tower grounding and soil ionisation.pdf

8/14/2019 tower grounding and soil ionisation.pdf

1/94

Tower Grounding and Soil Ionization

Report

Technical Report


2/94


3/94

EPRI Project ManagerA. Phillips

EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA800.313.3774 650.855.2121 [email protected] www.epri.com

Tower Grounding and SoilIonization Report

1001908

Final Report, February 2002


4/94

DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS ANACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCHINSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE

ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I)WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, ORSIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESSFOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON ORINTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUALPROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S

CIRCUMSTANCE; OR

(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER(INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVEHAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOURSELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD,PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT.

ORGANIZATION(S) THAT PREPARED THIS DOCUMENT

EPRIsolutions

ORDERING INFORMATION

Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 WillowWay, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally, x5379 (925) 609-9169

(925) 609-1310 (fax).

Electric Power Research Institute and EPRI are registered service marks of the Electric PowerResearch Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric PowerResearch Institute, Inc.

Copyright 2002 Electric Power Research Institute, Inc. All rights reserved.


5/94

iii

CITATIONS

This report was prepared by

EPRIsolutions115 East New Lenox Rd.Lenox, MA 01240

Principal InvestigatorsP. White

J. AndersonK. King

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Tower Grounding and Soil Ionization Report, EPRI, Palo Alto, CA: 2002. 1001908.


6/94


7/94

v

REPORT SUMMARY

Deregulation of the powder industry has increased the need for greater reliability of thetransmission system. Unplanned outages can have significant financial implications, andlightning activity is often cited as one of the main reasons. To address this issue, EPRI isconducting research to increase understanding of the lightning performance of transmission lines.This report details the results of one such study.

Background

The historical challenge of providing reliable electrical service is becoming more important.With electronic equipment in almost all facets of life, even momentary outages and powerquality problems can adversely affect customers at home and work. Lightning causes many suchmomentary customer outages, and EPRIs TFlash program was developed to help utility

engineers evaluate the lightning performance of power systems.

ObjectivesTo provide more accurate grounding algorithms for the TFlash program.

ApproachPublished algorithms, and those generally used by the industry for computing surge currentdynamic resistance of ground rods and concrete foundations, provide wide divergences inpredicted values. Also, in spite of almost a century of experience, the actual dielectric propertiesof soils are still a matter of debate. These dielectric properties have a profound effect on thelightning performance of transmission lines. Therefore, the grounding research documented inthis report encompasses two fundamental activities: determining the dielectric properties of sometypical soils and selecting dynamic resistance models of ground rods and concrete foundations asa function of current in these soils. The project team concentrated on ground rods and concretefoundations because these grounding geometries are so prevalent on transmission anddistribution lines.

ResultsStudy results, together with previous EPRI research, are being used to develop more accurate

models to predict transmission line performance. These models, in turn, will be included inEPRIs state-of-the-art Transmission Line Lightning Performance Prediction Software, T-Flash.The results also will be used to develop guidelines for utilities on how to effectively design,

construct, and maintain transmission line grounding systems.

EPRI PerspectiveTFlash is a state-of-the-art design tool that allows engineers to analyze the effect of a specifiedlightning challenge on a given transmission line as well as specified mitigation techniques such


8/94

vi

as shielding, improved grounding, line arresters, and upgraded insulation. With this software,utility engineers can analyze the degree of protection an existing line has, define changes to theline to improve protection, or design a new line with economical lightning protection. As aresult, TFlash has potential to help utilities achieve cost-effective improvements in lightningprotection and customer reliability.

KeywordsLightningGroundingSoil ionizationTFlashLPDW (Lightning Protection DesignWorkstation)


9/94


10/94

viii

CIGR/Weck Algorithm.....................................................................................................4-11

Chisholm - Janischewskyj Algorithm..................................................................................4-12

TFlash Algorithm ...............................................................................................................4-13

Effect of Rod Shape and Artificial Streamers.....................................................................4-13

Response of Concrete Ground Electrodes ........................................................................4-17Chemical Enhancement ....................................................................................................4-18

Butt-Wrapped Poles ..........................................................................................................4-18

Computer Modeling of Dynamic Resistance......................................................................4-20

5SUMMARY .......................................................................................................................... 5-1

6GOOD GROUNDING PRACTICES ..................................................................................... 6-1

7RECOMMENDATIONS FOR FUTURE WORK.................................................................... 7-1

8REFERENCES .................................................................................................................... 8-1

ATHE CIGR CRITICAL CURRENT EQUATIONS ...............................................................A-1

BTHE LIEW-DARVENIZA ALGORITHM...............................................................................B-1

CTHE GND_ROD1 ALGORITHM..........................................................................................C-1

DTHE CHISHOLM-JANISCHEWSKYJ (C-J) MODEL...........................................................D-1

ETHE ROBBINS/TFLASH DYNAMIC RESISTANCE MODEL.............................................. E-1

FDRIVEN ROD RESISTANCES IN TWO-LAYER EARTHS.................................................. F-1

GADJUSTMENT OF ROD-TANK RESISTANCES TO ROD RESISTANCES IN ANINFINITE PLANE....................................................................................................................G-1


11/94

ix

LIST OF FIGURES

Figure 2-1 Dimensions for a single vertical ground rod............................................................ 2-2

Figure 2-2 The Wenner four electrode method of measuring earth resistivity.......................... 2-3

Figure 2-3 Dielectric breakdown of soil around rod electrodes. ............................................... 2-4

Figure 2-4 Reduction of rod resistance with time..................................................................... 2-5

Figure 2-5 Concrete foundation in moist soil. .......................................................................... 2-5

Figure 2-6 Replacing a ground rod with a conducting hemisphere. ......................................... 2-7

Figure 2-7 Dynamic resistance of a concrete foundation vs. time............................................ 2-8

Figure 2-8 Liew-Darveniza ground rod surrounded by concentric cylindrical shells ofearth...............................................................................................................................2-10

Figure 2-9 Modernized version of the Korsuncev Curve (from Oettle)....................................2-12

Figure 2-10 Relationship of Parameter S to Ionization Zones.................................................2-13

Figure 3-1 Rogowski Electrode Test Cell. ............................................................................... 3-2

Figure 3-2 Test Circuit for Soil Uniform Field Dielectric Tests.................................................. 3-2

Figure 3-3 Resistivity Test Cell................................................................................................ 3-3

Figure 3-4 Photo of the Concentric Cylindrical Test Cell. ........................................................ 3-5

Figure 3-5 Electrical Diagram of the Concentric Cylindrical Test Cell...................................... 3-6

Figure 3-6 Concentric Test Cell Voltage Waves Before and During Breakdown...................... 3-8Figure 4-1 Large Test Tank Dimensions and Circuitry............................................................. 4-2

Figure 4-2 Photo of Large Test Tank Installation..................................................................... 4-2

Figure 4-3 Unfiltered Data....................................................................................................... 4-4

Figure 4-4 Chebyshev Filtered Data........................................................................................ 4-4

Figure 4-5 Typical Voltage-Current Waves During Rod Tests. ................................................ 4-5

Figure 4-6 Initial Dynamic Resistance Computation. ............................................................... 4-6

Figure 4-7 Dynamic Ohms vs. Applied Current. ...................................................................... 4-8

Figure 4-8 Dynamic Resistance at Two Microseconds As a Function of Current Into One

30-Foot Rod or Two 15-Foot Parallel Rods Spaced 10 Feet Apart. ................................. 4-9Figure 4-9 Liew Darveniza Algorithm Comparison With Dynamic Resistance TestValues. ...........................................................................................................................4-10

Figure 4-10 CIGR/Weck Algorithm Comparison With Dynamic Resistance Test Values. .....4-11

Figure 4-11 C_J Algorithm Comparison With Dynamic Resistance Test Values.....................4-12

Figure 4-12 TFlash Algorithm Comparison With Dynamic Resistance Values........................4-13

Figure 4-13 Artificial Streamers Attached to a Ground Rod....................................................4-14


12/94

x

Figure 4-14 Dynamic Resistance With Artificial Streamers in Place. ......................................4-14

Figure 4-15 Dynamic Resistance With Artificial Streamers Removed.....................................4-15

Figure 4-16 Tested Bar Cross-Section. ..................................................................................4-15

Figure 4-17 Rectangular Bar Ground Rod Dynamic Resistance in sand. ...............................4-16

Figure 4-18 One Inch Diameter Round Rod in Sand. .............................................................4-16

