Want to get accurate results from your earthing software?
Hugh takes us on a dive into some of the common problems encountered when using earthing software of any kind, understand the problems around software selection and limitations, how built-in debugging tools can make your life easier, why the debugging tools are necessary and how to fix these issues if you come across them. If you missed it, you can watch a replay of the webinar here.
Hugh: Hello, everyone. Thanks for joining our webinar today, where we’ll discuss how to get the most out of your earthly model.
In this piece we will cover:
- Introduction
- What does the debugger look for?
- How to fix common bugs
- Important consideration
- Review
Introduction in XGSLabs
So XGSLab uses a computational method, that’s called the PEEC method. The partial element equivalent circuit method. Now, that’s how it determines how an earthing system will perform. But like any computational tool, it has limitations it has issues that would in a way limit the scenarios it can consider.
The debugging tools within XGSLab and any other earthing tool you might use. Simply highlight these situations where either you’ve created something that the tool cannot assess, or it points out things that might affect the accuracy of your model. So, what we’re going to talk about today is what these considerations are so that you can take them away and use them to get the best accuracy out of your models. whatever tool you use. Even though I’m talking about XGSLab today, all these considerations will apply to any other earthing tool you might use.
The PEEC Method
So, to explain what this partial average equivalent circuit model does, we’ll dig into that a little bit.
What the software does is it takes the conductor system you’ve drawn, and it divides that conductor system into these shorts elements to which one can then apply second kirkhhoff law.
As you build up your system of elements, you can then apply an analysis that assesses how all these elements work together.
Within that, there are certain rules that you must follow. This is because the equation assumes that all our elements must be long and thin.
What does that mean?
Well, our fragmentation must be homogenous, all our little elements should be around the same length. Our elements should also be thin, so the length should be significantly bigger than the diameter.
But also, our length should be smaller than certain parameters of the larger system we’re assessing:
- System size
- Mesh size
- Wavelength
So, that’s a very quick dive into the science behind modelling. If that’s something you want to find out a bit more about, feel free to get in touch for a one-to-one session, because for a lot of people it’s just a bit more in depth than they need.
How does the PEEC Method apply to the models we build as earthing designers?
The key simplification is that all our elements are long and thin. This means that the diameter of our element must be more than its length. For example, if we have an element that’s a cube, this computational method cannot be applied to it because the only thing it can assess is the potential difference from one end to the other and, therefore, the current flowing in that element.
What exactly the maximum length is a quite complicated thing to assess. But it primarily depends upon your soil resistivity and the frequency of your computation.
Soil Model Layer Transitions
We also can’t have soil models that have conductors that cross the layers within our soil model unless they do it vertically.
So, we can have a vertical earth rod, which crosses into two or three different layers of our soil model, which is fine because what we can do in XGS, we can simply have one element that stops at a point, and the next element which starts at another point.
Equally with the horizontal conductor, we can divide it into elements and they’re all in the same soil model layer. But where we have a diagonal conductor, it’s a lot more difficult for software to assess the fragmentation to divide it at these layer transitions. You might need to adjust how you draw certain conductors to get a model that’s able to run.
Obviously if we have fat buried conductors at the depth of a layer transition, that conductor is effectively sitting between two layers. And again, that’s not something that we can assess in an earthing modeling tool.
Licence-related issues
There is also a couple of licensing related issues (this is a bit more XGSLab specific). So, within XGSLab we have license levels that focus on people who only do earthing analysis. And so those are lower cost options for you.
One of the limitations behind these is that they say “you’re only going to look at earthing” so all you need to model is structures that are located below Z=0. If you have structures located above that, the software will ignore them completely, or not run the model depending on a certain set of situations.
You can also choose to buy a license level that has a limited number of elements. So, it can only consider smaller models. If you set your model fragmentation to such a level that exceeds these limits, then the debugger will stop you from running your model.
Another consideration is what’s called electrode numbering. In XGSLab when you set up your conductor system, you assign each conductor within that system, an electrode number and that gives XGSLab information about whether it should or shouldn’t be connected to the other elements within that electrode number. So, in the simplified tool GSA, all elements with the same electrode number are treated as being connected, even if you’re drawn system of conductors does not connect them. That’s because our model in GSA is equipotential and so the self-impedance of those conductors is not considered important.
In GSA FD and above this electrode numberings just to checking tool. So, it makes sure that if you have a mesh of conductors for example, that all the conductors within that mesh are interconnected. That can be an effective way to find any drawing mistakes that you’ve missed.
Overlapping Elements
The next consideration we might have to address is overlapping elements. This again, goes back to that long and thin element that we spoke about earlier. Because the software only knows the potential at each end of that element, conductors can only be joined at an endpoint.
If you have conductors that overlap, either wholly or just in part, again, that’s not something that the software can assess, you’ll get an error. And then you can use the debugging tools to find exactly where that is and edit your model to take away these overlaps. This is one of the limitations where if you have any of these overlaps, you won’t be able to run your model.
