Electromagnetic compatibility assessment in multiconverter power
systems – Conducted interference issues
q
Robert Smolenski
a,⇑, Piotr Lezynski
a, Jacek Bojarski
b, Wojciech Drozdz
c, Lok Choon Long
aa
Institute of Automatics, Electronics and Electrical Engineering, University of Zielona Gora, Licealna 9, 65-417 Zielona Gora, Poland
b
Institute of Mathematics, University of Zielona Gora, Licealna 9, 65-417 Zielona Gora, Poland
c
Department of Development and Functioning of the Regions, University of Szczecin, al. Wielkopolska 15, 70-451 Szczecin, Poland
a r t i c l e i n f o
Article history: Received 29 March 2020
Received in revised form 8 June 2020 Accepted 11 June 2020
Available online 23 June 2020 Keywords:
Electromagnetic compatibility Electromagnetic interference Aggregated interference Power electronic converters Photovoltaic power plant
a b s t r a c t
An assessment of electromagnetic compatibility is important from a technical as well as legal point of view. The harmonized standards should provide detailed guidelines enabling assessment of electromag-netic compatibility for the typical application of devices in systems. This paper presents the specific issues accompanying measurements of conducted electromagnetic interference in multiconverter sys-tems. The theoretical considerations have been confirmed by experimental results obtained in a labora-tory and in a real 1 MW photovoltaic power plant. The presented theoretical and experimental results might constitute the recommendations for practitioners dealing with conducted interference measure-ments, while the proposed approach can be used as a basis for the elaboration of reliable standards for electromagnetic compatibility assessment in multiconverter systems within the scope of the studied interference frequency range.
Ó 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Currently developed technological concepts such as Smart Grid, Smart City, Cyber-Physical Systems, etc., usually require connec-tion of power electronic interfaces with measurement and control
systems [12,15,21]. Power electronic interfaces enable efficient
energy utilization, coupling of renewable energy sources, etc. Mea-suring and control systems provide opportunity for flexible
realiza-tion of various system services[22,20], based on power electronic
interfaces. Unfortunately, the pulse mode operation of the convert-ers is inherently linked with a substantial level of generated
elec-tromagnetic interference (EMI) [24,13,16], while the measuring
and control equipment is sensitive to EMI. A well-known
exempli-fication of the problem is the influence of EMI, generated by power electronic converters, implemented in, e.g., drive systems or light dimmers, and on transmission reliability of Power Line
Communi-cation (PLC) [5,4,7], commonly used in Advanced Metering
Infrastructure (AMI) for smart power meters reading[18,9,8,2].
In the unanimous opinion of specialists the assurance of the ElectroMagnetic Compatibility (EMC) of the high emission convert-ers and susceptible measuring and control systems is one of the key factors conditioning development of modern power systems
[10]. From a legal point of view EMC assurance is the so-called
essential requirement enabling introduction of products or
sys-tems to the market. According to the EMC directive[23]definition
‘‘(4) ‘electromagnetic compatibility’ means the ability of equip-ment to function satisfactorily in its electromagnetic environequip-ment without introducing intolerable electromagnetic disturbances to other equipment in that environment;”. It is difficult for producers to assess such generally described EMC, hence the usage of stan-dards, harmonized with the EMC directive, is recommended. How-ever, some of the phenomena closely linked with EMC assessment, demanded by the EMC directive, are not addressed in the standards.
This paper has presented the problem connected with EMC assessment in multiconverter power systems. According to the EMC directive: ‘‘(14) Manufacturers of equipment intended to be connected to networks should construct such equipment in a
way that prevents networks from suffering unacceptable
https://doi.org/10.1016/j.measurement.2020.108119 0263-2241/Ó 2020 The Author(s). Published by Elsevier Ltd.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
qThis paper is part of a project that has received funding from the European
Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 812391 - SCENT as well as the National Centre for Research and Development in the frame of European Regional Devel-opment Fund (ERDF), program Smart Growth Operational Programme, action 1.2, project number: POIR.01.02.00-00-0241/17.
⇑ Corresponding author.
E-mail addresses:R.Smolenski@iee.uz.zgora.pl(R. Smolenski),P.Lezynski@iee.uz. zgora.pl(P. Lezynski),J.Bojarski@wmie.uz.zgora.pl(J. Bojarski), Wojciech.Drozd-z@usz.edu.pl(W. Drozdz),L.Long@iee.uz.zgora.pl(L.C. Long).
