2014
International Symposium on Power Electronics,
Electrical Drives, Automation and Motion
Experimental Characterization of Conducted EMI in
Three-phase Power Electronics System using
Terminal Model
Junsheng Wei, Dieter Gerling
Marek Galek
Institute of Electrical Drives and Actuators
Universitaet der Bundeswehr Muenchen
Neubiberg, Germany
Corporate Technology
Siemens AG
Munich, Germany
to the requirement on measurement setup to extract model
parameters, the modification and build-up of hardware
measurement setup are described. The experimental
characterization based on this setup is then carried out with
procedure described and the data obtained is processed to
calculate model parameters. At the end, model obtained is
used to predict the noise level at LISN when connection
conditions between LISN and converter is changed. Results are
compared with measurement.
Abstract— This paper investigates the relevant issues for
experimental characterization of conducted EMI resulting from
three-phase power electronics system. Terminal model is used to
describe the EMI behavior. The theoretical analysis and
equivalent terminal model for three-phase power electronics
system are conducted and proposed respectively. The
measurement setup which fulfills the requirement of parameter
extraction is then discussed. Measurement procedure is described
and the obtained data is processed to calculate the desired
parameters. Using the model and parameters from experimental
extraction, conducted EMI level can be predicted and the results
are compared with measurement to validate the correctness of
the characterization.
II.
Fig. 1 and Fig. 2 show the schematic and prototype of used
research object. The investigated object consists of three-phase
diode-bridge followed by boost converter and is used as a
representative for three-phase system. Conducted EMI is
usually measured in standard-defined environment, as shown in
Fig. 3 [9].
Keywords—conducted EMI; three-phase system; terminal
model
I.
INTRODUCTION
Conducted EMI has been important concern during the
design phase of power converters since certain limits are set by
international standards committee to ensure that the emission
will not endanger other equipment in the vicinity. In order to
comply with the standards, certain measures have to be adopted
and EMI filter is one of the most widely used methods [1].
Filter design has been a difficult task since long time ago and
the trial-and-error routine makes it even more costly as well as
time-consuming. There is a trend to design the filter in a virtual
way [2]. By combining the estimated filter characteristic with
models describing the EMI characteristic of power converter,
the effectiveness of noise suppression can be estimated with
very good accuracy. Because of the high frequency essence of
EMI, the models of power converters, either based on timedomain simulation [3, 4] or frequency-domain simulation [5,
6], require detailed knowledge on the converters, which are not
always easily accessible. Thus, terminal modeling has been
proposed as an alternative [7]. Most of the researches of
terminal modeling focus on single-phase system while threephase system plays also important role in industry and is
worthy to be investigated.
Fig. 1 Schematic of three-phase prototype
This paper is organized in such way that firstly the terminal
model of power converter with active source and passive
impedance matrix is presented based on analysis of the
measurement setup, as already proposed in [8]. Then according
978-1-4799-4749-2/14/$31.00 ©2014 IEEE
TERMINAL MODEL OF THREE-PHASE SYSTEM
Fig. 2 Photo of three-phase Prototype
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The passive admittance matrix in Boost part is a 2×2 matrix
and active source is described with a 2×1 matrix. For the
acquisition of LISN and cable matrix, it can be done with
impedance analyzer and no further measurement setup is
needed. As introduced in [8], the matrix of D6 can be obtained
with special modeling technique. The parameters in Boost part
must however be extracted based on terminal responses under
different tests.
III.
HARDWARE MEASUREMENT SETUP
To realize the parameter extraction for Boost part, several
tests should be carried out. In order to fulfill the test condition,
the LISN should be modified and according to the
measurement environment in Fig. 3, additional components are
needed.
Fig. 3 Measurement environment defined by standard
A. Terminal Model
Considering the conducted EMI measurement environment
and assuming that the line impedance stabilization network
(LISN) works as expected in the discussed frequency range, the
block diagram in Fig. 4 represents the flow of high frequency
current. The diode bridge (D6) is separately considered, apart
from other parts in converter. Reason is, under assumption that
D6 only affects as propagation path but not as noise source, its
time varying essence prevents it to be included in the passive
admittance matrix, which should be time invariant. The rest of
converter can be described with active source and passive
admittance matrix as in Fig. 5, according to Norton’s theorem.
A. LISN
The used LISN in the investigation is a commercial
product, which is designed to fulfill the requirement for
conducted EMI measurement in the frequency range from 9
kHz to 30 MHz as defined in [10]. The LISN has circuitry
schematic for each phase as shown in Fig. 6. The series
connected inductors are used to block high frequency current
and parallel connected capacitors are for the purpose of
bypassing noise. The LISN has generally two functions:
1.
Avoid noise from grid to affect measurement results;
2.
Provide constant impedance at different frequencies.
Fig. 6 Schematic of one phase in LISN
As discussed in [8], for the calculation of model
parameters, the high and low impedance terminations at
different ports should be applied under different test conditions.
