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 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 2
Hybrid system HV-power lines E-Motor Battery Inverter Radio/TV window antennas 3
EMC Sketch of Hybrid System Interference into vehicle antennas High voltage battery DC/AC Inverter Electric engine Engine ground strap Antenna amplifier 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 4
Shielded High Voltage Lines 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! Measurement vs Kley formula, optimized, large diameter 5
Transfer Impedance According to Various Formulas Transfer impedance Coroplast 9-2610 FLR2GCB2G 16 mm² according to various formulas Frequency [MHz] 6
Simplified hybrid system: Standard interference device (SID) 7 7
Simulation of shielded cables in FEKO 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 8
Indirect FEKO Calculation of the Hybrid System / SID Problem FEKO (Option MOM): Only independent coaxial cables allowed Hybrid system / SID: has coaxial cables with coupled inner conductors One possible solution Excitation of inner conductors by equivalent currents Superposition principle Transfer impedance concept 9
Steps towards a Solution I 1. Equivalent current excitation I 1 Z,l I 2 I 1 Z 1 Z 2 Z,l I 2 ~ U 1 U 2 U 1 U 2 State (U(x), I(x)) of conductor is the same for both excitations Voltages result uniquely 2. Superposition principle I 1 Z,l I 2 I 1 Z,l Z,l I 2 + 10
Steps towards a Solution II 3. Transfer function, Transfer impedance I 1 U s = I 1 Z s1 ~ U S 4. Superposition principle again I 1 I 2 U s = I 1 Z s1 + I 2 Z s2 ~ U S 11
Steps towards a Solution III 5. A shield changes the transfer function, but the linear dependence remains valid I 1 I 2 U s = I 1 Z s1 + I 2 Z s2 ~ U S 6. Application to a multi conductor system I 1 I 2 U s = 4 i=1 I i Z si I 3 I 4 ~ U S Core crosstalk neglected Coupling of originally coupled sources is comprised within complex amplitudes of current excitation 12
Summary of Superposition procedure 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 U s = m i=1 I i Z si 3. Result is a voltage U s (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. 13
How to Obtain Complex Current Spectra 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! 14
Modelling of the inner conductor system in SPICE Parasitic Elements EC SID 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 EC motor power lines 15
Voltage [dbv] Voltage [dbv] Comparison of Results of Measurement and Spice Model 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 spectrum of a line at BI end Simulation Measurement Noise Noise Simulation Measurement Frequency [Hz] Frequency [Hz] 16
Measurement SID on Table F-51 SID MI BI F-51 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 17
Model SID on table 18
Results: Total Current Battery Line at SID Total Current Battery Line at SID Simulation Measurement Noise 19
Results: Total Current at Motor Imitation Total Current at Motor Imitation Simulation Measurement Noise 20
Results: Voltage at Antenna HFH2-Z1 Voltage at Antenna HFH2-Z1 Measurement Simulation 21
Summary up to here + 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 22
Utilizing Current Probe Measurements Z (nxm) couples m currents I at inner conductor line ends to n thereof linear dependent quantities U (e.g. U 3 ). T (mxm), a special kind of Z, couples m currents I to m sheath currents J measureable outside the cable. Coefficients T ij, Z ij are determined by simulation. Calculation of currents J i J 1 = T 11 I 1 + T 12 I 2 J 2 = T 21 I 1 + T 22 I 2 Can be written as J = T I I = T 1 J Excitation I can be calculated from measured sheath currents J Current clamp i measures J i J 1 J 2 I 1 I 2 Calculation of voltage U i U i = Z i1 I 1 + Z i2 I 2 = Z i1 Z i2 I 1 I2 = Z i T 1 J Z i : i-th row of Z = Z i I U i ~ Voltage U i (and any other linear dependent quantity) can be calculated from measured sheath currents J 23
Shields don t matter 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 I i this can be written as Z ij = Z ij Z t,i For all transfer functions Z this can be expressed by a diagonal matrix D t, containing the transfer impedances Z t,i at the m cable ends associated to the m exciting currents I i. Z = Z D t Analog for matrix T, that couples the measured currents to the exciting currents T ij = T ij Z t,i T = T D t T 1 = D t 1 T 1 The dependent quantities U are calculated by U = Z I = Z T 1 J = Z 1 D t D t T 1 J = Z T 1 J This shows: The calculation of dependent quantities U by measured sheath currents J does not depend on the transfer impedance of the shields. 24
Summary 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 25