1. Definitions
In this document, the following words and expressions will have the following meanings:
I: RMS value of earth fault current (kA)
Cu: Earthed electrode in copper
TCAP: Thermal capacity per unit volume (J/cm³/°C)
Tc (ts): Fault current withstand capacity per conductor (°C)
ar: Thermal resistivity coefficient at the reference temperature (°C)
rr: Resistivity of the earth conductor at the reference temperature (μOhm-cm)
Ko: Reciprocal of the thermal coefficient of resistivity (°C)
Tm: Maximum admissible temperature (°C)
Qf: Final temperature (°C)
Ta: Ambient temperature (°C)
p: Soil resistivity (Ohm-m)
Ca: Corrosion allowance (mm²)
Kf': Division factor
V: System voltage
ps: Resistivity of the upper layer (Ohm-m)
Ig: Maximum fault current considered (A)
A: Section area of conductor (mm²)
d : Equivalent diameter (mm)
L: Length of switchyard (m)
B: Width of switchyard (m)
Ag: Equivalent area of switchyard (m²)
Ds: Conductor spacing (m)
Nx: Number of grids in the X direction
Ny: Number of grids in the Y direction
Lc:Total length of conductors (m)
H: Depth of burial (m)
hs: Depth of the upper layer (m)
n0: Weighting factor of the correction factor
SEV1 to 7: Soil resistivity surveys
2. Purpose
The purpose of this document is to describe the functional requirements for Substation Earthing and their integration into a substation.
3. Scope
This document states the functional requirements regarding Substation Earthing and integration into a substation, as well as items that need to be considered in the area surrounding the substation to ensure the safety of personnel and the general public. The scope includes all items in the field such as soil resistivity, ground resistance, rods, electrodes, potential risers, touch voltage, step voltage, etc.
4. Referenced Documents
The table below lists applicable legislation, standards, and referenced documents.
IEEE Std. 80/2000 : IEEE Guide for Safety in Grounding of Substations
ASTM G57-06 : American Society of Testing and Materials
IEEE Std. 81:2012 : Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System
IEEE Std. 837:2014: Standard for Qualifying Permanent Connections Used in Substation Grounding
AS/ISO 1000:1998: The International System of Units (SI) and its Applications
IEC/TS 60479-1:2016: Effect of Current on Human Beings and Livestock - Part 1 General Aspects
IEC/TR 60479-5:2007: Effect of Current on Human Beings and Livestock - Part 5 Touch Voltage Threshold Values for Physiological Effects
CIGRE Technical Brochure 535:2013
EMC within Power Plants and Substations
5. Functional Requirements
The functional requirements of an earthing and bonding system must ensure the following:
• Safety of people
• Protection of equipment
• Operational security
The functional requirements must satisfy the latest edition of IEEE 80-2000 and the listed standards.
5.1 Design Requirements
Input parameters for the design requirements include:
• Soil model in accordance with IEEE-81
• Fault level and contributions
• Fault clearing time
• Fault current
Designing an earthing system typically involves software tools that can simulate electrical grounding characteristics and ensure safety and compliance with standards. Here's a list of both open-source and commercial software commonly used for this purpose:
Open-Source Software:
• CDEGS (Current Distribution, Electromagnetic Fields, Grounding, and Soil Structure Analysis)
• EMTP-RV (Electromagnetic Transients Program)
• Neplan
Commercial Software:
• ETAP (Electrical Transient Analyzer Program)
• SKM Systems Analysis (Power Tools)
• PSCAD (Electromagnetic Transients including DC)
• Amtech ProDesign
• SAFE (Software for the Analysis of Faulted Electrical Networks)
Hybrid (Both Open and Commercial):
• MATLAB/Simulink
• DIgSILENT PowerFactory
In this study, we limit ourselves to the calculation of the substation earthing without simulation. A case study follows, based on Smath software.
5.2 Design Deliverables
The final earth grid design for a specific project must be detailed in an earth grid layout drawing showing the following details as a minimum:
• Conductor type, size, and depth of burial
• Gravel/bitumen thickness and design resistivity
• Ground electrode detail including location, depth, and length
• Earthing risers to the equipment and operator loops
• Earth grid design impedance
• Fault current level in which the earthing system is deemed safe and functional for its purpose
5.3 Constructability Requirements
Earthing systems involve the installation of horizontal and vertical electrodes buried or driven into the general mass of the earth. Key requirements include:
• Backfill material must surround the conductor by at least 150 mm and must be between pH levels of 6 and 10. The resistivity of fill material must be similar to or less than that of the natural soil.
• Vertical electrodes must be driven in using appropriate tools. If they cannot be directly driven, they must be installed in 75 mm boreholes, which should be backfilled with a 50/50 bentonite and gypsum slurry mix.
5.4 Operability Requirements
The earthing and bonding system must operationally:
• Provide a low resistance path to remote earth to ensure fault currents are directed to ground and dissipated for all earthed items during the fault duration.
• Limit the earth potential rise to reduce the likelihood of hazardous voltage exposure to personnel.
• Maintain all conductive plant and equipment at the same potential.
• Not create potentially hazardous conditions to external third-party metallic structures.
• Provide a low impedance path to earth for lightning strikes.
• Offer two independent earthing connections to equipment structures, each capable of carrying the maximum expected fault current for the longest expected clearing time.