Jordningsberäkningar : Implementering av tvålagers jordmodell i Python

Sammanfattning

For a safe and reliable grounding system design, the accurate estimation of the soil resistivity is essential. There are different soil models that can be used to calculate the required parameters for the dimensioning of the grounding system of a power system. The simplest one is when the ground is assumed to be uniform and is therefore assumed to have the same resistivity though the entire region. This is usually not consistent with real-world conditions, as the resistivity varies with depth. In this thesis, the two-layer soil model based on Annex B IEEE std 81-2025, was implemented in a Python script to improve the accuracy and the efficiency of grounding calculations at Sweco. The model applies an iterative parameter estimation to calculate an upper layer resistivity, a lower layer resistivity and the depth of the upper layer. The data that the iterations are based on is measured with Wenners method as it is required in the two-layer model. The Python script was validated with both data from IEEE std 81-2012 and an old scientific report that had investigated the two-layer soil model. The results were in good agreement with both references and had a fast convergence, which implies a well implemented model. Results using Sweco’s example data gave a reasonable result for the parameters of the upper layer resistivity and the depth of the upper layer. According to the results and the analysis the resistivity of the lower layer did not seem as reasonable. The two-layer model was not as sensitive to the lower layer, as it was for the upper layer and the depth of the upper layer. That implies that the result for the lower layer is less reliable than the results for the upper layer and the depth of the upper layer. Comparing the resistance between the uniform soil model and two-layer soil model resulted in the uniform model giving a larger one. This is mainly because the soil resistivity is chosen from the data with a safety margin, while the resistivities are exactly calculated according to the two-layer model from the same data. The two-layer model can be used to roughly determine the soil types of the studied ground. Different soil types have different resistivity, then based on the results of the two-layer soil model, the soil types can be determined.