Thank you for your question regarding the design of earth mats at high voltage substation switch yards, it is our pleasure to help.
These high-voltage substations are very dangerous places for personnel to work in. There are numerous electrical hazards to be concerned about, including Arc-Flash hazards, Step & Touch Voltage hazards, and more. The earthing grid is an important part of not only providing a safe working environment, but also in providing an effective electrical system.
2,800 ohm-meter soil is very resistive which can be both good and bad. It’s good for corrosion as your system will suffer very little from the corrosive effects of conductive soil. It can also be very good for Step Voltage Hazards, however it can be very bad for Touch Voltages. We’re assuming that when you say you have 2,800 ohm-meter soil you are referring to the very top layer of earth down to some very shallow depth? Do you know how deep this layer of soil goes? What about as you go deeper in to the earth? Does the soil get more conductive or less? You should really have an understanding of your soil conditions down to a depth of at least as wide as your substation is at its widest point (the diagonal distance). Obviously, the conductivity of the soil will change as you go deeper into the earth, for either the benefit of your earthing system, or for its detriment.
For your facility (70m X 33m with a diagonal distance of 78m) you should have conducted a series of Wenner 4-point soil resistivity tests with “A” spacing’s out to at least 78-m (256-ft) which would be a linear run of 234-m (768-ft), with a multitude of spacing’s ranging from less than a 1-m all the way out to the end. Please see the link below:
The data collected from the Wenner soil resistivity tests can then be modeled using computer software to determine where the electrically conductive soil layers lay at your facility. It could be that down 20 meters in the earth you have some great soil. If that is the case, your earth grid design should include earth wells. You won’t know this of course until you conduct the soil resistivity test and use computer modeling software to analyze the data.
Once you have an understanding of the soil, you can now start the process of designing a earthing grid. You will also need to know the physical site plan of the substation (fences, gates, blockhouse, transformers, towers, etc.), and the electrical fault data of the busses, including clearing time and X/R ratio factors. So how do you provide proper protection? It is a combination of removing electrical energy from the grid (electrodes), balancing electrical energy (grids), bringing electrical energy closer to people and equipment in certain cases (bonding), and taking it away in other cases (insulation). Frankly, it can get very complex. Much more than we could ever really cover in a simple blog.
The bottom line is, you need highly sophisticated software capable of modeling an electrical fault on your designed earthing grid in your local soil conditions. There are many software packages available, but only a few that accurately model Step & Touch voltages. We recommend the CDEGS software package from Safe Engineering Services ltd in Canada (www.sestech.com). To our knowledge, they have the only software package that accurately calculates the individual Step & Touch Voltages, all other programs only provide a theoretical maximum that inevitably leads to over engineering in the safer areas, and under engineering in the dangerous areas.
For example, the Touch Voltage hazard is far greater for a person who is actually touching a transformer, than it is for a person touching the block house. Both places do in fact have touch voltages, but obviously the transformer is far more hazardous. To our knowledge, only CDEGS properly addresses this issue. All other programs merely provide a touch voltage based on a theoretical maximum across the entire grid, and do not have the ability to even find these “hot spots”. In fact most of these programs calculate touch voltages at all points across the grid, when in fact touch voltages are only viable at 1-meter from any object. This is why other programs show touch voltages in open areas of the compound, where there is actually nothing to touch! Areas beyond one-meter can only be impacted by step voltages!
One of the quickest and easiest ways to know if your software is accurately calculating Step & Touch Voltage hazards is in the time it takes the computer to make the calculations. The inaccurate programs perform the calculations in the blink of an eye, seconds of calculating time at best. A proper analysis can take a high-powered computer from 10-15 minutes to several hours of calculating time! Obviously one program is really doing an analysis, the other is just giving you some maximums which can result in serious life-threatening errors.
The bottom line is this, to properly design a earthing grid for a switch yard, you need a good electrical fault computer simulation software program. We here at E&S Earthing Solutions are of course more than happy to help you design your earthing grid, but if not us, please get someone with the proper software and experience to help you with this critical life-saving process.
Feel free to give us a call at 310-318-7151 California time, and someone will be happy to speak with you about your project, free of charge.
The Engineering Team at E&S Earthing Solutions
Photo credit: E&S Earthing Solutions