Giancarlo Leone.pdf

Substation Grounding 101

Giancarlo “GC” LeoneProtection and Controls Dept. Manager

Stanley Consultants, Inc.

•What?•Why?•Basic Concepts•Case Study 1•Case Study 2 (time allowing)

Agenda

What is Grounding?

Grounding is the art of making an electrical connection to earth.

Why is it Needed?

IEEE‐80‐2000 states that:“a safe grounding design has the following two objectives:‐To provide means to carry electric currents into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service.‐To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock.”

Why is it Needed?

• Discharge currents into the earth.

• Lower the grounding resistance.

Equipment and

Personnel Safety

Goals for Effective Grounding Design:

Reduce the Ground Potential Rise (GPR)

Reduce the Ground Potential Difference (GPD)

Reduce Touch and Step Voltages Below Safe

Thresholds

So, what are GPR and GPD?

GPR = IG*Rg

What are Step and Touch Voltages?

IEEE‐80‐2000 defines Step Voltage as: “The difference in surface potential experienced by a person bridging a distance of 1 m with the feet without contacting

any other grounded object.”

IEEE‐80‐2000 defines Touch Voltage as:“The potential difference between the ground potential rise (GPR) and the surface potential at the point where a person is standing while at the same time having a hand in contact with

a grounded structure.”

What Major Factors Affect Grounding System Performance?

System network (e.g., transformer connections, shield & neutral wires,

etc.)

Fault current (SLG or DLG) & X/R

Surface material resistivityFault duration

Grounding system area and geometryLocal soil resistivity

Grounding System Design Steps

Grounding Grid Design

Soil model

Current division factor (CDF)

Fault magnitude, X/R, and clearing

time

Validation and interpretation of soil testing results

Soil testing

Soil Testing

Some Testing Methods

Wenner 4‐pin Method

Schlumberger 4‐pin Method

Unipolar Wenner Method

Soil Testing

Wenner 4‐pin Method

Same pin spacing “a” at each interaction

Preferred for the design of short

horizontal or vertical electrodes

Typically used in the power industry.

Soil Model Development

A two‐layer soil model is generally sufficient for modeling substation and transmission line grounding systems.

Exercise caution when interpreting soil measurement values!

Fault Current

•Magnitude often provided by the planning department.• It should account for future system growth.

• Clearing time often provided by protection/relay department.

Fault Magnitude

and Duration

Fault Current

• Ratio of the system reactance to resistance.

• It is indicative of the rate of decay of any DC offset.

• A large X/R ratio corresponds to a large time constant and a slow rate of decay.

X/R

Current Division Factor (CDF)

It is common practice to use the total fault current as the current discharged by the grounding system. This approach might lead to uneconomical and overdesigned grounding systems.

In most cases, alternative paths exist, so portion of the fault current will flow back to the remote source(s) through:

• Shield wires• Neutral conductors• Other metallic paths connected to the grounding system

Current Division Factor (CDF)

CDFNumber of

transmission lines with shield wires tied to the ground grid

Substation grounding resistance

Number of power source and non‐power source terminals

Soil resistivity

Self‐impedance of shield wire

Tower footing resistance.

Mutual impedance between faulted

phase conductor & shield wires

Substation Grounding Design StepsSoil

TestingSafety Criteria (Step & Touch)

Initial Design

Grid Resistance(Rg)

Current Division

Grid Current (Ig)

GPR=Ig*Rg

Safety criteria met?

ModifyDesign

Refine Design

MeshDesign(iterative)

0

100

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800

0 20 40 60 80 100 120 140 160

Volts S1

GPR S1


0

100

200

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500

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800

0 20 40 60 80 100 120 140 160

Volts S4

GPR S4


0

100

200

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500

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0 20 40 60 80 100 120 140 160

Volts S16

GPR S16


0

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0 20 40 60 80 100 120 140 160

Volts S64

GPR S64

FictitiousSubstation

Grounding Design Case 1

115 kV Total SLG Fault

Magnitude = 10 kA

DesignCriteria

SLG Fault X/R = 10

Three T‐Lines with Shields Tied to Grid

Fault Clearing Time = 0.5 seconds

Two‐Layer Soil Model: 250 ohm‐m (4 feet); 25

ohm‐m (infinite)

Grid Dimension

= 250’x250’

Surface Layer 3” of

3,000 ohm‐m

4/0 AWG Stranded Copper

Conductor

Burial Depth = 18 inches

GroundingDesignCase1‐ Iteration#1

TouchVoltage

250’x250’PerimeterGrid4EquallySpacedMeshesConductorlinearlength=2,500’

Rg (Ω)

GPR (V) CDF

GridCompression

Factor

0.452 4,518 0 1.0

StepVoltage*



Rg (Ω)