Figure 4-19 Voltage-Current Waves on a Concrete-Encased Rebar. .....................................4-17

Figure 4-20 4.5-Foot Ground Rod Surrounded With Eight Inch Diameter GEM Material. .......4-18

Figure 4-21 Photo of Bottom End of Butt-Wrapped Pole. .......................................................4-19

Figure 4-22 Dynamic Resistance of a Butt-Wrapped Pole in Loam. .......................................4-19

Figure 4-23 Comparison of Lowest Dynamic Ohms Calculated by Korsuncev and Liew-Darveniza Algorithms. ....................................................................................................4-20

Figure 4-24 Single Rod Korsuncev Dynamic Resistance vs Rod Length and SoilResistivity.......................................................................................................................4-21

Figure 4-25 Dynamic Resistance of Two ground Rods in Parallel vs. Rod Length and

Separation Distance (Korsuncev). ..................................................................................4-22Figure 4-26 Variation of Double Rod Low Frequency Resistance With Spacing. (1-in.-Diam. Rod, 3 Feet Long, =100 ohm-meters).................................................................4-23

Figure A-1 A thin soil resistance shell surrounding an embedded conductinghemisphere. ....................................................................................................................A-1

Figure A-2 Gradient at the surface of a conducting hemisphere.............................................A-2

Figure B-1 Liew-Darveniza ground rod model surrounded by concentric shells of earth.........B-1

Figure C-1 Collision of Soil Resistance Shells for Two Rod Case ..........................................C-1

Figure D-1 The Korsuncev Curve for Ground Electrodes .......................................................D-2

Figure D-2 Application of the Characteristic Dimension S. .....................................................D-2

Figure E-1 Expanding ionization zone around a rod...............................................................E-1


13/94

xi

LIST OF TABLES

Table 3-1 Uniform Field Dielectric Tests.................................................................................. 3-4

Table 3-2 Breakdown Vs Waveshape: Kiln-Dried Sand........................................................... 3-7

Table 3-3 Effect of Moisture and Tail Time.............................................................................. 3-8

Table 4-1 Large Scale Dynamic Resistance Tests. ................................................................. 4-3


14/94


15/94

1-1

1OVERVIEW

This report discusses work performed in 2000 and 2001 on lightning grounding research. Thework was initiated by the clear necessity of providing better grounding algorithms for the EPRITFlash transmission line lightning performance program. In 1982 EPRI published a firstcomprehensive report (Reference 7) on transmission line grounding, but this report was directedprimarily to ac grounding and said little about ground electrode dynamic response underlightning surge currents. Published algorithms and those generally used by the industry forcomputation of surge current dynamic resistance of ground rods and concrete foundationsprovide wide divergences in predicted values. Also, in spite of almost a century of experience,

the actual dielectric properties of soils are still a matter of debate, and these dielectric propertieshave a profound effect on the lightning performance of transmission lines. Therefore, groundingresearch in 2001 has encompassed two fundamental activities: determination of the dielectricproperties of some typical soils, and the selection of dynamic resistance models of ground rodsand concrete foundations as a function of current in these soils. In 2001, attention wasconcentrated on ground rods and concrete foundations because these grounding geometries areso prevalent on transmission and distribution lines. This report is organized in the followingsections:

Section 2of this report reviews present theories and limitations of ground electrode dynamicresistances under high impulse currents. This review is to acquaint the reader with the dynamicsof the electronic processes involved.

Section 3reports results of small-scale experiments to clarify the dielectric properties of varioussoils, particularly sand, clay and loam, and how these results can apply to large scale electroderesponse.

Section 4shows some results of large-scale high current experiments on rods and concreteelectrodes (including conductivity enhancement chemicals) in a special soil-containment tank,and how these results fit various theoretical algorithms.

Section 5 then sums the results and makes recommendations for algorithms to apply in theTFlash program.

Section 6 makes recommendations - based on the results of the present investigation - for bettertower grounding strategies and grounding enhancement procedures.

Section 7 outlines needed future work, including extension of the present work to concretefoundations and substations, rods in two-layer soils and development of better data acquisitionmethods to determine soil constants on rights-of-way that control the dynamic response of towergrounds.


16/94


17/94

2-1

2THEORY OF GROUND ELECTRODE RESPONSE

Introduction

The simplest practical ground electrode used on transmission and distribution lines is a verticalground rod. Often this rod penetrates into two or more layers of earth with different resistivities,such as a layer of loam above a layer of clay or glacial till. The total rod resistance to earth is theparallel combination of the rod resistances in each layer. Earth is a poor dielectric, and at highlightning surge currents electrical streamers and glow discharges develop in the soil around a rod

as the soil breaks down, effectively enlarging the rod electrical radius. Electrical streamers alsopenetrate down into the earth off the bottom tip of the rod, thereby effectively increasing the rodlength. In addition, as current is injected into a ground rod, magnetic energy is stored in theearth and in the air just above the rod, and at lightning frequencies this energy is evidenced as aninductance in series with the rod. Finally, the relative dielectric constant of earth can be 10 ormore, so a rod has a capacitance to earth in parallel with its resistance that is sometimes takeninto account at lightning frequencies.

The inductance and capacitance are usually assumed constant for a given rod and independent ofearth resistivity. However, clearly the rod capacitance increases as the streamers increase the roddiameter. As for inductance, the conductivity of the metal rod is so much higher than theconductivity of the streamers in an axial direction parallel to the rod that inductance is assumed

independent of streamer development. This ignores the fact that each streamer is carryingcurrent and also sending magnetic energy into the soil and that the current in the rod is greater atthe earth's surface than down at the rod tip.

Even for the simplest case of a single vertical rod, the entire electrical event when a strokecurrent surge enters the rod is very nonlinear. Calculation of rod dynamic resistance vs. time isfurther complicated by a lack of information on earth resistivities around the rod anduncertainties as to the earth dielectric strength and its ionization time constants. Concretefoundations are in some ways, simpler because of the large surface area of the concrete and itsslow variation of resistance with soil moisture. Theories of ground electrode dynamic resistanceabound in the literature, and some of the many references are listed at the end of this report.

Reference 2 by Mousa provides a particularly comprehensive list of grounding referencesrelevant to this technology.

Because of the complexity of the process and the many uncertainties about the soil electricalenvironment around any transmission tower electrode, this report has, insofar as possible, tried tosimplify the dynamics, and to not describe with great mathematical precision that which is in factknown only very approximately, if at all. The simplified models thus developed should besufficient until more precise information of soil electrical properties are available.


18/94

Theory of Ground Electrode Response

2-2

Ground Rod Resistance

When a single vertical rod is driven into the earth (Figure 2-1), its resistance, R oto the flow oflow frequency low amplitude current is given by Eq. 2-1 by Dwight:

d

L

Figure 2-1

Dimensions for a single vertical ground rod.

= 1

8log

2 d

L

LRo

Equation 2-1

where:

Ro= low frequency, low current resistance, ohms

= earth resistivity, ohm-meters

L = rod length, meters

d = rod diameter, meters

This resistance Ro is to a uniform infinite earth. It assumes that no skin-effect exists near thesurface of the earth in spite of the high frequencies contained in any lightning surge currentflowing into the rod, no dielectric breakdowns of soil around the rod caused by high voltages onthe rod, no effect of retardation time (the current has spread to infinity instantly - an obviousimpossibility), no variations in earth resistivity with frequency or with depth, and no effect of thedielectric constant of the earth around the rod. However, in spite of its shortcomings for fasthigh transient currents, Eq. 2-1 is the standard Dwight equation frequently used in transmissionline lightning simulation programs for rod resistance before any soil ionization processes begin,and is used as a starting point for most dynamic resistance calculations.

Eq. 2-1 can also be used to determine earth resistivity in some situations. This is done by drivinga rod into the earth, measuring its Rowith a ground resistance megger and then rearranging Eq.2-1 to calculate the equivalent earth resistivity that results in the observed resistance R o.However, as will be noted in more detail later, driving a ground rod will - in some soils - vibratethe rod sufficiently so that the hole containing the rod is slightly enlarged, making only partial


19/94


2-3

electrical contact on some of the rod surface areas, and thereby producing a significant resistanceerror. This effect has been encountered in EPRI tests of ground rods in sand. Also, if varieswith depth, the calculated resistivity will not indicate the resistivity at any particular depth.

I

x

v

Figure 2-2

The Wenner four electrode method of measuring earth resistivity.

