The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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A Comprehensive Approach to Real Time Power Cable Temperature Prediction and Rating in Thermally Unstable Environments
Robert John Millar
Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S1 at Helsinki University of Technology (Espoo, Finland) on the 13th of November, 2006, at 12 noon.
Dissertation in PDF format (ISBN 951-22-8416-2) [2817 KB]
Dissertation is also available in print (ISBN 951-22-8415-4)
Abstract
Electricity utilities are being forced to maximise the use of their major assets while maintaining or improving availability of supply. Most urban environments have ageing underground cable networks that are extremely expensive and disruptive to replace. Original steady-state ratings are often being approached or even exceeded. As is generally known, transient rating methods that acknowledge that load transfer is usually less than the peak value on which steady-state ratings are based can give extra transfer capacity and thus extend the useful life of cables. Real-time temperature prediction based on actual loads can increase the utilisation of cables still further. It is imperative, however, that real-time temperature prediction methods allow for changing environmental parameters such as overall moisture content, the movement of moisture away from highly loaded cables, ambient temperature and the effect of external heat sources, because the inherent safety margin of steady-state rating is lost.
This thesis takes a direct and inclusive approach to these issues, using a real-time formulation of a summation of exponential terms to provide a simple but consistent framework to model cables and their installed environment. Methods are given to extend the use of a thermal ladder circuit to cover the entire environment rather than just the cable itself because the nodal solutions in the environment support the prediction of moisture movement in a transient adaptation of the 2-zone approach to moisture migration used in the standards, where the backfill or native soil surrounding cables is assumed to dry when a stipulated critical temperature rise has been exceeded. One feature of the work is that the movement of moisture can be slowed down, an especially important attribute when cables are cooling after extended high temperature operation. Measurements from a cable-scale heating tube validate this approach.
The main content of the thesis is implemented in an algorithm that consists of two parts. The first part analyses the environment of a buried cable system and generates the governing exponential equations. The coefficients and time constants of these equations consist of moisture and moisture migration dependent polynomials. The second part of the algorithm consists of the real-time implementation, with full dependence not only on the position of the critical isotherm delineating dry from wet regions during moisture migration but also the overall moisture content of the environment. The algorithm is validated by comparison with Finite Element Method simulations and standard based computations.
The thesis also contains overviews of how the approach can cope with installations in composite plastic tubes and external sources. While the main application of the algorithm is to predict conductor temperatures in real time from current measurements and a realistic knowledge of the thermal environment of a cable, it is realised that operating margins can be more safely reduced if temperatures are monitored. A full thermal analysis of the installed cable system can lead to highly accurate algorithms predicting the conductor temperature from current and surface or sheath temperature measurements, and these plus less accurate but 'universal' algorithms are also presented, as a development of earlier work.
Keywords: cables, underground cables, moisture migration, ampacity, real-time rating
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© 2006 Helsinki University of Technology
Last update 2011-05-26