Designing a substation grid with IEEE Std 80-2013 pursues two goals: that the grid dissipates the fault current without exceeding equipment limits and that no person is exposed to a critical electric shock. The method starts from a preliminary design, calculates the current entering the soil and compares the mesh and step voltages with the tolerable ones until they comply.
Short answer
Designing a substation grid per IEEE Std 80-2013 is an adjustment process: a grid is proposed, the current that actually enters the soil is calculated, and the mesh and step voltages are compared with those a person can tolerate. If they do not comply, the spacing between conductors is tightened or rods are added and the calculation is repeated. The inputs that weigh most are the maximum grid current, the fault duration, soil resistivity, the resistivity of the surface layer and the grid geometry. The limits being compared come from the tolerable current of the human body.
Design objectives
The standard sets two objectives that any substation grounding system must meet, in both normal operation and fault:
- Dissipate the current into the earth without exceeding operating limits or equipment limits.
- Protect the people near the grounded facilities, so that they are not exposed to a critical electric shock.
The design procedure is meant to ensure safety against step and touch voltages inside the substation and just outside its fence. Voltages transferred from other points can exceed the ground potential rise of the substation itself; the standard treats them separately, in its clause on special areas, and the final design must review them.
Why the mesh voltage is used
In a grid with equally spaced conductors, the touch voltage rises from the center toward the corner. That is why the standard uses the mesh voltage as the reference: the touch voltage at the center of the corner mesh, which is normally the worst touch voltage inside the substation.
The difference between the corner and the center grows with the size of the grid. In the three cases of Table 11 of the standard, square grids without rods and with uniform spacing, the ratio between the corner mesh voltage and the center one is 2.71 for a 10 × 10 mesh grid, 5.55 for a 20 × 20 and 8.85 for a 30 × 30.
The corner stops being the most critical zone in specific cases: a grid with projections or an L shape, with rods on the perimeter, or with very uneven spacing. There the touch voltage can be higher at another point.
The step voltage is less dangerous than the mesh voltage. Even so, it must be compared with its limit once the grid meets the touch criterion. If safety inside the grounded area depends on a high-resistivity surface layer that does not extend beyond the fence, the step voltage outside it can become dangerous.
Critical design parameters
The standard identifies six site parameters with an important influence on the result:
| Parameter | What it represents | Remark in the standard |
|---|---|---|
| Maximum grid current (IG) | The fault current that flows from the grid into the soil | Must reflect the worst fault, the decrement factor and future system growth |
| Fault duration (tf) and shock duration (ts) | How long current flows through the person | Taken as equal unless there are successive shocks, as with reclosing; typical values between 0.25 s and 1.0 s |
| Soil resistivity | How well the ground conducts | Varies horizontally and vertically; measured with the Wenner method |
| Surface layer resistivity | The gravel or floor material | Adds resistance to the body and reduces the current through it |
| Grid area | Surface covered | The most important geometric factor in grid resistance |
| Conductor spacing and depth | Internal geometry | Together with the area, they weigh most in the mesh voltage |
Conductor diameter and surface layer thickness have less influence. For conductors between 2/0 AWG (67 mm²) and 500 kcmil (253 mm²), the diameter hardly changes the mesh voltage.
Typical conductor spacings run from 3 m to 15 m, and typical depths from 0.5 m to 1.5 m. Trench excavation limits the spacing to about 2 m or more.
Procedure steps
The standard arranges the design in a sequence that is repeated until the mesh and step voltages fall below the tolerable ones.
- Starting data. Site plan, area to be grounded and measured soil model
- Conductor and tolerable voltages. Size for the maximum fault and the clearing time; touch and step limits
- Preliminary grid and its resistance. Perimeter loop with cross conductors
- Grid current and potential rise. With the worst fault and future system growth
- Mesh and step voltages of the proposed grid. By approximate equations or by computer program
- Are the tolerable limits met? Yes: continue; No: back to step 3.
- Design with refinements. Conductors for equipment, rods and review of special areas
Two remarks on that order:
- If the potential rise of the preliminary grid is below the tolerable touch voltage, no further analysis is needed: only the conductors to connect the equipment are added.
- To avoid oversizing, the calculation uses only the part of the fault current that flows through the grid to remote earth, not the total fault current.
If the grid does not comply
When the touch or step limit is exceeded, the standard raises several options: reduce the spacing between conductors, add rods, bury two or more parallel conductors around the perimeter at increasing depths, divert part of the current through other paths (shield wires of the lines, lower tower-footing resistance), restrict access to limited zones, or raise the tolerable limits with a shorter clearing time or a more resistive or thicker surface layer. The calculation of resistance is detailed in grounding grid resistance.
Limiting the fault current would reduce the potential rise and all the gradients proportionally, but it is rarely practical, and if it is achieved at the cost of slower clearing the result can get worse. The calculation example for a square grid without rods shows a grid that fails at the first iteration and how it is redesigned.
When a computer program is used
The approximate equations of the standard were compared with program results for square, rectangular, triangular, T-shaped and L-shaped grids, with areas from 6.25 m² to 10,000 m² and uniform soil. They offer good accuracy within those limits.
A program that models every conductor and every rod is justified when:
- the parameters fall outside the limits of the equations,
- a soil of two or more layers is advisable because of large resistivity variations,
- the conductors or rods are not uniformly spaced,
- the hazardous points need to be located in more detail,
- there are buried metallic structures or conductors that are not bonded to the grid.
The accuracy of the program depends on the soil model and the geometry reflecting the reality of the site.
What changes when the substation grows
The grid current must include planned expansions. If the system grows and current division is ignored, the grid ends up oversized. A change such as disconnecting the shield wires of a line can increase the current entering through the grid and leave it inadequate. That is why the design is reviewed when the short-circuit level grows or the protection times change, as explained in the guide on the ground grid.
You can check a simple design with the free Grounding resistance tool.
How we do it at Tiza Group
Design with real data needs a measured soil model, the fault current from the short-circuit study and the clearing time of the protections. At Tiza Group we do the design and verification of the grid for medium-, high- and extra-high-voltage substations and industrial plants according to IEEE 80, remotely and worldwide. See our engineering design and studies service or write to us.
Related guides
- IEEE 80: Substation Grounding Safety
- IEEE 80 Grid Calculation Example: Square Grid Without Rods
- Maximum Grid Current (IG) and Split Factor
- Substation Grounding Grid: Design and Testing
- Short-Circuit Study: Sources, IEC 60909 and ANSI/IEEE
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding, clauses 9 and 16.
- IEEE Std 81, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System.
Frequently asked questions
Which voltage is compared first with the tolerable limit in a grid design?
The mesh voltage, which is the touch voltage at the center of the corner mesh. The standard takes it as the baseline because it is usually the worst touch voltage inside the substation, not counting transferred voltages. Once it complies, the step voltage is checked as well.
Why can the grid of a small substation comply and that of a large one not, in the same soil?
The area covered by the grid is the geometric factor that weighs most in its resistance: the larger the area, the lower the resistance and the lower the ground potential rise. But in large grids the corner mesh voltage grows a lot compared with the center, so the design must check exactly that zone.
When should a computer program be used instead of the approximate equations?
When the soil has very different layers, when the spacing of conductors or rods is not uniform, when there are buried metallic structures not bonded to the grid, or when the hazardous points need to be located in more detail. In those cases the simplified equations of the standard are not enough.