Hi Bradley, thank you for contacting us about your question, it is our pleasure to help. Ground Potential Rise or GPR, is a phenomena that occurs when large amounts of electricity (current) enters the earth causing an increase in voltage. While many factors are involved in what the ultimate Ground Potential Rise will be in any given area, the overall Ground Potential Rise can be simply calculated by using Ohm’s Law. Being that the fault-current at your substation will be a fixed number of amps, the only thing that can be done is to reduce the resistance (impedance) of your ground grid. To do this, you will need to increase your grid size, and as space is limited, you really will only be able to drive deep electrodes. It is not uncommon to see 100-meter (300+ ft) deep electrodes at some sites.
But is reduction of Ground Potential Rise really your goal? Often in substations, we are more concerned with human safety factors (IEEE Std-80) generated by Ground Potential Differences (GPD) such as Touch-Voltage or Step-Voltages. If reduction of hazardous Step & Touch Voltages is your goal, then Ground Potential Rise reduction really shouldn’t be your concern. A well designed grounding grid can eliminate Step & Touch hazards regardless of how high the Ground Potential Rise voltage is. Just like how a bird can land on a power line without being electrocuted, properly designed substation grounding grids can do the same thing.
Good substation design is difficult, and the grounding grid is one of the most challenging aspects. A good ground design demands computer modeling that can simulate the effects of an electrical fault on a 3D model of your substation, taking into account such factors as conductor spacing and depth, X/R, soil resistivity, human fibrillation currents, ground coverings, etc. This takes powerful software to accomplish. While your average electrical coordination study software programs will make ground grid recommendations, they are really not designed to properly model a substation to IEEE 80 standards.
We recommend that you contact a professional engineering group specializing in electrical grounding with the proper computer software systems. We are of course happy to help, but if not us, please find someone who can help you with these challenging human safety issues.
The E&S Grounding Solutions Engineering Team