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
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0 20 40 60 80 100 120 140 160
Volts S1
GPR S1
MeshDesign(iterative)
0
100
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0 20 40 60 80 100 120 140 160
Volts S4
GPR S4
MeshDesign(iterative)
0
100
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0 20 40 60 80 100 120 140 160
Volts S16
GPR S16
MeshDesign(iterative)
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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*
GroundingDesignCase1‐ Iteration#1
250’x250’PerimeterGrid4EquallySpacedMeshesConductorlinearlength=2,500’
Rg (Ω)
GPR (V) CDF
GridCompression
Factor
0.452 1,807 0.6 1.0
TouchVoltage StepVoltage*
GroundingDesignCase1‐ Iteration#2
250’x250’PerimeterGrid4EquallySpacedMeshesConductorlinearlength=2,500’25‐10’Rods
Rg (Ω)
GPR (V) CDF
GridCompression
Factor
0.304 3,039 0 1.0TouchVoltage StepVoltage*
GroundingDesignCase1‐ Iteration#2
250’x250’PerimeterGrid4EquallySpacedMeshesConductorlinearlength=2,500’25‐10’Rods
Rg (Ω)
GPR (V) CDF
GridCompression
Factor
0.304 1,824 0.4 1.0TouchVoltage StepVoltage*
GroundingDesignCase1‐ Iteration#3
250’x250’PerimeterGrid8EquallySpacedMeshesConductorlinearlength=4,500’
Rg (Ω)
GPR (V) CDF
GridCompression
Factor
0.316 3,160 0 1.0TouchVoltage StepVoltage*
GroundingDesignCase1‐ Iteration#4
250’x250’PerimeterGrid8EquallySpacedMeshesConductorlinearlength=4,500’81‐10’Rods
Rg (Ω)
GPR (V) CDF
GridCompression
Factor
0.203 2,031 0 1.0TouchVoltage StepVoltage*
GroundingDesignCase1‐ Iteration#4
250’x250’PerimeterGrid8EquallySpacedMeshesConductorlinearlength=4,500’81‐10’Rods
Rg (Ω)
GPR (V) CDF
GridCompression
Factor
0.203 1,625 0.2 1.0TouchVoltage StepVoltage*
GroundingDesignCase1‐ Iteration#5
250’x250’PerimeterGridConductorlinearlength=4,500’81‐10’Rods
Rg (Ω) GPR (V) CDF
GridCompression
Factor
0.2004 1,604 0.2 0.7TouchVoltage StepVoltage*
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
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StepVoltage– NoCrushedRock
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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.