A more accepted on-site method of measuring earth resistivity (valid for a uniform earth) isshown in Figure 2-2. An alternating current I is fed between the two outer rods, creating avoltage drop along the surface of the earth, resulting in a voltage V between the two inner rods.The indicated earth resistivity is then:

I

Vx 2= Equation 2-2

where:

x = distance between the two inner rods, meters.


Wenner resistivity measurements (Eq. 2-2) are often complicated by stray earth currents and bythe fact that both V and I are often small in field measurements. The problem with many on-sitemeasurements of earth resistivity is that the earth often consists of an upper layer of sand, loamor clay only a few meters thick on top and another layer of rock, gravel or earth underneath witha much different resistivity. Aerial surveys at frequencies from 10 to 100 kHz can provide usefulresistivity data in multiple layer soils. For a two-layer case, Eq. 2-2 will indicate different

resistivities as the spacing x is changed, and this variation has been used (Reference 7) to inferthe nature of the resistivity layers underneath A special EPRI Fortran program RESIST wasdeveloped for this purpose. However, it still required substantial interpretive skills to infer thenature of the subsurface strata.

The dynamic resistance of a ground rod will usually be much less than its low-frequency low-current resistance Ro, and this dynamic resistance is used by TFlash to evaluate transmission linelightning performance. If a surge current of 50 kiloamperes flows into a single ground rod


20/94


2-4

having a resistance Ro of 20 ohms (a condition easily occurring in service) a voltage of 1000 kVwill appear on the rod if Roremains constant. On the rod - particularly at its deep end and alongits wall surfaces- the electric gradients will reach magnitudes far greater that the dielectricstrength of any soil, and the soil will start ionizing and failing electrically. This ionizationconsists of a set of radial electrical streamers and glow discharges accompanied by strong voidionizations between soil particles extending out beyond the electrode (Figure 2-3).

I I

Figure 2-3Dielectric breakdown of soil around rod electrodes.

This ionization has the effect of enlarging the electrical diameter of the rod, and by Eq. 2-1 isequivalent to reducing the rod resistance. Note in Figure 2-3 that for two rods, the streamers

and/or ionization between the two rods will be shorter that the outer streamers because both rodsare at the same potential. The reduction in rod resistance can be dramatic, and reduces thelikelihood of insulator flashovers on any tower or pole connected to the rod. Ionization of thesoil can continue after voltage crest has been reached, so the rod resistance can continue to dropout on the tail of the surge current wave (Figure 2-4) and then gradually return to a valuesomewhere near its predischarge value as the soil deionizes. Experiments have shown(Reference 5) that the streamer velocities are quite slow, but ionization in the soil voids canproceed rapidly, making the latter play a dominant role in the resistance reduction. Afundamental objective of this EPRI research is to select the best mathematical model thatdescribes how this resistance changes with current and time.


21/94


2-5

kA

Ohms

kA Ohms

Time Miliseconds Figure 2-4Reduction of rod resistance with time.

Soil moisture plays an important role in values of electrode resistivity with time. Moisturecontent near the surface can vary very significantly with weather conditions, while moisture

content at depths of several meters will change only slowly with time since it tends to reach anequilibrium condition with long-term environmental conditions. Although two-layer or multi-layer resistance calculations in various grounding strategies can improve mathematical precisionin line flashover estimates, in reality for ground rods over 10 feet in length, the uncertainties ingeneral knowledge of soil moisture with time can make the additional precision illusionary.Also, there is an implicit assumption in practically all lightning calculations that soil resistivitycaused by moisture content is constant for all frequencies, whether 60 Hz or lightning transientcurrent frequencies. One of the concerns in this investigation has been to determine the bestadjustment to make in TFlash for soil dielectric response to transients of different waveshapesand moisture content.

Concrete FoundationsSoil moisture also has an interaction with the resistance of concrete foundations. In Figure 2-5,concrete surrounds the rebars and steelwork electrically connected to a tower leg.

H O2 H O2

Figure 2-5Concrete foundation in moist soil.


22/94


2-6

When the concrete is poured, its water content will initially be much higher than the surroundingsoil. However, present theory (supported by a substantial amount of research ( Refs. J-M) is thatafter an initial resistance increase, the moisture in the concrete gradually reaches a roughequilibrium with moisture in the soil, so that the reinforcing steel in the concrete eventually actsas if it is more or less embedded electrically in the soil without the concrete present, albeit a"soil" with a different resistivity. To the extent that this is true it still ignores the fact that thereinforcing steel in soil alone would emit electrical streamers, whereas any streamers in theconcrete tend to be inhibited, and - if they occur - can conceivably cause damage by creatingpuncture paths. The National Electrical Code adopted a recommendation that a copper electrodenot less than #4 in diameter and not less that 20 feet in length be embedded in a concretefoundation along one or two of the external walls. This would reduce electrical stresses insidethe concrete. It is important to reexamine how concrete foundations compare with driven rods asgrounding electrodes, and more research work is indicated.

Experience in the former Soviet Union (Reference 13) during nine years of impulse and 50 Hztesting of concrete footings found the following:

A ratio of maximum to minimum annual range of 50 Hz resistance = 1.4 caused by variationof soil moisture. The maximum resistance occurs in the winter due to freezing.

After stabilization of the concrete over several months, the ratio of impulse impedance to acimpedance ranged from 0.92 to 1.0 up to 1.8 kA impulse current and this ratio drops to about0.7 at 10 kA. However, for multiple foundations - because of inductance effects - the ratiocan reach unity or even higher.

Even in very dry surface conditions, the concrete foundations retained most of their moisturethrough capillary attraction from sub-surface soil.

As an example of foundation resistance magnitudes, in a soil of approximately 1000 ohm-meters, four concrete foundations spaced 7.5 meters apart and having a length of 2.8 meters

and resting on a buried concrete plate had a combined parallel ac resistance of 11.5 to 14.5ohms.

As far as the authors are aware, the only EPRI design curves for concrete-encased electroderesistances is that reported in Reference 7, and these are inadequate for foundations with platesor pyramidal foundations and take no account of the dynamics of the impulse resistance. A setof design curves in Reference 9 can be used for ac resistance only. Development work isindicated to incorporate concrete foundation dynamics in TFlash.

CIGR Dynamic Resistance Models

Electrical experiments of ground electrode performance have always been limited by the abilityof impulse generators to force high surge current magnitudes into the earth. Since the earlyresearches of Bewley, Bellaschi and others, a substantial literature has evolved to attachmathematical models to the observed voltage-current relationships for different soils andelectrode configurations. However, the process is electrically complex and simplifications canbe perilous. An example is the present CIGR mathematical model for a single rod (Reference19). As Figure 2-6 shows, a ground rod is represented in ionized soil - not as a rod - but as ahemisphere embedded at the earth's surface.


23/94


2-7

I

Eo

Figure 2-6Replacing a ground rod with a conducting hemisphere.

The justification for this concept lies in the argument that streamers from the rod project out intothe soil and soil ionization occurs in a kind of radial cloud whose boundary can be assumed toapproximate a hemispherical surface for high currents. Neglecting for the moment its validity, ahemispherical ionized cloud with negligible internal resistivity is easy to represent analyticallyand makes an algebraic representation possible (rather than a digital one). The problem thenbecomes one of finding suitable parameters for the various soil types and electrode geometriesthat make an acceptable fit of this model to experimental results.

If one accepts a spherical representation as a rough working model, Appendix A shows that thelow-frequency low-current resistance Roof a hemispherical electrode (Figure 2-6) is:

rRo

2= Equation 2-3

where Ro = resistance, ohms

= soil resistivity, ohm-meters

r = sphere radius, meters

Note that this model assumes that the soil inside this hemisphere is completely ionized so that the

soil resistivity in that region can be assumed zero. Making this assumption, Appendix A of thisreport then derives the CIGR Weck equation (Eq. 2-4) for the critical current I 0required tocreate a soil critical ionization gradient Eo at the surface of the equivalent hemisphericalelectrode, this electrode having low frequency resistance Roof the ground rod it replaces:

20

2 R

Eo

oI

= Equation 2-4


24/94


2-8

As derived in Appendix A, using Iothe equation for dynamic resistance of a foundation that canbe represented by a spherical electrode is:

o

o

I

I

RR= Equation 2-5

where R = instantaneous dynamic resistance, ohms with the proviso that it can never

be greater than Ro.

