Diapositivas de Puesta a Tierra-Donohoe

Department of Electrical and Computer Engineering

Ground Resistance Computations Using theNumerical Electromagnetics Code (NEC)

J. Patrick Donohoe, Ph.D., P.E.Professor

Dept. of Electrical and Computer EngineeringMississippi State University

Steel Distribution Pole ForumReston, Virginia

May 17, 2002


Objectives:

(1) Use the Numerical Electromagnetics Code(NEC-4) to accurately determine the ground resistance of steel poles.

(2) Compare the computed ground resistanceof a steel distribution pole to that ofcommonly used grounding electrodes(ground rods, concrete piles).


Numerical Electromagnetics Code (NEC-4)

l Code Description!Designed for analyzing antennas and scatterers!Method of moments solution!Electric field integral equation (frequency domain)

l Code Features!Versatile geometry input

(straight segments for wires, flat patches for surfaces)!Solution algorithm applicable to electrically small

structures (low frequency) !Allows for perfect conductors and/or conductors of

finite conductivity!Allows for lossy ground planes and conductors that

penetrate the ground plane


DefinitionsANSI / IEEE Std 81-1983 IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System

Ground Resistance (of a grounding electrode) – the ohmic resistance between the grounding electrode and a remote grounding electrode of zero resistance.

A remote grounding electrode is sufficiently distant that the mutual resistance between the two electrodes is zero.

Mutual Resistance (of grounding electrodes) – the voltage change in one electrode produced by a change of one ampere of direct current in the other, expressed in ohms.


Components of the Electrode Ground ResistanceIEEE Std 142-1982 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems

1. Resistance of the electrode

2. Contact resistance between the electrode and the soil

3. Resistance of the soil from the electrode surface outward

• Resistance components (1) and (2) are very small in comparison to component (3).

• Resistance components (1) and (3) are included in NEC-4 analysis.


Components of the Electrode Ground Resistance(Equivalent Current Carrying Capability)

Steel Pole

Wall Thickness

Cross Sectional

Area*

Resistance per unit length

Equivalent copper

conductor 40 ft. Class 5

(40G5) 0.10 in. 12.127 cm2 107.20 µµΩΩ/m 350 MCM

(19 strand) 45 ft. Class 3

(45SX3) 0.12 in. 142.127 cm2 87.63 µµΩΩ/m

400 MCM (19 strand)

65 ft. Class 2 (65EP2) 0.162 in. 19.414 cm2 66.96 µµΩΩ/m

500 MCM (37 strand)

0.507 V

* Cross sectional area is determined 5 ft. from the pole top


Methods of Measuring Ground ImpedanceANSI / IEEE Std 81-1983

1. Two-Point Method2. Three-Point Method3. Ratio Method4. Staged Fault Tests5. Fall-of-Potential Method

Modeled with NEC- 4


Fall-of-Potential Method

+

VoI

+

Groundelectrode

z

x

s

Currentelectrode

Voltageprobe

V(x) IV(x)

R(x) =


Code Validation

• NEC-4 is used to determine the ground resistance of a 10 ft. steel ground rod (5/8 in. diameter) for different soil types. The computed ground resistances are compared with results obtained using the analytical formula.

• 62% Rule - Using the fall-of-potential method under ideal conditions, the measured resistance should match the theoretical ground resistance at a distance of 0.618s.


-5 0 5 10 15 20 25 30 350

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x (m)

V(x

) (V

olt

s)

0.618s=18.84m (61.8 ft)

V(0.618s)

Current electrodelocation

s=30.48m (100 ft)

Ground rodlocation

Voltage probelocation

Fall-of-Potential Method 10ft. (5/8 in. diameter) Ground Rod


Alternative Fall-of-Potential Method

+

VoI

+

Groundelectrode

z

x

s

Currentelectrode

Voltageprobe

V(x) IV(x)

R(x) =


-125 -100 -75 -50 -25 0 25 500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x (m)

V(x

) (V

olt

s)

