DEGREE PROJECT IN MECHANICAL ENGINEERING,
SECOND CYCLE, 30 CREDITS
STOCKHOLM, SWEDEN 2021
Evaluation of thermal expansion in
busbars used for battery electric
vehicles
FREDRIK LARSSON
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT
Evaluation of thermal
expansion in busbars used
for battery electric vehicles
FREDRIK LARSSON
M.Sc Mechatronics
Date: September 14, 2021
Supervisor: Andrii. Berezovskyi.
Examiner: Martin Edin Grimheden.
Host company: Scania AB
Swedish title: Utvärdering av termisk expansion i busbars för
implementering inom batteridrivna fordon
© 2021
Fredrik Larsson
Abstract | i
Master of Science Thesis MMK TRITA-ITM-EX 2021:470
Evaluation of thermal expansion in busbars
used for battery electric vehicles
Fredrik Larsson
Approved
2021-07-01
Martin Edin
Grimheden
Examiner
Commissioner
Scania
Andrii
Berezovskyi
Contact person Arikson
Heraldus Panji
Supervisor
Abstract
Thermal expansion can be an issue in solid busbars, the expansion is caused
by several factors and can cause plastic deformation in connection points or
structure around it. The expansion occurs due to temperature differences in
the busbar as a result of altered ambient temperature and/or joule heating. The
environment where a vehicle is used can be harsh and varying in temperatures a
lot. For future fast charging systems, a high amount of current will be passed in
the conductors. In a stationary installation, this could be solved by increasing
the cross-section area. In vehicles, the weight, cost, and space limitations call
for optimization of the conductor.
In this thesis, there are several geometrical alterations done to the busbar
to investigate the possibility to reduce the amount of stress acting on the
connection points. The main geometrical evaluation is to compare a straight
busbar to a U-shaped busbar. In the U-shape, the height, bend radius, and
cross-section shape are investigated. To investigate this issue a simulation
model was developed using Comsol, this software was used to evaluate stress
values, max temperature, losses, and displacement.
The results from the simulation showed that the U-shape has a large potential
to reduce the amount of stress. Also, the cross-section shape tests showed
that the steady-state temperature was lower for the more flatter shaped busbar.
ii | Abstract
This is true both for the U-shape and straight busbar. This resulted in
reduced amount of thermal expansion causing lower amount of stress, without
adding any weight. The weight parameter is extremely important for vehicle
implementation.
The last test is looking at the busbar material where nickel-plated copper is
compared to anodized aluminum. This test is divided into two parts, the first
one is looking at an aluminum busbar compared to a copper busbar of the
same geometry. This test showed that the losses in the aluminum busbar were
much higher, but the steady-state temperature and max stress were lower. The
second part of the test investigated the compensated aluminum busbar, this one
is modeled by compensating the cross-section area for the higher resistance
value of aluminum. The results from this busbar compared to the standardshaped busbar showed a substantially lower stress, temperature and weight.
But the overall dimensions are larger due to the compensated cross-section
area. Having this larger Cross section area might hinder the implementation
of aluminium busbars in parts of the vehicle where there is a lack of space,
like in a battery box.
Keywords
Busbar, Battery electric vehicle, Thermal expansion, Cross-section shape,
Sammanfattning | iii
Examensarbete MMK TRITA-ITM-EX 2021:470
Utvärdering av termisk expansion i solida metallskenor
för användning inom batteridrivna fordon
Fredrik Larsson
Godkänt
2021-07-01
Martin Edin
Grimheden
Examinator
Uppdragsgivare
Scania
Andrii
Berezovskyi
Kontaktperson Arikson
Heraldus Panji
Handledare
Sammanfattning
Termisk expansion i solida busbars är ett vanligt problem vid kraftig temperaturvariation.
Problemet ökar med längden av busbaren och kan leda till plastisk deformation
i infästningen av busbaren. Temperaturvariationen kan ske genom varierad
omgivningstemperatur eller genom resistiv uppvärmning. Om en busbar ska
användas i ett fordon för kraftöverföring är arbetsmiljön mycket påfrestande.
Den termiska uppvärmningen går normalt att motverka genom att öka tvärsnittsarean,
men i ett fordon där vikt, kostnad och platsbrist minskar möjligheten för ökad
tvärsnittsarea blir optimering av ledaren extra viktig.
För att undersöka problemet utvecklades en simuleringsmodell med hjälp av
Comsol. Denna programvara använder för att utvärdera spänningskoncentrationer,
maxtemperatur, förluster och utböjningar i busbaren. För att undersöka eventuella
lösningar togs det fram flera geometriska variationer till busbaren, där möjligheten
att använda en “U-form” utgjorde basen i en jämförelse mot en vanlig rak
busbar. För U-formen undersöktes U-höjden, böj-radien samt tvärsnittsformen.
Även en jämförelse mellan nickelpläterad koppar och anodiserad aluminium
genomfördes för att urskilja eventuella för och nackdelar med materialen.
Resultaten från simuleringarna visade att U-formen gav klart lägre spänning i
kontaktpunkterna. Även tvärsnittsformen påverkade temperaturen och spänningen
i busbaren, där den plattare varianten presterade bättre på alla parametrar som
iv | Sammanfattning
undersöktes i simuleringen.
För utvärderingen av materialet utfördes två tester, det första testet jämför en
busbar i aluminium mot en i koppar med exakt samma geometri, detta test
visade att temperaturen samt spänningen blir lägre i aluminiumvarianten, dock
ökar förlusterna kraftigt då aluminium har högre resistans än koppar. I den
andra testet användes en kompenserad aluminiumbusbar där tvärsnittsarean
har ökats för att ge samma resistans som kopparvarianten. Denna busbar fick
en mycket lägre sluttemperatur, spänning och vikt. förlusterna blev detsamma.
Den högre tvärsnittsarean ger dock en fysiskt större busbar.
Acknowledgments | v
Acknowledgments
I would like to thank the UECB team at Scania for allowing me to work on
this thesis topic together, special thanks to Heraldus Panji Arikson Pardede
for all the support and rewarding conversations around the topic of busbars
and cables. I would also like to thank Andrii Berezovskyi for the phenomenal
support throughout the whole project.
Additionally, I would like to thank Martin Edin Grimheden for giving vital
feedback on the thesis.
And finally, I would like to thank Fredrik Asplund for helping me steer this
project in a proper direction suitable for a master’s thesis.
Stockholm, June 2021
Fredrik Larsson
vi | CONTENTS
Contents
1
2
3
Introduction
1.1 Problem . . . . . .
1.2 Research questions
1.3 Purpose . . . . . .
1.4 Goals . . . . . . .
1.5 Delimitations . . .
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Background
2.1 Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Skin effect . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Connectors . . . . . . . . . . . . . . . . . . . . . .
2.2 Megawatt charging system . . . . . . . . . . . . . . . . . .
2.3 Related work area . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Busbar for inverter . . . . . . . . . . . . . . . . . .
2.3.2 Thermal expansion in long busbars . . . . . . . . .
2.3.3 Mechanical modelling of busbars under short-circuit
conditions . . . . . . . . . . . . . . . . . . . . . . .
2.3.4 Copper busbars . . . . . . . . . . . . . . . . . . . .
2.3.5 High Thermal Conductivity Insulation . . . . . . . .
Method
3.1 Quasi-experiment . . . . . . . . . . . . . . . . . . .
3.2 Research Paradigm . . . . . . . . . . . . . . . . . .
3.3 Research Process . . . . . . . . . . . . . . . . . . .
3.4 Data Collection . . . . . . . . . . . . . . . . . . . .
3.5 Assessing reliability and validity of the data collected
3.5.1 Mesh refinement analysis . . . . . . . . . . .
3.5.2 Validity of method . . . . . . . . . . . . . .
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Contents | vii
3.5.3
3.5.4
3.5.5
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Reliability of method . . . . . . . . . . . . . . . . . . 17
Data validity . . . . . . . . . . . . . . . . . . . . . . 18
Reliability of data . . . . . . . . . . . . . . . . . . . . 18
Implementation
4.1 Hardware . . . . . . . . . .
4.1.1 Cross Section Shape
4.1.2 U-height . . . . . .
4.1.3 Bend radius . . . . .
4.1.4 Material . . . . . . .
4.2 Simulation platform . . . . .
4.3 Simulation data . . . . . . .
4.3.1 Losses . . . . . . . .
4.3.2 Temperature . . . .
4.3.3 Stress . . . . . . . .
4.3.4 Displacement . . . .
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Results and Analysis
5.1 System documentation . . . . . . . . . . . . . . . .
5.2 Test protocol specifications and visual representation
5.3 Test 1: Cross section shape . . . . . . . . . . . . . .
5.4 Test 2: U-height . . . . . . . . . . . . . . . . . . . .
5.5 Test 3: Bend radius . . . . . . . . . . . . . . . . . .
5.6 Test 4: U-shape . . . . . . . . . . . . . . . . . . . .
5.7 Test 5: Material . . . . . . . . . . . . . . . . . . . .
5.8 Reliability Analysis . . . . . . . . . . . . . . . . . .
5.8.1 Mesh refinement . . . . . . . . . . . . . . .
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Conclusions
51
6.1 Research questions conclusion . . . . . . . . . . . . . . . . . 52
6.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.3 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
References
55
viii | Contents
LIST OF FIGURES | ix
List of Figures
2.1
Busbars, a stationary installation . . . . . . . . . . . . . . . .
