Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013
Influence of a Traction Battery’s Input Impedance on
Conducted Emissions of an Automotive HV Inverter
M. Reuter1*, S. Tenbohlen1, W. Köhler1
1
Institute of Power Transmission and High Voltage Technology (IEH), University of Stuttgart, Germany
*
Martin.Reuter@ieh.uni-stuttgart.de
impedance and the value of the load impedance [6]. As the
disturbance sources within a component are constant at a given
operating point, the load impedances dominate the amplitudes
of conducted emissions in HV systems [2]. Thus, comparable
conducted disturbances on component and system level are
only achieved, if the load impedances of the disturbance
sources within the tested device are similar. But the load
impedances in the standardized test setup differ significantly to
the impedance situation within the automotive HV system’s
environment. This contribution quantifies the impedances of
both situations and their difference in measured conducted
emissions of an automotive traction power inverter.
Abstract — Conducted emissions of automotive high voltage (HV)
components recently are measured according to CIPSR 25. If the
tested device conforms to specific limits on component level (set
by customer and supplier agreements) it is inserted into the
automotive system. Thus, the EMC performance of the
investigated device has to be independent of the test level, e.g. on
component or vehicle level. Comparing the standardized test
setup with an automotive HV system, the main difference consists
in the termination of the cable harness. In a component level
EMC test setup the HV harness is terminated with line
impedance stabilization networks (LISNs), whereas implemented
in a vehicle the cables are connected to a traction battery.
This contribution investigates the difference in the conducted
emissions of an automotive power inverter, when it is introduced
into an electromagnetic environment that is similar to vehicle
level. Therefore, the input impedances of a HV battery are
characterized and modeled. This battery emulation is introduced
into the component level test setup and the deviations in the
conducted emissions of a traction inverter are determined.
II.
In lack of standardized EMC test setups for HV systems,
CISPR 25 is recently also applied to determine the conducted
emissions of HV components. This standard prescribes a
measurement setup consisting of the device under test (DUT),
its cable harness with a rest bus simulation and LISNs for
termination of the supply cables [7]. It was developed to
emulate the impedance situation within a typical low voltage
(LV) harness, based on impedance measurements [8]. DUT
and cable harness are laid up as close as possible to their later
configuration in the vehicle, and the LISNs are used to emulate
a standardized car environment.
Keywords – Conducted Emissions, CISPR 25, Traction Battery,
Automotive HV Network Impedance, Electric Vehicles.
I.
INTRODUCTION
The electromagnetic compatibility (EMC) of electric
traction systems is a major task in the development process of
electric driven cars. Most electric vehicle (EV) concepts
position the required traction battery underneath the rear trunk,
whereas power electronics and electric machine are placed in
the front motor compartment. Thus, a transmission of electric
power is required through the entire car, with a cable harness of
about 3 – 4 m length. An increase of the operating voltages up
to 120 – 400 VDC reduces ohmic losses of the power
transmission. The battery, power electronics and cable harness
form a high voltage traction network, which is assembled
entirely shielded to reduce electromagnetic interferences
(EMI). The applied coaxially shielded HV cables act as
transmission lines in the EMC frequency range [1]. Thus, the
termination of the traction harness effects the conducted
emissions of HV components [2].
The LISNs are needed for three reasons: First to power the
DUT and simultaneously providing a radio frequency (RF)
decoupling of the supply from the cable harness. This is
required to prevent the DUT’s disturbances from propagating
along the supply cables. The supply lines are usually much
longer than the harness itself and their lengths differ in each
EMC laboratory. Therefore, only with an effective decoupling,
comparable test results can be achieved, independent of power
source and supply lines. Second task is to emulate the input
impedance of a LV cable harness having a length of
approximately 5 m. LV cable harnesses consist of unshielded
wires, bundled close to each other. Their termination does not
impact the input impedance of the harness above a length of
ℓ > λ/6 because of high series inductance and crosstalk [9]. In
contrast, the coaxially shielded HV cables act as transmission
lines in this frequency range and transform the termination
impedance into the cable’s input impedance. Thus, the
termination affects significantly the input impedance of the HV
harness [2]. The third reason for using LISNs is that they
provide a voltage measurement port for EMI test receivers.