Debug Tools
In XGSLab there’s the debugging section of the software gives us tools to get the best out of the debugging process.
So that’s a quick overview of the things the debugger tool picks up.
Computation Time
Computation time for modules can become problematic if you’re looking at larger systems, particularly large photovoltaic sites, for example. We’re going to look at the factors that come into play to affect how long your computation takes.
I’ve picked up these formulae from the from the XGSLab manual that just explains how long your computation might take for the different modules of XGSLab that are available. These aren’t necessarily computations you can evaluate yourself. But this allows you to understand the factors and how they will affect your computation time.
The computation time depends on:
- Which model you use
- Computer specification (p)
- Number of elements (n)
- The soil model (c – which depends on the number of layers and difference in resistivity between layers)
- (For *_TD) The number of frequencies being processed (N)
Important Consideration
The most important consideration – Are you using the correct tool for the job?
GSA vs. GSA_FD
GSA uses a simplified equipotential model
- Simplification ignores the self and mutual impedance of conductors for computational efficiency
- GSA – and other equipotential modelling tools – can only be used for small earthing systems where the self and mutual impedance is not relevant.
GSA_FD (and above) use a full-wave model
- Suitable for all sizes of earthing system
- Computation times are longer
- More accuracy of input data required
The debugger does not tell you when you are reaching the limitations of GSA!
Here’s a graph from the XGSLab developers on exactly when you can use GSA.
So, what we have is along the x axis, we have our soil resistivity divided by our model frequency. For example, one would represent a soil resistivity of 50 Ωm at a power system frequency of 50 Hertz.
You can see that for that condition (a poorly meshed earthing system), we can have an earth electrode with a diagonal dimension of about 100/150 meters.
If we have an even mesh, we can have a large earthing system of 300 meters in dimension. Often our earthing systems are not able to be well meshed on larger sites. So, keeping an eye on this lower dotted line is normally where that that transition point is.
If you’re looking at particularly low resistivity scenarios where you’re in the range, you can see that we can only consider earthing systems with a with a dimension of 50 or 60 meters and so that that rules out potentially larger transmission substations as well.
What does the difference look like?
Here we have a 100 x 100-meter grid with a 10 x 10 mesh within it, and we’ve injected our Earth fault into the corner of the earth grid.
While in GSA we can’t do that because the point of focus is irrelevant. You can see that
as we look at the potential within the earthing system, it’s level across the entire earthing grid. But obviously, it drops off between those conductors (in GSA without potential assumption).
If we then move to GSA_FD with both self and mutual impedances considered, you can see that the rate of change of potential within this electrode is significant. As we move away from the point of fault there’s a potential difference as we move across the grid.
If we had known about that when we were designing our earthing system, we might have made different choices about where to bond certain pieces of equipment and where to add density. Fundamentally, we can say that the touch voltages in the right model will be vastly different from the touch voltages in the left model because the rate of change of voltage across the grid is completely different.
I’m not saying that you shouldn’t use GSA, there’s a whole load of scenarios where it’s entirely the right tool for the job. But you need to be aware that there are also scenarios where it’s the wrong tool for the job. And you need your full wave model to get the most accurate results.
Review
So, that’s a very quick summary of earthing modeling’s limitations and how to fix them. We’ve also discussed the limitations of potential modeling tools.
GSA is obviously the XGSLab Equipotential tool, but all the other earthing modeling tools will have an equipotential option as well.
Q&A
Is there the same difference on CDEGS MALT/MALZ?
So, when we speak about this difference between GSA and GSA FD within the CDEGS suite you’re given three levels to choose from:
- MALT – Base level equipotential model
- MALZ – Intermediate level (considers the self-impedance of conductors but not the mutual independence between conductors)
- HIFREQ – considers both
In the XGSLab suite there isn’t that intermediate option, you just have extra potential or wave and nothing in the middle.
Does this apply to the lightning rolling sphere versions?
This is just for the models where we’re looking at the current distribution within the model, if we’re going to look at earthing when we’re looking at the earth potential rise, step and touch voltages, or if we’re looking at lightning or something like that, where we’re assessing our lightning impulses.
In that case, if we’re looking at any sort of impulse, you’d need the full wave version of the model to consider that high frequency effect.
But where you’re using a rolling sphere tool, slightly different limitations apply. And that’s not something we were going to cover today, but we’ll touch on that a bit more in future webinars.
What if you had to look at above ground level items?
Within the XGSLab suite, GSA and GSA FD are only for structures below grounds. And if you want to look at the how things work above ground, you need to use XGSA FD.
Within the CDEGS suite, you can only look at conductors above ground in HIFREQ (as far as I know). Again, there are other tools out in the market that will have similar limitations.
So, on that note thanks for listening everyone, and we’ll see you soon.
