Download High-Current, Low-Voltage Power Net

High-Current, Low-Voltage Power Net
Florian Bachheibl and Dieter Gerling
Institute of Electrical Drives
Universitaet der Bundeswehr Muenchen, D-85577 Neubiberg, Germany
e-mail: [email protected]
Abstract—The ISCAD drive is the first low-voltage electrical
machine applicable to automotive traction applications.
Supplying the drive with high current at low voltage is both a
challenge and an opportunity. This paper investigates highcurrent, low-voltage power nets for electric vehicles and presents
possible realizations. The focus lies on aspects of security,
feasibility and cost-effectiveness of a power net with a primary
bus voltage lower than 60V.
Keywords—High-Current, Low-Voltage, Automotive Power
Net, Safety Concept, ISCAD, Electromagnetic Fields,
Electromagnetic Compatibility
I. INTRODUCTION
For several reasons, car manufacturers generally rely on
high-voltage power nets to supply traction motors in BEV or
HEV applications, using bus voltages of 400V and above. First
of all, conventional drive topologies reach their maximum
efficiency at voltages between 200 and 300V for low power
demand and higher voltages for high power vehicles [1].
Secondly, it is easier to integrate a high-voltage system into the
stationary electric grid, since there is only a small voltage-gap
to be overcome by DC-DC-converters or by transformers.
Thirdly, regardless of isolation problems, the effort for
operating a high-voltage power net appears to be lower than the
effort for the distribution of high currents. In the light of a new
machine topology with a stator-cage-winding [2], the first
argument for high-voltage power-nets has lost its validity. The
stator cage machine first presented during a tutorial at E|DPC
2014 requires a DC supply voltage of 24V and a peak current
of 15kA and is very promising in terms of overall drive-cycle
efficiency and cost-effectiveness. This paper investigates
several aspects linked to supplying a machine with the given
electrical characteristics. In section II, feasibility of carrying
currents of that magnitude is proven realistic and in section III
a general safety concept for a high-current system is
investigated. EMC is investigated in section IV and general
operation parameters such as losses and voltage drop across the
conductors are investigated in section V. Finally, a conclusion
is drawn summarizing the findings presented in the sections
mentioned and showing that a low-voltage power net is a
viable alternative to the high-voltage state of the art.
978-1-4799-6075-0/14/$31.00 ©2014 IEEE
II. FEASIBILITY STUDY
A. Current-carrying capacity
A low voltage system opens up the opportunity to use the
car’s ground or the battery case as conductor of the main power
bus. This allows economizing at least one cable, thus cutting
down costs, weight and size of the system. In Tesla’s Model S
sedan, the battery pack is protected against ballistic impacts by
a 6.3mm thick “ballistic shield” of solid aluminum [4] which is
1550mm in width. This yields a cross-section area of
1007.5cm² - enough to carry currents up to 50kA at a current
density as low as 5A/mm². If the same current density is
assumed for one discrete conductor as shown in the middle of
Fig. 1 and a current of 15kA has to be transmitted, a crosssection of 30cm² and a corresponding mass of 22kg would be
sufficient. However, transmitting current inside the battery is
only half the challenge and linking battery and machine can be
performed in a straightforward way using a structure as shown
in Fig. 2. The connection between the coaxial structure and the
DC-bus rings of the machine may be made by using bond
wires. This allows inserting a fusible component between
machine and power net by designing the bond wires such that
they disconnect when overcurrent occurs. This solution already
exists in a similar way in the battery pack design by Tesla
Motors for the connection of each individual battery cell [10].
Single Conductor, A=30cm²
Tesla “Ballistic Shield“
Fig. 1. Possible geometry of battery and conductor
Fig. 2. Motor-battery link
B. Evaluation of HV and LV power nets
The safety requirements to automotive power nets are
defined in the regulation ECE R100 [6]. According to this
standard, voltages beneath 30V AC and 60V DC are
considered safe and no protection is needed. For high-voltage
systems, on the other hand, several layers of protection are
necessary:
 A service switch which disconnects the HV-battery
needs to be present and accessible.
 HV-isolation must be monitored.
 When inactive, both poles of the HV-battery must be
disconnected from the power net.
 All HV-components must be located out of reach of
users without special tools.
In addition to these safety requirements, it is technically
necessary to realize a galvanic isolation of the battery
management bus, introducing further complexity into the
HV-system. Using Tesla’s Model S as an example, the cost of
a HV-power-net is compared to the presented LV-solution in
TABLE 1. It is worth noting that, although a battery disconnect
switch is not required legally for a low-voltage system, it is
necessary to foresee a mechanism which allows a controlled
disconnection for several failures. This topic will be discussed
in the following section.
TABLE 1: Comparison of Costs
Item
Internal cabling of battery (unshielded)
Cost HV
13.5€
External cabling (shielded)
3.6€
External cabling (unshielded)
-
Cost LV
170€
85€
Main battery disconnect switch
112€
tbd.
Isolation monitoring device
100€
-
BMS optocouplers
46€
-
Total
275€
255€
III. POWER NET SAFETY CONCEPT
The targeted voltage of 24V DC lies well beneath the limits
set in DIN-VDE0100 (120V DC) [7] or in ECE-R100 (60V
DC) [6] above which significant protection is needed.