Ro= low frequency low current resistance, ohms

I = instantaneous foundation current, kA

Io= a critical current when ionization starts, kA

As an example, Figure 2-7 shows a conceptual foundation dynamic resistance vs. time. Havingmore bulk and a larger diameter that a rod electrode, its dynamic resistance reduction is less. Itshould be understood that the critical gradient Eoin Eq. 2-4 is not necessarily the actualbreakdown strength of the soil. More realistically, it can be considered a soil critical ionizationgradient at which the voids in high dielectric strength soils start to ionize, but it might alsorepresent breakdown strength in poor dielectric soils. More generally, it should be considered aworking constant that best fits any model that uses it .

Critical Ionization

Current = 22.5 kA

0 10 20 30 40 50 60 70 80

10

0

5

15

20

25

DynamicOhms*

Ro= 20 Ohms o= 300 kV = 300 Ohm-meters S = 1.5 meters

* Dynamic Ohms Calculated Using the Korsuntcev Curve.

Instantaneous kA

Figure 2-7Dynamic resistance of a concrete foundation vs. time.


25/94


2-9

Mousa (Reference 2), in a quite extensive analysis of the electrical dynamics of soils suggests aconservative value of 300 kV/m for Eo. IEEE (Reference 17) has suggested a value of 400 kV/m,but Oettle in her researches in South Africa has proposed values as high as 1000 kV/m. Liewand Darveniza (Reference 1) found a value of Eo = 300 kV/m to fit several of their tests. If Eoistaken as the dielectric (puncture) strength of soil, Oettle (Reference 5) has shown that there is

only a very weak correlation between the resistivity and E0. Her proposed equation for thisrelationship is:

215.0241=oE Equation 2-6

Also, the effect of water content in the soil can have an erratic effect on the resistivity, since thechemical composition of the soil (such as salt) will interact with the moisture present. In theEPRI model to be described later, Eowill be the constant that best fits experimental results,recognizing that it should not be higher than the puncture strength of the soil in a uniform orquasi-uniform field.

For a rod, Weck (Reference 18) recognized that ionization starts off the tip of the rod at very lowsurge currents, so that nonlinear resistance is almost continuously present. For this rod case(Appendix A) he proposed a modification of Eq. 2-5 above to:

o

o

I

I

RR

+

=

1

Equation 2-7

where Iois a critical ionization current of Eq. 2-4.

Note that neither of these CIGR models have time as a dynamic. The greatest resistance

reduction always occurs at the instant of maximum current, whereas many tests have shown that- at least for rods - the resistance can continue to fall for several microseconds after crest current,and these few microseconds can be important in establishing the flashover performance oftransmission lines.

Liew-Darveniza Dynamic Resistance Model

The CIGR assumption that one can replace a 20-foot ground rod with an equivalent embeddedhemisphere and have the same dynamic resistance performance is a long stretch of theimagination, and Liew and Darveniza (Reference 1) used concentric shells of earth around the

rod as a better representation (Figure 2-8). Assuming relatively uniform flow of current out ofthe rod in Figure 2-8 (not very likely) the current density flowing through each shell can easilybe calculated. If this current density is sufficient to create a critical ionizing gradient Eo acrossthe wall of any shell,

Liew and Darveniza assume that the shell wall starts ionizing and its resistivity starts droppingexponentially with time. Note that Eo is assumed everywhere constant. As the resistance of ashell near the rod starts dropping, that impresses more voltage across shells farther away andthey start ionizing also. When the surge current decays sufficiently, the current density in some


26/94


2-10

shells falls below the critical ionizing value, and those shells start deionizing (againexponentially with time) and the resistance starts increasing towards its original low frequencyvalue. Appendix B describes the Liew-Darveniza model in more detail. While a digitalsimulation of the process is relatively straightforward and can be incorporated in TFlash, it isdifficult to represent analytically the reduction in resistance versus time and instantaneouscurrent with a couple of equations as is possible for the CIGR model.

r

I

L d r

Eo

Figure 2-8Liew-Darveniza ground rod surrounded by concentric cylindrical shells of earth

While in many ways the Liew-Darveniza algorithm seems more realistic than the CIGRapproach and at low currents provides Rovalues that match theoretical formulas quite well, thismodel also has some dubious characteristics, among them:

The assumption of uniform current density in each shell wall at high currents and ignoringindividual streamer penetrations of any shell walls.

The assumption of uniform exponential ionization and deionization.

Uncertainty of values of the ionization and deionization constants and Eoto use for eachelectrode and soil type.

The assumption of an idealized hemispherical bottom end for each shell.

For any ground electrodes other than rods, describing the shell geometries becomes difficult

unless they are assumed to be hemispheres.However, dynamic resistance in the Liew-Darveniza model usually continues to drop after thesurge current has passed crest and is decaying. This is in conformance with many testobservations, including tests by EPRIsolutions described in this report. Conversely the CIGRmodel reaches its lowest dynamic resistance when the current reaches its maximum value, aphenomenon unlikely for high currents into ground rods.


27/94


2-11

If multiple ground rods are located within a few meters of one another, the shells in the Liew-Darveniza model will collide and no current will flow across the collision interface. Again thisis complicated analytically, but relatively easy to evaluate digitally, and a special programGND_ROD1 was written to calculate the total low frequency resistance of any set ofasymmetrically located ground rods using the Liew-Darveniza algorithm (Appendix C) andallowing for shell collision.

The Chisholm-Janischewskyj Model

A dynamic ground resistance model (Reference 3) developed by Chisholm and Janischewskyj(hereafter referred as the C-J model and described in detail in Appendix D) has the advantage ofbeing applicable to a wide range of ground electrode geometries and makes use of a similaritymethod originated by Korsuncev (Reference 4). In 1958, A.V. Korsuncev published an analysisof research results on dynamic resistances of several different ground electrode configurations,and in 1987, Oettle (Reference 5) extended the Korsuncev analysis to include recentexperimental results. Korsuncev plotted experimental ground electrode test results in terms of

two dimensionless ratios 1and 2, where:

sR=1 Equation 2-8

22 sE

I

o

= Equation 2-9

where:

s = a characteristic distance from the center of the electrode configuration to its outermostpoint(meters).

R = the electrode dynamic resistance, ohms


I = instantaneous current, kA

Eo= critical soil gradient, kV/m

Figure 2-9 is a modernized version of the Korsuncev curve from Reference 5.


28/94


2-12

0.01 0.02 0.05 0.1 0.2 0.5 1 2 5 10 20 50 1000.01

0.02

0.05

.1

.2

.5

1

I P

S2 o

R

SP

2222

1111

Figure 2-9Modernized version of the Korsuncev Curve (from Oettle).

It should be clear that the values of , Eo, R and s used to make this curve were probably subjectto large errors, and yet it is remarkable that the scatter of the plotted test points is not larger thanit is. Eq. 2-10 fits the straight line in Figure 2-9:

(log 1) = -0.342 (log 2) -1.515 Equation 2-10

As described in Appendix D, the C-J algorithm assumes a constant electrode resistance as long

as 1is below an initial value. Above that value, 2is calculated using the current I in Eq. 2-9,then using the 2 and the Korsuncev curve or Eq. 2-10, 1can be calculated, and from 1thecorresponding instantaneous resistance R can be found. The algorithm then works its way upand down the Korsuncev curve as the electrode current rises and falls, calculating new values ofinstantaneous R at each time step until the algorithm terminates.

The parameter s in Eqs. (2-8) and (2-9) deserves further explanation. Since s is the distance fromthe center of the electrode configuration to the farthest point on the electrode, for a single rod s issimply the length of the rod in meters. Figure 2-10 shows the application of this parameter forsome more complicated electrode shapes. If two electrodes are far apart, then s is the greatestdistance at each electrode only. However, if they are sufficiently close, their ionization areas

will overlap, and s becomes the distance using the combined set. This complication is explainedin more detail in Appendix D. The existence of this parameter s permits the algorithm to beapplied to a wide variety of foundations, ground rod configurations and even butt-wrapped poles.


29/94


2-13

S

S S

Figure 2-10Relationship of Parameter S to Ionization Zones.

A very important contribution of Chisholm and Janischewskyj in Reference 3 is theirclarification of intrinsic ground electrode inductance. Based on their experimental results withnanosecond geometrical model measurements and time domain reflectometry, they show thateven if the ground were made of solid copper, lightning surge currents entering the ground planewill see an inductance in series with whatever ground electrode resistance exists. This is becausethe finite velocity of light makes the currents "pile up" at the tower base and directelectromagnetic coupling exists between tower currents and those out in the ground plane.Therefore this inductance is influenced by tower height. Their experimental results combinedwith an analytical analysis indicate that this footing inductance is:

T

crest

Tfooting

TL

log60= Equation 2-11

where:

Lfooting= footing inductance, microhenrys

T= tower travel time, microseconds .