3.1
A flowchart depicting the steps of the research process. . . . . 15
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
Test1: Busbar, cross section view.
Cross section shape difference . .
U-height steps . . . . . . . . . . .
Bend radius steps . . . . . . . . .
Comsol interface . . . . . . . . .
Losses in a busbar . . . . . . . . .
Stress plot . . . . . . . . . . . . .
Busbar displacement . . . . . . .
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5.1
5.2
5.3
5.4
5.5
CSS1, CSS2 and CSS3 from left to right. . . . . . . . . . .
Test 2: Height options for the U-shape . . . . . . . . . . . .
Test 2: Bend radius options required for the U-shape . . . .
A straight busbar used as a reference in test 4. . . . . . . . .
Test 5: Compensated aluminium busbar next to standard
copper busbar. . . . . . . . . . . . . . . . . . . . . . . . . .
Test 5: Test protocol summary specifications. . . . . . . . .
Test1: Busbar with CSS1 . . . . . . . . . . . . . . . . . . .
Test1: Busbar with CSS2 . . . . . . . . . . . . . . . . . . .
Test1: Busbar with CSS3 . . . . . . . . . . . . . . . . . . .
Stress plot, forced temperature, CSS1 . . . . . . . . . . . .
Stress plot, forced temperature, CSS3 . . . . . . . . . . . .
Test 2: Displacement UH1, Magnified 15×. . . . . . . . . .
Test 2: Displacement UH3, Magnified 15× . . . . . . . . .
Test 3: Current density, Bend radius 1 . . . . . . . . . . . .
Test 3: Current density, Bend radius 3. . . . . . . . . . . . .
Test 4, Straight busbar displacement, Magnified 15× . . . .
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5.6
5.7
5.8
5.9
5.10
5.11
5.12
5.13
5.14
5.15
5.16
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6
x | LIST OF FIGURES
5.17
5.18
5.19
5.20
5.21
5.22
5.23
5.24
5.25
5.26
Test 4, Straight busbar stress plot . . . . . . . . . . . . . . .
Mesh analysis base settings . . . . . . . . . . . . . . . . . .
MRF = 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
MRF = 12 . . . . . . . . . . . . . . . . . . . . . . . . . . .
Losses calculation for different MRF values. . . . . . . . . .
surface temperature difference between MRF value 1 and 12.
Stress difference between MRF value 1 and 12. . . . . . . .
Stress difference between MRF value 5 and 12. . . . . . . .
Final mesh settings. . . . . . . . . . . . . . . . . . . . . . .
Mesh as a result of final settings. . . . . . . . . . . . . . .
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List of acronyms and abbreviations | xi
List of acronyms and abbreviations
AC Alternating current
BEV Battery Electric Vehicle
BR Bend radius
CSA Cross section area
CSS Cross section shape
DC Direct current
EMC Electromagnetic compatibility
ICE Internal combustion engine
MCS Megawatt charging system
MRF Mesh refinement factor
UH U-height
xii | List of acronyms and abbreviations
Introduction | 1
Chapter 1
Introduction
In this thesis, the problem of thermal expansion in busbars for implementation
in automotive applications are investigated. A busbar is a metal rod that is used
to carry high amounts of current and can in some cases replace cables. The
surrounding environment of the busbar is assumed to be in a Battery Electric
Vehicle(BEV). The difference between a stationary installation of busbars and
vehicle installation is mostly related to higher temperature variations together
with varying performance parameters like weight, losses, and more. In this
thesis, geometrical alterations to the busbars are investigated together with
varying the material between copper and aluminium.
BEV are on the rise all over the world, and most companies are actively
working on it or planning to do so[1]. The change from the combustion engine
to propulsion by electric machine is a vital step in reaching a sustainable
mode of transportation. In this project, the use of a busbar for transferring
power within a BEV will be evaluated. The current solution is mostly to use
cables[2], but when the current demand is high, the resistive losses within
the cable result in the need for thicker conductors. This leads to expensive
and very bulky cables that are complicated to install. Another method is to
use busbars, these allow a more effective and compact way of transferring
DC power. An issue with using a solid busbar is that the high temperature
differences that can be present in a vehicle together with the resistive heating
of the conductor might result in high forces acting on the connection points
due to thermal expansion. These phenomena are investigated in this report.
2 | Introduction
1.1
Problem
The main issue with using a solid busbar within a BEV is that the solid
busbar will experience a length difference when its temperature varies[3]. This
temperature is affected by the external temperature around the vehicle, heat
generated from nearby components, and the heating that occurs as a result of
transferring high current through a conductor. The expansion can result in high
stresses on the connections of the units that are connected and the surrounding
mounts. The cross-section area of the busbar, dimension variations, total
length, and material will all affect the final stress values due to the thermal
expansion.
1.2
Research questions
RQ1: Can the change of the busbar shape to the U-shape sufficiently mitigate
the thermal expansion effects caused by high temperature differences without
introducing negative effects of increased length and current density heat issues
in the bends required for the U-shape?
RQ2: How does the dimensional change of the cross section affect the thermal
expansion for a rectangular shaped busbar?
RQ3: What material is most suitable to allow high current transfer together
with a low amount of thermal stress in the environment of a battery-electric
truck?
1.3
Purpose
Having a good understanding of how varying several base characteristics of
a busbar affect the final result will be a vital step in reaching higher charging
speed and power output in a BEV. From a wider perspective, the ability to
reduce the charging time is one of the most important problems facing BEV,
and the issue is even greater when looking at transportation and construction
vehicles where "uptime" is a key factor to reduce the cost and increase the
efficiency for the customer.
Reducing the amount of ICE-based vehicles and replacing them with BEVs,
Introduction | 3
fuel cells, or similar will be an important step in the fight against global
warming and air pollution.
The busbar could also be used to simplify complex systems where the high
heat generated within cables calls for the use of water cooling, active cooling
increases both complexity, cost, maintenance requirements and could reduce
the efficiency of the power transfer considering the power demand to drive the
system.
Having a good understanding of this topic can also aid the efficiency of battery
packs, as the busbars are in general more space-efficient.
1.4
Goals
The goal of this project is to create a simulation model that can be used to draw
conclusions regarding the thermal expansion behaviour for implementation
within a battery electric vehicle.
Subgoal 1: Create a simulation model that is sufficient to answer the defined
research questions.
Subgoal 2: Evaluate the usability of a U-shaped busbar.
Subgoal 3: Deliver simulation data that can be applied in practical applications.
1.5
Delimitations
The simulation model is looking at only one conductor, thus the interaction
between the conductors’ positive and negative sides is not investigated. If the
conductors are mounted close to each other, the heat generated by the busbars
might interact with each other to increase the total temperature.
Another delimitation is the modeling of the isolation material around the
conductor. This isolation can heavily affect the max temperature that will be
reached in the busbar. The reason this was not added is that the isolation
material would not be affected by the choice of material or variations of
geometry. Thus, this would not affect differences in results, altering the
geometry and material.
4 | Introduction
The terminal connectors are also not investigated in this report. The applied
solution in the simulation environment is similar to using a bolt and nut
connection, the current passing through the bolt is "solidly" connected to the
busbar.
There are also limitations in geometry, the length is defined by Scania as
the maximum length of the busbar, note that the thermal expansion issue
is more prominent the longer the busbar is. For the bend radius, U-height,
and cross-section area, the values are chosen to be reasonable from a BEV
implementation perspective.
