Existing Grounding Systems Rob Schaerer

7/28/2019 Existing Grounding Systems Rob Schaerer

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April 7, 2011

Substation Grounding Systems

, . .


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Outline of Topics

Why grounding is important

What are we looking at

Basic grounding system design process

Considerations for existing substations

Previous analysis

Verification of previous study and data

Mitigation

Testin

Maintenance Plans


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Personnel Protection

IEEE 80-2000

Provides guidance onlimits based onscenaros presen ein a substation

environment and

body when subjectedto an electric shock

Additional concernsinclude equipment

fault conditions)


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Personnel Protection Voltages

Touch/ste volta es

Touch voltage

ground at your feet

Typically limited to a reach distance of three feet (or one meter)

Step voltage

Voltage difference in ground between your feet as you are standing

Typically limited to a stride of three feet (or one meter)


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Personnel Protection Voltages

SLG Fault SLG Fault

200Volts 100Volts

Voltageat Foot

Voltage1000 V

Voltageat Foot

Voltageat Foot

Grid1000 V

800 V

Grid1000 V

800 V900 V


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Behavior of Substation Under Fault Conditions

Ground Potential Rise

V = I * R

Fault current into the grounding system times resistance toremo e ear

Voltage magnitude determines grounding system performance

rmar y use or personne comp ance

Can also damage equipment

Basis for determinin touch and ste volta es

Fault Current Return Path


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Behavior of Substation Under Fault Conditions

Ground Potential Rise (GPR) = I * R


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Process of Designing a Grounding System

Gather and analyze soil data

Obtain fault data Develop preliminary grounding system design

Analyze design for touch and step voltage performance,

plus impedance and GPR Perform mitigation until touch and step voltage

compliance are met

a ona equpmen spec c groun ng Test the installed grounding system to verify performance

eexamne e groun ng sys em n e u ure


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Considerations for Existing Substations

Was an analysis ever preformed?

Many older substations were built on rules of thumb

If not, most practical approach is to analyze before acting

Is the previous analysis still valid?

Has the system changed? Did the study use accurate data?

Testing to validate performance

Maintenance plans


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Examining Previous Analysis - Fault

Fault currents

Maximum single-line-to-ground fault

Often increases with time as the system strengthens

,

proportionally

Clearing time (backup)

Protection failure

Consider worst case scenario (longest clearing delay time) anduse for grounding analysis


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Examining Previous Analysis - Soil

Soil information has large impact on overall result, but is

Soil data often collected by geotechnical or other firms that havelimited understanding of how data is used for grounding analysis

Measurements are often insignificant (not enough data measured)

Data collection process often produces errors that may not beexpected by experienced engineers or testers

Examining the raw data can help validate the measurements

Analysis of soil data measurements is both an art and a science Older techniques often involved a uniform soil approximation, or

sometimes a two layer model that may be insufficient


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Soil Resistivity Tests

Characterize soil by the electrical resistivity

v y y u

system for a specific performance objective All soil conducts electrical current

Some soils have good electrical conductivity while themajority has poor electrical conductivity

Varies widely throughout the country and world Can changes dramatically within small areas

Soil resistivity is mainly influenced by:

The type of soil (clay, sand, rock, etc.) Moisture content and temperature

Amount of electrol tes (minerals and dissolved salt)


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Soil Testing Resistivity Test Set


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Wenner Resistivity Test Set-Up

Source

Black lines are current injectedRed lines are volta es measured

As the probes are spread out further, the deeper the measurements will go


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Why Soil Data Is So Important

Example Full Data ubstation ize is 300 by 300


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Why Soil Data Is So Important (cont.)

Soil model with all data:103

s)

Measured DataComputed Results CurveSoil Model

ty(O

hm-meter Measurement Met hod. . : Wenner

RMS er r or . . . . . . . . . . . : 10. 46%

Layer Resi st i vi t y Thi cknessNumber ( Ohm- m) ( Feet )====== ============== ==============

Ai r I nf i ni t e I nf i ni t e2 227. 3146 16. 94758

102

arentResistivi . .