Voltage probelocation

Current electrodelocation

Ground rodlocation

Alternative Fall-of-Potential Method 10ft. (5/8 in. diameter) Ground Rod


Length Radius Soil Type

V I Rcomputed Ranalytical

10 ft 5/8 in sand 0.443 V 1.46 mA 304 ΩΩ 311 ΩΩ

10 ft 5/8 in clay 0.443 V 33.7 mA 13.1 ΩΩ 13.4 ΩΩ


8 ft 5/8 in clay 0.489 V 30.9 mA 15.8 ΩΩ 16.2 ΩΩ


8 ft 1/2 in clay 0.500 V 30.4 mA 16.4 ΩΩ 16.8 ΩΩ 0.507 V

Grounding Resistances of Typical Ground Rods


Current Density Surrounding a 10ft. (5/8 in. diameter)Steel Ground Rod in Sand (total current = 1A)


10.4 m (34 ft)

1.83 m (6 ft)

Outside diameter at groundline0.303 m (11.93 in)

Outside diameter at pole base0.333m (13.11 in)

Outside diameter at pole top0.133 m (5.25 in)

Steel thickness3.05 mm (0.12 in)

40 ft. Class 3 Steel Pole


NEC-4 Steel Pole Modeling Issues

l Pole Taper! NEC allows for tapered conductors.! The conductor radius must be constant for conductors

that penetrate the air/soil interface.

The steel pole is modeled as a straight conductor with radius equal to the mean radius of the tapered pole below ground[6.26 in. (0.159m) for the 40 ft. class 3 steel pole].

l Below Grade Protection (Insulation)! NEC allows for insulated conductors. ! NEC does not allow for insulated conductors below

the air/soil interface.

An upper limit on the grounding resistance of the coated steelpole is determined by modeling only the bare portion of polebelow ground.


Grounding Resistance of a Coated Steel Pole

I

Coated pole(Rcoated)

Rcoated < Rbare

I

Bare pole(Rbare)

Coated portion

Bare portion

db

db


Length of bare

portion (db)

Soil Type Rbare

Length of bare

portion (db)

Soil Type Rbare

6 ft sand 213 ΩΩ 6 ft clay 9.20 ΩΩ





1 ft sand 465 ΩΩ 1 ft clay 20.3 ΩΩ 0.507 V

Computed Grounding Resistances (Upper Bounds) for Partially Coated Steel Poles


NEC-4 Concrete Pile Modeling Issues

l Inhomogeneous Ground (Concrete/soil)! NEC does not allow for an inhomogeneous ground.! The conductivity of concrete is lower than the average

conductivities of sand or clay. The conductivity of concrete is comparable to dry sand.

A lower limit on the grounding resistance of the concretepile is determined by computing the grounding resistanceof the bare conductors in homogeneous soil.


Grounding Resistance of a Concrete Pile

I

Concrete pile(Rpile)

Rbare < Rpile

I

Steelconductors in

concrete

Steelconductors in

soil

Bare Conductors(Rbare)


Soil Type Rbare

sand 243 ΩΩ

clay 10.6 ΩΩ 0.507 V

Computed Grounding Resistances (Lower Bounds) for the Concrete Pile

Concrete Pile DetailsAll conductors – ½ in. diameter steelVertical conductors – 6 ft. in lengthHorizontal conductors – 1 ft. diameter circles


Sand Clay


Conclusions

l A 40 ft. class 3 steel pole with between 3 and 4 feet of buried bare length offers equivalent groundingcapability to a 10 ft. (5/8 in. diameter) ground rod.

l A 40 ft. class 3 steel pole with approximately 2 feet of buried bare length offers equivalent groundingcapability to an 8 ft. (5/8 in. diameter) ground rod.

l A 40 ft. class 3 steel pole with between 1 and 2 feet of buried bare length offers equivalent groundingcapability to a 8 ft. (1/2 in. diameter) ground rod.

l A 40 ft. class 3 steel pole with between 4 and 5 feet of buried bare length offers equivalent groundingcapability to the concrete pile.


Conclusions

lThe steel pole grounding equivalencies computed here are very conservative given the assumptions made in the steelpole and concrete pile modeling.

lMore precise numbers for the steel pole grounding equivalencies could be computed using a custom code developed especially for this purpose.

Diapositivas de Puesta a Tierra-Donohoe

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