The GPR is the maximum potential that the grid of a substation reaches with respect to remote earth during a fault. IEEE 80 defines it as the maximum grid current multiplied by the grid resistance. If the GPR is below the tolerable touch voltage, no further analysis is needed; if it exceeds it, the mesh and step voltages must be calculated.
Short answer. Ground potential rise, or GPR, is the maximum potential that the grid of a substation can reach with respect to a distant point assumed to be at remote-earth potential. IEEE Std 80-2013 defines it as the maximum grid current multiplied by the grid resistance. It is the first figure compared with the tolerable touch voltage to know whether the design requires further analysis.
What the GPR is
Under normal conditions, grounded equipment operates at a potential almost equal to zero, like that of the grounded neutral and of remote earth. During a ground fault, the part of the current that the grid carries into the soil raises the grid potential with respect to remote earth. That rise, at its maximum value, is the GPR. The potential that leaves the substation through pipes, cables or fences is described in transferred voltage and transferred potentials.
The definition is that of clauses 3 and 8.1:
GPR = I_G × R_g
where I_G is the maximum grid current in amperes and R_g the grid resistance to remote earth in ohms.
How the two factors are obtained
The maximum grid current. It is not the whole fault current. Only the part that flows between the grid and the surrounding soil counts. The guide obtains it in two steps: the symmetrical grid current is the ground fault current multiplied by a current division (split) factor, and the maximum grid current is that current multiplied by the decrement factor for the fault duration. It must reflect the worst type and location of fault and the future growth of the system. If there is a shield wire, a neutral, a gas-insulated bus or underground feeders, part of the current returns to the source through that path and the GPR is lower.
The grid resistance. It depends mainly on the soil resistivity and on the area the grid occupies. In clause 16.2.5 the guide points out that the area is the geometric factor with the most influence: the larger the grounded area, the lower the resistance and the lower the GPR.
An example from the standard itself
In the Annex B example, the ground fault current at the 115 kV bus is 3180 A, the current division factor is 0.6 and the decrement factor is 1.0. The maximum grid current comes out as 1908 A. The grid resistance is 2.78 Ω. The GPR is then about 5304 V, far above the 838 V tolerable touch voltage of the example. That is why the analysis must continue and the mesh and step voltages must be calculated.
The example shows another detail: at the 13 kV bus the fault is larger, 6814 A, but the grounded-wye winding of the 13 kV transformer is a local source of fault current and does not contribute to the GPR. The maximum grid current is based on the 3180 A of the 115 kV bus.
What the potential profile says
The current leaving the grid into the soil produces equipotential curves around it. The potential is highest over the grid and drops with distance, down to the remote-earth value. The difference between that profile and the grid potential is what people can receive in touch and step.
| Quantity | How it relates to the GPR |
|---|---|
| GPR | Grid potential with respect to remote earth |
| Touch voltage | Difference between the GPR and the potential of the surface where the person stands |
| Mesh voltage | The maximum touch voltage within a grid mesh |
| Step voltage | Potential difference at the surface between two points 1 m apart |
| Transferred voltage | Special case of touch in which the potential is carried to or from a remote point, and can equal the full GPR |
When more verification is needed
In the design procedure of clause 16.4, step 7 compares the GPR of the preliminary design with the tolerable touch voltage. If the GPR is lower, no further analysis is needed: only the additional conductors to give access to equipment grounds. If it is higher, the mesh and step voltages of the proposed grid are calculated and compared with the tolerable ones.
There is one more reason to look at the GPR even if the mesh voltages comply. The potential rise is transferred through pipes, cables, fences and communication circuits to the outside, where the soil is at a different potential. To see how it is diagnosed in the field, see high step and touch voltages in the grid.
How to reduce the GPR
There are two paths: reduce the grid resistance (the most effective way is to increase the area it occupies; if space is limited, deep driven rods or wells that reach lower-resistivity layers) and reduce the current entering the soil, diverting part through the overhead ground wires of the lines or by lowering tower footing resistances, as clause 16.6 indicates. Reducing the clearing time does not lower the GPR, but it raises the tolerable voltage with which it is compared.
We carry out your grounding study
We calculate the maximum grid current of your substation from a short-circuit study, estimate the grid resistance with your soil resistivity and verify the GPR against the tolerable voltages. For a first estimate with your data you can use our free grounding resistance tool. See the engineering design and studies service or write to us through contact.
Related guides
- IEEE 80: Safety in Substation Grounding
- Transferred Voltage: Cables, Pipes and Fences
- Ground grid resistance: IEEE 80 formulas
- High step and touch voltages in the ground grid
- Short-circuit study: contributions, IEC 60909 and ANSI/IEEE
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding: clause 3 (definitions of GPR, maximum grid current and voltages), 8.1 (definitions of the criteria), 15.1 (maximum grid current; equations 68 and 69), 16.1 and 16.2 (criteria and critical parameters), 16.4 (design procedure, step 7) and Annex B, example 1.
Frequently asked questions
What is remote earth?
It is the distant point to which the grid potential refers, assumed to be at zero potential. Under normal conditions grounded equipment operates near zero with respect to it; during a fault, the current the grid discharges raises its potential with respect to that point.
Does a high GPR mean the substation is unsafe?
Not necessarily. It means a more detailed comparison is needed. If the GPR exceeds the tolerable touch voltage, IEEE 80 asks for the mesh and step voltages to be calculated and verified not to exceed the limits. A high GPR also implies risk for circuits leaving the substation, such as communication circuits.
Which current is used for the GPR: the whole fault current?
No. Only the part of the fault current that enters the soil through the grid is used, the maximum grid current. Another part returns through shield wires, neutrals or other metallic paths. The guide recommends not using the total current so as not to oversize.