• analyzed and the effectives of various
ground improvement methods evaluated in a
range of uniform soil resistivity values
covering the most commonly encountered
difficult soil conditions.
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Configurations modeled
Method for improving ground resistance
Single-Pole
structure (Steel
Pole)
Structure Type
H-Frame
Lattice structure
structure with
on four legs
and without guys
1
no treatment
var soil resistivities
var soil resistivities
var soil resistivities
2
radial counterpoise
var soil resistivities
var soil resistivities
var soil resistivities
3
var soil resistivities
var soil resistivities
var soil resistivities
4
loop counterpoise
continuous
counterpoise
var soil resistivities
var soil resistivities
var soil resistivities
5
vertical well
var soil resistivities
var soil resistivities
var soil resistivities
- Uniform soil resistivity values in the simulations [Ωm]: 300, 1000,
2000, and 5000.
- Relative permittivity of the soil: 10
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Software packages
• Low-frequency resistance of the electrodes computed with
the standard CDEGS MALTZ package.
• High-frequency impedance computed with the CDEGS
HIFREQ module, and with NEC-4.
• CDEGS software used to study single-layer versus two and
multi-layer soil results.
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Modeling methodology
Numerical equivalent of the EPRI ZedMeter® test
method:
it simulates a lightning-like impulse injection into the
transmission structure base and measures the resulting
potential rise relative to a remote ground
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Transient impedance of transmission line towers
0.8
Current [A]
Injected current
1
0.6
dwir
e
0.4
0.2
0
-0.2
-0.2
0
0.2
0.4
0.6
0.8
Time [microseconds]
1
14
Vtower-grd.ref
12
Ground rod
10
Voltage [V]
Voltage to remote earth
n
Grou
8
6
ntial
Pote
V
4
2
e
C u rr
nt le
ad
Tower foundation
0
Impulse injection
-2
-0.2
0
0.2
0.4
0.6
0.8
Time [microseconds]
1
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lead
Transient impedance of transmission line towers
Typical results obtained using NEC-4
model for a ground resistivity of 1000 Ωm
[Ohm]
40
calc
35
30
100
80
/I
calc
25
Tower Impedance: V
Tower Impedance: V
calc
/I
calc
[Ohm]
Typical results obtained using NEC-4
model for a ground resistivity of 50 Ωm
20
15
10
5
0
-0.2
0
0.2
0.4
0.6
0.8
Time [microseconds]
40
20
0
-0.2
1
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0
0.2
0.4
0.6
0.8
Time [microseconds]
1
Example: H-frame/guyed structure
Adjacent
structure
H-frame structure model;
height = 18.4 m
2 x OHGW
span length = 200 m
Current injection lead;
length = 150 m
Potential lead;
length = 100 m
Adjacent structure
H-frame guyed structure reference case (no treatment)
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Example: H-frame/guyed structure
2 x radial
counterpoise,
each 12 m
long, installed
at a depth of
0.5 m
4 x radial
counterpoise, each 30
m long, installed at a
depth of 0.5 m
a) 2 x 12 m radial counterpoise 12 m
b) 4 x 30 m radial counterpoise
H-frame guyed structure radial counterpoise treatment
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Example: H-frame/guyed structure
Loop counterpoise total length
30.7 m, installed at a depth of
0.5 m
H-frame guyed structure loop counterpoise treatment
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Example: H-frame/guyed structure
Continuous counterpoise
installed at a depth of 0.5 m and
an offset of 7.5 m from the
center of the ROW
H-frame guyed structure continuous counterpoise treatment
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Example: H-frame/guyed structure
current injection lead;
length = 150 m
potential lead;
length = 100 m
Vertical 152 mm dia. well 60 m
deep
H-frame guyed structure vertical well treatment
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Results example: H-frame guyed structure results in uniform
