IEEE Presentation Sep 11

transcript

Grounding and Ground Testing

GROUND RESISTANCE TESTING

What is a ground?

“An electrical connection intentionally made

between an electrical body or system and a

metallic body in the earth.”

Source: NBS Technical Paper 108; June, 1918

Low resistance Connection to Earth to

Drain Away Energy and Engage Protective

Devices

What is a ground?

“A conducting connection, between an electrical

circuit or equipment and the earth, or to some

conducting body that serves in place of the

earth.”

Source: NFPA 70-1981; National Electrical Code

Ability to Drain Away Energy in Sufficient

Manner is also Important. Not Simply

Making a Ground Connection

What is a ground?

“A ground is a conducting connection by which

an electrical circuit or equipment is connected

to the earth or some conducting body.”

Source: IEEE Standard 81

Ground Standards

There is not one standard ground resistance threshold

that is recognized by all agencies.

NFPA and IEEE have recommended a ground resistance

value of 5.0 ohms or less.

The NEC has stated to "Make sure that system impedance

to ground is less than 25 ohms” specified in NEC 250.56.

The Telecommunications industry has often used 5.0 ohms

or less as their value for grounding and bonding.

Communication requires lower signal level

with higher frequency characteristics than

60 Hz Utility requirements

Loose Neutral Effect on other Phases

What defines a good ground?

A conductor with resistance low enough to

dissipate fault currents, lightning strikes, etc. into

the earth.

Does a good ground ensure good power

quality??

Voltage imbalance variation is in

amplitude (peak – rms).

Balanced load,

no neutral current

required

3 Phase System, Zero Sequence

60 Hz, balanced with no zero sequence

current

Ground Plane

Net Current Flow

5th Harmonic (300 Hz), no neutral current

Percents of Fundamental

• 3rd = 91%

• 5th = 74%

• 7th = 56%

• 9th = 35%

Fundamental RMS Current

• 0.13A

Fundamental RMS Current

Plus RMS of Harmonics

• 0.23A

Example of 140% TDD

Switch Mode Power Supply – Square Wave Generation

3rd Harmonic – Neutral Current is 3 Times

Phase Current – When Load is Balanced

Neutral Current

Neutral Phases have become working

Phases

For example - a

90% electronic load

will require the

neutral to carry 1.6

times the respective

phase current –

even when all three

phases are balanced

Neutrals have become working phases;

Not simply connections for emergency

situations.

“McMinnville”

Analogy of Water usage. Large pipe for human

service usage. < Quality is Acceptable

Human Consumption Pipe. Quality Necessary

Consider Ohm’s Law:

V = R x I

Where:

- V is Volts

- R is the resistance in Ohms

- I is the current in Amperes

Is Determining a Ground

Quality simply sticking a

DMM in the Dirt?

Components of System Ground How far do we need to run the DMM leads?

1) Electrical properties of Grid up to ground rod

2) Ground Rod Sphere of Electrical Influence

3) Infinite Earth plane

1) Facility Ground Plane

(copper is assembled)

A facility can spend a large amount of resources to

establish a good “ground” in a substation location -

Metal to Metal grid is constructed

Low resistance ohm meter used to

quantify grid resistance and

construction quality

2) MEASURING GROUND to EARTH

RESISTANCE

Bottle Neck of all Grounds

R = r L

a Where:

r is the resistivity of the earth in ohm-cm

L is the Depth of the conducting path

a is the cross-sectional area of the path

System Resistance

Effective electrical conductivity area is minimal at

connection point where ground rod contacts soil.

Facility Ground Plane Ground Rod

Resistance

Flow At some point

the area is so

great, R adds

little

Move from a

very good

conductor, to

a poor

conductor

with a small

conduction

R = ρ L

Earth Shells

Earth Resistance Lowers by Increasing

Shell Areas Next layer

of ground

increased

surface

Where metal meets the dirt (rod interface)

1‟ Deep; 1”

Diameter

Ground Rod

Surface Area at

Ground Rod

Interface (G) is

0.27 SQ FT

Electrons Move from Metal bus work to

Dirt (Bottle Neck Begins)

First Layer of Soil, 1” in Thickness

Area at Ground (1”) of

soil shell thickness is

0.90 SQ FT

R = 5.13 Ω

Add 5.13 Ω to all

prior bus work

for 1” of soil

Ground Resistance at 2” of Soil Thickness

Area at Ground Rod (4) 2” of soil shell is

1.66 SQ FT; Ground Rod was 0.27 SQ FT

6 Fold increase in surface area for 2”

distance.

