Tech Briefs Magazine - October 2021 - 11a

SIMULATION-BASED PRODUCT DEVELOPMENT
FIGURE 3       
        
      
Apart from being major project
investments, cables are demanding to test
experimentally as well (Figure 1). In fact,
cables researched by NKT, a global cable
supplier, have been tested experimentally
for many years, and it has been both time
and resource consuming. " Cable losses
are a complex thing to measure, " says Ola
Thyrvin, senior analysis engineer at NKT.
One tool that can help in this regard?
Electromagnetics modeling, which enables
the NKT team in Karlskrona, Sweden, to
test cable designs virtually, visualize how
different cable parameters affect armor
losses, and predict cable performance in
different installation conditions (Figure
2). With cable costs as steep as they are,
designers can, with simulations, analyze
the cable losses and reduce the amount
of required conductor size and, thereby,
cable cost. However, they need to be
absolutely confident that their modeling
tools can perform the analyses they need
- and give them the correct results, since
the cable cannot be measured until it is
manufactured after the design is sold.
» BYPASSING LIMITATIONS
IN CABLE MODELING
One issue when it comes to testing cable
designs is that the standards are a bit
outdated. In fact, some IEEE and IEC
standards for cables are still based on
analytical expressions that were derived
about 80 to 100 years ago and simplified
to enable hand calculations. Over the
last decade, several publications have
provided measurements that show that
the formulas in the standard overestimate
the armor losses. For some cases, the
losses are around 50% of what the IEC
standard gives. As the possible current a
cable can carry is limited by a maximum
allowed conductor temperature,
a reduction of the losses enables a
possibility to reduce the conductor size. A
reduced conductor size means less copper
or aluminum, which are expensive metals,
and therefore cost savings for the cable
project.
It is possible to measure the armor
losses accurately with methods
developed ten years ago, but it requires
that you have the cable. Almost all
high-voltage offshore cables are custom
made and therefore not available
to test before a project is sold and
manufacturing starts, and cables need
to be designed already in the tender
phase. With the adoption of numerical
analysis, the study of cables and armor
wires became easier, but still left a lot to
be desired. In fact, the first 3D models of
a cable were created less than a decade
ago. Even more inhibiting: Models of
this kind, up until recently, could take
several days to a few weeks to run on
a supercomputer. Advancements in
both computer hardware and modeling
techniques have made cable design
and analysis quicker, easier, and more
robust. A cable model that used to
require a supercomputer, for example,
can now be run on a standard laptop
and take minutes instead of days. These
enhancements have opened up new
possibilities for NKT's research.
» MODELING AN
ARMORED CABLE IN 3D
Part of the work at NKT involves the
electrical simulation of cables as well as
calculating their temperature distribution
and corresponding losses. In an armored
cable, it is difficult to calculate the losses in
the magnetic steel armor. This is because
of a complex interaction between active
and passive conductors, combined with
nonlinear material properties (hysteresis)
and temperature dependence. Further,
the geometry of an armored cable model
(Figure 3) includes small, detailed features,
like the narrow gaps between the armor
wires, leading to a large number of
mesh elements, long computation times,
and increased memory requirements.
To address these challenges, NKT set to
find out if they could use a coarse mesh
FIGURE 4           
         
COMSOL NEWS 11a
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Tech Briefs Magazine - October 2021

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