Another aspect that is not considered is the potential cooling effect of having
the vehicle moving, this could, depending on implementation, cause something
similar to air-cooling. In this simulation model, there is no cooling of that sort
implemented. one reason is that the charging that represents the worst case
scenario is done stationary, thus no high amount of cooling would occur.
Another limitation is in the materials chosen, there could be other materials
that could be investigated, also the surface treatment is limited to one for
copper and one for aluminum.
Background | 5
Chapter 2
Background
BEV are not new, but the magnitude and type of implementation have changed
a lot in the last years. The most common way of transferring power is through
cables, there are several reasons for using cables, but the main benefit is the
flexibility. As cables are used to connect several components together the
tolerance of installation and cable does not have to be that fine to allow the
connection of cables. The use of busbars are more common in high-power
installations where the distances are small and the current high. It’s also
common in high power electronic units as a way of allowing a compact and
efficient way of transferring power.
When looking at busbar research, the most common topics are related to
inter-electronic use where short distances heavily reduce the consequences of
thermal expansion. Instead, problems are related to AC implementation and
EMC issues.
For long busbars where the length is much greater than the width and height,
the temperature-driven expansion will mostly occur in the length direction.
In reality, a volume-based expansion is present but the difference in the
dimensions other than length is very small. In this thesis, a volume-based
simulation model was developed using Comsol Multiphysics (from hereon
Comsol).
The difference between a stationary installation of busbars in a power station
and an installation within a BEV is related to isolation material, surface
treatment of the conducting material, and quality attributes that vary. For the
isolation material, many stationary installations use no isolation around the
conductors as can be seen in figure 2.1.
6 | Background
Figure 2.1: Busbars, a stationary installation
For the BEV implementation, the environment is harsh, the weight and cost
are important and the service life is long. This promotes the use of heavily
optimized power-transfer systems.
2.1
Cables
The most common way of transferring high power within a vehicle is by
using cables. The benefits are flexibility, that they are easy to connect and
the connectors are heavily standardized. The main principle is using multistrand cable, where several strands of copper or aluminium are tightly packed
in isolation material,this is what gives the flexibility of the cable.
There are also solid cables where there is only one strand of a high thickness
(depending on current demand). But these are unusual for high power
applications as they easily become hard to bend.
Depending on the application, the design of cables change heavily. For a DC
application, the resistance can be calculated using the following equation:
R = (RA L)/A,
where RA is the resistivity of the matierial, L is the length of the conductor
and A is the CSA.
As can be seen, the total CSA directly affects the resistance, and with that
the geometry of the cable. In the case of high power charging, the amount
of current will be high, this makes it inevitable to use thick conductors
Background | 7
and/or water-cooled cables. Using thick conductors results in cables that are
extremely hard to bend, and prices increase accordingly.
Using a liquid-cooled cable has been shown to solve thermal issues in charging
cables connecting to the vehicle inlet [4], but it affects the efficiency of the
power transfer as the generated heat is the result of losses. Also, the cost and
complexity of adding a cooling system to the power transfer should in itself be
a reason to look for alternative solutions.
2.1.1
Skin effect
Another important aspect to consider is what type of power is being transferred.
If the cable will transfer DC power, then the CSA and length is the most
dominant aspect to consider, but if the power is a high frequency AC, then the
skin effect will limit the design options and cause lower efficiency [5]. Skin
effect will cause the current to travel closer to the surface of the conductor, this
increases the resistance in the conductor causing higher losses and inefficient
transfer of current. the higher the frequency the greater the issue.
Using a cable with many individual conductors will mitigate this issue, but
looking at a busbar for a replacement will require redesign of the busbar by
using several thin layers or a wide flat busbar to allow a efficient power transfer.
2.1.2
Connectors
One benefit of using cables is that there are many standardized connector types,
this allows for rapid design when designing power-transfer for an electric
vehicles. One issue with connectors is that the connectors usually have higher
resistance than the cable that is connected. This leads to heat being generated
in and around the connector. Thus, the design of the connector becomes vital
when looking at the whole power transfer in the vehicle. The goal should be to
have as low resistance as possible to reduce the risk of damaging the isolation
material close to the connector. The connector design is not covered within
this thesis, and could potentially be a good next step for further development.
8 | Background
2.2
Megawatt charging system
The transport sector in the United States is responsible for 28 % of the total
greenhouse gas emissions. Among this sector, medium and heavy duty trucks,
boats and planes are responsible for 34 % [6].
These values show that not only the passenger cars needs to be overhauled,
but also the truck sector. One of the important factors to consider when
investigating heavy duty trucks is uptime. This is important as the customers
want their equipment to be ready to be used as much as needed, on demand. At
the same time, large heavy duty truck has to hold batteries many times larger
than any personal vehicle. These requirements indicate that the truck industry
cannot rely solely on the same charging infrastructure (delivering the same
power) as what is used for passenger cars, as the maximum charging power
would not be sufficient.
"For perspective, electrifying one large truck on the road is equivalent to
electrifying 25 passenger vehicles"[6]
One solution could be the Megawatt charging systems (MCSs). The goal is to
develop a charging connector that can deliver charging power in the megawatt
range, this would reduce the charging time substantially. In the preliminary
specifications for the MCSs the amount of current required would be between
1000 and 3000 ampere[6]. As this amount of current has to be transmitted in
the truck on the way to the battery, the development of the power transfer had
to be performed at the same time as the MCSs to reduce the "time to market".
2.3
Related work area
2.3.1
Busbar for inverter
There has been other research that investigate how a busbar can be used, how
to optimize it both with the help of simulations and practical experiments. One
paper is investigating the use of a busbar in an inverter [7]. In this article there
are several aspects that are investigated, and busbar designs are compared,
one of the aspects is how sharp cornering bends can affect the amount of eddy
current close to the bend, and that these bends can increase the losses and thus
the temperature of the busbar. This aspect is investigated in this thesis as the
Background | 9
bends radius is altered in three steps. The difference is that in this thesis the
DC, instead of the AC power transfer will be evaluated.
This article is also looking at simulation results and comparing them to
measurements of the stray inductance and stray capacitance. The results
between the simulations and the measurements are very similar with only few
percent difference for the capacitance, the values for the inductance were more
unreliable on lower frequencies, the simulations conducted in this article are
more complex than what is presented in this thesis, this is due to the use of high
frequency AC power being transferred in the busbar, this requires modeling of
the skin effect. In this thesis the simulations are not as advanced, this implies
that the validity of the simulation has potential to be of higher accuracy.
2.3.2
Thermal expansion in long busbars
Another paper is investigating the thermal expansion behaviour for long busbar
[3]. The focus is not on the geometry but rather on connection points, isolation,
interaction of using several busbars, and how to support the busbar when
mounting it to a construction. The amount of current is also considerably
higher at 200kA compared to 1KA. As the amount of current is so dominating,
the use of flexible and welded plate joints are investigated heavily. The results
show that the connection points of the busbar are the weak point of the
construction, and that extra care has to be taken to these. Another interesting
part is that this paper is also using FEM based simulation to obtain the results.
2.3.3
Mechanical modelling of busbars under shortcircuit conditions
This paper is investigating the electrical, mechanical, thermal, and magnetic
effects of having a busbar in a short-circuit load case[8]. The simulationdriven analysis is also using the Comsol software to perform the analysis. The
main difference is the load case as the short-circuit peak current is taking place
after 0.01 seconds.
Another difference is that in this paper, the busbars are used in a three-phase
system, where all the conductors are modeled in the simulation environment.
The short circuit is causing Lorentz forces to act on the busbars in the shortcircuit moment. This effect is not analyzed in this thesis.
10 | Background
Other similarities are busbar size and nominal current, the busbar is used with
a current of 1650 ampere, but the short circuit current is as high as 50KA.
The cross-section shape is 120x10 mm on each conductor In this paper, a
comparison between the simulation results and manual calculations on the
deflection showed a 0.9% difference. As this model is set up in a similar way
to the model in this thesis the reliability of the simulation results increases.
2.3.4
Copper busbars
In this guide for the design of busbars, there is plenty of general practical
information[9]. In the maintenance section, the issue with copper and
aluminium natural oxidization is described. The first issue with oxidized
copper is that the surface is really good at isolating heat in the busbar. This
promotes the use of any kind of plating in order to protect the copper surface.
The section also describes the phenomenal thermal properties of the natural
oxidization of aluminium. An issue with the high conductivity of aluminium
oxide is the potential risks connected to arcing. According to this guide,
oxidized copper is much less likely to sustain arcing than oxidized aluminium.