4 515. 6556 i nf i ni t e

Ap

Grounding System Impedance is 1.07 ohms

10-1

100

101

102

103

Inter-Electrode Spacing (feet)


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Why Soil Data Is So Important (cont.)

Soil model with only the first 50 of measurements:

103

s)

LEGEND

Measured DataComputed Results CurveSoil Model

ty(O

hm-mete Measur ement Met hod. . : Wenner

RMS er r or . . . . . . . . . . . : 8. 26%

Layer Resi st i vi t y Thi cknessNumber ( Ohm- m) ( Feet )

====== ============== ==============Ai r I nf i ni t e I nf i ni t e2 209. 4786 25. 52040

102

arentResistivi

3 15. 40446 i nf i ni t e

Ap

Grounding Impedance is 0.28 ohms (~1/4 of actual)

10-1 100 101 102

Inter-Electrode Spacing (feet)

10


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Examining Previous Analysis - Conductor

Conductor size and ampacity

For a given fault duration and X/R ratio, grounding conductor

can only carry a given amount of fault current without fusing

#1/0 AWG)

Lar est concern is the e ui ment leads stin ers

One may carry full fault current

Once the current is in the main grid, the current splits in multiple


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Examining Previous Analysis - Conductor

From IEEE Standard 80-2000


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Examining Previous Analysis - Surfacing

Crushed rock surfacing

Adds additional impedance to current flowing through body

Increases allowable touch and step voltages

SLG Fault


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Examining Previous Analysis - Surfacing

Crushed rock surfacing, cont.

Also provides clean surface for preventing vegetation, etc.

Washing the material of fines improves performance

Typically a crushed rock or gravel (3/4 2)

Thickness of 3-6 inches is typical Should extend beyond the substation fence and gate swings

Must be maintained over time to keep free of contamination,


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Mitigation of Non Compliant Designs

Basic design approaches

Check fault current distribution Optimal selection of mitigation approaches

Horizontal ground conductors

Ground rods

Surfacing improvements


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Grounding Design Basics

Grounding layout basics

Entire substation area should be encompassed

Minimize resistance of system (proportional to area of groundingsystem)

Layout should extend 3 feet beyond substation fence, includingoutward swing of gates


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Grounding Design Basics (cont.)

Conductor Spacing/Layout

Typically laid out in a square grid covering station Typical spacings vary from 10 to 50

Depends on soil, fault current, station size

Large areas without equipment can be left uncovered if thereare no ste volta e issues

A denser grid towards outside of substation is more effective

Worst case touch voltages occur at the corners of the grid


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Grounding Design Basics (cont.)


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Fault Current Distribution (FCD)

Most conservative case is to assume all current

Not a practical representation in many cases

Faut current w ta e any pat ava a e

Transmission line shield wires

Distribution neutrals Other metallic paths tied to grounding system

These other paths are in parallel with the ground

In turn, lowers the substation GPR

Effects are most significant where poor soil exists at thesubstation (resulting in a high grounding systemimpedance)


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Fault Current Distribution Example

Fault current returns through all

Both directly on shield wires andthrough tower grounds

TransmissionLineswithShieldWireFaultedSubstation

e urns o sourcesubstation(s)

SourceSubstation


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Optimal Mitigation Selection

Ground rods vs. grid vs. ground wells

Primary goal is low impedance, therefore mitigation shouldtarget the lower resistivity soil

Horizontal ground grid

Works well when lower

layers are higher

Install most copper inupper, low resistivity,la e

Keeps surface closer toequipotential

Trenching around existingequipment difficult attimes


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Existing Yard Installation


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Optimal Mitigation Selection(cont.)

Ground rods vs. grid vs. ground wellscon .

Ground rods

os e ec ve w en op ayer s g erresistivity and fairly lower resistivity layers arebelow

Can be useful where water table is < 20 deep

Can also extend effective size of substationand pull touch voltages down

Typically should not be placed closer togetherthan length of rod as effectiveness decreases

Also can be difficult to install around existingequipment


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Optimal Mitigation Selection(cont.)