Therefore, one possible reaction to a short circuit in the power
net may be the controlled depletion of all energy stored in the
batteries, using the resistances occurring in a short circuit.
However, when a battery chemistry is used that becomes
unstable during deep discharge [8], this concept is no longer an
option and circuit breakers or fuses are needed. Tesla’s solution
of using bond wires to both connect and fuse the individual
cells [10] is also applicable to a highly parallel battery pack.
However, there are several fault scenarios which cause currents
high enough to heat up parts of the car until ignition whilst not
causing a battery current high enough to trigger the fuses.
These scenarios require a solution to actively disconnect the
battery which could be realized by a detonating fuse, e.g. an
ABB Is-Limiter [12]. The operation principle of using
pyrotechnical fuses is already in use in automotive applications
[9]. Another way of actively disconnecting the battery pack
from the power net could be to use low-voltage MOSFETswitches which can be realized in a very loss-optimized way
for such low voltages [3]. The safety-concept of a low-voltage
power net can be kept relatively simple when it comes to touch
protection and other measures against harming humans by
direct contact. The high currents involved, however, require a
thorough analysis of emitted radiation which will be performed
in the following section.
IV. ELECTROMAGNETIC COMPATIBILITY
The relevant law for EMC tolerances of vehicles in the
European Union is the Commission Directive 2004/104/EC
[11], which states maximum permissible amplitudes for highfrequency AC-emissions. There are no limitations concerning
DC-emissions of magnetic fields in the aforementioned
directive. Concerning biological safety, the International
Commission on Non-Ionizing Radiation Protection (ICNIRP)
issued Guidelines [13] which state an exposure limit of 400mT
for the general public. However, when movement or presence
of medical devices are involved, a much more restrictive limit
of 50mT is suggested. A more general directive on EMC of the
European Union (2004/108/EG), which is transferred to
German Law in [15] defines a Flux density exposure limit of
500μT for the general public, which shall be the base for the
following considerations. The setup displayed in Fig. 1 is an
obvious solution as it does not require much modification of
the existing Tesla battery and it allows for a simple connection
between battery and machine. However, its EMCcharacteristics do not satisfy the requirements imposed by EUStandards. Fig. 3 shows the flux density distribution for the
setup proposed in Fig. 1. Thereby all areas displayed in red
face a flux density higher than 500μT, disqualifying the
geometry as not compliant to flux density standards. An
alternative solution relies completely on the protective shield of
the battery by using both halves as conductors for the positive
and the negative side of the DC-bus, respectively. However,
this solution also radiates too high values of flux density into
the environment, as shown in Fig. 4.
Fig. 3. Distribution of flux density for one central conductor using the
baseplate as second conductor
Fig. 4. Distribution of flux density for a setup using both halves of the
ballistic shield as conductors for the DC-bus
The only realistic option is therefore to modify the baseplate,
creating a sandwich-like structure which consists of at least
three layers. In this way, one conductor is encapsulated by the
other conductor, creating a shielding effect for the whole DCpower net. As Figures 5 and 6 show, a complete encapsulation
of the inner conductor, including the sides is both more
complicated to manufacture and less effective in shielding as
there remains a high value of stray flux at the edges.
Fig. 5. Distribution of flux density for a sandwich-like setup and complete
encapsulation of the second conductor
Fig. 6. Distribution of flux density for a sandwich-like setup without
encapsulation of the front edges
Fig. 7. Distribution of flux density for a sandwich-like setup with shielding
of the sides
In Fig. 6, the region of high flux density is very limited and can
be further reduced by either increasing the number of layers, by
choosing a more complex structure of the edge or by using
magnetic steel to shield the edge by short circuiting the
magnetic flux path, as shown in Fig. 7. However, the easier
solution of Fig. 6 would already comply with standards, as the
region of high flux density cannot be reached by humans
during operation.
At the current state of development of the system, it is not
possible to calculate conducted EMI of the inverter and
therefore to include it into the EMC-Concept. Yet, another
source of high-frequency emission is the vehicle passing a
stationary measurement point at high speeds. To model this
effect, the car is assumed to pass a measurement point in a
distance of 0.2m at a speed of 200km/h. A constant flux density
of 0.337μT, which has been calculated by FEM, is assumed to
be constant along the battery length with a decay rate of 1/r³ at
both ends, yielding a Flux density waveform as displayed in
Fig. 8. The chosen decay rate is a general value for magnetic
fields [5],[16].
The resulting spectrum is plotted in Fig. 9 against the human
exposure limits proposed by ICNIRP [14]. Although a very
conservative approach has been chosen, the results show
clearly that the emitted radiation is marginal.
Conductive Shield
Battery Pack
Flux density [T]
decay ~1/r³
Constant Bmax
decay ~1/r³
M
V=200 km/h
Fig. 8. Model used to calculate high-frequency spectrum
lbatt
x
Length of battery pack
Fig. 10. Schematic drawing of battery, protective shield and electrical
machine
Assuming that the current passing through the cross-section of
the conductive shield is:

x 
I(x)  I0  1 

 lbatt 
(1)
With I0 being the DC-link current of the machine, the resulting
voltage drop across the length-axis x is:
U  lbatt  
lbatt


  I  x   A dx  I
0
Fig. 9. Human exposure limits and calculated spectrum
V. LOSSES IN THE POWER NET
The very reason for using high-voltage systems despite the
dangers involved lies in the fact that power can be transmitted
with lower currents, therefore causing less Joule-Loss. The
purpose of this section is to show that the degrees of freedom
obtained by using a safe voltage level can easily be used to
reduce the Ohmic Resistance so drastically that Joule-Loss is of
no more concern. As already mentioned above, the protective
shielding at the bottom of the traction battery can be used as
conductor for the currents inside the battery. A possible way to
save material could be thinning out the profile towards the end
of the battery in order to maintain the same current density all
over the battery. This concept however has several
disadvantages:

The voltage drop remains constant over the whole
length of the battery.

A minimum thickness of 6.3 mm is required anyway
to achieve the shielding effect.

Thinking of cars propelled by more than one motor,
this concept becomes obsolete.
0
 l batt
A 2
(2)
In a similar manner, power loss in the battery pack can be
calculated:
P  lbatt  
lbatt

Ix 
0
2

 lbatt
dx  I02
A
A 3
(3)
From equation ( 3 ) it becomes obvious that the effective
resistance of the conductors inside the battery is:
R eff 
 lbatt
A 3
(4)
The length and width of the battery pack are chosen to be equal
to Tesla Model S’ battery pack. It has a width of 1556mm and
a total length of 2730mm, leaving the thickness of the
conductive shield plate as only variable. In order to assess the
necessary values for the thickness, a drive-cycle-based analysis
was performed. The drive cycles chosen for this task are the
ARTEMIS Road, NEDC 2000 [17] and WLTP Class 3 [18],
whose speed vs. time-characteristics can be seen in Fig. 11.
Therefore, the cross-section of the baseplate remains constant
along the length axis of the battery for the following
considerations. The battery is assumed to be so highly parallel
that the error made by distributing the current supply
continuously along the length is negligible.
Fig. 11. Speed vs. time for widespread driving cycles
A good approximation for the calculation of required power
from the drivetrain can be made by focusing on acceleration
power and aerodynamic loss. According to [19], the
aerodynamic drag force can be calculated by:
FW  c W A x
Air 2
vF
2
(5)
PW  c W A x
Air 3
vF
2
(6)
The values of Table 2 are inserted into ( 3 ) in order to obtain
the average losses per drive cycle for different values of
thickness, as shown in Fig. 13. The vertical line drawn at a
thickness of 6.3mm marks the already available thickness of
Tesla’s ballistic shield. Notably, a thickness up to this point
does not have any effect on the battery cost if compared to the
actually existing vehicle.
and therefore:
The instantaneous power of acceleration at a given speed can
be obtained by
Pa  m  a  v
(7)
The aerodynamic parameters of the Model S are known to be
0.24 for cw and 2.34m² for the projected surface [20]. The curb
weight amounts to 2108kg [21], allowing to calculate the
required power to follow each of the driving cycles and the
current needed to supply that power at a voltage of 24V, as
shown in Fig. 12.
Fig. 13. Loss in conductive plate for various driving conditions
In addition to the losses, it is very important to guarantee a very
small voltage drop across the conductor since it is not only
used to transfer current to the machine but also to connect the
cells in parallel. Equation ( 2 ) is used to calculate the values
displayed in Fig. 14.
Fig. 12. Cycle-based current consumption
The current waveforms of Fig. 12 are simplified by calculating
their effective values according to:
1T