Tcrest = tower surge current time to crest, microsecs

While this equation was derived for a straight-rising front, it should be roughly applicable forother waveshapes. It holds provided the time to crest of the tower surge current is several timesthe tower travel time. A 120-foot tower would have a travel time Tat the velocity of light ofapproximately 0.12 microseconds, and for a tower surge current cresting at 2 microseconds,Lfootingwould be 20 microhenrys. If the front had a rate of rise of 25 kA/microsecond ( not anunexpected value) the footing voltage would be 500 kV, even though the footing resistance werezero. Prior to the researches of Chisholm and Janischewskyj, this inductive component wasusually ignored.


30/94


2-14

The Robbins/TFlash Model

The present (2001) dynamic resistance model for rods in TFlash was developed by DavidRobbins of EPRIsolutions and is described in detail in Appendix E. It is basically an expandingrod electrode model of the Liew-Darveniza type. It utilizes tables of default soil characteristics

for sand, loam, clay, gravel and stone, including default resistivities, critical ionization gradients,ionization and deionization constants, and a special IonConstant variable to adjust dynamicresistances to fit test data.

Other Models

Several other dynamic resistance models have been proposed. Examples are the Oettle model(Reference 5) and the Geri model (Reference 15, 16). It was not possible in the allotted time in2001 to adapt these models to existing data, but it is recommended that these models also becarefully examined in any future experimental work, particularly the Geri model.


31/94

3-1

3SMALL-SCALE PRELIMINARY EXPERIMENTS ON

ELECTRICAL PROPERTIES OF SOIL

Introduction

The dynamic resistance of any tower ground electrode depends not only on surge current andtime, but also on several fundamental soil properties such as dielectric strength, resistivity,moisture content and morphology. Particular concerns include the effects of moisture andwhether a breakdown path - once initiated - dominates subsequent breakdowns. All

mathematical models make assumptions about these effects, and it was important to verify theelectrical characteristics of our test soils on a small scale before proceeding to full-scaleelectrode tests. The basic soils available were sand, clay, loam and crushed rock. Electricalcharacteristics to be determined included uniform field dielectric strength under lightningimpulse voltages, effects of moisture and the permanence of breakdown paths.

It should be recognized at the outset that soils occur in infinite varieties and mixtures ofparticulates, and neither small-scale or full-scale tests can possibly cover all the electricalpeculiarities that exist on rights-of-way; only a general idea is possible.

Uniform Field Dielectric Strength of SoilsOne of the fundamental electrical characteristics to determine for the soils under scrutiny was theintrinsic dielectric strength versus type of soil and moisture content. This measurement requiresthe soil to be placed in a uniform field gap, and lightning surge voltages applied in increasingmagnitudes until the soil fails dielectrically.


32/94

Small-Scale Preliminary Experiments on Electrical Properties of Soil

3-2

Figure 3-1Rogowski Electrode Test Cell.

A Rogowski gap (Figure 3-1), centered in a wooden soil-containment box, was built for thispurpose. The electrodes were constructed of solid aluminum, and gap spacings could be set byturning the threaded shaft at the high-voltage end. To fill the cell, the containment box was laidon its side with the fill-door uppermost. Soil was slowly poured into the box and between theelectrodes, carefully compacting the soil as it was poured. The cell was then turned upright andconnected to the test circuit shown in Figure 3-2.

4 cm Gap

Voltage

Divider

Test Cell

Impulse

Generator

U

Rch

Rf

Rt

Cg

Sg

HVDC

+

_

_ +

Load

Capacitor

Figure 3-2Test Circuit for Soil Uniform Field Dielectric Tests.


33/94


3-3

The grounded electrode was covered with a thin sheet of paper before filling took place. Bycounting punctures in this paper, the number of separate-path breakdowns occurring during a testcould be determined and compared with the total number of breakdowns oscillographically. Notwo breakdowns ever took the same path. Each breakdown path healed with at least thedielectric strength it had before breakdown.

This result is indicative that soils heal after a transient lightning event. Once it was establishedthat dielectric breakdowns in soils follow separate paths, it was not necessary to use the paper.

Soil Resistivity

A separate cell was used to measure soil resistivity (Figure 3-3). It consisted of two metal platesseparated by a ring of PVC plastic with an inner diameter of 7.875 inches and a height of 2.0inches.

1/4" X 8" X 8 " Steel Plate

1/4" X 8" X 8 " Steel Plate

2" X 8" PVC Tube

Figure 3-3Resistivity Test Cell.

The ring was placed on the bottom plate and overfilled with soil. The top plate was then appliedand pressed and turned until it just contacted both the top of the ring and the soil inside the ring.

Then by measuring the resistance between the two plates, the soil resistivity is given by:R618.0= Equation 3-1

where:


R = measured resistance, ohms

Table 3-1 provides results of soil tests using the Rogowski uniform field test cell and lightning

transient waveshapes.


34/94


3-4

Table 3-1Uniform Field Dielectric Tests

SoilType

Resistivity(ohm-meters)

Dielectric Strength(kV/meter)

DrySand

17E+6 1346

DampSand

2900 787-1102

Clay 124 759-955

Loam 1050 1312

DryStone

22E+6 892

WetStone 48E+3 761

Note that in these tests the dielectric strength of wet or dry stone is not much different fromdamp sand. The stone itself is a reasonably good dielectric, but the high dielectric constant ofthe stone enhances the electric gradients in the air voids between the stone particles. It is the airin these voids that is breaking down.

Details of the tested soil materials are as follows:

Dry Sand:Kiln dried sand to have a material with very high resistivity and dielectricstrength.

Damp Sand:24 parts kiln dried sand thoroughly mixed with one part well water by volume.Sand would clump together when squeezed.

Clay:Compacted clay dug from the EPRI Lenox Center test site. Natural state, not sifted.

Loam: Purchased and compacted top-soil, sifted to remove vegetable matter.

Dry Stone: One-half inch mesh glacial till.

Wet Stone: Dry stone sprayed with water and allowed to drip to remove any excess.

When normal dielectric breakdown occurs, the voltage wave at the instant of breakdown chopscleanly to or through zero. Notable in these tests was the absence of such a clean chop at the

moment of breakdown except for the stone.

As Mousa (Ref. 2) points out, these small sample dielectric strengths can be misleading in thaton a long rod electrode, the minimum dielectric strength on the rod is likely to be substantiallyless than an average small sample measurement, and it is at the minimum dielectric strengthlocation where streamers will initiate. It appears that the soil minimum dielectric strength isroughly half the average small sample measured value for a typical soil.


35/94


3-5

Non-Uniform Field Dielectric Strengths of Soils

While the uniform field tests above can provide fundamental dielectric information on the testedsoils, additional information is needed to characterize the process of breakdown in the non-uniform radial fields around rods or counterpoise wire. A concentric cylindrical electrode test

cell is ideal for this purpose since it can simulate rod/wire radial fields and its electric fieldgradients are easily computed. Figure 3-4 shows the concentric cylindrical test cell devised forthese tests, and Figure 3-5 displays the equivalent circuit. The outer electrode was a metal barrel22.5 inches inside diameter with metal guard rings on each end to keep divergence of the electricfields at the ends as small as practical. The center electrode was a 0.625 inch copper rod withcorona spheres on each end to inhibit streamer discharges off the ends of the rod. The guardrings in Figure 3-5 were separated from the active center of the test cell by solid 1-inch plasticseparating panels, and soil was packed tightly on both sides of the panels. Impulse voltages wereapplied to the center rod, and the center cylinder electrode was grounded through a high-frequency current transformer.

Figure 3-4Photo of the Concentric Cylindrical Test Cell.


36/94


3-6

1 1.06

Impulse

Generator

Concentric Test

Cell

Voltage

Divider

Load

Capacitor

Rx Polypropylene

Rc

h

Rf

Rt

Cg

Sg

HVDC+

_

_+

U

Figure 3-5Electrical Diagram of the Concentric Cylindrical Test Cell.

Dimensions of the cell are as follows:

D1=0.625" (0.0159 meters)

D2=22.5" (0.57 meters)

L=35" (0.89 meters)

The electric charge-free gradient Eoon the surface of the inner rod for any applied voltage is:

=

1

21 log

2

D

DD

VEo Equation 3-2

where:

Eo = electrode gradient, kV/meter

V = applied voltage, kV


37/94


3-7

The low frequency resistance between the center rod and the test cell wall is:

=

1

2log2 D

D

LRx

Equation 3-3

where:

Rx= cell resistance, ohms.


