Tech Briefs Magazine - October 2021 - 12a

SIMULATION-BASED PRODUCT DEVELOPMENT
FIGURE 5              
            
FIGURE 6          
for their cable model (Figure 4) while
still accurately describing the nonlinear
magnetic behavior of the steel material,
a strongly magnetic soft steel with high
permeability and large hysteresis losses.
The group turned to the COMSOL
Multiphysics®
simulation software, as well
as the add-on AC/DC Module, which is
especially suited for cable analysis. This
software enables the 3D modeling of
an armored cable in order to analyze
the magnetic fields and compute the
armor losses (Figure 5). Going back to
the computational expense of cable
modeling, Ola Thyrvin mentions a feature
from the COMSOL®
software that he
found particularly helpful: the Periodic
boundary condition, which enabled
the team to model a small piece of the
cable, keeping it as short as possible.
The reduced size of the model saves
on computational time and memory
requirements that are specific to this
application area, while also ensuring that
12a COMSOL NEWS
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all of the relevant physics are captured
in the model. " The model needs to
capture one conductor meeting one
armor wire up until they meet again, "
says Thyrvin. Another memory-saving
modeling approach is the use of infinite
elements, which lets the designers include
a sufficient amount of air around the
cable in the modeling domain, while still
limiting the required mesh and memory.
» INCREASED PERFORMANCE,
ACCURATE COMPUTATIONS
The NKT team's modeling approach
involved three main stages. First, they set
up a current-driven model with predefined
temperatures. The current is not affected
by the cable impedance or variations in
temperature and is instead controlled by
the system load. Next, the team calculated
the eddy current losses as losses that
are induced by local currents flowing
in the armor wires at the predefined
temperature. They found that the losses
are dominated by the screening currents
around the armor wire perimeters, in the
wire sections near the phase conductors.
Third, they calculated the magnetic
hysteresis losses by integrating a function
of the magnetic B-fields over the armor
wire volume (Figure 6).
In their 2019 paper " Fast Modelling
of Armour Losses in 3D Validated by
Measurements " at the 10th
International
Conference on Insulated Power Cables
in 2019 (Ref. 4), NKT demonstrates
additional ways to increase performance
without significantly harming accuracy.
First, even without resolving the skin
depth in the armor, they have discovered
that with the proper geometric
correction factors and fitted material
parameters, it is still possible to compute
realistic loss values - typically more
realistic than what the IEC standard
provides, and in several cases, within the
measurement accuracy.
Furthermore, while running the model
with a coarse mesh, they used a uniform,
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Tech Briefs Magazine - October 2021

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