Power Cable Rating Examples for Calculation Tool Verification

Introduction

Generations of engineers have developed their own spreadsheets and tools or use commercial software applications for computing cable current ratings. At present, there are no means to verify the accuracy of the results obtained with various computational tools. To address the need for such verifications, CIGRE Study Committee B1 decided to launch a dedicated Working Group (WG) with the task of providing guidance and test cases to allow the verification of calculation techniques. That Working Group, WG B1.56, now delivers its results.

The calculated current rating depends on the choices and assumptions made by the engineer that performs the calculations. This has a direct impact on the design of a new cable system or on the cable system loading limitations of an existing installation. In contrast with the voltage withstand capabilities of a power cable, the current rating is not physically tested. The only evidence that a certain current rating is indeed achieved, is provided by the calculation itself. The verification of the calculation is, therefore, essential. Recommended methodologies were already advised in CIGRE TB 640 [1], and the means to perform the verification of current rating calculations will soon be published in a Technical Brochure prepared by CIGRE WG B1.56.

In that TB, detailed guidance points and case studies are provided with the goal of verifying techniques and tools used for the calculation of the current rating of power cable systems. The TB is intended to be used by cable specialists who perform calculations themselves or request calculations from others. With the help of the TB, the user can verify a calculation technique, calculation tool or software tool before using it, which provides a good basis for a reliable power cable current rating, as was recommended in CIGRE TB 640.

In this paper, the background and verification methodology developed by the Working Group will be introduced. Also, some key guidance points will be shared to overcome pitfalls, interpretation issues and missing elements when calculating the current ratings of power cables. Furthermore, the various case studies that have been developed will be introduced. The paper will conclude with a description of future work.

Background

The current rating of a power cable sytem depends not only upon the electrical and thermal parameters of the cable itself, but also upon the thermal parameters of the environment in which it is laid. It is well known that the different thermal resistivities of soil (chiefly depending on the density and moisture content) may strongly vary along the route and unless extensive measurements are taken, these parameters cannot be known with great detail. Also, the ambient temperature of the cable environment is often only known to a certain limited accuracy.

Therefore, the first key point of this paper is that there must be a good balance between the quality of the starting points and the quality of the current...