Articles

Comparison between XGSLab and CDEGS

In this blog, I’m going under to hood, so to speak. Comparing CDEGS with XGSLab to show you some real-world examples where CDEGS and XGSLab give almost identical results.

Therefore, in all of these examples, we have completed and verified the earthing design for these sites and found good agreement between the modelled results and the post-installation measurements. On this basis, we can say that XGSLab matches CDEGS for accuracy, and both tools have good agreement with the real-world measurements.

Comparing CDEGS With XGSLab - General Introduction

As a result, the 3 examples presented compare results from CDEGS HIFREQ against results from XGSA_FD. These are the high-end packages, offering the best agreement with the real world, particularly in low soil resistivity situations, such as those in example 2. So, for each example, we give an outline of the project and then present the EPR at the point of fault, together with the EPR contour plot provided by each tool.

For consistency, the same soil models are used for both tools, generated using CDEGS RESAP. A future blog will look at the comparison between CDEGS RESAP and XGS SRA (Soil Resistivity Analyser)

Example 1 - Wind Turbine Project

This is a wind turbine project, located in the countryside near Cardigan in Wales. The Wind Turbine connects to the 11 kV distribution network via a wood pole line with a small package substation to the bottom left. A transformer located at the base of the turbine feeds a Farmhouse (not included in the model. A key requirement on this project was to keep the hot contours away from the Farmhouse and other 3rd parties.

Soil Model

XGSA_FD

In the latest release, XGSA_FD takes 45 seconds to perform the first phase of modelling and 30 minutes to run the 2nd phase

CDEGS HIFREQ

Another note is that the first phase of the model, where CDEGS determines the current distribution (and therefore the voltages) in the conductors, takes 10 minutes to run on my PC. The second phase, where it determines the voltages at the soil service takes 15 minutes

Comparison

Example 2 - 33kV Substation

This is a 33 kV substation that was constructed as the point of connection for 2 large scale solar farms near Bristol.

Soil Model

XGSA_FD

N/A

CDEGS HIFREQ

N/A

Comparison

Example 3 - Waste Site

This is energy from a waste site located near London. There are 2 generators located on-site, each with a dedicated grid connection, with the point of connection on an isolated earth electrode. This model simulates an earth fault in the larger building.

Soil Model

XGSA_FD

N/A

CDEGS HIFREQ

N/A

Comparison

Conclusion

In these example cases, we’ve shown that XGSLab and CDEGS agree extremely well. The team at SINT Ingegneria has produced papers showing that XGSLab produces results matching the standard examples in IEEE 80.

So the difference we found between XGSLab and CDEGS

Comparing XGSLab and CDEGS, a key point of difference is in conductor segmentation. The default CDEGS segmentation can lead to poor accuracy, particularly in low resistivity soil structures like those in example 2. In comparison, XGSLab makes assigning segmentation for the whole model a 1 click process, and the default segmentation option leads to much better accuracy. Obviously, any increase in segmentation level is likely to lead to an increase in computation time. However, getting the accuracy right is more important, and in my experience, a few minutes increase in computation time is not as important as getting things right the first time. The increased difference in example 2 can likely be put down to the difference in segmentation approaches

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Comparison between XGSLab and CDEGS