Energy Efficiency Made Simple Vol IV

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65 ENERGY EFFICIENCY MADE SIMPLE 2015
• They require high level knowledge to build the cable models into the database
• It has been shown [5] that cables can be modelled in FES programs and that
the results for steady state and transient ratings compare favourably with the
results obtained from traditional mathematical methods
When carrying out computer simulations based on the FE method, the cable is
drawn and specified in the program in its finest detail for analysis. The only differ-
ence between specifying the cable in detail and using a general model as far as
the software program is concerned is the time that it takes to do the analysis. It is
also obvious that the degree of accuracy of the results done with FE analysis is a
function of the detail specified in the model used. With the availability of powerful
computers the analysis time for the most detailed (and accurate) simulations is
reduced to minutes. A typical cable model that was used for steady state as well
as transient simulations is shown in Figure 1.
The main problemwith the determination of the installed ratings of cable systems
is that the engineer responsible for the cable network normally does not have
general access to the methods and tools used to determine the steady state and
emergency ratings of the installed cables under the varying conditions. These
conditions include ambient temperature, soil temperature, soil thermal resistivities
and other variables that will influence the temperature of the cable.
Although a number of FE simulation programs are freely available, it requires
a good understanding of the specific program, and experience in the application
of the program, to be able to do advanced thermal studies as is required for cable
installations.
This article address these factors and describes a model that can be used
by engineers and utility operators to determine the load (constant and variable)
under which a cable can operate under non-standard installation conditions without
exceeding the maximum allowable conductor temperature.
The proposed model has the benefit that utilities and municipalities would not
be required to purchase the simulation program. This reduces the costs dramati-
cally and makes it affordable for the small municipalities and clients.
The system is based on a third party providing the simulation software
and other programs to perform the required actions as required by the client.
The service provider will have the combined experience and knowledge of the
thermal behaviour of cables as well as the required expertise in the application
of the specific simulation software. The service provider will install and maintain
the software on a server and will ensure that the latest versions of the software
are used for the simulations.
The simulation model will be developed by the service provider based on the
information supplied by the client. The information required from the client will be
SHEATHING 3,2mm
ARMOURING ø 3,15mm
ø 61,7 ø 65,1 ø 71,4 ø 77,8
BEDDING 1,7mm
FILLER
CORES
IEC60287-1-1. 2006. Electrical cables – calculation of the current
rating – current rating equations and calculation
of losses.
IEC 60287-2-1, 2006. Electrical cables – calculation of the current
rating – thermal resistance.
Figure 1: Cross section of the cable used for the simulations [5].