Thévenin's theorem states that any combination of linear
electrical circuits is equivalent to one ideal voltage source and
a series resistor [3], [4]. Applied to the field of EMC, the
sources of conducted emissions can be approximated by an
ideal voltage source and a series impedance [5], [6]. Therefore
the amplitudes of conducted emissions are determined by three
factors: The amplitude of the ideal voltage source, its source
978-1-4673-4980-2/13/$31.00 © 2013 IEEE
MEASUREMENT OF CONDUCTED EMISSIONS ON
COMPONENT LEVEL
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Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013
III.
local minima, the transmission coefficients reach local maxima
(5, 20, 30 MHz). Thus, the HV battery can be described as a
symmetrical and reciprocal two-port device.
SCATTERING PARAMETERS OF AN AUTOMOTIVE
TRACTION BATTERY
As the main difference between standardized test setups
and automotive environment is the termination of the HV
cables, this termination impedances needs to be characterized.
The input impedance of LISNs is known and prescribed in
CISPR 16 [10]. The RF properties of an automotive HV battery
are determined with the test setup described in [1]. A photo of
the measurement setup is shown in Figure 1.
c
DUT
IV.
CALCULATION OF BATTERY IMPEDANCES
Scattering parameters are able to describe the complete RF
behavior of a DUT as black box. They are not suitable to
develop a physical model, as they do not visualize internal
structures of the tested device. To receive a physical model
suited for EMC issues, a calculation of common mode (CM)
and differential mode (DM) impedances is more useful.
Figure 3 shows a simplified EMC equivalent circuit of the HV
battery with its CM impedances ZG1 and ZG2 to ground and one
DM transmission impedance ZX.
d
a
b
Figure 1. Measurement setup to determine the scattering parameters of an
automotive HV battery, consisting of DUT, coaxial adapter (a), DC
block (b), battery controller (c) and Network Analyzer (d) [1]
The tested device is an automotive HV traction battery with
an operating voltage of 130 VDC, a power rating of 25 kW and
a total capacity of 9.6 Ah. A DC blocking device (b) enables
the measurement of the RF behavior of the DUT using a
Network Analyzer (d) in the frequency range of 300 kHz –
200 MHz. The coaxial adapter (a) allows a connection of Port 1
of the VNA to HV- at the battery and Port 2 to HV+. Figure 2
shows the resulting scattering parameters of the HV battery.
Figure 3. EMC equivalent circuit of the battery and calculation methodology
of CM and DM impedances using Z- parameters
The measured scattering parameter matrix [S] of the tested
HV battery can easily be converted into an impedance
parameter matrix [Z] as e.g. shown in [11]. The impedance
parameters [Z] of the simplified circuit in Figure 3 are known
as function of the equivalent circuit impedances
, ,
. An inversion of this equation system leads to
the CM and DM impedances in dependency of the ZParameters as displayed in Figure 3. The calculation result of
CM and DM impedances is plotted in Figure 4.
0
S11, S22
-5
1k
|S| / dB
-10
-15
S21, S12
|Z| / Ω
-20
-25
-30
1
ZG1, ZG2
100
10
f / MHz
10
1
100
0.1
Figure 2. Measured scattering parameters [S] of investigated HV-Battery
The scattering parameter curves of the HV battery in
Figure 2 present the reflection coefficients S11 and S22, which
are very similar to each other. Thus, the battery is a symmetric
device up to frequencies of ~30 MHz. Above this frequency the
reflection coefficients differ, but show similar trends. In the
transmission parameters S21 and S12 there are no visible
deviations in the considered frequency range. A comparison of
reflection and transmission parameters presents that they have
a diametrically opposed behavior: At frequencies, where the
reflection coefficients peak (3.5, 11, 25 MHz) the transmission
coefficients minimize. Vice versa where the reflections have
ZX
1
10
f / MHz
100
Figure 4. Common mode impedances (ZG1 / ZG2) and differential mode
impedance (ZX) of the investigated HV Battery; a main series
resonance at 5 MHz occurs
The CM impedances ZG1 and ZG2 have a series resonance at
fR = 4.5 MHz. ZG1 and ZG2 show capacitive behavior below the
resonance frequency fR and an inductive trend above. The DM
impedance ZX starts at low resistive values, increasing to a
maximum at ~3.5 MHz before minimizing to the main series
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Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013
resonance at 4.5 MHz. Above 5 MHz several minor resonances
can be observed, overlaying a mainly inductive trend.