Therefore, at the stage of LISN, it should be modified so that
these two terminations are realized and easily changed from
one to the other. The two circuit connections chosen for
achieving these two terminal conditions are shown in Fig. 7
Fig. 4 Block diagrams for high frequency current
In the low impedance termination case, only the second
capacitor leg is disconnected. This is mainly for the easier
switch of impedance states. In the remaining two capacitor
legs, the first on the grid side, which has high capacitance
value, has actually small impact on overall impedance since the
two series-connected inductors have a total inductance of 300
mH and effectively block the high frequency current. The
LISN has a load of 50 Ω in each phase. The load is low
compared to the other possible impedance in the propagation
path of high frequency noise and thus it can be kept in low
impedance termination case together with the series connected
capacitor.
Fig. 5 Equivalent model of Boost part
B. Model Parameters
As shown in Fig. 4 and Fig. 5, in order to describe the EMI
behavior of the system, the passive matrix of LISN, cable, D6,
Boost part and the active source matrix of Boost part must be
known. The passive matrix of LISN is a 3×3 matrix while that
of cable is 6×6 matrix and D6 is represented with a 5×5 matrix.
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(1)
Fig. 9 Termination impedance with cable (green: high, blue: low)
Low impedance termination
A possibility to solve this problem is using high impedance
between input cable and power converter, to ensure that the
impedance seen from power converter is still high. Because the
parasitic capacitance effects only at high frequency, the
impedance of the additional component is not necessary high in
low frequency and the use of inductor with low inductance
value should be enough.
(2)
There will be power current flowing through the blocking
inductors which could be critical since with DC-bias the
permeability of magnetic material may be influenced. The
power current is in the range of several Ampers depends on
operation point. The impedance of blocking inductor is
therefore tested under DC-bias in this range. It is however not
possible to directly measure the impedance with impedance
analyzer and thus the measurement setup using S-Parameter
measurement is used as shown in Fig. 10.
High impedance termination
Fig. 7 Circuit connections in LISN for realization of test
conditions
In the high impedance termination case, besides the second
capacitor leg, the third one is also disconnected. In order to
facilitate the switch between impedance terminations, BNC
connectors are used at the point of disconnection. The
measured impedances at low as well as high impedance
termination are shown in Fig. 8.
Fig. 10 Measurement setup for impedance of blocking inductor under DC-bias
S-Parameters are measured with network analyzer and
based on characteristic impedance the impedance of device
under test is calculated. Fig. 11 shows the impedance of the
inductor under different DC-bias and even though the resulting
impedance is lower with increasing DC-bias, the impedance
value is still high enough for the application and therefore the
test inductor is used as blocking inductor.
Fig. 8 Measured impedances at different terminations (blue: low
termination impedance, green: high termination impedance)
B. Blocking Inductors
The measurement setup in Fig. 3 shows the use of input
cable and in reality, an input cable of about 2 meters is used to
supply power from LISN to power converter. Because of the
length of cable, the parasitic effects, including parasitic
inductance and parasitic capacitance, cannot be neglected in the
frequency concerned. Especially the parasitic capacitance
induces problem for realizing high impedance termination,
since it provides a low bypass path in high frequency range and
reduces the effectiveness of high impedance. Fig. 9 shows the
comparison between high and low impedance termination if
cable is taken into account.
Fig. 11 Measured impedance of blocking inductor under DC-bias
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considering the voltage level, input capacitance and if it is
differential or not. Those possibilities are considered and tested
to find out suitable probes for the purpose of conducted EMI
characterization as shown in following.
Comparing with Fig. 9 which shows the comparison
between low and high impedance with cable, Fig. 12 shows the
comparison between high termination impedance considering
the cable and blocking inductor. Improvement of high
impedance in high frequency range is obvious.
The voltage probes are tested firstly. The tests have been
made at the same source and with FFT the spectrums from two
voltage probes are calculated. The oscilloscope with better
performance as shown in last paragraph is used. Fig. 14 shows
the spectrums.
Fig. 12 Termination impedance with cable and blocking inductor
(green: high, blue: low)
C. Oscilloscope
The oscilloscope is used to capture waveform in time
domain, which is then post-processed and transformed into
frequency domain by using Fast Fourier Transformation (FFT).
From the theory of FFT, it is known that in order to achieve
good resolution in the spectrum at high frequency, the recorded
waveform must contain small steps and include accurate
change in short time. These requirements on waveform can be
interpreted as requirements on oscilloscope on two aspects: the
storage capability and the resolution of Analogue-DigitalConverter (ADC). In the investigation, two oscilloscopes with
different storage capabilities and ADC resolutions are used.
Fig. 13 shows the spectrums calculated by FFT from recorded
waveforms of the two oscilloscopes with the same source.
Fig. 14 Calculated spectrums with different voltage probes
From the spectrums, the resolution in high frequency range
is obvious. The better performance is obtained using voltage
probe with smaller capacitance and lower voltage rating.