Note that this guide is produced by the copper alliance, so they might be biased.
The paper is covering many important aspects, including the skin effect and
how different shapes, skin dept, the proximity of poles, and more. Also, the
CSS is mentioned under section "shape factor for rectangular bars" where the
following sentence can be found;
"There are no analytic formulae for this case. Resort must be made to
numerical methods such as finite elements, finite differences or current simulation,
i.e. particle elements"
This sentence argues the use of simulations to figure out the effect of altering
the CSS, which is covered in this thesis.
2.3.5
High Thermal Conductivity Insulation
This paper investigates different isolation materials to combat thermal issues
in electric vehicle cables[10]. This aspect is not covered in this thesis but
shows the importance to reduce temperatures in the conductor, for one of the
materials the thermal conductivity was enhanced by 90%. This should be
seen as a compliment and not an alternative solution to reducing conductor
Background | 11
temperatures, this paper was covering cable isolation but there should be no
issue in deploying the same materials for busbars. it can even be easier on
busbars as the isolation does not have to be flexible.
Another important aspect to consider is the safety of these isolation materials
as they will work in a harsh environment, and has to last for a very long time.
This is not covered in this article.
12 | Background
Method | 13
Chapter 3
Method
3.1
Quasi-experiment
To be able to answer the research questions, a quasi-experiment approach was
chosen. The reason is that a full-scale experiment was not viable due to several
reasons.
In the tests performed, the geometrical factors are altered in three steps. The
size of these steps are chosen from the perspective of implementation.
The only factor that is altered during each test is the factor corresponding to
that specific test. This is done to make sure the result is isolated from other
factors. All combinations of variables are not analyzed by a "full factorial
experiment" the approach can not be considered as a standard experiment but
rather a Quasi-experiment.
In this thesis, the variables can be divided into independent variables, dependent
variables, and control variables.
The dependent variables are what is affected by the independent variables, the
dependent variables are stress, temperature, losses, and displacement.
Control variables are current, ambient temperature, and initial temperature.
Depending on what test is performed, the geometries or materials that are not
altered will also be a part of the control variables. For example, in test 1,
the CSS is altered, then it becomes an independent variable, where the other
factors covered in the other tests) become control variables.
14 | Method
3.2
Research Paradigm
The research paradigm in this thesis is Positivism. It can be described
as "Positivism (objective), "it assumes that the reality is objectively given
and independent of the observer and instruments. The researchers test
theories, usually in a deductive manner, to increase predictive understanding
of a phenomenon. The view of positivists is used in projects that are of
experimental and testing character" [11].
3.3
Research Process
The approach of this thesis can be summarized in figure 3.1.
The literature study was conducted to find related work both on cables and
busbars, but also on the environmental challenges of implementation in a
vehicle. The use case was important to define in order to limit the evaluation
for the scope of this thesis. This process was carefully conducted together with
personnel at Scania.
The goal was to find a generalization of a case that that would be representative
of the real-world. The results from this study gave the initial geometrical
boundaries. By combining the literature study and the use case, the framework
of the worst-case study could be set. The detailed worst case scenario were also
defined together with Scania personnel working with cable harness.
The next step was to develop the design options that would be evaluated by
the use of multiphysics simulations. This led to several iterations in order to
refine the CAD models and the simulation platform. When the models were
working properly the final simulations results could be derived.
Method | 15
Figure 3.1: A flowchart depicting the steps of the research process.
16 | Method
3.4
Data Collection
In order to perform a large amount of tests, and derive relevant data, a
simulation-based approach was preferred. The software Comsol was chosen
as it is widely accepted as a proper simulation tool for both educational and
professional use. Before the final values were derived, a mesh analysis was
conducted to make sure the mesh is not affecting the precision of the results.
3.5
3.5.1
Assessing reliability and validity of the
data collected
Mesh refinement analysis
One way of investigating how fine the mesh has to be is to perform a mesh
refinement analysis, this can be done by altering the parameters defining the
mesh manually or by the use of a parameter sweep. The first step is to
define a base set of values, in the interface of Comsol, there are predefined
recommended values depending on the required precision. To alter the mesh,
a Mesh refinement factor is added to the max/min element size. increasing
the value will lower the max/min element size. by analyzing the simulation
output the user is able to determine approximately how fine the mesh has to
be. In this report, both the actual values of simulation data output and a visual
representation of the difference of altering the MRF will be shown.
3.5.2
Validity of method
As the tests are performed by only altering one factor at a time (independent
variables), the interaction between the different independent variables are not
analyzed, and the control variables are kept constant in all cases. The external
validity is considered high as the effect of thermal expansion is important in
both small and large components, the power transfer does not have to be as high
as represented in this report. The important factors are the proportion between
the amount of current and conductor size. This will define if the expansion
issue is to be considered or not. In general, the results can be translated in
most busbar design issues, where heat is an important factor.
Method | 17
The internal validity is harder to review, in order to increase the internal
validity practical tests should be conducted to evaluate how accurate the results
are. One Way the internal validity is analyzed is by separating the two effects
caused by altering the CSS. These are differences in inertia and larger surface
area.
Construct validity is also considered to be high as the simulation software
is straightforward in what it measures, and how it is done. These types of
simulations are not unusual and many guides can be found in order to make
sure the software is set up correctly.
Conclusion validity can be broken down into two questions, first, is there a
chance that the concluded relationship is in fact true?
Secondly, is there a risk that there is a relationship that is not shown/covered
in the tests.
As the testing is limited within a certain range of geometry (for example the
bend radius used) there could be other conclusions drawn if the smallest and
largest bend radius were expanded. Having for example a 90-degree turn of the
busbar is not analyzed. Also, altering the length of the busbar could potentially
show or hide effects that are covered/uncovered in this thesis.
3.5.3
Reliability of method
The simulation output data is heavily affected by the control variables that are
initial and ambient temperature, current, surface emissivity
The values that are presented in section 5.1 are based on a really pessimistic
premises. To use the simulation model in order to derive practical values
with higher precision the busbar specifications needs to be defined with higher
precision. The goal of the simulation model has been to set it up as "real to
life" as practically possible, but in order to fine tune the simulation parameters,
some practical tests could be done to increase the validity of the simulation
parameters.
The reliability of the simulation model in itself is very high, the type of
simulations that are covered in this thesis are not unusual.
The largest threat to validity in this thesis is probably the comparison between
the aluminium and the copper, the reason is that the surface treatment is heavily
affecting the end temperature, and thus also the amount of expansion resulting
18 | Method
in the maximum stress. In order to increase the validity of the comparison
between the aluminium and the copper, a further investigation into different
surface treatments in combination with different isolation materials could be
conducted.
Another aspect to investigate is the interaction of positive and negative side of
busbar being routed close to each other, this cross interaction might result in
higher temperature.
3.5.4
Data validity
As the input data for the simulation is not based on any real worlds measurements
the validity is hard to interpret. The data used is based on a worst case
scenario derived together with personnel at Scania. The data derived from the
simulation model is reliable as the method for setting up the model is known.
But to increase the reliability a physical experiment should be performed in
order to compare the simulation results with real world experiments. The
results from the physical measurements can be used to fine-tune the simulation
parameters.
3.5.5
Reliability of data
As the tests are performed in a simulation environment the repeated simulation
gives the same results as expected. The mesh settings are the same in all the
tests covered in this thesis.
Implementation | 19
Chapter 4
Implementation
4.1
Hardware
The first mission was to analyze the system to understand what challenges
and possibilities there were, and why the standard solution of cables were
insufficient in some cases. This information, together with contact persons
at Scania led to the busbar design space specifications. These specifications
are both covering dimension limitations, environment temperature span, max
current and more. This information was used to decide what parameters to
change and what to alter in the simulation environment.
From a design perspective for implementation in a BEV the geometrical
limitations result in higher demand for optimization in the power transfer
medium. In this report three main geometrical alterations are evaluated, these
are cross section shape, U-shape busbar and bend radius, these are compared
to a standard straight busbar with the same length and cross section area.
4.1.1
Cross Section Shape
The first reason CSS is analyzed is due to the change of force required to bend
an object with an altered cross section inertia. Note that the cross section area
is not changed as this would alter the total resistance of the conductor. The
CSS inertia can be manually calculated using the following formula:
20 | Implementation
J = (W H 3 )/12
where "W" is the width, and "H" is the height of the busbar cross section, as
can be seen in figure 4.1.