URLs:http://www.iee.uz.zgora.pl(R. Smolenski),http://www.iee.uz.zgora.pl(P. Lezynski),http://www.wmie.uz.zgora.pl (J. Bojarski),http://www.usz.edu.pl(W. Drozdz),http://www.iee.uz.zgora.pl(L.C. Long).
Contents lists available atScienceDirect
Measurement
degradation of service when used under normal operating condi-tions. Network operators should construct their networks in such a way that manufacturers of equipment liable to be connected to networks do not suffer a disproportionate burden in order to pre-vent networks from suffering an unacceptable degradation of ser-vice. The European standardisation organisations should take due account of that objective (including the cumulative effects of the relevant types of electromagnetic phenomena) when developing harmonized standards”.
In spite of the evaluation of the cumulative effects being an obligatory requirement, our EMC laboratory measuring practice indicates that the producers have no support in the harmonized standards and cannot find detailed instructions on how to perform this type of evaluation. Generally, for EMC assessment in multicon-verter systems the standard CISPR 11 (EN 55011) can be used. In this standard ‘‘semiconductor converters” described in detail as ‘‘switched mode power supplies and semiconductor converters (when not incorporated in an equipment), semiconductor recti-fiers/ inverters, grid connected power converters (GCPC)” have been classified as group 1 ISM (Industrial, Scientific and Medical)
equipment. In the current version of the standard[3]a lot of space
was devoted to ‘‘Statistical assessment of series produced equip-ment against the requireequip-ments of CISPR standards” included in Annex H. Such evaluation should be performed on a sample of not less than five and not more than twelve pieces of equipment.
Such evaluation is similar to those presented inFig. 1and is related
to emission level differences between individual pieces of equip-ment selected from series production. However, there is no requirement concerning multiple final measurement for a single
selected frequency. The experimental results presented in Fig. 2
andFig. 3have indicated that due to dispersion of emission level values, measured in multiconverter systems, EMC assessment based on single final measurements might be unreliable.
The novelty of the paper consists in mathematical description of the phenomena accompanying aggregation of sinusoidal compo-nents of similar frequencies, which might be used for elaboration of EMC assessment procedure for multiconverter systems. Theoret-ical considerations have been supported by experimental results obtained in a laboratory system as well as in a 1 MW photovoltaic (PV) power plant. The authors have not found any descriptions of the measurement problems linked with EMC assessment in
multi-converter systems nor any explanation of phenomena causing any indicated problems.
2. Standardized measurements of aggregated interference generated by a group of DC-DC converters
In earlier paper[1,14]we have shown some results presenting
the difficulties linked with assessment of electromagnetic compat-ibility in the conducted EMI frequency range. We used standard-ized measuring arrangement based on the superheterodyne EMI receiver with normalized intermediate frequency bandwidths (IF BW) and detectors. Detailed descriptions of IF BW and detectors with mathematical descriptions and parameters are included in a
standard CISPR 16-1-1[11].Fig. 1 andFig. 2show the results of
1000 measurements, taken using the peak detector, which is the
Fig. 1. Box-and-whisker plots of peak detector measurements for single converter.
Fig. 2. Box-and-whisker plots of peak detector measurements for single converter and groups of converters.
Fig. 3. Box-and-whisker plots of average detector measurements for single converter and groups of converters.
most dynamic among EMI detectors for identifying the changes of measured values. Each of the measurements was taken during measurement period equal to 1 s, typical for final measurements, which according to standards are performed using averaging
detectors, such as quasi-peak and average[6,19]. InFig. 1, in the
form of box-and-whisker plots, there are presented the results for five DC-DC converters, which have been built to be as similar as possible.
Based on the conducted EMI standards one final measurement for one selected frequency is sufficient. The distribution of the results obtained for each converter confirms that such an approach is reasonable. The differences between values measured for indi-vidual converters are also located in the 1 dB range. However,
Fig. 2shows the results for a group of converters, indicated as 1,
2, 3, 4, and 5 inFig. 1. In spite of the fact that the impedance of
the interference flow remained unchanged during all tests the measurements obtained for groups of converters differ signifi-cantly. The differences between individual measurements attained 28 dB, which makes assessment of EMC in multiconverter systems, based on the single 1 s measurements, unreliable.