For current probes, tests have also been conducted and their
spectrums are shown in Fig. 15. As the figure shows, neither
the method of shunt resistor nor the hall-effect sensor has
enough accuracy, considering that the current transformer is a
well-calibrated device for high frequency application.
Therefore, only the current transformer will be used for
acquisition of current information.
Fig. 13 Spectrums using two different oscilloscopes
From the spectrums, it is obvious that the used oscilloscope
has determinant influence on the accuracy of data acquisition,
especially in the high frequency range.
Fig. 15 Calculated spectrums with different current probes (red: Shunt
resistor, blue: current transformer, green: Hall-effect sensor)
Till now all the used hardware devices have been
determined.
D. Voltage Probe and Current Probe
In order to transfer the electrical signal from measurement
object to oscilloscope, voltage and current probes are used.
There are different possibilities of measurement methods and
different types of devices to be chosen. For instance, in order to
measure the current, possible methods include shunt resistor,
current transformer and hall-effect sensor. For voltage
measurement, the choice of voltage probe is various
IV.
MEASUREMENT PROCEDURE AND DATA PROCESSING
With the measurement setup and devices chosen from
analysis as described in last chapter, the measurement
procedure can be carried out. As explained in [8], to calculate
the parameters in terminal model, the following tests must be
conducted:
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• High impedance termination at all ports;
• Low impedance termination at Port 1 and high
impedance termination at all other ports;
• Low impedance termination at Port 2 and high
impedance termination at all other ports.
With the hardware setup, the tests can be described as
modification on measurement setups:
• All BNC connectors at LISN are disconnected and all
phase lines are fed through blocking inductors;
• BNC connector at Phase 1 is connected and line of
Phase 1 bypasses blocking inductor while other phases
stay the same as first test;
• BNC connector at Phase 2 is connected and line of
Phase 2 bypasses blocking inductor while other phases
stay the same as first test.
Fig. 17 Phase alignment for waveforms (top: before, bottom: after)
Since as discussed in [8], for each of these tests, all the
terminal behaviors including terminal currents and voltages are
needed for the derivation of model parameters while the
oscilloscope as common devices has only four input ports, five
measurements should be conducted for each test. The input
ports of oscilloscope are arranged as following: one for voltage
measurement, one for current measurement with high
frequency transformer and two for current measurement with
hall-effect sensor. The organization is shown in Fig. 16. The
use of two current measurements with hall-effect sensors is
mainly for time domain aligning of all the measurements and
thus the accuracy of such measurement in high frequency is not
critical.
Fig. 18 Measured waveforms and zoomed-in waveform in
two-diode conduction mode
Once the frequency domain information about terminal
behaviors in different tests is known, the following equations
are used to calculate the model parameters, as explained in [8].
[YBoost ] = [Yall ] − [YLISN +Cable+ D 6 ]
[ IsBoost ] = [ Isall ]
(1)
(2)
⎡ I p1 ⎤
⎡ u p1 ⎤
⎢ I ⎥ − [ IsBoost ] = ([YBoost ] + [YLISN +Cable+ D 6 ]) ⋅ ⎢u ⎥ (3)
⎣ p2 ⎦
⎣ p2 ⎦
Fig. 16 Arrangement of oscillation input ports and overall measurement
setup
To validate the accuracy of model, it is used to predict the
conducted EMI under different connection conditions and
results are shown in Fig. 19 in comparison with measurement.
If it is assumed that the usable range should have deviation
within 6 dB, the accuracy of the model suggests the use
availability up to 10 MHz. The reason for such accuracy
limitation can be located at the realization of termination
impedance. As shown in Fig. 12, at about 10 MHz, the
Fig. 17 shows the measured waveforms before and after
phase alignment. As can be seen, the waveforms are
overlapped in time domain with DC deviation which will not
influence the calculation of spectrum in frequency domain. The
phase alignment is however critical for the correct calculation
of model parameters. Fig. 18 shows the measured waveforms
of current and voltage at Phase 1 in test 1, with zoomed
waveforms in the time frame of two-diode conduction mode.
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parameters in terminal model are derived and the accuracy of
model is validated by comparison between predicted and
measured conducted EMI level under different connection
conditions. Thus the model is most suitable for the design of
EMI filter and can help to estimate the attenuation brought by
filters.
difference between high and low impedance termination is
small. The better agreement between predicted and measured
results in scenario 2 results from the fact that connection
condition in scenario 2 is similar to those used in
characterization procedure.
REFERENCES
[1]
(1)
Predicted noise level in scenario 1
(2)
Predicted noise level in scenario 2
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Fig. 19 Spectrums using two different oscilloscopes
V.
CONCLUSION
This paper investigates the relevant issues of experimental
characterization for terminal modeling of conducted EMI in
three-phase power electronics converters. The investigation
provides analysis and suggestions on hardware, measurement
devices and the overall setup. Different possibilities of
measurement devices are considered in order to guarantee best
performance in data acquisition. Issues that should be paid
attention to during data processing are also addressed. With
measurement data from different test conditions, the
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