Figure 4.1: Test1: Busbar, cross section view.
note that the "H" is the length acting in the same direction the bending is
present in. For a rectangular shaped busbar experiencing thermal expansion
this equation explains why the busbar is only bending in one direction. This
effect is exploited in this thesis to analyze if altering CSS can be used to
mitigate thermal expansion issues.
Another benefit of using a more flat shaped busbar is that the total surface area
is greater, maintaining the CSA. This ensures that more heat is radiated away
from the surface, this effect is described by Stefan-Boltzman law:
Pterm = σ(T 4 − T04 )A
where referes to the emissivity value of the material, σ is the StefanBoltzmann constant and A is the total surface area. T is the temperature of
the busbar and T0 is the temperature of the surrounding environment.
One interesting aspect of altering the CSS is that the stress will be reduced
Implementation | 21
due to both effects, and it might be hard to figure out if the potential benefits
of altering the CSS is due to the altered inertia or the lower temperature as
a result of the larget total surface area. One solution to this is to isolate the
altered inertia effect by lowering the current to zero and increase the ambient
temperature, this ensures that the temperature difference is zero, and thus no
effect from the change in surface area is noted. The isolated effect of altered
inertia can now be shown.
In this report three different CSS are investigated, the three geometries are
chosen to be reasonable for implementation in a BEV. The difference between
the least and most flat-shaped busbar can be seen in figure 4.2.
Figure 4.2: Cross section shape difference
4.1.2
U-height
The second parameter to be analyzed is the height of the "U". The idea is to
investigate if the height can be used to reduce stress by having a longer leaver
arm for clamping of the "U". It is not obvious that a tall "U" is a viable option
as the total length of the busbar will increase, this directly results in a higher
total resistance as can be seen in the following equation:
R = (ρL)/A
where ρ is the resistivity of the material, L the total length of the conductor
and A the cross section area.
This increased total length together with the higher resistance will lead to
a longer total free length expansion. This expansion is explained by the
following equation:
∆L = αLO ∆T
22 | Implementation
where ΔL is the change in total length, α the coefficient of thermal expansion,
L0 the initial length, and ΔT the difference in temperature acting on the object.
Note that this equation is accurate for parts where the length is much greater
than the other dimensions of the object.
The three steps of U-height can be seen in figure 4.3
Figure 4.3: U-height steps
4.1.3
Bend radius
Altering the bend radius is interesting from several perspectives. One of the
issues with having a sharp corner in any electrical conductor is that the current
density in the bend increases, as a result, local hot spots can occur. Another
issue with having a bend that is too sharp is that it can be hard and/or expensive
to manufacture. In this thesis, the bend radius is investigated to see how it
affects the different parameters that are simulated. The three analyzed bend
radius steps can be seen in figure 4.4
4.1.4
Material
For the main part of the thesis, a copper busbar is used, the reason is that the
copper material is the most common to use in busbars. An aluminum busbar
will also be compared to the standard copper busbar. Aluminum is a lighter and
cheaper than copper but has a higher electrical resistance. For both materials,
Implementation | 23
Figure 4.4: Bend radius steps
the standard values regarding conductivity, thermal expansion coefficient and
more are used, but when looking at emissivity the surface treatment of the
material will affect how the busbar radiates heat to the environment. For
the simulation of the copper material, a nickel-plated copper was chosen as
it works well in challenging environments [12]
For aluminum, an anodized surface treatment is widely used as it does not
rely on any external material being put on the surface but rather uses the
aluminum’s natural oxidization to create a thin, hard, and durable surface
suitable for use in a vehicle. [13].
Two main tests were performed on the aluminum busbar, the first test was done
by using the aluminum with the same Cross section areas (CSAs) and general
shape as the copper busbar. The main reason is to get a good overview of the
differences between the materials considering the same geometry. This will
make it easier to decide on the proper solution if the design space is limited.
The second test will be performed by compensating the CSAs for the higher
resistance value of the aluminum. This implies that in order to reach the same
resistance as a copper busbar the CSAs must be larger. But how should the size
increase be done in a proper way? The approach in this report is to compensate
the CSS to reach the same inertia as the standard copper busbar. This is done
by solving a system of equations for the inertia (J) and the resistance (R) where
the common parameters are the Width (W) and height (H):
24 | Implementation
J = (W H 3 )/12
R = (pL)/(W H)
this ensures that the difference in performance parameters are not a direct result
of a generally altered inertia.
4.2
Simulation platform
For the simulations, Comsol was chosen as the preferred simulation platform.
The software is widely used in both educational and professional environments.
In the Comsol simulation platform, there are many types of physics that can
be modeled. For this thesis a multi-physics simulation could be developed
using three main physics nodes, these are "Electric current", "Heat transfer in
solids", and "Solid mechanics".
In the "Electric current" node, the connectors, and current are defined. For the
"heat transfer in solids" node all the temperature-related information is present,
both initial temperature, ambient temperature, surface emissivity, heat flux,
and material properties.
The solid mechanics node holds all the mechanical constraints, for this
simulation. The connectors are defined as "fixed constraint" this assumes that
the connectors do not flex or move as they are experiencing force acting on
them. Note that the constraint is working on a surface and not in a volume.
The simulations are performed as “stationary”, this implies that the simulation
will run until the simulation data outputs are not being altered by a defined
margin. This gives simulation data that is representing the "end" scenario,
and as the input data is constant over time we will have the worst-case scenario
presented in the output data.
The interface of the Comsol software can be seen in figure 4.5
In order to set up this simulation model the base physic modules must be
added. these are "Electric current", "Heat transfer in solids" and "solid
mechanics". the next step is to define the control variables, these are ambient
temperature, current, and initial temperature. these variables are not altered
Implementation | 25
Figure 4.5: Comsol interface
in the performed tests and can be globally defined in the "parameter" node.
the next step is to define the material, and to make sure the material properties
are sufficient to perform the simulation. If any property is missing it can be
manually added.
The next step is to define all geometrical, mechanical and electrical boundaries.
These are both connection points for the current, fixed mechanical constraints,
"surface to ambient radiation" and "heat flux". The defined values can be seen
in the section 5.1 system documentation.
The "surface to ambient radiation" is heavily affected by the surface treatment
of the metal, this is why the copper and aluminium surface treatments were
specified, and the treatments were suitable to be used in harsh environments
as is the case with a heavy duty vehicle.
4.3
Simulation data
As the Comsol software is a multiphysics platform a huge amount of data can
be derived from the simulations. In this thesis, the following areas have been
investigated to be able to answer the research questions.
26 | Implementation
4.3.1
Losses
The volumetric losses in the busbars are calculated to derive a value of the
losses at the steady-state of the model. These losses are in the form of Joule
heating, and as a result, the temperature of the busbar is increasing. Keeping
this value as low as possible increases the total efficiency of power transfer in
the busbar. If we consider a standard conductor, this value is mostly affected by
material properties, CSAs and length. Another aspect to consider is how the
resistance is changed by temperature. A material’s resistance is not constant
but increases with temperature. This change of temperature can be calculated
using the following formula:
R = Rref (1 + α(T − Tref ))
where "R" is the resistance at a given temperature defined by "T", Rref is the
resistance at a defined temperature, usually at 20 degrees Celsius. α, is the
material-dependent temperature coefficient. The change in resistance is taken
into account by the simulation software. As a result a more reliable steady
state result can be expected.
To get a better understanding of the losses a visual representation can be seen
in figure 4.6, note that the losses are not uniform in the busbar. The reason
is that the current density is not the same due to the current taking the closest
path from input to output.
Figure 4.6: Losses in a busbar
Implementation | 27
4.3.2
Temperature
The temperature of the busbar is investigated as it is one of the most important
factors to consider when modeling a conductor. If the busbar will be used for
high power charging keeping the temperature low will allow a high charging
capacity over a long period of time. The temperature presented in chapter
5 is the temperature that the busbar reaches at a steady state. The reason
the temperature is increasing is due to a difference in initial temperature and
ambient temperature, as well as joule-heating due to current being transmitted
in the busbar. The thermal power heating up the busbar is a direct effect of the
losses. These losses can be calculated by using the following formula:
P = RI 2
where P is the loss power, "R" the total resistance of the busbar and "I" is the
current passed in the busbar. Note that this is the same loss covered in section
4.3.1.
