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Calculation of RF-Interference from Coupled Shielded Hybrid Cables Utilizing Current Probe Measurements
Dr. Peter Hahne, Ingenieurbüro Dr. Peter Hahne
Dr. Martin Aidam, Daimler AG,
Andreas Ludwig, Daimler AG,
Xiaofeng Pan, Daimler AG,
Dr. Markus Schick, Altair GmbH
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Overview
2
• EMC of hybrid vehicle electric drive (hybrid system)
• Hybrid system coaxial power lines
• Simplified hybrid system: Standard interference device (SID)
• Simulation of shielded cables in FEKO: Restriction to decoupled inner conductors
• Solution method for calculation of multiple single-shielded cable systems: Superposition procedure
• Determination of current spectrum, acting as excitation
• Measurement SID
• Comparison of measurement and simulation
• Utilization of measured currents
• Summary
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Hybrid system
3
Battery
E-Motor
Inverter
Radio/TV window antennas
HV-power lines
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EMC Sketch of Hybrid System
4
Interference into vehicle antennas
Cause of interference
• PWM-conversion generates high frequency current and voltage pulses
• A small fraction of the accompanying electromagnetic fields pass through the cable shields, radiate into the antennas and cause an interference voltage there
High voltage battery
DC/AC Inverter
Electric engine
Antenna amplifier
Engine ground strap
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Shielded High Voltage Lines
5
Measurement vs Kley formula, optimized, large diameter
1 inner conductor
2 Isolator
3 braided shield, 3a foil shield
4 outer insulation
The cable properties, especially the braid geometry determines the transfer impedance of the cable.
Alternative: measurement
Not so simple, unfortunately!
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Transfer Impedance According to Various Formulas
6
Transfer impedance Coroplast 9-2610 FLR2GCB2G 16 mm² according to various formulas
Frequency [MHz]
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Simplified hybrid system: Standard interference device (SID)
7 7
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Simulation of shielded cables in FEKO
8
Uses the concept of transfer impedance:
• Coupling of inside and outside by transfer impedance and transfer admittance only
Adjustments FEKO
• Option MOM, radiating.
• transfer impedance: predefined| Kley for braided shields| Schekulnoff for massive shields
Restrictions für braided shields, option MOM
• Coaxial cables allowed only
• It is not enabled to calculate a system of coaxial lines directly, whose inner conductors are electrically coupled
~ ~ ~ ~ ~
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Indirect FEKO Calculation of the Hybrid System / SID
9
FEKO (Option MOM): Only independent coaxial cables allowed
Hybrid system / SID: has coaxial cables with coupled inner conductors
Problem
One possible solution
• Excitation of inner conductors by equivalent currents
• Superposition principle
• Transfer impedance concept
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Steps towards a Solution I
10
1. Equivalent current excitation
• State (U(x), I(x)) of conductor is the same for both excitations • Voltages result uniquely
2. Superposition principle
I1 I2 I1 I2 Z,l Z,l
≡
Z,l
+
~
I1
U1
I2
U2
Z,l I1
U1
I2
U2
Z,l
≡
Z1 Z2
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Steps towards a Solution II
11
3. Transfer function, Transfer impedance
I1
US ~
𝑈𝑠 = 𝐼1𝑍𝑠1
4. Superposition principle again
I1 I2
US ~
𝑈𝑠 = 𝐼1𝑍𝑠1 + 𝐼2𝑍𝑠2
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Steps towards a Solution III
12
5. A shield changes the transfer function, but the linear dependence remains valid
I1 I2
US ~
𝑈𝑠 = 𝐼1𝑍′𝑠1 + 𝐼2𝑍′𝑠2
6. Application to a multi conductor system
US ~
𝑈𝑠 = 𝐼𝑖𝑍′𝑠𝑖4
𝑖=1
I3 I4
I1 I2
• Core crosstalk neglected • Coupling of originally coupled
sources is comprised within complex amplitudes of current excitation
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Summary of Superposition procedure
13
Superposition procedure (SP)
1. Calculation of all transfer functions 𝑍′ from inner conductor currents
at cable ends to a sink by simulation
2. Weighting of transfer functions with complex current spectra at cable ends and summation
𝑈𝑠 = 𝐼𝑖𝑍′𝑠𝑖𝑚
𝑖=1
3. Result is a voltage 𝑈𝑠 (or current, electrical field strength, …) at the
sink (antenna, current clamp, field sensor, …)
For all sinks the weighting process can be expressed as
𝑼 = 𝒁′ 𝑰 ,
with vector 𝑼 (𝑚x1) containing all requested quantities,
matrix 𝒁′ (𝑛x𝑚) containing all transfer functions and
vector 𝑰 (𝑚x1) containing all exciting currents, where 𝑚 is the number of
exciting currents and 𝑛 is the number of sinks.
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How to Obtain Complex Current Spectra
14
How are complex current spectra obtained?
• Approach here: Modelling of the inner conductor system in SPICE, simulation in time domain
• Adjustment of model to measurements of the inner conductor system, i.e. by adding and dimensioning parasitic elements
• Complex current spectra obtained by simulation in time domain and subsequent Fourier transformation
The accuracy of the final result depends linearily of the accuracy of the complex current spectra!