Similar results have been obtained using the average detector, during a 1 s final measurement. The dispersions of the 1,000 results
presented inFig. 3for multiconverter arrangements (1–2, 1–3, 1–4,
1–5) confirmed that single final measurement, using the average detector, should not be used for comparison with conducted EMI limits specified in the standards. In fact, a 16 dB (six times) differ-ence between results obtained for multiconverter arrangements makes the evaluation procedure based on a single measurement definitely unreliable.
3. Low frequency envelopes of EMI in multiconverter systems The individual switching frequency harmonics of power elec-tronic converters might be treated as sinusoidal components of a Fourier series. Thus, considering a multiconverter system, includ-ing commercially available converters, with a switchinclud-ing frequency adjustment resulting from accuracy of real components, the phe-nomena accompanying aggregation of sinusoidal components with slightly different frequency should be examined.
The appearance of slow-changing envelopes superimposed on aggregated sinusoidal signals of similar, but not the same, frequen-cies is relatively well-known, especially in acoustics, as frequency
beat[17]. According to frequency beat theory, the sum of harmonic
vibrations with frequencies f1and f2of amplitudes equal to 1 can
be expressed by: S2ðt; ff 1; f2gÞ ¼ sin 2ð
p
f1tÞ þ sin 2ðp
f2tÞ ¼ 2 cos 2p
f1 f2 2 t sin 2p
f1þ f2 2 t : ð1ÞThe frequency beat effect appears when fj 1 f2j f1þ f2. In
such conditions the absolute value Env2ðt; ff 1; f2gÞ ¼ 2 cos 2
p
f1 f2 2 t ð2Þis the envelope of aggregated signal. It is possible to observe that the period of the envelope depends on the difference between fre-quencies of aggregated signals, not the frefre-quencies of the
compo-nents, which is presented inFig. 4andFig. 5.
Fig. 4presents signal, which arose from the aggregation of
sinu-soidal components f1¼ 10 Hz and f2¼ 9 Hz, whileFig. 5shows the
aggregation of components for frequencies f1¼ 100 Hz and
f2¼ 99 Hz. It is possible to note that in the case of the same
fre-quency difference equal to 1 Hz the period of the envelope is the same 1 s, in spite of the different frequencies of aggregated sinu-soidal components.
For a higher number of harmonic vibrations of slightly different
frequencies (f1; f2; . . . ; fn) it has been found that the total sum can
be considered as a linear combination of frequency beats: Snðt; ff 1;...;fngÞ ¼ Xn i¼1 sin 2ð
p
fitÞ ¼ 2 n 1 X 26i6n 16j<i cos 2p
fi fj 2 t sin 2p
fiþ fj 2 t : ð3ÞThe frequency beat effect appears when: max 26i6n 16j<i fi fj min 26i6n 16j<i fiþ fj :
In such case the envelope can be expressed as: Envnðt; ff 1; . . . ; fngÞ ¼ 2 n 1 X 26i6n 16j<i cos 2
p
fi fj 2 t : ð4ÞThe period of this envelope is the inverse of the largest common
divisor of all pairs fi fjfor 26 i 6 n; 1 6 j < i.
As an exemplification,Fig. 6andFig. 7present the envelopes of
high frequency signals formed by the aggregation of sinusoidal components of slightly different frequencies and amplitudes equal
to unity. InFig. 6the individual signals have been formed by the
addition of a various number of sinusoidal components with ran-dom initial phase. We assumed the frequency of sinusoidal signals equal to 15 kHz with a random variation of 0.02%. The smaller the frequency differences are, the slower the changing envelopes of the aggregated signals are observed. Unfortunately, from the measure-ment point of view the more precise the switching frequency of the
Fig. 4. Envelope of signal resulting from aggregation of two sinusoidal components of frequencies: f1¼ 10 Hz and f2¼ 9 Hz.
Fig. 5. Envelope of signal resulting from aggregation of two sinusoidal components of frequencies: f1¼ 100 Hz and f2¼ 99 Hz.
converters is, the slower the changes in the EMI envelopes are which have to be taken into account and the longer the measure-ments are which are required for reliable EMC assessment. This
phenomenon is illustrated in Fig. 7, which shows the envelopes
of the aggregated signal for six sinusoidal components, which depend on frequency tolerances expressed in ppm values, and which are typical for crystal oscillators used for generation of power electronic converter clock signals. For an assumed switching frequency of 15 kHz, the accuracy of 5 ppm means that the
fre-quencies of the sinusoidal components (inFig. 7) differ from each
other less than 0.075 Hz.
