Thank you for your question regarding substation design using CDEGS and IEEE Std-80 formulas. It is our pleasure to help.
IEEE Std-80 is the IEEE Guide for Safety in AC Substation Grounding. It was originally published in 1961 and has been updated four (4) times, in 1976, 1986 & finally in 2000. IEEEE Std 80 is by far and away the most controversial standard in the IEEE library, and is considered one of the most difficult standards to understand. This is mostly because the standard is trying to accomplish two functions. First, it is trying to provide a reasonably safe environment for personnel working in high-voltage substations. Secondly, it is trying to show how to calculate and mitigate hazardous Step & Touch Voltages. Frankly, the IEEE would be doing everyone a favor if they split Standard-80 into two separate standards, as the calculation and mitigation of hazardous Step & Touch Voltages applies to many other areas outside of AC substations. But that said, it is always important to remember that IEEE Std-80 is a voluntary standard. The real enforcement is under Title 29 of the Code of Federal Regulations part 1910.269. This Federal Law (29 CFR 1910.269) is the true regulatory authority governing Human Safety in High-Voltage environments.
To your question, there is no doubt that computer modeling can save you time and money, not only in copper wire and installation costs, but simply in the time it would take to pay an engineer to do the hand-calculations. Surely, CDEGS is a cost-saver in engineering time alone.
But the real issue here is in accuracy. Using IEEE Std-80 to do hand calculations will NOT provide you with an accurate understanding of actual hazards at the substation. There are simply far too many equations. For example, there are many improvements in the calculations found in the 2000 edition over the previous 1986 edition of the standard that dramatically add complexity. Not only were the formulas regarding permissible body current changed for Step & Touch Voltage criteria’s, but the surface layer Derating Factor was dramatically changed. This Derating Factor change also impacted the calculation of foot resistances, by adjusting the formula from a simplified expression to being based on a rigorous series expression, with each term being a surface integral. Another new issue is the Decrement Factor which requires you to take into account the effects of the DC offset current generated due to the asymmetrical fault current. But the biggest issue in the new standard is the change from uniform soil to multilayer soils.
Chapter 13 of the 2000 edition of IEEE Std-80 deals with the soil structures and the selection of a soil model. The new edition introduced an erroneous method for the calculation of a 2-layer soil structure, and only briefly discusses multi-layer structures. This new method uses a uniform soil structure that claims to provide accurate 2-layer soil equivalences. Numerous studies have shown that this method presented in Annex E of the standard is erroneous. In some cases when the grid is increased in size, the resistance increases. In others, when the soil resistivity increases, the resistance of the grid decreases. These are both backwards from the laws of physics.
The real problem with the IEEE 80 and soil structure selection is that Uniform and 2-layer models seldom exist in nature. Three, four and five layer models are more common. The reason the IEEE 80 uses uniform model equivalents, is that it is virtually impossible to hand calculate 2-layer models, let alone 3 or 4 layer systems. That is why the standard doesn’t even try, it simply tries to ‘fudge’ a single-layer (uniform soil) that it thinks will be close to a 2-layer. Unfortunately for the IEEE, it simply doesn’t work that way, and you get grid resistance errors as mentioned above.
Another issue is the Grid Current Calculation. The single line-to-ground fault current is actually divided and re-divided as it passes through the grid, reducing the actual current at any given point in the system. Annex C of the standard uses a graphical method to approximate the grid current. And that is about the best that can be said, it is an approximate. The standard does not even take into account voltage drops across the conductors, leakage current into the soil, magnetic and electric fields, impedance imbalances, or any number of a dozen other parameters caused by material differences, grounding rods penetrating different depths, etc. Nor does IEEE 80 take into account overhead ground wires (OHGW) from transmission lines, or current flowing in distribution feeder neutrals. In one study it was shown that the IEEE 80 calculations actually overrated the fault current by 13% more than in reality.
We could go on and on as there are numerous other issues (particularly calculating the touch voltages for the exterior fences) that we could discuss. But by now it should be obvious that hand calculating these variables would be a daunting task. In fact, if you were to print-out all the calculations that a good computer model performs, it would look like a doctorial thesis. And even if the hand-calculations were done according to the standard, you could still end up with either an over engineered substation, or worse yet, an unsafe substation.
In our opinion computer modeling is the only viable and ethical way to design a substation grounding grid. CDEGS is a computer-based electrical engineering modeling software program that uses IEEE 80 calculations as a base, but additionally takes into account multi-layer soils and all the other engineering factors mentioned above. In our opinion CDEGS is the best grounding software on the market, and was in fact was used by the IEEE Std-80 committee for the generation of the 2000 edition.
The bottom line is that in 1961 hand calculations was the best method possible. In today’s computer age and under 29 CFR 1910.269, a jury may consider hand-calculations to be criminal negligence.
In conclusion, we do indeed firmly believe that using CDEGS will not only save you engineering time, but it will also save you in construction costs by ensuring that the proper amount of copper is used to get the job done. CDEGS is also the only way to accurately conduct the proper IEEE 80 calculations.
We are of course more than happy to help you with a proper substation design using CDEGS. However, if not us, please get someone to do the mandatory Human Safety study to ensure the mitigation of hazardous step and touch voltages. Feel free to contact one of our engineers directly and they will be happy to discuss your project free of charge.
The Enginerring Team at E&S Grounding Soltuions
Photo Credit: E&S Grounding Solutions