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Verification with TB 880

Grøft Design® (GRØFT) is verified against the Technical Brochure 880​1​ issued by CIGRE Working Group B1.56 (TB 880). The purpose of the validation is to ensure that the current ratings in the software are computed consistently and in accordance with the IEC 602872 standard. The software is benchmarked with the reference case studies supported by GRØFT for which a description of the modelling technique and guidelines for users of GRØFT, are provided.

IEC 60287 and TB 880

The international standard IEC 602872 provides a widely accepted method for calculating the current ratings of power cables using analytical formulas. Engineers rely on this standard for their calculations, and organizations often require that these calculations adhere to the recognized guidelines. However, IEC 60287 is limited in scope, covering only certain cable types and installation methods. Additionally, developing a theoretical model of a cable according to the standard requires specialized expertise in cable rating calculations. Even highly qualified engineers, with the aid of computer-assisted analysis tools, may struggle to apply the standard’s theoretical principles to analyse complex cable installations or accurately represent the many design features of modern cables. This can lead to incorrect cable ratings, resulting in either overrating or underrating the cable installation.

To address these issues CIGRE Working Group B1.56 released Technical Brochure 880 (TB 880)1. TB 880 discusses the verification of current rating calculation tools. It provides detailed guidance and case studies with the goal of verifying techniques and tools used for the calculation of the current rating of a power cable. TB 880 is intended to be used by cable specialists who perform calculations themselves or request current rating calculations from others.

Summary of Verification Report “Verification of Grøft with TB 880”

The verification shows that GRØFT can reproduce with high accuracy the results found in TB 880. As a matter of fact, GRØFT result in more accurate cable ratings than TB 880. This is proven with the validation of GRØFT carried out with the empirical models. This is due to the COMSOL Multiphysics® engine used by GRØFT. COMSOL Multiphysics® is a simulation software that utilizes the finite element method (FEM).

The software does not yet support analysis of steel armoured cables, therefore the subsea cables found in TB 880 were excluded from the analysis. The support for armoured cables, in GRØFT, will be developed in the INCA research project.

Verification of GRØFT against TB880 includes several study cases that consider heat transfer by means of radiation and convection (see CASE #0-2, -3 and -5). In TB 880, which follows the IEC analytical approach, heat transfer is only calculated based on the thermal resistance T4. In order to approximate the characteristic of the heat dissipation from cables placed in ducts, troughs or air, the simplified formulas for the appropriate representation of T4 were developed by the IEC. In the verification of GRØFT with TB880, it was found that the IEC models are, however, a rough approximation of the expected, realistic conditions. Therefore, GRØFT has recently introduced the feature to calculate the heat transfer by means of convection, which follows the FEM calculations. The validation of GRØFT model was conducyed using empirical models, COMSOL Multiphysics® models and experiments carried out by SINTEF3.

Although this functionality was considered for the verification of GRØFT with TB 880, it is not yet available to users. The non-linearity of convection may require further adjustments by users to achieve accurate results. We are currently working to make this feature as simple and intuitive as possible before its release.

Verification Report

The complete report, “Verification Of Grøft Design With TB 880”, is found here.

Sources

Footnotes

  1. ​CIGRE, TB 880: Power cable rating examples for calculation tool verification, WG B1.56, 2022. ↩ ↩2

  2. IEC 60287. ↩ ↩2

  3. S. M. Hellesø and E. Eberg: Simplified Model for Heat Transport for Cables in Pipes ↩