This document discusses ground grid systems used in substations. Ground grids are used to carry and dissipate currents from events like lightning strikes and faults. They help ensure human and animal safety and allow protective relays to operate properly. Designing an effective ground grid involves analyzing soil resistivity, sizing the grid area, determining fault currents, and using methods like the IEEE or finite element approaches to verify touch and step voltages are within limits. Key steps include soil testing, sizing conductors, placing rods, and iterating the design to meet safety criteria.

1. ©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Ground Grid Systems

2. Slide 2©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Need 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

3. Slide 3©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Objectives
• Human and animal safety
• Carry and dissipate current into earth under
normal and fault conditions
• Grounding for lightning impulses and surges
• Low resistance to ground for protective
relays

4. Slide 4©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Construction

5. Slide 5©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Common Definitions
• Earth Current
• Ground Fault Current
• Ground Potential Rise
• Step Voltage
• Touch Voltage

6. Slide 6©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Step 1 – Soil Analysis
• Done at a number of places in the substation
• Several layers with different resistivity
• Lateral surface changes are more gradual
than vertical changes
• Wenner Four-Pin Method

7. Slide 7©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Wenner Four-Pin Method
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8. Slide 8©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Step 2 – Grid Area
• Area should be as large as possible
• Increasing area is more effective than
adding additional conductor to reduce grid
resistance
• Outer conductor should be placed on the
boundary of substation
• Fence should be placed a minimum of 3 feet
inside
• Square, rectangular, triangular, T-shaped, or
L-shaped grids

9. Slide 9©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Step 3 – Ground Fault
Currents
• L-G fault on substation bus or transmission
line
• Interested in maximum amount of fault
current expected to flow into the ground grid
• Determine maximum symmetrical rms fault
current

10. Slide 10©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Ground Fault Current

11. Slide 11©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Ground Fault Current

12. Slide 12©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Ground Fault Current

13. Slide 13©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Ground Fault Current

14. Slide 14©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Symmetrical Grid Current
• Io = Symmetrical rms value of Zero
Sequence fault current in amperes
• Transmission Systems – Model Maximum Io
for L-G fault for present and ultimate
configuration
• Distribution Systems – Model future fault
current with suitable growth factor (1.1)
)3(* ofg ISI

15. Slide 15©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Decrement Factor
• Accounts for the asymmetrical fault current
• AC component does not decay with time but
remains at its initial value
• Calculated from time duration of fault and X
over R ratio
• Transmission Systems – Use fastest
clearing relay + breaker time
• Distribution and Industrial Systems – Use
worst case backup clearing time

16. Slide 16©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Typical Shock Situations

17. Slide 17©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Design Procedure Summary
• Use network of bare conductors buried in the
earth
• Encompass all area within the substation
fence 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

18. Slide 18©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Design Procedure Summary
• Size conductors
• Conductor should be buried a minimum of
18 inches to 59.1 inches
• Vertical ground rods should be at least 8 ft.
long
• Determine if touch and step voltages are
below tolerable limits
• Few iterations may be required to determine
correct grid design

19. Slide 19©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Ground 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

20. Slide 20©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
IEEE Methods
• Empirical method; limited applications
• Handles 2 layers plus protective surface
material (1 layer for touch potential)
• Rectangular and triangular shapes only, with
vertical and horizontal conductors
• One ground grid only
• Rods; but arrangements are not flexible
• Calculates required parameters

21. Slide 21©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Finite Element Method
• Handles 2 layers plus a protective surface
material
• Any shape
• Multiple interconnected ground grids
• Rod location modeled in detail
• Calculates required parameters at all points
• Graphic potential profile

22. Slide 22©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Typical IEEE Grid

23. Slide 23©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
IEEE Grid Description
• 40 ft. X 40 ft. square grid with 8 conductors
along 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

24. Slide 24©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
FEM Grid Example

25. Slide 25©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Step Potential Profile

26. Slide 26©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Touch Potential Profile

27. Slide 27©1996-2010 ETAP/Operation Technology, Inc. – Workshop Notes: Ground Grid Systems
Absolute Potential Profile