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Modelling of the inner conductor system in SPICE
15
EC SID
EC motor power lines
Parasitic Elements
Part of equivalent circuit
• Parasitic elements dominate the frequency spectrum of the lines up from 15 MHz
• Modelling for higher frequency is troublesome therefore
• Not shown EC‘s of:
• motor imitation (MI),
• battery imitation (BI),
• battery lines,
• coaxial lines
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Comparison of Results of Measurement and Spice Model
16
• Obtained by FFT measurement data and simulation data
• Dynamic range of measurement unsatisfactory
• Satifactory agreement up to 15 MHz
• Above 15 MHz no statement possible about validity of SPICE model
• Motor side modelled better than battery side
Voltage spectrum of a line at MI end
Voltage [
dBV]
Frequency [Hz]
Simulation
Measurement
Voltage spectrum of a line at BI end
Voltage [
dBV]
Frequency [Hz]
Simulation
Measurement
Noise
Noise
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Measurement SID on Table
17
Measurement with 4-channel scope, bandwidth 2.5 GHz Current clamp F-51, 3 positions Antenna R&S HFH2-Z2 150kHz bis 30MHz, distance 1m Antenna R&S HL-562 30kHz -1GHz, distance 3m
SID MI
BI
F-51
F-51
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Model SID on table
18
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Results: Total Current Battery Line at SID
19
Simulation
Measurement
Total Current Battery Line at SID
Noise
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Results: Total Current at Motor Imitation
20
Simulation Measurement
Total Current at Motor Imitation
Noise
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Results: Voltage at Antenna HFH2-Z1
21
Simulation
Measurement
Voltage at Antenna HFH2-Z1
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Summary up to here
22
+ Superposition procedure (SP) works fine
+ Acceptable results up to 15 MHz
Inaccurate results up from15 MHz Reason:
a) SPICE modelling of inner conductor system to obtain current spectra troublesome due to parasitic elements -> fair approximation only up to 15MHz
b) No general rule known for the applicability of the various formulas (Kley, Vance, Tyni, Demoulin) for the prediction of shield transfer impedance
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… Utilizing Current Probe Measurements
23
I1 I2
Ui ~
Current clamp 𝑖 measures 𝐽𝑖
J1 J2
Coefficients 𝑇′𝑖𝑗 , 𝑍′𝑖𝑗
are determined by
simulation.
𝑻′ (𝑚x𝑚) , a special kind of 𝒁′ , couples 𝑚
currents 𝑰 to 𝑚 sheath currents 𝑱 measureable outside the cable.
𝒁′ (𝑛x𝑚) couples 𝑚 currents 𝑰 at inner
conductor line ends to 𝑛 thereof linear
dependent quantities 𝑼 (e.g. 𝑈3).
𝐽1 = 𝑇′11𝐼1 + 𝑇′12𝐼2 𝐽2 = 𝑇′21𝐼1 + 𝑇′22𝐼2
𝑱 = 𝑻′𝑰 → 𝑰 = 𝑻′−1𝑱
Calculation of currents 𝐽𝑖
Can be written as
Excitation 𝑰 can be calculated from
measured sheath currents 𝑱
𝑈𝑖 = 𝑍′𝑖1𝐼1 + 𝑍′𝑖2𝐼2
= 𝑍′𝑖1𝑍′𝑖2𝐼1𝐼2= 𝒁′𝑖𝑰
= 𝒁′𝑖𝑻′−1𝑱
Calculation of voltage 𝑈𝑖
Voltage 𝑈𝑖 (and any other linear
dependent quantity) can be calculated
from measured sheath currents 𝑱
𝒁′𝑖 : i-th row of 𝒁′
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Shields don‘t matter
24
A transfer function can be separated into a shield dependent part and a part depending on all other factors (mainly geometrical ones).
For a transfer function 𝑍′𝑖𝑗 acting on current 𝐼𝑖
this can be written as
𝑍′𝑖𝑗 = 𝑍′′𝑖𝑗
𝑍𝑡,𝑖
Analog for matrix 𝑻′ , that couples the measured currents to the exciting currents
𝑇′𝑖𝑗 = 𝑇′′𝑖𝑗
𝑍𝑡,𝑖 → 𝑻′ = 𝑻′′ 𝑫𝑡
→ 𝑻′−1 = 𝑫𝑡
−1 𝑻′′−1
= 𝒁′𝑻′−1𝑱
The dependent quantities 𝑼 are calculated by
= 𝒁′′𝑻′′−1𝑱
This shows: The calculation of dependent
quantities 𝑼 by measured sheath currents
𝑱 does not depend on the transfer
impedance of the shields.
For all transfer functions 𝒁′ this can be expressed by a diagonal matrix 𝑫𝑡 , containing the
transfer impedances 𝑍𝑡,𝑖 at the 𝑚 cable ends associated to the 𝑚 exciting currents 𝐼𝑖.
𝒁′ = 𝒁′′ 𝑫𝑡
𝑼 = 𝒁′ 𝑰
= 𝒁′′ 𝑫𝑡 𝑫𝑡−1 𝑻′′
−1𝑱
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Summary
25
• Intention: EMC simulation of hybrid vehicle electric drive system
• Simplified model: SID and battery/motor imitations
• Inner and outer system: coupled by transfer impedance of cables
• Simulation needs:
• Sources (obtained by measurement + SPICE modelling)
• Transfer impedance (obtained by formula)
• Geometry of SID setup
• Decoupled inner conductors, computational requirement (accomplished by superposition procedure)
• Comparison with measurement: Results ok up to 15 MHz, inaccurate above.
• Reason: Inaccurate SPICE modeling of sources due to parasitic elements
• Remedy: Calculate source currents by measured sheath currents using the superposition procedure
• Replaces SPICE modelling
• Transfer impedances do not contribute
• Not measured yet