Ieff    I 2dt 
T 0

(8)
which yields the values of TABLE 2 for Ieff.
TABLE 2: Effective currents for different driving cycles
Driving Cycle
Effective Current at U=24V
ARTEMIS Road
724.6A
NEDC 2000
389.3A
WLTP
548.0A
Peak Power (360 kW)
15.0kA
Nominal Power (70 kW)
2.91kA
Fig. 14. Voltage drop in bottom plate
VI. CONCLUSION
A power net concept for a low-voltage, high-current drive
system has been proposed. Feasibility of such a system was
analyzed and validated considering cost, material expense,
safety and EMC. It is worth noting that the cost projection of
Table 1 would be much more in favor of a low voltage system
if a sandwich-structure for the conductors had been used and
the conductor inside the battery had not been necessary.
Further analyses need to be done with respect to a solution for
actively disconnecting the battery in an emergency case.
VII. REFERENCES
[1]
R. De Doncker and D. U. Sauer, “Annual Report 2011”, Institut für
Stromrichtertechnik und Elektrische Antriebe (ISEA), Aachen, 2011.
[2] G. Dajaku and D. Gerling, “Low Costs and High Efficiency
Asynchronous Machine with Stator Cage Winding”, IEEE International
Electric Vehicle Conference, Florence, 2014, to be published.
[3] A. Patzak and D. Gerling, “Design of a Multi-Phase Inverter for Low
Voltage High Power Electric Vehicles”, IEEE International Electric
Vehicle Conference, Florence, 2014, to be published.
[4] P.D. Rawlinson, “Vehicle Battery Pack Ballistic Shield”, U.S. Patent
020120312615A1, August 13, 2012.
[5] S. Brandt and H.D. Dahmen, Elektrodynamik, 4th ed. Heidelberg:
Springer, 2005.
[6] United Nations, “Agreement – Concerning the Adoption of Uniform
Technical Prescriptions for Wheeled Vehicles”, Addendum 99,
Regulation No. 100, 1995.
[7] K-H Krefter and H. Schmolke, DIN VDE 0100, 3rd ed. Berlin:
VDE-Verlag, 2012.
[8] S. Levy and P. Bro, Battery Hazards and Accident Prevention, New
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[9] W. Hentschel, “Fuse for a Motor Vehicle Power Line”, U.S. Patent
20130009745A1, January 10, 2013.
[10] J. Straubel et. al., “System and method for fusibly linking batteries”,
U.S. Patent 20070188147A1, August 16, 2007.
[11] Commission Directive 2004/104/EC, Official Journal of the European
Union Series L, 2004.
[12] ABB Product Sheet. (2013, April 11). Is-limiter – The world fastest
limiting
and
switching
device
[Online].
Available :
http://www05.abb.com/global/scot/scot235.nsf/veritydisplay/1e8df815dc
e50e6bc1257b4a0048e083/$file/2493%20Is-Limiter%20GB.pdf
[13] “ICNIRP Guidelines on limits of exposure to static magnetic fields”,
Health Physics, 96(4):504-514, 2009.
[14] “ICNIRP Guidelines for limiting exposure to time-varying electric and
magnetic fields (1Hz-100kHz)”, Health Physics 99(6):818-836, 2010.
[15] Bundesamt für Strahlenschutz, “26. Verordnung zur Durchführung des
Bundes-Immissionsschutzgesetzes
(Verordnung
über
elektromagnetische Felder – 26. BlmSchV)”, 2013.
[16] K. Küpfmüller et al., Theoretische Elektrotechnik. Berlin Heidelberg:
Springer, 2013.
[17] T.J. Barlow et al., “A reference book of driving cycles for use in the
measurement of road vehicle emissions”, TRL Limited, 2009.
[18] M. Tutuianu et. al., “Development of a World-wide Worldwide
harmonized Light duty driving Test Cycle (WLTC)”, UNECE Informal
document GRPE-68-03, 2014
[19] W-H. Hucho, Aerodynamik des Automobils, Wiesbaden:Springer, 2013.
[20] D. Sherman, ”Drag Queens: Aerodynamics Compared“, Car and Driver,
Ann Arbor, 2014.
[21] Tesla Motors Website, Specificactions of Tesla Model S [Online].
Available: http://www.teslamotors.com/models/specs