soil grouped by soil resistivity
35
30
25
20
15
10
5
0
H-Frame Guyed Tower - 1000Ωm
120
29
25
23
17
12 11
1515
4
6
11
5
3
5
2
8
Z [Ω]
Z [Ω]
H-Frame Guyed Tower - 300Ωm
NEC4
CDEGS LF
100
80
60
40
20
0
96
49 44
56
49
35
9
20
34 31
11 6
7
28
1716
CDEGS LF
CDEGS HF
CDEGS HF
H-Frame Guyed Tower - 2000Ωm
H-Frame Guyed Tower - 5000Ωm
250
150
100
50
600
192
72 65
112
99
50
40
16 13
Z [Ω]
Z [Ω]
200
53 50
1613
4335
22
NEC4
CDEGS LF
0
CDEGS HF
500
400
300
200
100
0
479
281
247
103 94
70
100
26 23
78 76
233215
658732
NEC4
CDEGS LF
CDEGS HF
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NEC4
Page 18
Experimental Results
Test data Rf vs. Z for compact electrodes
[Chisholm et al, 2010]
Test data Rf vs. Z for distributed electrodes
[Chisholm et al, 2010]
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Conclusions
• Transient impedance Z has a non-linear variation
with the soil resistivity. In other words, the degree of
improvement offered by the various methods of
treatment, judged from the transient impedance point
of view, varies with the soil resistivity.
• The crossover from low-frequency to high-frequency
impedance was described using an impulse
coefficient, that was typically less than unity for
compact electrodes and greater than unity for
distributed electrodes, up to certain limits of length
and resistivity.
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Conclusions
• In most of the cases, the results obtained with both
programs are showing a decrease of calculated impulse
coefficient with the increase of soil resistivity. This
indicates that the electrode is becoming more efficient in
dissipating the lightning currents with the increase of the
soil resistivity compared to what the low-resistance may
suggest.
• The two different high frequency models were in close
agreement for some electrodes but tended to differ on the
degree of reduction of high-frequency impedance for
continuous counterpoise and deep-well electrodes.
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Conclusions
• Best practices for improving the ground
resistance/transient response include the use of four
radial counterpoise. For the lattice structure case the
CDEGS HIFREQ and NEC-4 models did not agree
on the relative ranking of loop electrodes, four radial
counterpoise and continuous counterpoise, and this
discrepancy should be addressed by field tests.
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Future work
• A number of areas of interest for future research work
include the investigation of the ground electrical
parameters, their frequency-dependence, and the impact on
the calculated transient response of the transmission line
structures, and a comprehensive testing program that will
establish experimental values of impulse coefficient for
transmission line structures and wind turbines.
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Possible implementation of soil-parameter frequency
dependence using commercial software packages
Example: NEC-4
CM and CE comment cards CM Sim# I1-C-L01-GR-GW-50
CM Number of Frequency Loops: : 512 (Fmax: 100MHz)
CM --- Do not delete the above comment lines --CM Start of geometry
EPSR(f), SIG(f)
CE
the expressions
CM Impulse lead
derived from
GW cards define model geometry GW1,6,4.039,4.039,0.1,4.039,7.039,0.1,1.8e-3
experimental
....
results and
....
published
by
....
Visacro et al
End geometry input data: GE -1 0
10 0.02
Ground parameters card: GN 2 0 0 0
IS 0 1 0 0
3 1.0E-10 2.5E-3
IS 0 2 0 0
3 1.0E-10 2.5E-3
IS 0 5 0 0
3 1.0E-10 2.5E-3
IS 0 6 0 0
3 1.0E-10 2.5E-3
LD 0 1 3 3
50
0
0
LD 0 5 1 1 1.0E+07
0
0
Set frequencies card: FR 0 512 0 0 0.1953125 0.1953125
EX 0 1 3 00 1.0000 0.00000
XQ 0
EN
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Thank you!
emanuel.petrache@kinectrics.com
alex.mogilevsky@ceati.com
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