R = 3.37 Ω additional (2” – 1” shell)

8.50 Ω Total resistance (5.13 Ω + 3.37 Ω )

Resistance at 5” of Ground Soil Thickness

Area at Ground Rod (G5) 5”

of soil shell is 4.74 SQ FT;

R = 1.53 Ω additional (5” @

4” shell);

G5 5” from ground rod edge

Ground resistance at 5” of Soil

13.28 Ω Total resistance

through first 5” of soil

2" 5.13

3" 3.37

4" 1.85

5" 1.54

6" 1.40

13.28 Ω

Shell Resistance per 1” Increments

Area at Ground Rod (7) 7” of

soil shell is 10.3 SQ FT;

R = 1.17 Ω additional (7” – 6”

shell); 3.75 Ω

7” of Thickness

1" 5.13 0.90

2" 3.37 1.66

3" 1.85 2.56

4" 1.54 3.58

5" 1.40 4.74

6" 1.28 6.03

7" 1.18 7.44

15.75 Ω

Total Resistance through first foot of soil

1" 5.13

2" 3.37

3" 1.85

4" 1.54

5" 1.28

6" 1.18

7" 1.09

8" 1.01

9" 0.94

10" 0.87

11" 0.81

Total 19.07 Ω

Measured Ground Value – 33

1 2 3 4 5 6 7 8 9 10 11

Resistance Verses Distance

Combination of all three Ground components

Example: 300 feet from ground,

shell area is 128,800 SQ FT or 9

football fields of area

R= 26 µΩ; Resistance contributes

to overall ground resistance (1”

shell)

R = 33.07 Ω + 0.0000266 Ω

0 5 10 15 20 25 30 35 40 45 50

3) Infinite Earth Ground Potential

Once the electron fights its way through the Ground

Rod Sphere of Influence. Infinite Earth potential

exists. Once an electron leaves the zone of

influence of a ground, there is no effective

resistance of that electron to move across the

Earth.

For testing purposes, need to measure ground rod

up to the infinite Earth barrier. The distance after

that does not matter, why??.

Infinite Earth Potential

How can an electron moving 50„ - have a

net resistance of 50 Ohm, yet have zero

resistance from one side of the planet to

the other, once in the Earth Potential

Deep Space Asteroid

Gravitational force is equal to mass, Inverse

Distance Squared

An object never truly leaves the sun‟s field of

influence, but its effects are dampened by distance

Diminished Field or Free Space

An electron will behave like the

asteroid. Once the distance is so

far from the sun, other bodies have

an equivalent gravitational effect –

close neighbor asteroids, near

planets such as Pluto, even other

stars and galaxies.

Our electron never leaves the ground sphere of influence,

it is just the influence is dampened to the point where

other influences are just as prominent (the ground

becomes noise).

TESTING METHODS

Most Popular Testing

Methods:

Fall of Potential Method

(Wenner)

- Full

- Simplified

Slope Method

And others

4-Terminal Earth Test

(2 Terminals of 4 – Current)

Circuit is complete, Current

Flows, Voltage is known –

therefore can read a resistance

value – RIGHT?

Using Ohms Law; Δ V and I are known.

Variables –ground sphere of influence

(Ground of interest) & Test rod C2

Two variables, one equation

C2 Needs to be eliminated somehow

Δ V= I * (GEuT + C2)

Additional 2 wires (Potential Wires)

4 WIRE TEST

Physical Outline of Test Procedure

Equipotential

Circles

Should You Accept This Result

A location has been known to have an infinite Earth

distance of about 60 Feet. A crew comes back

after testing with the following set up & results;

Ground value 72 Ohms, Measurement distance of

65 Feet. Do you accept the result?

Ground

Under Test

Temporary Test

The Problem of Limited Distance/Space

Distance of Potential Probe from X (dp)

Current Probe (C)

Potential Probe (P)

Ground Electrode

Under Test (X)

Do not want

to be in

Current

Probes Zone

of Influence

Theoretical Background - Fall of Potential

Current Probe

Position

Distance of Potential Probe from X (dp) Ground

Electrode Position

Current Probe (C)

Potential Probe (P) Positions

Ground Electrode

Under Test (X)

Want to

determine this

Fall of Potential Method - Advantages

Extremely reliable: - Results can be checked by testing at different probe spacings.