Ground capacitance
This impedance behavior can be explained with the internal
structure of the HV battery: At low frequencies the CM
impedance is dominated by the capacitance to the shielded
housing. The capacitance and the line inductances of the
cabling form a series resonance circuit. Thus, for high
frequencies the line inductances dominate the CM input
impedance. As these parasitic elements are distributed parts,
they can be diverted into smaller elements, which form further
resonances in the higher frequency range. But these resonances
are way less dominant influencing the CM input impedance.
T+
Model of
aggregated
single cells
HV connector /
main contactor
TGround capacitance
The DM impedance ZX is low resistive for low frequencies.
Underneath the investigated frequency range it will converge to
the DC resistance of the battery [12]. Each single battery cell
can be modeled as plate capacitor parallel to a conducting
electrolyte [12]. All cells are interconnected in series. Thus, the
total wiring inductances add, whereas the cumulative
capacitance diminishes. With increasing frequency the
electrolyte becomes less conductive. This leads to an increasing
trend of the DM impedance. Above frequencies of 1 MHz the
electrolyte does not influence ZX any more, as the velocity of
the electrolyte molecules is too low. The effect was verified
using a battery dummy without electrolyte fluid, having the
same internal assembly as the investigated battery. The main
series resonance of the CM impedance also appears in the DM.
This is caused by the two interconnected CM impedances,
which are more conductive than the series interconnection of
the battery cells at the CM resonance frequency fR. Above the
series resonance frequency the DM impedance also shows
inductive behavior, caused by the internal wiring of the battery
cells. In the higher frequency range overlaying minor
resonances can be observed, again caused by smaller parts of
the distributed wiring structures.
V.
Internal wiring
External
wiring
Figure 5. SPICE Simulation Model of HV Battery with external connection
inductivities
The battery model of Figure 5 consists of a ground
capacitor (CG = 3.8 nF) in series to the wiring inductances with
a total value of LS = 370 nH. The aggregation of all single cells
is modeled as a total capacitance of CC = 6.7 μF and a series
inductance of LC = 320 nH. The cumulative wiring inductances
represent cables of a length of approximately ℓ ~ 1.06 m. As
within the main contactor the HV conductors are nearby, a
small capacitor (CMC = 330 pF) is needed to reproduce the
capacitive cross coupling. As a simulation of presented SPICE
model showed similar RF properties as the HV battery, the
model’s network was built up to emulate the battery within a
component level test setup.
VI.
BATTERY EMULATION FOR EMC TESTS OF HV
COMPONENTS
As HV battery prototypes are very rare, expensive and
dangerous to handle, a battery emulation is used instead. Using
the SPICE simulation model of the investigated HV battery, a
network of concentrated elements is built up. It simulates the
input impedances of the automotive HV battery within a
component level EMC test setup. Thus, for the measurement of
conducted emissions, it behaves in a similar manner as a HV
battery, without having the drawbacks of real HV batteries:
There are no flammable chemicals inside and no energy
storage. When the power supply is switched off, the device is
inherent safe. As the system is powered by an external supply,
there is no recharging required and the state of charge (SOC)
does not change within the duration of the test. Also, there is
no need for a CAN communication or rest bus simulation,
whose emissions would interfere with the test requirements.