GPR (V) CDF

GridCompression

Factor

0.452 1,807 0.6 1.0

TouchVoltage StepVoltage*


250’x250’PerimeterGrid4EquallySpacedMeshesConductorlinearlength=2,500’25‐10’Rods

Rg (Ω)

GPR (V) CDF

GridCompression

Factor

0.304 3,039 0 1.0TouchVoltage StepVoltage*



Rg (Ω)

GPR (V) CDF

GridCompression

Factor

0.304 1,824 0.4 1.0TouchVoltage StepVoltage*



Rg (Ω)

GPR (V) CDF

GridCompression

Factor




Rg (Ω)

GPR (V) CDF

GridCompression

Factor




Rg (Ω)

GPR (V) CDF

GridCompression

Factor



250’x250’PerimeterGridConductorlinearlength=4,500’81‐10’Rods

Rg (Ω) GPR (V) CDF

GridCompression

Factor


InConclusion:

• ReducegridresistancetoreduceGPR.

• ReducegridspacingtoreducelargeGPDgradients.

• Beawareoffaultmagnitude/durationandcrushedrockonvoltagethresholds.

• Utilizeadvancedtechniquestomoreaccuratelyaccountfortruecurrentdistribution.

Grounding Design Case 2

Expansionof138/46kVSubstation

(timeallowing)

70Existing Substation Grounding Plan

46 kV Yard46 kV Yard

138 kV Yard138 kV Yard

NewEquipment & Expansion

Area

NewEquipment & Expansion

Area

Cap BankCap Bank

138/46 kV

XMFR

138/46 kV

XMFR

Shunt Cap Bank

Shunt Cap BankControl

BuildingControl Building

138 kV Total SLG Fault

Magnitude = 8.5 kA

SLG Fault X/R = 6.77

Existing & New

Ground Rods = 8’‐5/8” Ø

Fault Clearing Time = 0.2 seconds

No extra resistance by means of shoes or

gloves.

Existing conductor = 250 MCM

Surface Layer 4” of

3,000 ohm‐m.

New conductor = 4/0 AWG

Burial Depth = 12 inches

Design Criteria

Initial DesignExistingDesign InitialDesign#1

Safety Criteria

Ground Resistance

Rg = 2.26 Ω

CDF Calculation 138 kV Fault

2 Ω

~40% Currentingrid≈40%perIEEE80graphicalmethod*fordeterminingcurrentdivision.

Grid Current & GPR

Ig = 8,500A*0.5 = 4,250A

• Extra10%wasaddedforconservatism.

• TotalCurrentingrid≈50%

GPR = 4,250 A*2.26 Ω = 9,605 V

TouchVoltage– CrushedRock

``

`

` `

StepVoltage– NoCrushedRock

`

Do you think the 138 kV fault is the

worst case?

Let’s take a look at a 46 kV fault outside the sub as well.

138 kV Fault• Fault current = 8,500 A

• X/R = 6.7633• Clearing Time = 0.2 seconds• 138 kV transmission lines shield wires are electrically connected to the substation ground, so some CDF can be used.

46 kV Fault• Fault current = 7,214 A (500 feet away from the substation)

• X/R = 4.68• Clearing Time = 0.44 seconds• The 46 kV transmission lines do not use shield wires, so CDF = 0

Auto Transformer Current Distribution

138 kV Fault• Current in Grid = 4,250 A• X/R = 6.7633• Clearing Time = 0.2 seconds• CDF = 0.5

46 kV Fault• Current in Grid = 5,648.6 A• X/R = 4.68• Clearing Time = 0.44 seconds• CDF = 0By inspection, this is

clearly worse!!

Additional changes to the design:

The grounding of the first 138 kV transmission tower is removed. The ground conductors connecting the substation to the tower grounding could not be verified by the contractor.

Modify DesignInitialDesign#2InitialDesign#1

Safety Criteria

Ground Resistance

Rg = 2.17 Ω

Grid Current & GPR

Ig = 5,268 A

• TotalCurrentinGrid=100%

GPR = 5,268 A*2.17 Ω = 12,257 V

TouchVoltage– CrushedRock

There is no grounded, contactable equipment in the yellow areas.

StepVoltage– NoCrushedRock

References

[1] W. Ruan, R.D. Southey, S. Fortin and F.P. Dawalibi, "Effective Sounding Depths for HVDC Grounding electrode Design: Wenner versus Schlumberger Methods", IEEE/PES T&D 2005 Asia Pacific, Dalian, China, August 14 ‐ 18, 2005

[2] C. Li, X. Wei, Y. Li and F. P. Dawalibi, "A Parametric Analysis of Fault Current Division between Overhead Wires and Substation Grounding Systems", Proceedings of the Seventh IASTED International Conference on Power and Energy Systems, Clearwater Beach, FL, USA, November 28 ‐ December 1, 2004.

[3] IEEE Guide for Safety in AC Substation Grounding, ANSI/IEEE Std 80‐2000.

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Giancarlo Leone.pdf