L = length of center electrode, meters

Since the cell resistance Rxcan be easily measured, the soil resistivity can be extracted fromEq. 3-3 to be:

=

1

2log

2

D

D

LRx Equation 3-4

Tests in kiln-dried sand (Table 3-2) provided the 17E+6 ohm-meters resistivity. There was apronounced volt-time effect in breakdown voltages (Table 3-2):

Table 3-2Breakdown Vs Waveshape: Kiln-Dried Sand.

Waveshape BreakdownCrest kV

Square Wave 90

20 x 70 259

By Eq. 3-2, the free-field gradient on the inner rod at 259 kV applied voltage would be 9091kV/meter or 231 volts/mil. This is in the range of transformer oil impulse strength for smallsphere-plane gaps and far exceeds the Rogowski dielectric strength of dry sand in Table 3-1,indicating that streamers started across the gap long before crest voltage was reached. Neitherthe CIGR dynamic resistance model (Appendix A) nor the Korsuncev curve (Appendix D)

make any allowance for volt-time effects, whereas the present TFlash model (Appendix E) andthe Liew-Darveniza model (Appendix B) do.

Soils in the concentric cylindrical test cell showed a significant reduction in dielectric strength assoil moisture was increased. Table 3-3 shows both effect of moisture and tail time on dielectricstrength of sand in the concentric test cell.


38/94


3-8

Table 3-3Effect of Moisture and Tail Time.

Material Breakdown Crest kV Tail Time - Microseconds

Dry Sand 245 70

Dry Sand 221 70

Dry Sand 112 Long tail

Damp Sand 61 Long tail

In Table 3-3, a long-tailed wave (approximately 200 microseconds) decreased the dielectricstrength of dry sand to approximately half its 70 microsecond value and adding moisture made asignificant further decrease. These tests confirm the need to incorporate volt-time ionizationeffects in any digital dynamic resistance model such as TFlash, and underscore the influence ofground moisture in lightning performance of transmission structure grounds.

As in the uniform field tests, "breakdown" was not accompanied by an abrupt chop of thevoltage wave to zero. Rather it consisted of a sudden increase in current and a pinhole punctureof the paper lining on the inside of the outer electrode. Figure 3-6 shows a set of "beforebreakdown" and "breakdown" voltage waves.

200.0

160.0

120.0

80.0

40.0

0.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Time - Microseconds

kV

Breakdown

Withstand

Figure 3-6Concentric Test Cell Voltage Waves Before and During Breakdown.

Apparently-at least in sand-the soil surrounding the breakdown path helps cool the arc andincrease the gas pressure sufficiently to increase the resistance substantially above that of asimilar path in air and to encourage a gradual drop in resistance as postulated by Liew-Darveniza(Ref. 1).


39/94

4-1

4LARGE-SCALE EXPERIMENTS ON DYNAMIC

RESISTANCES OF GROUND RODS

Introduction

Much oscillographic information on surge voltage and current response of ground rods has beenpublished, and Reference 1 provides a comprehensive set of oscillograms from a variety ofsources. However, several practical aspects of ground rod performance vital to EPRI membershave not been considered in published full-scale experiments, including:

The efficacy of ground rods or counterpoise wires with sharp edges vs. conventional roundrods or wires.

Dynamic resistance characteristics of butt-wrapped pole grounds as an effective alternative todriven rods.

Efficacy of chemical grounding enhancement materials in handling high frequency highamplitude lightning surge currents.

Dynamic resistance characteristics of tower concrete foundations.

Determination of the percentage of dynamic ionization currents flowing off the ends of

ground rods vs. that flowing off the side-walls. This is of particular importance when rodends penetrate into the lower layers of two-layer earths.

Finally, it was of particular importance to build into TFlash a dynamic resistance model that bestsimulates large-scale impulse response of ground rods and tower foundations. For this,comparative dynamic resistance data were needed to supply soil constants and ionizationcoefficients.

Test Configurations and Soils

The ground strata at EPRI Center in Lenox, MA has principally a two-layer morphology, with an

upper layer of mixed loam-sand and a lower layer of clay interspersed with a water table. Thiscomplex condition does not permit controlled testing of soil dynamic response of ground rodsdriven into the EPRI center yard, particularly if a ground rod penetrates into the underlying watertable. For this reason, an above ground steel test tank was built into which various kinds of soilsand ground electrodes could be experimentally investigated. Figures 4-1 and 4-2 describe thetank in detail. The tank was eight-feet high and six-feet in diameter. Any ground electrode to betested was located at the center of the cylinder to depths ranging from three to five feet,depending on the test desired. Currents collected by the tank walls and by the tank bottom could


40/94

Large-Scale Experiments on Dynamic Resistances of Ground Rods

4-2

be measured separately with high-frequency Pearson current transformers Voltages weremeasures with a compensated resistance divider located so as to minimize voltage induction inthe measuring circuit by circulating surge currents. Soils and electrode configurations tested aredisplayed in Table 4-1:

Impulse Generator

U

Rch

Rf

Rt

Cg

Sg

HVDC+

_

_ +

Voltage Divider

Bottom Plate

8'

6'

1" Ground Rod

Figure 4-1Large Test Tank Dimensions and Circuitry.

Figure 4-2Photo of Large Test Tank Installation.


41/94


4-3

Table 4-1Large Scale Dynamic Resistance Tests.

Electrode Dry Sand Damp Sand Loam Number ofImpulse

Tests

TestCurrents

(kA)

Round Rod - 3 ft. X 5 1 12


Round Rod - 5 ft. X X X 26 1 12

Round Rod - 5.5 ft. X X 14 1 12


Rectangular Bar - 5.5 ft. X 54 1 11

Concrete Encased - 5 ft. X 15 1 5.6

Rod With Streamers X 12 8 13

Butt-Wrapped Pole X 17 4 14

Rod Next to Pole X 13 5 14

Chemical Ground X 10 1.3 4

While these tests do not simulate long ground rods, they are sufficient to examine the dynamicsof ground electrode resistance on a large scale. The six-foot diameter of the test tank limitedstreamer distances to no more than three-feet from any center rod to the tank wall. This wassufficient for most streamer clearances but did not simulate the infinite distance to zeroresistance experienced by a rod driven into an infinite ground plane. The net effect is thatresistances between rod and tank wall will be lower than to an infinite environment. Todetermine the correction factor to adjust rod-to-tank resistance measurements, a program calledTANKOHMS was written, and is described in Appendix G. Computed resistances to an infiniteradial distance were found to be approximately 44% higher than resistances to the tank wall andthis correction was used to adjust all tabulations to an infinite condition.

Digital FilteringDuring high voltage measurements in outdoor environments, good electromagnetic shielding isnot possible, and extraneous electrical noise can seriously distort measurements made withmodern transistorized low-signal oscilloscopes. Some digital filtering is often necessary tosmooth the waveshapes. The waveshapes in this series of experiments were smoothed with aChebyshev Type 1 Filter:


42/94


4-4

-400000

-350000

-300000

-250000

-200000

-150000

-100000

-50000

0

50000

-0.000001 0 0.000001 0.000002 0.000003 0.000004 0.000005 0.000006 0.000007

Time-Microseconds

Voltag

e

-5000

-4000

-3000

-2000

-1000

0

1000

2000

Curren

t

Voltage Current

Figure 4-3

Unfiltered Data.

-350000

-300000

-250000

-200000

-150000

-100000

-50000

0

50000

-0.000001 0 0.000001 0.000002 0.000003 0.000004 0.000005 0.000006 0.000007

Time-Microseconds

Voltage

-5000

-4000

-3000

-2000

-1000

0

1000

2000

Current

Voltage Current

Figure 4-4Chebyshev Filtered Data.

Voltage and Current Waveshapes

In the large-scale tests it was desirable to apply a current wave to any electrode in the tank thatwould at least roughly approximate in shape a lightning transient in the field. Because of thenon-linearities between voltage and current, an approximate double exponential rod current waverequired a very distorted voltage wave. Fig 4-5 displays an example.


43/94


4-5

Figure 4-5Typical Voltage-Current Waves During Rod Tests.

There was usually some ringing on the current wave caused by the long lead inductance,generator inductance and rod inductance, but this ringing was not severe, and did not appearsignificantly in the measured dynamic resistance.