MODELING OF BATTERY IMPEDANCES
The main advantage of regarding CM and DM impedances
instead of scattering parameters, is that the modal impedances
allow a “grey box” modeling approach. Whereas a complete
model of all internal structures is highly complex and very
difficult to achieve, a black box model does not represent
physical expressions of the internal structures. A compromise
of these two opposed modeling approaches is to recognize
dominant structures and to abstract less influent parts. This
leads to a “grey box” model, which is less complex and faster
to achieve, but has physical explanations for the major
structures of the modeled device. In the case of presented HV
battery a simplified model was developed, that represents the
capacitances of the cells, the internal wiring, the capacitance to
ground and the capacitive coupling within the HV main
contactor. Figure 5 shows this simplified RF model of the HV
battery, showing similar impedance properties as the tested
device. This model is intended to emulate the HV battery’s
input impedances in a frequency range of 300 kHz – 30 MHz.
The developed battery emulation supplies the tested device
with DC currents up to 20 A and voltages up to 400 V. It also
decouples the supplying power source and DUT within the
EMC test frequency range. Additionally it enables the
measurement of disturbance voltage with an EMI test receiver,
which is not directly possible using a real HV battery. A photo
of the battery emulation is shown in Figure 6.
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Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013
The CM input impedances of HV battery and battery
emulation are equal up to a frequency of ~10 MHz. Above this
frequency the higher order resonances of the distributed
elements could not be represented with the simple circuit
shown in Figure 5. Nevertheless, the dominant inductive trend
of the CM impedance is obtained. A comparison of the
battery’s DM impedances and battery emulation shows, that
the battery impedances can be emulated up to ~9 MHz. Again,
the higher order resonances cannot be represented because of
the simplified network model.
Within a component level EMC test setup the DUT is
connected via coaxially shielded HV cables to the LISNs.
These cables act as wave guides at frequencies where the
cable’s length is more than ℓ > λ/6 [1]. Thus, their termination
impedance is transformed according to the transmission line
theory to the cable’s input impedance. In this frequency range a
higher accuracy of the battery emulation is not required, as
long as the impedance trend is represented. Figure 8 shows a
comparison of the HV cable’s input impedance, when it is
terminated with a HV battery and the battery emulation.
DUT
T+
a
Supply
Filter
Battery impedance
emulating network
a
Supply
HV-
DUT
T-
Figure 6. Battery Emulator inside a shielding box. It consists of three
functional blocks: A supply filter for decoupling of power supply
and DUT, battery impedance emulating network and two
impedance converters (a) for voltage measurements on the traction
lines (T+/T-).
Figure 6 shows the battery emulation device with its three
functional blocks. On the left hand side there is a supply filter
to decouple the power lines from the conducted disturbances of
the DUT. The supply filter provides an attenuation of > 50 dB
up to 80 MHz between power supply inputs and DUT output
ports. Thus, there is no influence of the supply’s input
impedance on the battery emulation’s input impedance.
Additionally, the filter yields an effective decoupling of RF
disturbances between supply and DUT, respectively the
measurement port. The battery impedance emulating network
is comparable to the schematic shown in Figure 5. To quantify
the conducted emissions, a measurement port is required. This
measurement device is connected in parallel to the battery
impedance emulating network. To avoid a change of the
emulation’s input impedance, the measurement device needs a
high ohmic input. For RF measurement purposes a 50 Ω source
impedance of the measurement port is required. Thus an
impedance converter circuit (a) was used for each traction
power line (T+ and T-). The 3 dB corner frequency of the
measurement port lies at ~67 MHz, because of a limited gainbandwidth product of the applied operational amplifiers.
Figure 7 shows a comparison of the CM and DM impedances
within the HV battery and its emulation.
1k
10
100
|ZDM| / Ω
|ZCM| / Ω
100
1
10
f / MHz
100
10
f / MHz
|ZIn,DUT| / Ω
10
100
HV
Battery
1
.
0,1
1
10
f / MHz
100
Battery
Emulation
10
1
.