4.3.3
Stress
To evaluate the mechanical stress as a result of the thermal expansion, the
von Mises stress is derived from the model. As the conducting material is
isotropic, the von Mises stress value will be sufficient to compare different
design options. If the material is experiencing a von Mises stress value higher
than the yield strength of the material, it can be assumed that the material is
experiencing plastic deformation. In the simulation environment, the exact
position of the maximum value is presented, an example of a stress plot can
be seen in figure 4.7.
Understanding what affects the amount of stress is important to reduce the risk
of mechanical failure in the busbar or the surrounding components.
4.3.4
Displacement
The displacement of the busbar is investigated to get a better understanding of
how the different design options affect how the busbar expands.
28 | Implementation
Figure 4.7: Stress plot
To see how the busbar moves during thermal expansion, a displacement
magnification can be added. An example of this can be seen in figure 4.8
Figure 4.8: Busbar displacement
The expansion of the busbar can also be important from an implementation
perspective. As the BEV is exposed to high amounts of vibrations, support of
the busbar will have to be added in many scenarios. These supports can have a
negative effect on the maximum stress if it restricts the busbar from expansion
in certain positions. Optimizing the support placement will allow the U-shape
to work unrestricted.
Results and Analysis | 29
Chapter 5
Results and Analysis
5.1
System documentation
The general load case scenarios can be seen in table 5.1. These factors were
determined during the worst case analysis. The low initial temperature could
be reached when the truck is operating in low temperature environments. The
high ambient temperature could be reached by heat generating components
mounted close to the busbar. The simulation input represents a worst case
combination of these extremes.
Load type
Value
Unit
Current
1000
Ampere
◦
Initial temperature
-40
C
◦
Ambient temperature 80
C
Table 5.1: Load case specifications
The geometrical base specifications can be seen in table 5.2. These values
were also derived together with Scania personnel.
Geometry specification
Value
Unit
Busbar length
700
mm
Connector diameter
25
mm
Cross section area
600
mm2
Table 5.2: Load case specifications
30 | Results and Analysis
Note that both the load case and geometrical specifications were not altered as
a part of this thesis.
5.2
Test protocol specifications and visual
representation
The tests are divided in 5 parts.
The first test is investigating three types of CSS. These can be seen in figure
5.1, and the specifications in table 5.3. Note that the CSAs is not altered.
Figure 5.1: CSS1, CSS2 and CSS3 from left to right.
CCS type
Dimension Unit
CCS1
20×30
mm
CCS2
10×60
mm
CCS3
5×120
mm
Table 5.3: Cross section dimensions
Results and Analysis | 31
The second test is looking at the effect of altering the height of the "U". The
test options can be seen in figure 5.2 and the specifications can be seen in table
5.4.
Figure 5.2: Test 2: Height options for the U-shape
U-height
Dimension Unit
options
UH1
50
mm
UH2
100
mm
UH3
150
mm
Table 5.4: U-height specifications
The third test is looking at the effect of varied BR for the bends required to
model the U-shape of the busbar. The three options are represented in figure
5.3 and table 5.5.
Bend radius Dimension Unit
options
BR1
20
mm
BR2
30
mm
BR3
40
mm
Table 5.5: Bend radius specifications
32 | Results and Analysis
Figure 5.3: Test 2: Bend radius options required for the U-shape
Test 4 consists of a comparison between the U-shape busbar with a standard,
straight busbar. The length between the connectors are the same as for the
U-shape busbar. The CSS is varied in the same three steps as for the U-shape
busbar. In figure 5.4 a straight busbar can be seen.
Figure 5.4: A straight busbar used as a reference in test 4.
The last test is investigating the use of aluminum instead of copper in the
busbar. As the aluminum has a higher resistance, two options were analyzed.
One with the same geometry as the standard copper busbar, and one where
the CSAs and inertia were compensated. The standard busbar and the
Results and Analysis | 33
compensated one can be seen in figure 5.5, the specifications are present in
table 5.6
Figure 5.5: Test 5: Compensated aluminium busbar next to standard copper
busbar.
Aluminium
busbars
CrossUnit
section
area
AL1
600
mm2
AL2
922,3
mm2
Table 5.6: Aluminum busbar specifications
In order to compensate the resistance to reach the same value for the copper
and aluminium busbar, the following equations were used:
RAlu = (ρAlu L)/AAlu
RC op = (ρC op L)/AC op
these equations are set equal to reach the final equation for the aluminum CSA:
AAlu = (ρAlu AC op )/ρC op )
34 | Results and Analysis
in order to find an inertia value the following equations were used.
The first step is the general equation for the inertia (J) of a rectangular shape:
JAlu = (bAlu HAl 3u )/12
this can be rewritten as:
JAlu = (AAlu HAl 2u )/12
where b is the width of the busbar and H is the height.
As the aluminum CSA was calculated and the inertia is set to be the same as
for the copper busbar the value of H can be calculated.
After this value is defined the last step was to calculate the value for b using:
bAlu = AAlu /HAlu
the final cross section dimensions for the compensated aluminum busbar is
8,07 ×114, 34(mm).
The total combination of values in order to perform each test can be seen in
figure 5.6, As can be seen, only one of the factors are changed in every test.
Figure 5.6: Test 5: Test protocol summary specifications.
Note that in most cases the parameters that are not varied as part of the tests
are chosen as the middle value. For example, in test 1, the UH2 and BR2 are
chosen and not altered in that test. The only exception is in test 2 where the
BR2 and BR3 could not be combined with UH1,due to geometrical limitations,
thus the BR1 value had to be used. As a result, the BR1 value was present in
Results and Analysis | 35
all UH tests to make sure that no difference in the results was caused by factors
not covered by the specific test.
5.3
Test 1: Cross section shape
The first test was performed by only altering the CSS. The results from the
tests can be seen in table 5.7
CCS type
CSS1
CSS2
CSS3
Stress
[Mpa]
1001
768
617
Temperature Losses
Displacement
[◦ C]
[W]
[mm]
153,2
34
1,68
133,6
32,6
1,41
112,0
31,7
1,14
Table 5.7: Test1, CSS results
Weight
[Kg]
4,578
4,578
4,578
The results from the stress analysis show that the altered CSS reduce the stress
quite dramatically. The stress reduction from CSS1 to CSS2 is around 32 %.
and from CSS1 to CSS3 38 %. This analysis showed the benefit of altering
the shape of the conductor. In the temperature section the effect of having a
flatter busbar construction results in dramatically reduced temperatures. This
difference is one of the reasons why the stress becomes lower in CSS3. The
differences in Losses are very small, the reason why there is a difference is
due to how the simulation model is constructed. One material factor that
was altered was the resistance of the material. A fine approximation is to
assume that the resistance is constant within a span of temperature, but a more
robust approximation is to alter the resistance with the given temperature of the
busbar. This is the reason why we see a higher loss power in CSS1. As higher
temperature increases the resistance of the material the losses will increase.
The three stress plots can be seen in figure 5.7, 5.8 and 5.9.
36 | Results and Analysis
Figure 5.7: Test1: Busbar with CSS1
Figure 5.8: Test1: Busbar with CSS2
Figure 5.9: Test1: Busbar with CSS3
The result differences in the stress and temperature are interesting as it shows
a strong effect on important performance parameters, without adding weight
Results and Analysis | 37
to the busbar. But how can we be sure what is causing the lower stress? There
are two main effects at work, altered inertia and temperature difference causing
lower thermal expansion. One way to investigate this is to neglect the current
and force the temperature to values in the range of the result values. This will
result in neglected effect of having a temperature difference, and thus, only
the difference in inertia would have the chance to alter the results. The results
from this sub-test can be seen in table 5.8.
CCS type
Stress[MPa] Temperature [◦ C]
CCS1
761,6
120
CCS2
682
120
CCS3
673
120
Table 5.8: Forced temperature, stress results.
The results are interesting and show that the maximum stress that occurs in
the connectors are being reduced, but not by any large margin. But, analyzing
the plot of the stress shows that the stress in the "U-bend" is being reduced
dramatically, as can be seen in figure 5.10 and 5.11.
One way these results are interesting is in the case where an active cooling
system is present if the system allows a certain max temperature and has the
ability to maintain the busbar at that temperature, then the difference in max
stress between the CSS are mainly affected by the inertia of the busbar. The
last part of the test shows that altering only the inertia will reduce the stress,
but mostly at the U-bend and that the max stress will not be affected by any
large margin.
In general, the effect of changing the CSS has a strong effect on the busbar’s
main performance parameters, stress, and temperature. Having a larger surface
area as a consequence of modeling the busbar in a less square shape helps to
increase the convection and heat radiation.