Initial Dynamic Resistances

When the first large scale impulse test was carried out on a driven rod in dry sand in the testtank, it was immediately apparent that the dynamic impedance at low surge currents was muchlower than the meggered rod low-frequency resistance. Part of this difference is caused bycapacitance charging currents on the front of the applied voltage wave, but it also becameapparent that the low frequency meggered resistance was made excessively high by poor soilcontact to the rod surface and possibly by electrolytic effects. When a rod is driven - particularlyinto compacted sand - it vibrates, and this vibration slightly enlarges the hole around the rod,making for poor contact between rod and soil at various segments along its length. However, onapplication of impulse currents even low magnitudes of surge currents created the few hundredvolts necessary to bridge the very small gap between rod wall and the adjacent sand, making agood connection. (It might be noted that when electrolytic tanks were used to calculate electric

fields before the advent of digital field plotting, it was found that copper and aluminum did not"wet" as well as iron, making for substantial resistance measurement errors if the former wereused as electrodes in water at low voltage levels).


44/94


4-6

To adjust for this poor soil contact, the early beginnings of the surge voltage and current waveswere used to derive a meaningful initial dynamic resistance. Figure 4-6 illustrates the initialrelation between surge current and dynamic ohms:

Figure 4-6Initial Dynamic Resistance Computation.

Effectively, the highest dynamic resistance encountered (invariably during the surge currentinitial rise time) was used as a "working" initial dynamic resistance, and any resistances lower

than this value (due to noise) that occurred prior to that value were adjusted to the "working"value. An additional constraint was utilized rejecting the voltages and currents at the first twotime steps in making this calculation. This was necessary because of noise levels at these twosteps were sometimes strong enough to override the true voltage-current signals, even withfiltering, and dividing observed voltages by observed currents to obtain impedances at low signallevels creates major errors.

This working initial dynamic resistance evaluation did not include effects of capacitancecharging currents on the front of the applied voltage wave. Rod to tank wall capacitance was ofthe order of 200 picofarads for a soil relative dielectric constant of 10. For a voltage rise time of500 kV/microsecond, this corresponded to a charging current of 100 amperes. Since most of the

surge currents into the rods were in the order of kiloamperes, the charging currents wereneglected in the initial impedance evaluations.

An effort was also made to evaluate rod inductance. The rod inductance could not be resolvedreliably from the voltage and current oscillograms because the dynamic resistance effectsobscured the inductive contribution. This inductance is determined by rod to tank geometry andis roughly independent of soil dielectric characteristics. However, in our test configuration anyindicated inductance is distorted by the tank iron, by the voltage divider measuring loop and by


45/94


4-7

inductance of the grounding cables leaving the tank. A separate set of outdoor tests in 2002 withdriven rods in a uniform earth is recommended.

Equation (4-1) from Grover (Reference 14) for inductance of a rod whose length is much greaterthan its radius is:

= 12

log2 r

SSL

Equation 4-1

where:

L = rod inductance, henrys

= soil permeability 4x 10-7henrys/meter

S = rod length, meters

r = rod radius, meters

However this equation assumes that the current in the rod is constant throughout its length,whereas the actual rod current varies along the rod as it is drained out into the soil. Eq. 4-1yields a value of 5.7 microhenrys for a five meter long one-inch diameter rod. This is roughly 38percent of the classical inductance of a 100 foot tubular steel pole, and this inductance plays amajor role in determining insulator voltages. Even if the inductance is reduced by half bycurrent leakage into the soil it is still very significant. Further research is indicated to evaluateinductances of ground rods. Historically this inductance has usually been neglected.


46/94


4-8

Dynamic Ohms vs. Applied Current

Even at a few kA, the dynamic ohms dropped rapidly with current for all soils tested. Figure 4-7illustrates a typical relationship between the dynamic ohms of a three-foot rod in sand and the

instantaneous current fed into the rod.

Figure 4-7Dynamic Ohms vs. Applied Current.

A substantial drop in impedance starting at about one kA is displayed. The small loop at thebottom of the trace is from electrical noise. The trace was cut off at six microseconds. If it hadbeen permitted to continue, it would have eventually returned to the initial value as the currentdied to zero.

Effect of Rod Length and Separation

The low-frequency low-current resistance of a single rod in uniform soil can be determined fromthe Dwight Equation (2-1). In the test tank however, it was not possible to directly compareseparations of rods. But as will be shown later, the Liew-Darveniza algorithm matches the tank

test values quite well when corrected for the finite distance from rod to tank wall, and thisalgorithm does permit analysis of separation of pairs of rods.


47/94


4-9

0 10 20 30 40 50 60

5

0

10

Dynam

icOhms

Total Surge Current - Crest kA

15

20

70 80

(2) 15 Foot Rods 10 feet Apart

(1) 30 Foot Rod

Figure 4-8Dynamic Resistance at Two Microseconds As a Function of Current Into One 30-Foot Rodor Two 15-Foot Parallel Rods Spaced 10 Feet Apart.

Figure 4-8 uses the Liew-Darveniza algorithm (Appendix B) to compare the dynamic resistanceof a single 30-foot ground rod with two 15-foot rods separated by 10 feet. Soil resistivity wasassumed 300 ohm-meters, the ionization constant Ti was 1.0 microsecond, and the soil criticalionization gradient was 300 kV/meter. The crest surge current was that into either one rod orinto the pair of rods, and the surge current waveshape was 2-50 microseconds. The lowfrequency resistance of the single rod was 35.1 ohms versus 35.7 ohms for the pair (essentiallythe same). As shown in Figure 4-8, the single rod is always better by having the lower resistance,regardless of the separation distance between the two rods. Part of he reason for this involves theionization dynamics. For the same voltage applied, each of the two rods receives only half thecurrent fed into the single rod, making the ionization envelopes around two rods less than that for

the single rod. There is also mutual coupling between the two rods, and streamers traveling fromone rod in the direction of the other are inhibited because the two rods are at the same potential(Figure 2-3).

While it was not possible to evaluate rod length effects greater than 5.5 feet in the test tank, thetest tank observation that dynamic resistance of the rods and tested soils followed the Liew-Darveniza algorithm (Appendix B) reasonably well made it possible to use this algorithm toexamine rod length effects up to 30 feet.

Comparison of Tank Test Values with Dynamic Algorithms

Dynamic resistance values versus time were measured in the test tank for sand, damp sand andloam for ground rods ranging from 3.0 feet to 5.5 feet, the range of lengths permitted by thedimensions of the test tank. The results were then compared with that predicted by variousdynamic resistance algorithms. (Nevertheless this limited range of rod lengths was sufficient tobring into play all the ionization effects that occur in practice, and to permit comparisons to bemade with predicted values). In the following examples, all observed dynamic resistances werecorrected for the proximity effect of the tank wall, and the initial (low current) resistance wasthat extracted from the front of the voltage and current waves, since - as pointed out above -


48/94


4-10

meggered low frequency values were in error because of slight gaps created between rod wallsand the surrounding sand when the rods were driven.

Liew-Darveniza Algorithm

Figure 4-9 shows an example of a rod surge current, observed dynamic resistance and predicteddynamic resistance using the Liew-Darveniza algorithm (Appendix B).

Figure 4-9

Liew Darveniza Algorithm Comparison With Dynamic Resistance Test Values.

Using a critical ionization gradient of 300 kV/meter as recommended by Mousa, the actualversus predicted dynamic resistances are quite close.


49/94


4-11

CIGR/Weck Algorithm

Figure 4-10 shows an example of the same comparison as in Figure 4-9, but using theCIGR/Weck algorithm (Eq. 2-7 ). The critical ionization current was 0.8 kA as is apparent

from the observed current and resistance waveshapes.

Figure 4-10CIGR/Weck Algorithm Comparison With Dynamic Resistance Test Values.

The theoretical and observed values do not compare very well. However, if instead of thecritical ionization current of 0.8 kA indicated by the oscillograms, the value proposed by Eq. 2-4had been used (0.21 kA), the observed and calculated values would have been quite good. Notehowever that the calculated dynamic resistance is dropping prematurely.


50/94


4-12

Chisholm - Janischewskyj Algorithm

Figure 4-11 compares the same rod dynamic resistance with that predicted by the Chisholm-Janischewskyj (C-J) algorithm (Appendix D).

Figure 4-11C_J Algorithm Comparison With Dynamic Resistance Test Values.

