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

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

Hybrid system

3

Battery

E-Motor

Inverter

Radio/TV window antennas

HV-power lines

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

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!

Transfer Impedance According to Various Formulas

6

Transfer impedance Coroplast 9-2610 FLR2GCB2G 16 mm² according to various formulas

Frequency [MHz]

Simplified hybrid system: Standard interference device (SID)

7 7

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

~ ~ ~ ~ ~

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

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

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

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

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.

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!

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

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

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

Model SID on table

18

Results: Total Current Battery Line at SID

19

Simulation

Measurement

Total Current Battery Line at SID

Noise

Results: Total Current at Motor Imitation

20

Simulation Measurement

Total Current at Motor Imitation

Noise

Results: Voltage at Antenna HFH2-Z1

21

Simulation

Measurement

Voltage at Antenna HFH2-Z1

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

… 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′

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

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