substation grounding design challenges and lessons learned · institute of electrical and...

OVERVIEWDESIGN CHALLENGES

1. GROUND WELLS2. WINTER SOIL MODELING3. STEP VOLTAGE OUTSIDE THE FENCE4. RIGHT-OF-WAY COUNTERPOISE

LESSONS LEARNED1. SOIL RESISTIVITY TESTING2. CRUSHED ROCK RESISTIVITY TESTING3. GROUND ROD LENGTH SELECTION4. GROUND GRID TESTING

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GROUND WELLS

•WELLS USED TO REACH CONDUCTIVE DEEPER SOILS•BEDROCK DEPTH: 15’ – 40’•WATER TABLE DEPTH: 40’ AND BELOW 3

GROUND WELLS•6” STEEL CASINGS TO AUGER REFUSAL DEPTH•¾” X 10’ GROUND RODS INSTALLED TO 100’•FILLED WITH BENTONITE SLURRY•ROBUST DRILLING RIG TO DRILL THROUGH

BEDROCK

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WINTER SOIL MODELINGSUMMERSoil Layer Resistivity

(Ohm-m)Thickness

(feet)1 ≈50 ≈12 ≈100 ≈63 ≈30 ≈304 ≈50 Infinite

WINTERSoil Layer Resistivity

(Ohm-m)Thickness

(feet)1 ≈2000 ≈12 ≈1000 ≈23 ≈300 ≈24 ≈100 ≈25 ≈30 ≈306 ≈50 Infinite

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GENERAL SETTINGS WINTER CONDITIONS

WINTER SOIL: INPUT PARAMETERS

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SUMMER VS.

WINTER SOIL

MODELS

Summer Winter Summer WinterSoil

LayerResistivity Thickness Resistivity

Multiplier(Ohm-m) (feet)1 ≈50 ≈2000 ≈1 40

2-4 ≈100 ≈1000 ≈6 ≈2 10≈300 ≈2 3≈100 ≈2 1

5 ≈30 ≈30 16 ≈50 Infinite 1

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WINTER SOIL: AGGREGATE RESISTIVITYMoisture Condition

As-Received

Optimum / SSD

Saturated

Moisture Content (%)

≈0.5 ≈1.5 ≈3

Resistivity (Ohm-m)

≈12,000 ≈3,000 ≈800

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WINTER SOIL: TOUCH VOLTAGESSUMMER SOIL ≈ 40% SAFETY MARGIN WINTER SOIL ≈ 90% SAFETY MARGIN

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STEP VOLTAGE OUTSIDE THE FENCE

• CRUSHED ROCK ENDS5 FEET OUTSIDE FENCE

• STEP VOLTAGES ARE AT MAXIMUM LEVELS APPROX. 8 FEET OUTSIDE FENCE

• HIGHLY CONDUCTIVE TOP SOIL LAYER

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STEP VOLTAGE OUTSIDE THE FENCE –PROPOSED SOLUTION

• ADDED 2ND COUNTERPOISE AROUND 4 CORNERS OF FENCE, BURIED 4’ DEEP.

• EXTENDED CRUSHED ROCK 8’ OUTSIDE FENCE.

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STEP VOLTAGE OUTSIDE THE FENCEBEFORE AFTER (WITH 2ND COUNTERPOISE)

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RIGHT-OF-WAY COUNTERPOISE• COMMUNICATION TOWER SITE• LAND CONSTRAINTS• LIMITED ACCESS TO EXPANDING THE

GRID• USE UTILITY RIGHT-OF-WAY FOR

TRENCH GROUND

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SOIL RESISTIVITY TESTING• WENNER 4-PIN METHOD

• PREVENT INTERFERENCE:KEEP TEST LOCATIONS AWAY FROM

• OVERHEAD POWER LINES• UNDERGROUND PIPELINES• GROUND GRIDS• OTHER METALLIC OBJECTS (FENCES, RAILROADS)

• PIN SPACING: LENGTH OF SUBSTATION DIAGONAL

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SOIL RESISTIVITY

MEASUREMENT LOCATIONS

LOCATION 1

LOCATION 2

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RESISTIVITY OF VARIOUS SOIL TYPES

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SOIL RESISTIVITY MEASUREMENTSResistivity (ohm-m)

Spacing (ft) Location 1 Location 2 Ratio2 3,355 94.3 36 3 2,871 87.7 33 6 2,259 30.5 74

10 2,535 14.9 170 20 3,522 6.9 511 30 4,249 6.3 672 60 5,498 7.5 736

100 5,841 7.7 763 150 7,598 11.5 661 200 10,131 11.5 882 300 148,691 11.5 12,940

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CRUSHED ROCK RESISTIVITY TESTING• MINNESOTA SUBSTATION• PRE-CONSTRUCTION ROCK TEST RESULTS WERE

HIGHER THAN 3,000 OHM-M• AFTER GRID WAS INSTALLED, ROCK FROM A

DIFFERENT QUARRY WAS BROUGHT IN• INSTALLED ROCK WAS TESTED TO BE BELOW

3,000 OHM-M• GRID WAS REDESIGNED

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CRUSHED ROCK RESISTIVITY TESTING• MODIFIED MILLER BOX TEST• TESTING CONDITIONS

• AS RECEIVED• FULLY SATURATED• SATURATED SURFACE DRY (SSD)

• IS IT 3,000 OHM-M IN THE SSD CONDITION?• FINISH STONE SPECIFICATION

• SIZE: 1 ½” STONE• COLOR: DARK

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CRUSHED ROCK RESISTIVITY TESTING• CALIFORNIA SUBSTATION

• 3,000 OHM-M CRUSHED ROCK WAS DIFFICULT TO FIND.