0,1
HV
Battery
1
10
f / MHz
100
Figure 8. Input impedances of HV battery and battery emulator before and
after impedance transformation
The curves of Figure 8 show the dominance of the
impedance transforming behavior above 10 MHz, where the
HV cables begin to act as transmission lines. Although the
battery emulation is not able to reproduce the battery
resonances at frequencies higher than 20 MHz, the input
impedance of the HV harness is similar. Both devices, which
terminate the cable, are similarly mismatched to the wave
impedance and therefore result in a similar HV cable’s input
impedance. Only smaller deviations can be observed at
~10 MHz and around 15 MHz. In this region of frequency the
HV cable begins to behave as wave guide and its impedance
transforming behavior is not yet dominant enough to cover the
differences.
HV
Battery
1
ZIn,Cable
1k
Battery
Emulation
100
1
.
0,1
HV cable
ZL = 18.5 Ω, ℓ = 3.7 m
ZIn,DUT
1k
Battery
Emulation
10
HV
Battery
1
.
0,1
1k
Battery
Emulatio
HV Battery
/ Battery
Emulation
|ZIn,Cable| / Ω
Supply
HV+
As the mentioned deviations are very small, it can be stated
that the battery emulation represents a valid HV battery model
within a component level test setup. The emulation meets the
battery’s impedance in the lower frequency range. Above
10 MHz it is sufficient to reproduce the impedance trend of the
battery for the tested device.
100
Figure 7. Comparison of CM impedance (left) and DM impedance (right) of
HV traction battery and battery emulation
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Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013
resonances following. In contrast, the input impedance of the
cable terminated with a HV battery starts at ~100 Ω declining
to the main series resonance of the battery at 3 MHz. This
series resonance is followed by a parallel resonance at 9 MHz
at ~400 Ω and regular line resonances in the higher frequency
range. Above 40 MHz there are no visible deviations in the
input impedance of the cable, as for both devices the
termination impedance of the cable is higher than the wave
impedance. Below 10 MHz there are significant deviations
between 20 log(ZLISN/ZBat) = 33 dBΩ at 3 MHz and -25 dBΩ at
7.5 MHz.
VII. INFLUENCE OF THE BATTERY IMPEDANCE ON
CONDUCTED EMISSIONS OF A TRACTION INVERTER
To investigate the influence of the battery impedance on
conducted emissions of HV components, a power inverter for a
hybrid driven car is tested. Figure 9 shows a photo of the test
setup according to CISPR 25.
HV LISNs /
Battery Model
DUT:
Inverter
M
In Figure 11 the recorded disturbance voltages of the power
inverter depending on the HV cable termination are plotted.
Cable Harness
LV LISNs
120
Figure 9. Measurement setup on component level according to CISPR 25 for
a HV traction inverter
U / dBμV
The setup consists of the inverter (DUT), which generates
three phase AC to supply a synchronous machine (M). The
inverter is fed with DC via two HV traction cables of ℓ = 3.7 m
length. The HV and LV cables are bundled into one cable
harness and the supply lines are terminated with LISNs. The
electric traction system is operated at a typical working point
driving a generator with a mechanical power of Pm = 2.8 kW
(outside the anechoic test chamber). To investigate the EMC
performance of the inverter on system level, the HV LISNs are
replaced with the presented battery emulation. The conducted
emissions are recorded with an EMI test receiver (IFBW
120 kHz, peak detector).
|ZIn,Cable| / Ω
100
By changing the cable termination to the battery emulation
at the same operating point of the inverter a different spectrum
occurs: At low frequency range the disturbances are reduced by
5 – 10 dB. But at the battery’s main resonance (3 MHz) the
disturbance voltage peaks and superposes the disturbances
according to CISPR 25 by ~10 dB. This peak is followed by a
disturbance minimum, where the voltage emissions are up to
33 dB (at 6.3 MHz) lower as measured with LISNs. From
10 MHz the voltage disturbances emerge with increasing
frequency. Above 40 MHz the disturbance voltages are below
the noise floor due to the limited bandwidth gain product of the
used operational amplifiers within the impedance converters.
Because the load impedances of both cases are identical above
~40 MHz the conducted emissions in this frequency range will
be similar.