Ground rods vs. grid vs. ground wells (cont.)

Ground wells

Most expensive option

Involves drilling a hole (typically 6) to a significant depth (can varyfrom 50-500+ feet)

May use a steel casing or be free standing (in stable/firm soils)

, ,

concrete/bentonite slurry)

Typically installed near the edges of the substation, away fromu


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Optimal Mitigation Selection - Surfacing

Crushed rock can be added if not alread resent

Existing rock can be washed or thickness increased

Asphalt is occasionally used

Provides much better electrical performance


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Testing

Fall-of-Potential (FOP)

Measured grounding system impedance

Touch and Ste Volta e Checks

Measured grounding system impedance

Point-to-Point Resistance Tests Check continuity of conductors

Validate data if in question


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Testing - Grounding System Impedance

Fall-of-Potential (FOP)

Measures resistance of grounding system after installation Inject a current into grid and collect in remote current return probe

placed at 3-6.5x the system diagonal (6.5x preferred)

Voltage probe distance varied from 10% - 100% of current probe

distance

Resistance (V/I) of each point plotted versus distance

Curve flattens around 61.8% (demonstrating grounding system

resistance) with a 0 degree test Point varies based on soil structure (Standard 81-1983)


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Fall-of-Potential Test Set-Up

Substation Grounding System Impedance

6

4

5

(OHMS

1

2

3

RESIST

ANC

0

0 500 1000 1500 2000 2500 3000 3500 4000

PROBE SPACING


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Testing Touch and Step Voltages

Touch and step voltage check

Performed rarely, generally requires injection of significant testcurrent to provide reasonable voltage levels

Can be done in conjunction with FOP test

Inject a current similar to the FOP test

For touch voltages, measure voltage between any equipment ofconcern and a test probe placed just into the soil surface 3 feet fromthe equipment

For step voltages, measure the voltage between any two points inthe substation with probes placed just into the soil, separated bythree feet


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Testing Connectivity

Point-to-point resistance check

Used to verify that all equipment is attached solidly to the maingrounding system

Select a reference point (often a piece of equipment with multiplegrounds) and measure resistance between all grounded objects

and the reference

Since resistance is primarily of the lead, value should be verylow (less than one ohm)

If resistance is very high, a second equipment connection can beadded to the main grid, or the existing can be replaced.


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Maintenance of Existing Grounding

Maintenance of grounding systems after installation isoften overlooked

In 2005, a IEEE PES task force surveyed utilities

Key Findings/Recommendations

~80% evaluate the grounding systems after they are installed os o s examna on ony occurs a er a pro em appens, or

when expanding the substation

By proactively examining the grounding system (which very few do),many of these problems could be prevented in the first place

Recommends a review of the grounding system regularly Those who have a plan perform it every five to ten years, and more

often should there be concerns

n erva o es ng a ec e y age o e groun ng sys em ancharacteristics of the soil (for example very low resistivity soils

can be corrosive and degrade buried conductors over time)


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Maintenance Plan

Typical plan involves:

Visual inspection of all above grade connections

Point-to-point resistance test

Fall-of-potential grounding system impedance test

Surface layer visual inspection Thickness and cleanliness (resistivity test if needed)

Reanalysis of design when significant system changes occur


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Example Unique Substation


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Example Unique Substation


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Example Soil Resistivity Test


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Example Point-to-Point Test


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Example Point-to-Point Test

GROUND SYSTEM LAYOUT

39 49 56

34 36 40 42 44 45 51 53 57 59

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50 30 2232 24 18 15

31 23

25 19 16

29 26 20 17

2813,R3

14 21

12

11

8,

R27 6

5 4 3 2,

R1


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Conclusions

Substation grounding is critical for protection ofpersonnel and equipment

Some older substations were built with little analysisand/or data

Grounding can degrade over time

Systems (fault current) change over time

the grounding system continues to serve its purpose

Q i /C


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Questions/Comments

Rob Schaerer, P.E. POWER Engineers, Inc. (858) 503-5975 ext. 2237 [email protected]

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Existing Grounding Systems Rob Schaerer