• COMPACTED ASPHALT IS 10,000 OHM-M (WET)

• 2” LAYER OF COMPACTED ASPHALT COVER WAS USED AS A HIGHLY RESISTIVE REPLACEMENT FOR CRUSHED ROCK.

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GROUND ROD LENGTH SELECTION

•SOIL CHARACTERISTICS: SOFT•SOIL BORING DEPTH: 25’•GROUND ROD LENGTH: 32’

(FOUR 8’ SECTIONS)

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SOIL MODEL

Soil Layer

Resistivity (Ohm-m)

Thickness (feet)

1 ≈400 ≈52 ≈100 Infinite

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GROUND ROD

LENGTHS

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GROUND ROD LENGTHS27

GROUND GRID TESTING •FALL OF POTENTIAL METHOD•MEASURED RESISTANCE

SIGNIFICANTLY HIGHER THAN THE CALCULATED RESISTANCE

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ERROR RANGE BASED ON PROBE SPACING•LONGEST PIN SPACING:

•80 FEET.•GRID LENGTH:

•OVER 300 FEET.

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GROUND GRID TESTING • SOIL MODEL REVISED TO SIMULATE

SOIL WITH HIGHER RESISTIVITY• ALIGNED CALCULATED AND

MEASURED GRID RESISTANCE VALUES

• GROUND WELLS ADDED

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RECOMMENDATIONSDESIGN CHALLENGES

1. GROUND WELLS

2. WINTER SOIL MODELING

3. STEP VOLTAGE OUTSIDE THE FENCE

4. RIGHT-OF-WAY COUNTERPOISE

LESSONS LEARNED1. SOIL RESISTIVITY TESTING

2. CRUSHED ROCK RESISTIVITY TESTING

3. GROUND ROD LENGTH SELECTION

4. GROUND GRID TESTING

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REFERENCES• INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS, IEEE GUIDE FOR SAFETY IN AC SUBSTATION GROUNDING, IEEE STD 80-

2013, IEEE POWER AND ENERGY SOCIETY, NEW YORK, NY.• INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS, IEEE GUIDE FOR MEASURING EARTH RESISTIVITY, GROUND IMPEDANCE,

AND EARTH SURFACE POTENTIALS OF A GROUNDING SYSTEM, IEEE STD 81-2012, IEEE POWER AND ENERGY SOCIETY, NEW YORK, NY.• INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS, IEEE RECOMMENDED PRACTICE FOR GROUNDING OF INDUSTRIAL AND

COMMERCIAL POWER SYSTEMS, IEEE STD 142-2007, IEEE POWER AND ENERGY SOCIETY, NEW YORK, NY.• CURRENT DISTRIBUTION ELECTROMAGNETIC INTERFERENCE GROUNDING AND SOIL STRUCTURE ANALYSIS, CDEGS APPLICATION

VERSION 14.4.7651.1, SESTECH INSTALLATION VERSION 15.4.8190.• “IDENTIFYING AND QUANTIFYING MATERIAL PROPERTIES THAT IMPACT AGGREGATE RESISTIVITY OF ELECTRICAL SUBSTATION

SURFACE MATERIAL” J. EDLEBECK AND B. BESKE, IEEE TRANSACTIONS ON POWER DELIVERY, VOLUME: 29, ISSUE: 5, OCT. 2014.

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PHOTO / VIDEO REFERENCES• HTTP://WWW.STEVENSONRESOURCES.CO.NZ/PROJECTS/INFRASTRUCTURE/TRANSPOWER-SWITCHYARD/

• HTTPS://WWW.GLOBALTESTSUPPLY.COM/CONTENT/AEMC-UNDERSTANDING-SOIL-RESISTIVITY-TESTING.HTML

• HTTPS://IAEIMAGAZINE.ORG/MAGAZINE/2018/07/30/THE-5-FT-GROUND-ROD-AND-ITS-LITTLE-KNOWN-USE-IN-THE-NEC/

• HTTPS://WWW.HAMMONDFARMS.COM/PRODUCTS/SANDS-AND-GRAVELS/CRUSHED-ASPHALT-MILLINGS/

• HTTP://ELECTRIALSTANDARDS.BLOGSPOT.COM/2014/07/EARTH-PIT-RESISTANCE-MEASUREMENT-METHOD.HTML

• HTTPS://MOBA-AUTOMATION.COM/MACHINE-APPLICATIONS/COMPACTORS/MCA-3000/

• HTTPS://WWW.YOUTUBE.COM/WATCH?V=TU5C4BARIKS

• HTTPS://WWW.YOUTUBE.COM/WATCH?V=XFZGV70O6DU

• HTTPS://WWW.YOUTUBE.COM/WATCH?V=CBLIEM_1MC4

• HTTPS://WWW.YOUTUBE.COM/WATCH?V=VDHDZZZ0JKQ

• CURRENT DISTRIBUTION ELECTROMAGNETIC INTERFERENCE GROUNDING AND SOIL STRUCTURE ANALYSIS, CDEGS APPLICATION VERSION 14.4.7651.1, SESTECH INSTALLATION VERSION 15.4.8190.

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