Methods for Improving Ground Resistance/Transient · PDF fileMethod for improving ground resistance Structure Type Single-Pole structure (Steel Pole) H-Frame structure with and without

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Methods for Improving Ground Resistance/Transient

Ground Impedance of Transmission Structures

Dr. Emanuel Petrache

Kinectrics Inc., Canada

CEATI Project: T113700 #3227

2014 IEEE PES Transmission & Distribution

Conference & Exposition

Chicago, April 14-17



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.



Single-Pole Structures: Possible Ground

Electrode Installations for Difficult Soil

Base Case

Short Rod Nearby, distance s from edge, length L

Long Rod Nearby, length below foundation L

Single Horizontal Wire, length L, buried at depth d

Two Wires, 180º, length L, depth d

Four Wires, 90º length L, depth d

Four Wires, 90° to 10 m then Bent 45° along ROW, length L, 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



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)

H-frame 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.

Guyed Structures: Possible Ground Electrode

Installations for Difficult Soil



Scope

1. Low-frequency resistance Rf

2. High frequency response of

transmission line structure footings.

?



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.



Configurations modeled

Method for improving ground resistance

Structure Type

Single-Pole

structure (Steel

Pole)

H-Frame

structure with

and without guys

Lattice structure

on four legs

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 loop counterpoise var soil resistivities var soil resistivities var soil resistivities

4

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



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.



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



-0.2 0 0.2 0.4 0.6 0.8 1-0.2

0

0.2

0.4

0.6

0.8

1

Time [microseconds]

Cu

rre

nt

[A]

Transient impedance of transmission line towers

Groundwire

Current lead

Potential lead

V

Impulse injection

Tower foundation

Ground rod

-0.2 0 0.2 0.4 0.6 0.8 1-2

0

2

4

6

8

10

12

14

Time [microseconds]

Voltage [

V]

Vtower-grd.ref

Inje

cte

d c

urr

en

t

Vo

lta

ge

to

re

mo

te e

art

h



Typical results obtained using NEC-4 model for a ground resistivity of 50 Ωm

-0.2 0 0.2 0.4 0.6 0.8 10

5

10

15

20

25

30

35

40

Time [microseconds]

Tow

er

Impedance:

Vcalc /

Icalc [

Ohm

]

Typical results obtained using NEC-4 model for a ground resistivity of 1000 Ωm

-0.2 0 0.2 0.4 0.6 0.8 10

20

40

60

80

100

Time [microseconds]

To

we

r Im

pe

da

nce

: V

calc /

Icalc [

Oh

m]

Transient impedance of transmission line towers



Current injection lead;

length = 150 m

Potential lead;

length = 100 m

Adjacent structure

2 x OHGW

span length = 200 m

H-frame structure model; height = 18.4 m

Adjacent structure

H-frame guyed structure reference case (no treatment)

Example: H-frame/guyed structure



a) 2 x 12 m radial counterpoise 12 m

b) 4 x 30 m radial counterpoise

4 x radial counterpoise, each 30 m long, installed at a depth of 0.5 m

2 x radial counterpoise, each 12 m long, installed at a depth of 0.5 m

H-frame guyed structure radial counterpoise treatment




Loop counterpoise total length 30.7 m, installed at a depth of 0.5 m

H-frame guyed structure loop counterpoise treatment




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




Vertical 152 mm dia. well 60 m deep

current injection lead; length = 150 m

potential lead; length = 100 m

H-frame guyed structure vertical well treatment




Results example: H-frame guyed structure results in uniform

soil grouped by soil resistivity

25

15

4

12

5

11

29

15

6

17

25

23

3

118

0

5

10

15

20

25

30

35

Z [Ω

]

H-Frame Guyed Tower - 300Ωm

NEC4

CDEGS LF

CDEGS HF

4935

9

34

11

28

96

49

20

56

617

44

7

3116

0

20

40

60

80

100

120

Z [Ω

]


NEC4

CDEGS LF

CDEGS HF

7250

16

53

1643

192

99

40

112

1335

65

13

5022

0

50

100

150

200

250

Z [Ω

]


NEC4

CDEGS LF

CDEGS HF

103 7026

7823

65

479

247

100

281

328794

2376

15 32

0

100

200

300

400

500

600

Z [Ω

]


NEC4

CDEGS LF

CDEGS HF



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]



• 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.

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.

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.

Conclusions



• 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.

Future work



Possible implementation of soil-parameter frequency

dependence using commercial software packages

CM and CE comment cards

GW cards define model geometry

End geometry input data:

Ground parameters card:

Set frequencies card:

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

CE

CM Impulse lead

GW1,6,4.039,4.039,0.1,4.039,7.039,0.1,1.8e-3

....

....

....

GE -1 0

GN 2 0 0 0 10 0.02

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

FR 0 512 0 0 0.1953125 0.1953125

EX 0 1 3 00 1.0000 0.00000

XQ 0

EN

EPSR(f), SIG(f)

the expressions

derived from

experimental

results and

published by

Visacro et al

Example: NEC-4



Thank you!

[email protected]

[email protected]

mailto:[email protected]

mailto:[email protected]

Documents

Methods for Improving Ground Resistance/Transient · PDF fileMethod for improving ground resistance Structure Type Single-Pole structure (Steel Pole) H-Frame structure with and without