The match is quite good for the final dynamic resistance at 5 microseconds, but - as is the casefor the CIGR algorithm - the dynamic resistance decays prematurely


51/94


4-13

TFlash Algorithm

Figure 4-12 compares the same rod dynamic resistance with that predicted by the algorithm thatis presently in TFlash (Appendix E). This algorithm was devised by David Robbins of

EPRIsolutions, and is based on the Liew-Darveniza algorithm, with the addition of a table ofscaling constants.

Figure 4-12TFlash Algorithm Comparison With Dynamic Resistance Values.

The match is again quite good with some premature drop in calculated dynamic resistancecompared with measurements.

In general the above trends were common throughout the range of rod lengths and soils tested.Any one of these algorithms can be made to match measured values reasonably well if theconstants are chosen properly, but the constants will be different for different algorithms.

Effect of Rod Shape and Artificial Streamers

If dynamic resistance is reduced as the number of streamers from a rod are increased, then it canbe reasonably assumed that the addition of artificial metallic streamers to a ground rod should


52/94


4-14

further reduce the dynamic resistance. Figure 4-13 shows a set of artificial streamers made fromcopper wire and attached to every six inches along the bottom three feet of a four foot rod. Therod was inserted at the center of the test tank, and loam carefully packed around it until the tankwas completely filled.

Figure 4-13Artificial Streamers Attached to a Ground Rod.

Figure 4-14 displays an oscillogram of dynamic resistance for a 10.5 kA peak current with thestreamers in place, and Figure 4-15 shows the same test result with the streamers removed.There is essentially no difference

Figure 4-14Dynamic Resistance With Artificial Streamers in Place.


53/94


4-15

Figure 4-15Dynamic Resistance With Artificial Streamers Removed.

The apparent reason for a lack of improvement using artificial streamers on a ground rod is thatthere are so many natural breakdown streamers and ionization pockets along the rod in any case,and they extend so far beyond the length of the artificial streamers, so as to mask out anyartificial additions.

This lack of improvement was also apparent in changes in rod cross-sectional shape. Tests were

made comparing the dynamic resistance provided by a rectangular bar with sharp edges ascompared to a round rod with no edges. Figure 4-16 shows the cross-sectional area of a testedrectangular copper bar with a total surface area of five inches.

2 "

1/2 "

Figure 4-16Tested Bar Cross-Section.


54/94


4-16

Figure 4-17 shows a dynamic resistance oscillogram of this electrode as compared with Figure4-18 for a round rod. The dynamic resistances of the rectangular rod with sharp edges and theround rod with substantially less surface area and no edges are essentially the same.

Figure 4-17Rectangular Bar Ground Rod Dynamic Resistance in sand.

Figure 4-18One Inch Diameter Round Rod in Sand.


55/94


4-17

As far as dynamic resistance is concerned, the cloud of streamers and ionization off either theround or rectangular rod masks the rod cross-sectional geometry, and the shape of the rod cross-section makes little difference in dynamic resistance. This suggests the following rule:

Select ground rod cross-section geometry for mechanical reasons. Variations in shape orsize will have little influence on the rod dynamic resistance.

Response of Concrete Ground Electrodes

In the six-foot diameter test tank, it was not possible to bury concrete tower foundations and stillmaintain sufficient clearance to the tank walls. However, an attempt was made to examine thetransient response of a concrete incased ground rod. Figure 4-19 shows one result of a set oftests on a 5/8 inch rebar five feet long incased at the center of an eight-inch cylinder of concreteand buried in damp sand.

Figure 4-19Voltage-Current Waves on a Concrete-Encased Rebar.

Until 2.4 microseconds the dynamic resistance process was progressing normally, but at thattime a breakdown started from a section of the rod outside the concrete which dived under the

sand near the surface to the tank wall. The current increase after 2.4 microseconds is the resultof this failure. Note that again the voltage does not chop suddenly to zero as it would do in air.The sand appears to increase resistance of the arc by cooling and compressing the failurechannel. There was no evidence of concrete fracture from the severe dielectric stresses in theconcrete around the rod. A useful calculated dynamic resistance could not be obtained from thistest because of the complete breakdown from the concrete to the tank wall.


56/94


4-18

Chemical Enhancement

The limited size of the test tank prohibited complete tests of effects of chemical enhancementmaterials on the dynamic resistances of ground rods. The chemicals appeared to migrate out intothe damp sand and initiate failure paths, obscuring dynamic resistance measurements. Figure

4-20 shows one example.

Figure 4-204.5-Foot Ground Rod Surrounded With Eight Inch Diameter GEM Material.

Dynamic resistance reduction does occur with time, but because of the limited size of the testtank a comparison with damp sand alone was postponed to a future date when an outdoor testcould be made.

Butt-Wrapped Poles

One set of tests was completed embedding a butt-wrapped grounding electrode in sand. Figure4-21 shows the butt-wrapped pole electrode. The butt-wrap was 14 turns of #6 stranded wire onthe butt of an eight-inch-diameter pole embedded in loam.


57/94


4-19

Figure 4-21Photo of Bottom End of Butt-Wrapped Pole.

A dynamic resistance oscillogram of this grounding arrangement is shown in Figure 4-22. Ingeneral, the dynamic resistance was roughly the same as for a five-foot ground rod.

Figure 4-22Dynamic Resistance of a Butt-Wrapped Pole in Loam.


58/94


4-20

Computer Modeling of Dynamic Resistance

It is clear from the above tests that relying on meggered low-frequency resistances of towers toaccess transmission line lightning performance can be very misleading. It is the dynamicresistance of grounding electrodes that governs the flashover process, not the meggered

resistance. It is clear from both the Korsuncev and Liew-Darveniza algorithms that major effortsto improve meggered resistances can - in some cases - have little effect on dynamic resistance,and conversely that in some cases, the dynamic resistance will be sufficient although themeggered resistance appears inadequate. Dynamic resistance changes with time, but by sixmicroseconds the dynamic resistance has usually stabilized for most lightning currents andflashover has taken place if it is going to. Dynamic resistances depend on soil resistivity, but sodo meggered resistances. Dynamic resistance depends on a critical ionization gradient of theearth around the electrode, but the analysis in this report on various soils was fairly successful ifa gradient of 300 kV/meter was used (as recommended by Mousa, Reference 2), rather than theIEEE 400 kV/meter value. If soil resistivity is known, the Liew-Darveniza algorithm canprovide the dynamic resistance of rods (Appendix B) and the Korsuncev C-J algorithm(Appendix D) can provide values for either rods or tower concrete foundations. The C-J

algorithm is very easy to use. A comparison of the Liew-Darveniza and Korsuncev C-Jalgorithms is shown in Fig.4-23 as a function of rod length.

0 2 4 6 8 10 12

10

0

20

LowestDynamicOhm

s

Rod Length - Meters

30

= 500/meter

= 100/meter

40

Lieu Darveniza

Korcuncev Lieu Darveniza

Korcuncev

Figure 4-23Comparison of Lowest Dynamic Ohms Calculated by Korsuncev and Liew-DarvenizaAlgorithms.


59/94


4-21

At a soil resistivity of 100 ohm-meters the six-microsecond dynamic resistance provided by theLiew-Darveniza algorithm matches the Korsuncev C-J algorithm quite well. At a higherresistivity of 500 ohm-meters, the C-J algorithm is more optimistic at long rod lengths. In thetests made in this report, both algorithms yielded similar results at six microseconds.

Based on the above, it was decided to examine how target dynamic resistances could be utilizedduring line construction. Because of our lack of concrete foundation data at this time theprocedure was restricted to ground rods. Figure 4-24 displays dynamic resistance (at 6microseconds) of single rods in uniform soil as a function of soil resistivity and rod length up to30 feet, using the C-J algorithm. One can interpolate between the curves to arrive at a targetvalue. If a single rod is not sufficient, or if two rods are required anyway (H-frameconstruction), Figure 4-25 provides improvement in percent if two rods are used instead of one.Note that for short rods, increased separation distance can make a substantial improvement, butfor 30-foot rods there is little improvement by driving two rods instead of one even if theseparation distance is 30 feet. In no case does two rods yield half the resistance of one rod.

0 5 10 15 20 25 30

10

0

20

Dyn

amicResistanceOhms

Rod Length - Feet

30

40

50

= 100/meter

= 50/meter

= 300/meter

= 500/meter

= 700/meter

= 1000/meter

32 kA 2 X 50 Wave, c==== 300 kV/m,

Figure 4-24Single Rod Korsuncev Dynamic Resistance vs Rod Length and Soil Resistivity.

8/14/2019 towe

tower grounding and soil ionisation.pdf

Documents