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Copyright 2003 Operation Technology, Inc.
Ground Grid SystemsGround Grid Systems
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 2
Need for Grounding GridsNeed for Grounding Grids
• Currents flow into the grounding grid from:
– Lightning Arrester Operations
– Switching Surge Flashover of Insulators
– Line-Ground Fault from Connected Bus
– Line-Ground Fault from Connected Line
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 3
ObjectivesObjectives
• Human and animal safety
• Carry and dissipate current into earth undernormal and fault conditions
• Grounding for lightning impulses and surges
• Low resistance to ground for protectiverelays
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 4
ConstructionConstruction
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 5
Common Definitions
• Earth Current
• Ground Fault Current
• Ground Potential Rise
• Step Voltage
• Touch Voltage
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 6
Step 1Step 1 –– Soil AnalysisSoil Analysis
• Done at a number of places in the substation
• Several layers with different resistivity
• Lateral surface changes are more gradualthan vertical changes
• Wenner Four-Pin Method
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 7
Wenner FourWenner Four--Pin MethodPin Method
2222 421
4
baa
baa
aRa
+−
++
= πρ
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 8
Step 2Step 2 –– Grid AreaGrid Area• Area should be as large as possible• Increasing area is more effective than
adding additional conductor to reduce gridresistance
• Outer conductor should be placed on theboundary of substation
• Fence should be placed a minimum of 3 feetinside
• Square, rectangular, triangular, T-shaped, orL-shaped grids
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 9
Step 3Step 3 –– Ground FaultGround FaultCurrentsCurrents• L-G fault on substation bus or transmission
line
• Interested in maximum amount of faultcurrent expected to flow into the ground grid
• Determine maximum symmetrical rms faultcurrent
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 10
Ground Fault CurrentGround Fault Current
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 11
Ground Fault CurrentGround Fault Current
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 12
Ground Fault CurrentGround Fault Current
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 13
Ground Fault CurrentGround Fault Current
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 14
Symmetrical Grid CurrentSymmetrical Grid Current
• Io = Symmetrical rms value of ZeroSequence fault current in amperes
• Transmission Systems – Model Maximum Iofor L-G fault for present and ultimateconfiguration
• Distribution Systems – Model future faultcurrent with suitable growth factor (1.1)
)3(* ofg ISI =
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 15
Decrement FactorDecrement Factor• Accounts for the asymmetrical fault current
• AC component does not decay with time butremains at its initial value
• Calculated from time duration of fault and Xover R ratio
• Transmission Systems – Use fastestclearing relay + breaker time
• Distribution and Industrial Systems – Useworst case backup clearing time
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 16
Typical Shock SituationsTypical Shock Situations
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 17
Design Procedure Summary
• Use network of bare conductors buried in theearth
• Encompass all area within the substationfence and extend at least 3 feet outside
• Perform soil resistivity test
• Surface material at least 4 inches
• Determine fault current using short circuit
• Determine maximum clearing time
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 18
Design Procedure SummaryDesign Procedure Summary• Size conductors
• Conductor should be buried a minimum of18 inches to 59.1 inches
• Vertical ground rods should be at least 8 ft.long
• Determine if touch and step voltages arebelow tolerable limits
• Few iterations may be required to determinecorrect grid design
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 19
Ground Rod LengthGround Rod Length• Three schools of thought
– Length of 10ft is adequate
– Length of 40ft is required to reach water table
– Longest possible rod depth should be used
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 20
IEEE MethodsIEEE Methods• Empirical method; limited applications
• Handles 2 layers plus protective surfacematerial (1 layer for touch potential)
• Rectangular and triangular shapes only, withvertical and horizontal conductors
• One ground grid only
• Rods; but arrangements are not flexible
• Calculates required parameters
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 21
Finite Element Method
• Handles 2 layers plus a protective surfacematerial
• Any shape
• Multiple interconnected ground grids
• Rod location modeled in detail
• Calculates required parameters at all points
• Graphic potential profile
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 22
Typical IEEE Grid
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 23
IEEE Grid Description
• 40 ft. X 40 ft. square grid with 8 conductorsalong X-axis and 8 conductors along Y-axis
• Depth = 1.5 ft., 4/0 copper-clad steel wire
• 1 rod in each grid corner, diameter = 0.5 in.,length = 8 ft. same material as conductor
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 24
FEM Grid Example
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 25
Step Potential Profile
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 26
Touch Potential Profile
Copyright 2003 Operation Technology, Inc. – Workshop Notes: Ground Grid Systems Slide 27
Absolute Potential Profile
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