38 | Results and Analysis
Figure 5.10: Stress plot, forced temperature, CSS1
Figure 5.11: Stress plot, forced temperature, CSS3
Results and Analysis | 39
5.4
Test 2: U-height
The second test is investigating the effect of altering the height of the "U". The
results can be seen in table 5.1.
CCS type
UH1
UH2
UH3
Stress
[Mpa]
799
760
781
Temperature Losses
Displacement
◦
[ C]
[W]
[mm]
133,2
29,2
2,64
133,6
33,3
1,42
134,0
37,4
1,07
Table 5.9: Test2, U-height results
Weight
[Kg]
4,13
4,67
5,21
Looking at how the stress changes with the height of the U, it becomes apparent
that altering the height of the U-shape does not necessarily result in a decreased
amount of stress. The amount of stress is being reduced by about 5 % between
the UH1 and UH2, but as the height is increased further, the stress starts to
increase again. There are also other issues with increased U-height. We can
see increased losses of about 22 % between UH1 and UH3. The reason why
we see this difference is due to the increased total length of the busbar. This
causes a higher total resistance and thus also increases the loss-power. One
might ask why this increased loss-power doesn’t cause a higher temperature
in UH3, the reason is that, as we increase the length of the busbar, we are also
increasing the total surface area, and thus increasing the relative convection
and radiation.
Analyzing the displacement shows that increased UH can reduce the amount
of displacement. The difference between UH1 and UH3 can be seen in figure
5.12 and 5.13.
When analyzing the displacement figures, the effect of having the U-shape
becomes visible, we can see the compression of the "U" that acts as a relief
for the thermal expansion that occurs as a result of Joule heating. If this
amount of deformation not causing plastic deformation depends on the specific
material, there are many types of aluminium and copper, and different types
of manufacturing methods that affect mechanical properties of the material.
40 | Results and Analysis
Figure 5.12: Test 2: Displacement UH1, Magnified 15×.
5.5
Test 3: Bend radius
In this test, the effect of altering the bend radius in three different steps are
investigated. The results can be viewed in table 5.1.
Bend
radius
type
BR1
BR2
BR3
Stress
[Mpa]
760
768
761
Temperature
[◦ C]
Losses
[W]
Displacement Weight
[mm]
[Kg]
133,6
33,3
1,42
133,6
32,6
1,41
133,6
32,0
1,41
Table 5.10: Test3, Bend radius results
4,67
4,58
4,49
As can be seen, the difference in results between the bend radius options are
minor. The biggest difference in the results are among the losses which are
smaller in BR3, but by a low margin. The difference is a result of having a
shorter busbar as the sharper bends cause a total shorter path for the current
to take.
When looking at the current density in figure 5.14 and 5.15, we can see how
the current density differs between BR1 and BR3. This difference in current
density does not seem to have any effect on the busbar with the bend radius
span covered within this thesis.
These results show that in the span of the bend radius analyzed in this
thesis, the effect difference is negligible. There could be other benefits of
altering the bend radius in AC applications when considering Electromagnetic
comparability issues, but that is not covered in this thesis. This also implies
that the manufacturer can adjust this value according to their available equipment
and requirements for the specific installation.
Results and Analysis | 41
Figure 5.13: Test 2: Displacement UH3, Magnified 15×
Busbar
type
U-shape
Straight
CSS1
Straight
CSS2
Straight
CSS3
Stress
[Mpa]
768
2160
Temperature
[◦ C]
133,6
152,2
Losses
[W]
32,6
27,8
Displacement
[mm]
1,41
2,35
Weight
[Kg]
4,58
3,77
1380
132,9
26,5
0,26
3,77
1135
111,1
25,9
0,22
3,77
Table 5.11: Test4, Straight busbar compared to the U-shape
The benefit of using the u-shape becomes quite apparent, the most fair
comparison is between the U-shape and the "straight CSS2" as they use the
same CSS. The difference in stress is 44%. The stress reduction, however,
translates into a significant increase in the losses (18% higher for the u-shape
busbar) Also, the weight is increased by around 18%. But this relative weight
increase would be reduced depending on the total length of the busbar, and by
decreasing the amount of U-height. This would also reduce the difference in
power loss while maintaining a lot of the stress reduction on the connectors.
The displacement of the straight busbar can be seen in figure 5.15 And the
stress in figure 5.15.
42 | Results and Analysis
Figure 5.14: Test 3: Current density, Bend radius 1
Figure 5.15: Test 3: Current density, Bend radius 3.
Results and Analysis | 43
Figure 5.16: Test 4, Straight busbar displacement, Magnified 15×
Figure 5.17: Test 4, Straight busbar stress plot
44 | Results and Analysis
5.7
Test 5: Material
The last test is investigating the effect of replacing the copper material with
aluminum. The aluminum’s surface is anodized. The copper is nickel-plated,
both these options are suitable for harsh environments as we have in a vehicle.
The results of this evaluation can be seen in table 5.1.
Busbar type
Stress
[Mpa]
768
591
445
Temperature
[◦ C]
133,6
123,9
97,2
Losses
[W]
32,6
50,2
31,4
Displacement
[mm]
1,41
1,76
1,35
Copper
Aluminum
Aluminum
compensated
Table 5.12: Test5, Material analysis, copper and aluminum.
Weight
[Kg]
4,58
1,39
2,13
The definition and calculations for the compensated aluminum busbar can
be seen in 5.2 Considering the standard-shaped aluminum busbar against the
same busbar using copper, the stress levels are reduced around 23%. Also,
the temperature is reduced by approximately 10 ◦ C. The biggest difference is
in the losses section, here we can see a huge increase of around 54%. As the
geometry is the same and the losses are higher in the aluminum busbar we
should expect higher temperatures, the reason why this is not correct is due
to the difference in surface emissivity. This difference is a result from having
different materials and surface treatments.
This cools down the aluminum busbar, in this case by such a large margin that
the end temperature is lower than the copper busbar. The reason why the losses
are higher in the first place is due to the higher resistance value of aluminum.
If we look at the compensated aluminum busbar we can see an even greater
reduction in stress, this is expected as the higher CSAs results in lower
resistance. The temperature value is very low and the first busbar configuration
that remains below 100 ◦ C. The losses do not exceed those of the reference
copper busbar, this value should be exactly the same as the resistance value
was compensated to match the copper busbar. The reason why we see a small
difference is due to how the resistance changes with temperature. We can also
see that even though the busbar is performing better on all parameters, we
have to remember that the aluminum busbar is much larger. Another aspect
to consider is the lower weight of the aluminum. Weight is an important
Results and Analysis | 45
performance parameter in vehicles, and especially in BEV. The size difference
can be seen in figure 5.5.
5.8
5.8.1
Reliability Analysis
Mesh refinement
The mesh refinement analysis is performed in order to ensure that the simulation
results do not contain unacceptably high computational errors. This will
ensure the validity of the simulation from a mesh perspective.
The base settings used for the mesh can be seen in figure 5.18
the MRF value is altered between (1) and (12) with steps of (1) in this analysis.
Figure 5.18: Mesh analysis base settings
The values that are not being altered by the MRF are standardized values in the
simulation software corresponding to a “really fine” mesh. This was chosen
to make sure that these values do not limit the max/min element size study.
The difference between MRF = 1 and MRF = 12 can be seen in figure 5.19
and 5.20. Note that even though the MRF = 1 results in a very coarse mesh
around the general structure, there are small elements that translate the round
shape of the connector in a proper way, this is due to the high precision values
46 | Results and Analysis
corresponding to the “maximum element growth” rate, “curvature factor”, and
“resolution of narrow regions”.
Figure 5.19: MRF = 1
Figure 5.20: MRF = 12
The next step was to look at how the varied MRF value affected the simulation
output. The first one can be seen in figure 5.21 and represent the total losses
through joul heating for the busbar. As can be seen, the change from MRF =1
to MRF =2 is pretty dramatic but after that, the value converges in a proper
way.
One way of visualizing the difference between using low and high MRF values
is to plot them together, one with MRF =1 and one with MRF =12.Difference
between the solutions can be seen in figure 5.22 (temperature) and figure
5.23 (Von mises Stress) in figure 5.22 the temperature variation between the
Results and Analysis | 47
Figure 5.21: Losses calculation for different MRF values.
solutions are neglectable as the difference is lower than 0.12 Kelvin, but as
expected the difference is higher in regions where there are bends as the
geometry is more complex. For figure 5.23 the difference in stress is huge
where the shape is more complex, note the high value on the sidebar. This
implies that stress calculations require a higher degree of mesh refinement.