Battery
Emulation
10
f / MHz
10
f / MHz
Figure 11 shows that the conducted voltage disturbances
measured according to CIPSR 25 begin at 110 dBμV. They
minimize at around 1 MHz with a value of 90 dBμV before
peaking at 6 MHz with a value of 115 dBμV. For higher
frequencies a declining trend can be observed with overlaid
line resonances caused by the termination impedance mismatch
of the HV cables [2].
LISN
1
1
Figure 11. Disturbance voltage of the power inverter with LISNs and battery
emulation as cable termination
10
0.1
Battery
Emulation
60
20
1k
1
80
40
The traction inverter is supplied via two HV cables, whose
input impedances dominate the conducted emissions of the
tested device. A comparison of the HV cable’s CM input
impedance within the test setup is plotted in Figure 10. These
impedances represent the load conditions for the disturbance
sources inside the tested power inverter.
100
LISN
100
100
Figure 10. CM input impedance of the HV cables according CISPR 25
(marked with LISN) compared to the presented battery emulation
as cable termination. This impedance loads the CM disturbance
sources within the tested inverter
As for radiated emissions the CM current is one dominant
influencing factor, a current clamp is used to determine the
current emissions of the power inverter. The results are plotted
in Figure 12.
CIPSR 25 specifies that the HV cables need to be
terminated with LISNs, resulting in the green impedance curve
of Figure 10. This load impedance for CM sources starts at
around 8 Ω (300 kHz) and follows the LISN’s input impedance
up to ~3 MHz. At higher frequencies the impedance decreases
to the first line resonance at ~15 MHz with regular line
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Proc. of the 2013 International Symposium on Electromagnetic Compatibility (EMC Europe 2013), Brugge, Belgium, September 2-6, 2013
100
80
I / dBμA
emissions on the traction lines. When a HV component, whose
EMC performance is validated according to the only existing
component level test setup, is connected to a HV battery its
EMC performance changes dramatically. The deviations are up
to 20 dB in the disturbance current and even 33 dB in the
voltage emissions.
Battery
Emulation
60
LISN
40
These changed conducted emissions, caused by the
changed impedance situation, can lead to ineffective EMC
countermeasures. To ensure the EMC of electric traction
systems the cable’s termination impedances need to be
considered in the development of EMC concepts.
20
0
1
10
f / MHz
100
ACKNOWLEDGMENT
Figure 12. Disturbance current of the power inverter with LISNs and battery
emulation as HV cable termination
The authors would like to thank the Robert Bosch GmbH
for funding and supporting the EMC measurements of an
automotive electric traction system.
Within the standardized setup at low frequencies the
disturbance currents are higher, because of the lower CM input
impedance. Above 600 kHz the currents with the battery
emulation exceed the ones of the LISN setup, because the input
impedance of the emulation is lower. A maximum deviation of
25 dB occurs at the battery’s main series resonance (3 MHz).
Above this frequency the difference become smaller, but still
there is a minimum deviation of 10 dB in a broad frequency
range of 900 kHz – 7 MHz. At 11 MHz there is a resonance
with 15 dB higher current amplitudes, but above the
disturbance currents equal each other, because the input
impedances become similar.
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The measurements show considerable differences in the
EMC performance of the tested device, although its operating
point is kept constant. The occurring deviations are explained
by changed load impedances of the disturbance sources within
the DUT. Therefore, the conducted emissions of HV
components differ substantially, when they are inserted in a
traction system. Thus, the same component at the same
operating point may emit a different disturbance spectrum, as
measured according to CISPR 25.
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The EMC performance of HV components is recently
tested according to CIPSR 25. This standard specifies the use
of LISNs as supply cable termination. But the input
impedances of HV batteries differ significantly from the input
impedance of LISNs. This causes a changed EMC performance
of the tested device, when it is inserted into the automotive
system environment.
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To quantify the amount of disturbance deviation, a battery
emulation has been developed, based on scattering parameter
measurements of an automotive traction battery. The battery
emulation replaces the LISNs within the component level EMC
test setup and the differences of the conducted emissions in the
HV harness of an electric traction system are recorded.
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Although the operating point of the traction system is kept
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