Conforms to IEEE 81; only approved method.

Operator has complete control of the test set-up.

Can be used to test any size system.

Highly accurate: - 4-wire configuration/no additional loop resistances included.

- Significant for low resistance (1-2) grounds.

Tester uses a unique source frequency, non 60 Hz, so active

power fields will not interfere with testing.

Fall of Potential Method – Disadvantages

Extremely time consuming and labor intensive. - Temporary probes must be placed.

- Cables must be run to make connections.

Space constraints can make it hard to place

remote probes.

Must disconnect individual ground electrodes to

measure them (only return must be ground)

Must “know” of other Grounds in system.

Substation Testing requiring low resistance values

can run 4000‟ or greater test lengths.

Field Tricks

A customer complaint about grounding quality

occurs. In order to test the ground, must throw

service cutout, terminate power, disconnect utility

feed ground from service ground.

Alternate – Place a new ground rod 6‟ from service

rod. Test the rod only to determine 3 or 4 wire

grounding value. When measured, attach new

ground to old in service ground. Ground is at least

that value (or better). No need to interrupt service.

Abbreviated 4 Wire – 3 Wire Test

V Potential

carries

current, so

it will have

its own

voltage

MEASURING GROUND RESISTANCE

(testing methods)

61.8% Rule/Method:

Based on the theory behind the full Fall of

Potential method.

Take measurement at only one point.

Quality check – “First Down Rule”

(testing methods)

61.8% Rule/Method:

Advantage: Extremely quick and easy.

Disadvantage: Assumes that conditions are

perfect (adequate probe spacing and soil

homogeneity). The ground behavior needs to

be known before testing begins.

“Kansas Test”

(testing methods)

Slope Method:

Based on the theory behind the Fall of

Potential method;

- for complex grounding systems and/or

- situations where lead lengths prohibitive

Use three measurements in calculation; can

take more; 40%, 50%, 60%

(testing methods)

Slope Method:

Advantage: Provides an approach for dealing

with complex systems.

Disadvantage: Makes assumptions about soil

resistance in region not tested

Ground Test Clamp on Method

Clamp-On/Stakeless Methodology

Based on Ohm‟s Law (R=V/I): - Apply known voltage to a complete circuit. Measure resulting current

- Calculate resistance of the entire circuit.

Apply signal and measure current without direct electrical

connection: - Grounds do not need to be lifted for testing

- Power does not need to be disconnected for testing.

Clamp includes transmit coil (applies voltage) and receive

coil (measures the current).

Measurement Loop is Directional.

It will first give continuity, to determine if a ground

exits to being with

Clamp-On/Stakeless Methodology

For accuracy, more return paths, the better results

For 1 return electrode = average of the two.

For 6 similar electrodes with a resistance of 10: - Rloop = 10 + 2 = 12

For 60 similar electrodes with a resistance of 10: - Rloop = 10 + 0.17 = 10.17

Governing Equation – Parallel Circuits

Rloop = RTest + (1/(1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5))

Note: The resistance of the ground under test will

always be higher than the actual ground resistance

value. Worst case approach leaves a safety margin

of error (if infinite Earth potential is reached)*

If Rtest is greater than acceptable, prove value with

4 wire test unit.

10 Transmission Tower Under Test

0 20 40 60 80 100

20 Tower Value 10.5

No basis for the test in standards – no objective

reference for the test results

Less effective for very “low” grounds: - Extraneous elements in reading become comparatively large.

There is no built-in proof for the method - results must

be accepted on “faith”.

The returns must be well clear of the Infinite Earth

Potential zone. This is the greatest cause of

testing result failure.

Must be aware of other grounds tied to the system,

that are in close proximity.

Clamp-On Method - Disadvantages

Recommendation. On a tower ground, 3 or 4 wire test

to determine distance to Infinite Earth Distance (note

the resistance verses distance measured value)

Assure that tower distance is at least 2X this distance.

Note that measured tower ground point values will

always be higher than actual.

If a tower is out of spec, 3 or 4 wire test to confirm if

rework is required.

Clamp-On Method - Strategy

Analogy – The length of a wire spool is to

be tested to determine length

A four wire test is analogous

to stretching the wire out, and

measuring. Know resistance

per foot, measure the

resistance.

The short cut measurement

method

Know – 1 per Foot.

