π Case Study
Substation Design: Ground Grid for 345kV GIS Switchyard with High Fault Current
120 kA asymmetrical fault current creating hazardous step potentials (>5 kV) across gravel-surfaced yard despite existing grid
ποΈ Project Overview
Transmission substation upgrade in Ohio connecting wind generation
π― Challenge
120 kA asymmetrical fault current creating hazardous step potentials (>5 kV) across gravel-surfaced yard despite existing grid
π§ Design Approach
Reinforced 12Γ12 m mesh grid with 70 mmΒ² bare copper, 0.5 m burial depth, 3 m perimeter conductor, and 32 vertical rods; integrated with station fence grounding
π Key Calculations
Mesh Voltage
Em = (Ο Γ Kβ Γ Kα΅’ Γ Iβ) / (Lβ Γ βt)
Result: 4.21 kV
Below 4.5 kV IEEE 80-2013 limit for 0.5 s clearing
Ground Grid Resistance
R = Ο Γ (1/Lβ + 1/(20βA))
Result: 0.63 Ξ©
Enables reliable relay operation and limits GPR to <12 kV
π Results
Step voltage reduced from 6.8 kV to 3.1 kV; achieved full compliance with NERC PRC-026-2 and FERC Order 888π‘ Lessons Learned
- β’GIS enclosure grounding must be bonded at *every* flangeβnot just at cable entries
- β’Gravel resistivity must be modeled as top layer (not assumed infinite)
- β’Thermal withstand of grid conductors verified via IEEE Std 80 Annex D iterative simulation
β Key Takeaways
- 1GIS enclosure grounding must be bonded at *every* flangeβnot just at cable entries
- 2Gravel resistivity must be modeled as top layer (not assumed infinite)
- 3Thermal withstand of grid conductors verified via IEEE Std 80 Annex D iterative simulation