Figure 5.22: surface temperature difference between MRF value 1 and 12.
48 | Results and Analysis
Figure 5.23: Stress difference between MRF value 1 and 12.
Altering the MRF value to 5 vs 12 yields a much more steady result. The
difference (or lack of difference) can be seen in figure 5.24
Figure 5.24: Stress difference between MRF value 5 and 12.
As the value of MRF starts to converge when exceeding (2) any value equal
or higher than that is good enough for this simulation model. Depending on
the computer hardware using a really high value will result in slow simulation
runs, and unnecessary energy consumption.
The final values that were chosen can be seen in figure 5.25, and the corresponding
mesh in figure 5.26.
Results and Analysis | 49
Figure 5.25: Final mesh settings.
Figure 5.26: Mesh as a result of final settings.
50 | Results and Analysis
Conclusions | 51
Chapter 6
Conclusions
The future of the propulsion systems for vehicles are most likely to be electric,
this could be hydrogen, battery-electric, or something else. The development
in the vehicles are not only connected to propulsion, we see a fast development
of autonomous driving, and implementation on small scale has already started.
The introduction to autonomous driving in the truck sector could lead to
around-the-clock operation as the driver is separated from the vehicle. If a
BEV is going to operate as much as possible the charging time becomes even
more important. Optimization in this field becomes vital for the successful
implementation of these kinds of vehicles. In this thesis, one way to handle
this has been investigated, but there are many ways to allow a higher efficiency
of the power transfer.
The future for the power transfer could lie in active cooling of the conductors,
optimization of shape and isolation material, controlled charging cycles with
situation-dependent feedback. Or most likely a combination of the above.
Implementing a simulation-driven optimization of the conductors in a vehicle
could aid the cable harness design of electric vehicles, the benefit of having
a simulation-driven approach is the reduction of losses, temperatures, and
weight. Even tho the thermal expansion would be solved by implementing
soft elements like a layered busbar the shape optimization will have a large
effect on the overall performance of the power transfer.
52 | Conclusions
6.1
Research questions conclusion
The first research question covers the evaluation of the U-shape busbar. The
analysis covered in this thesis shows that this reduces the stress in the busbar by
around 50% depending on the configuration. The drawbacks include a higher
losses as the conductor is longer (required to model the u-shape). There is also
increased weight, space requirement, and cost.
As the stress values are not reduced by more than 50% it is hard to say that the
thermal expansion issue is "solved" rather it is reduced significantly.
The evaluation of having a varied bend radius showed that no clear reduction in
performance could be found when decreasing the bend radius to the minimum
value covered in this analysis. This is great news as it gives the conductor
designer more freedom.
The U-height evaluation turned out to be more problematic, as there is a
nonlinear behavior between the amount of stress and U-height. But the benefits
of increasing the height from UH1 to UH2 implies that having a UH not too
small reduces stress values.
The second research question covers the CSS. Altering the CSS resulted in a
dramatic effect on the performance parameters. It is shown to be a factor of
great importance. The benefits of using a more flat-shaped busbar are heavily
reduced stress, temperature, and displacement. These benefits are obtained
without introducing negative effects on any parameter covered within this
analysis.
In a case where a temperature-controlled busbar (using active cooling) is used.
It is shown that the lower inertia, as a result of a flatter-shaped busbar, reduces
the stress. especially in combination with the U-shape busbar as the stress in
the bends are reduced dramatically.
The third research question is covering the material effect comparing nickelplated copper, to anodized aluminum. The issue with this evaluation is that
it only covers these materials with one type of surface treatment. To be able
to answer this research question fully, a larger evaluation of different surface
treatments would be needed. The main issue is the surface emissivity value as
it directly affects the temperature of the busbar resulting in a different amount
of thermal expansion. As the surface treatments covered in this thesis are
suitable for implementation in harsh environments the results are still valid
when comparing these materials, but if other material surface treatments are
Conclusions | 53
used the comparison will not be as valid. Note that the effect of altering the
Cross section shape will be reduced if the surface emissivity value is lower, as
the heat will be more isolated in the busbar.
6.2
Limitations
The main limitations of the results are the lack of isolation material around
the conductor, this would affect the end state temperature, the benefit is that
it would probably not increase the reliability when comparing the different
design shapes as the isolation material would have to be used in all scenarios.
Another aspect that limits the simulation is the connector boundary conditions.
As the current solution is to assume a rigid connection of the bolt connecting
the busbar, a more fine approximation would be to assume some elastic
deformation in the bolt as this would represent a real-life scenario better. This
would certainly decrease the maximum amount of stress in the connectors.
6.3
Future work
Next step would be to investigate other geometrical shapes that could be used
to mitigate the effects of thermal expansion, this could be a "S" shape instead
of the "U". Another aspect to consider is to simulate a layered busbar together
with the U-shpe (or similar) to see what effect it would have on the stress
values, how thin would the layers have to be? And is there a risk of local stress
concentrations that could happen due to the difference between compression
and stretching of the layers on the top and bottom side.
An additional aspect to investigate is sliding supports. This would, if implemented
in a proper way, allow the expansion to occur without having stress issues in
the busbar.
For the validation of the results presented in this thesis, a set of real-world
experiments would be needed. Measuring the temperature using a thermal
image camera to find hot spots, and to find the difference in steady-state
temperature between CSS1 and CSS3 would be interesting.
In future simulations of busbars, the use of a load scenario with current varying
with time could be used to better understand real world implementation.
54 | Conclusions
REFERENCES | 55
References
[1] 1, “Iea (international energy agency 2020 report,” 1, vol. 1, Jun.
2020. doi: 1. [Online]. Available: https://web.archive.org/web/
20210325142252/https://www.iea.org/reports/global-ev-outlook-2020
[2] R. Jackson, “E-mobility cables,” 1, vol. 1, Jan. 2021. doi: 1. [Online].
Available: https://web.archive.org/web/20210603160256/https://www.
emobility-engineering.com/focus-cables/
[3] D. R. Andre Felipe Schneider, Olivier Charette and C. Turcotte, “A
thermal-mechanical approach for the design of busbars details,” 1, vol. 1,
Jan. 2013. doi: 1
[4] E. Langer, “Liquid cooling for ev charging— what to know to keep
electric vehicles on the go,” 1, vol. 1, Jan. 2019. doi: 1
[5] 1, “Stray inductance in laminated busbar,” 1, vol. 1, Jul. 2014. doi:
1. [Online]. Available: https://ieeexplore.ieee.org/document/6603269?
arnumber=6603269
[6] ——, “Megawatt charging system set to rapidly reduce fuelling time
for commercial evs,” 1, vol. 1, Oct. 2020. doi: 1. [Online]. Available:
https://web.archive.org/web/20210508165011/https://electricautonomy.
ca/2020/10/30/megawatt-charging-system-commercial-vehicles/
[7] M. E. B. D. J. G. M. P. B. B. Alan Dorneles Callegaro, Jing Guo
and A. Emadi, “Busbar design for high power inverters — approximate
solution,” 1, vol. 7a, Jan. 2016. doi: 10,1109
[8] M. H. Gholamreza Kadkhodaei, Keyhan Sheshyekani, “Coupled
electric–magnetic–thermal–mechanical modelling of busbars under
short-circuit conditions,” 1, vol. 1, Jan. 2019. doi: 1
[9] D. C. . P. T. Norris, “Copper for busbars,” 1, vol. 1, May 2014. doi: 1
56 | REFERENCES
[10] J. L. B. X. Du, X. X. Kong and M. Xiao, “High thermal conductivity
insulation and sheathing materials for electric vehicle cable application,”
1, vol. 1, Aug. 2019. doi: 1
[11] A. Håkansson, “Portal of research methods and methodologies for
research projects and degree projects,” 1, vol. 1, Jul. 2013. doi: 1
[12] 1, “Surface finishing concepts for busbars,” 1, vol. 1, May 2019. doi:
1. [Online]. Available: https://web.archive.org/web/20210419084011/
https://holzapfel-group.com/en/aktuelles/news-detail-en/artikel/
beschichtung-von-stromschienen-und-sammelschienen-busbars.html
[13] ——, “anodizing aluminium,” 1, vol. 1, Jan. 2021. doi: 1. [Online].
Available: https://web.archive.org/web/20200925184759/https://www.
anodizing.org/page/anodizing-environmental-advantages
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