Measurement is 40 ; therefore 40‟ correct?

Only item knows from test results is there is at least

40‟ of wire, that is my minimum spool wire length.

There maybe more.

Outcomes from the Clamp on

Tester

Measure 15 at a meter entrance. Spec calls for

25 . The service is 15 or greater – known.

Measure 45 at a service. We know the ground

is 45 or greater. Remedial action required

Measure 250 at a pole mount. Know there is

continuity, along with potential rework.

Applications & Limitations – Service

Entrance/Meter

Incorrect Reading

Grounding

Conductor

Ground

Rod Butt

Utility

Applications – Pole Grounds

Applications & Limitations – Pad Mount

Transformer Facility with Multiple Ground Points

Underground buried concentric

Neutral

Ground

MEASURING SOIL RESISTIVITY

* 3 Meter rod, 33 Ω; 3‟ foot rod ~ 109 Ω

Super Ground PISA style helix

with extensions

Variables in Grounding Quality

Temperature

Moisture

Ionification (Salt Adders)

Ground Rod Diameter

Ground Rod Depth

Number of Ground Rods

Ground Resistance of Farming Clay Loom

ρ = 100 Ω – M

3’ Ground Rod

106 Ohms

0 10 20 30 50 60 40

40,000

35,000

30,000

25,000

20,000

15,000

10,000

Variation of Soil Resistivity

with Temperature

Soil Contained 18.6% Moisture

Temperature - Degrees F

Variation of Soil

Resistivity with

Temperature

Liquid Water

SOIL MOISTURE VERSES CONDUCTIVITY

0 10 20 30 35 50 55 60 40 45

400,000

300,000

200,000

100,000

80,000

Variation of Soil Resistivity

with Moisture Content

Red Clay Soil

Per cent Moisture in Soil

60,000

40,000

20,000

5 15 25 65

Variation of Soil

Resistivity with

Moisture Content

MEASURING SOIL RESISTIVITY

Effect of salt content on soil resistivity:

MONTHS OF YEAR

5 6 7 8 9 10 11 12 2 1 3 4 5 6 7 8 9

Chemical treatment reduces seasonal variations

5/8 x 8’ ROD

(SOIL UNTREATED)

5/8 x 8’ ROD

(SOIL TREATED)

IMPROVING YOUR GROUNDING SYSTEM

Solution of Proposed Items to this Point

Warm Soil

Moist Soil

Salted Soil

Great for ground electron conduction, not so

great for metallic existence

Ground Rod Diameter Effects

Ground Rod Length Effects

Number of Ground Rods Effects

0.50 0.75 1.00 1.25 1.5 1.75 2.00

IMPROVING YOUR GROUNDING

SYSTEM

Doubling Rod diameter, decreases

resistance by only 10%

10% increase for four times the

material usage. Increase diameter

used for mechanical strength & rod

survivability (time)

DEPTH OF ROD, FEET

0 1 2 3 4 5 6 7 8 9 10 11

IMPROVING YOUR GROUNDING SYSTEM

What steps can be taken if there is a problem

in the grounding system

Use longer ground rods.

Chemically treat the soil

Use multiple ground rods.

Effects of Multiple Ground Rods

What causes a ground system to deteriorate

(and become ineffective)?

Corrosion and weather influences exert mechanical

strain on ground rods and cause metallic corrosion

over time (as a ground rod corrodes, its resistance

rises and it loses its effectiveness) - TILLAMOOK

soil resistivity can vary considerably with changes

in climate and temperature

Water Tables

Last Variable – The Macro Environment

Results depend on time of year of test; Worst

Case or not

(testing methods) 61.8% Rule Method Probe Placement: •Determine depth of ground electrode to be tested

•Distance of C > 4 x Depth of electrode to be tested

•Place P probe at 61.8% of the distance of C

•Take the measurement

Maximum Ground Resistance Targets

Typical values for a power company: - Generating station: 1 maximum

- Large sub-station: 1 maximum

- Small sub-station: 5 maximum

Water pipe ground should be less than 3 and

frequently less than 1 .

An electron really doesn‟t move to China from Ohio,

with zero resistance. It is deposited in a infinite pool

of available electrons, while an electron is picked

from the pool for service in China. There is a net

movement of free space electrons but it is <<

negligble.

Anchoring

Lazy Spikes

Unique Test Frequency (105 to 160 Hz)

IEEE Presentation Sep 11

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