4. Standardized, laboratory EMI measurements
The EMC assessment might be performed using ‘‘the presump-tion of conformity of equipment” based on the fulfillment of
har-monized standards requirements [23]. In order to show the
influence of frequency beat on EMC assessment in a multiconverter system, standardized measurements have been taken in a labora-tory setup, in full compliance with EN 61800-3 standard for power
drive systems (Fig. 8). The measurements were taken using a
dig-ital EMI receiver, which provides CISPR 16-1-1 compliant measurements.
Fig. 9andFig. 10present waterfall spectrograms of electromag-netic interference measured using an average detector with IF BW
= 200 Hz in standard measuring setup, presented inFig. 8.
During measurements both converters were connected to the grid in order to provide unchangeable impedance of interference
current paths. However,Fig. 9presents a spectrogram for
measur-ing conditions where only one frequency converter fed drive was operated. In this case the 16 kHz harmonics, caused by the con-verter switching frequency, have relatively constant value. In the case, when two converters were operated, the low frequency envelopes have been superimposed on the converter switching
harmonics,Fig. 10. It is possible to predict that a standardized 1
s final measurement for 16 kHz would give a dispersion of results
similar to those presented inFig. 3.
According to Eq.4the frequency of the envelope depends on the
difference between beating frequencies. In the case of power elec-tronic converters the difference between switching frequencies of individual converters is multiplied by the number of harmonics.
Fig. 6. Envelopes of high frequency signals resulting from superimposition of various number of sinusoidal components.
Fig. 7. Envelopes of high frequency signals resulting from aggregation of six components of different frequency tolerances.
Fig. 8. Laboratory testing arrangement for EMI measurements of power drive systems, according to EN 61800-3.
Fig. 9. EMI spectrogram for one operated converter measured using average detector.
In the experimentally obtained spectrogram, presented inFig. 10, the frequency of the envelope of the second harmonic (32 kHz) is approximately two times greater than the frequency of the envel-ope of the first harmonic (16 kHz).
5. Case study measurements in photovoltaic power plant The results of laboratory measurements as well as theoretical and simulation analyses have been confirmed in a typical applica-tion area for multiconverter systems. The below presented results have been taken from the 1 MW PV power plant, at a common low voltage connection point of eight 125 kW PV power electronic
interfaces.Fig. 11schematically shows the PV power plant with
depicted converters, and measuring point.Fig. 12shows the
spec-trogram for frequency range 9 kHz--60 kHz and a period of 120 s, measured using digital EMI receiver type TDEMI X6 with quasi-peak detector and IF BW = 200 Hz. The low frequency envelopes
have been observed on the 16 kHz harmonics corresponding to the switching frequency of the PV interfaces.
The results obtained using a digital EMI receiver encouraged further long-term interference data recording using a power qual-ity analyzer type PQ BOX 300 equipped with a so-called
supra-harmonics analysis feature.Fig. 13shows the waveforms of active
power generated by a PV power plant with a 16 kHz component of EMI, averaged in 10 min time slots during three days with IF BW = 200 Hz. It is possible to observe that the EMI levels on individual days are similar in spite of the significantly different active power. In order to identify sources of interference, the spectra of the interferences for specific conditions of PV plant operation,
indi-cated by the active power waveforms in Fig. 13, were analyzed.
Fig. 14shows spectra measured for a maximum level of
interfer-ence, depicted as point A in Fig. 13. In the spectra it is possible
to distinguish harmonics of the frequency 16 kHz, which might be linked with the switching frequency of PV interfaces. This assumption is confirmed by measurement results taken when PV power plant did not work (point B - active power generated by a PV power plant equal to zero). In the spectrum measured for point
B, presented inFig. 15, 16 kHz harmonics, linked with PV interface
switching frequency, do not occur. In this spectra 17 kHz
harmon-Fig. 10. EMI spectrograms for two operated converters measured using average detector.
Fig. 11. PV power plant scheme with depicted measuring point.
Fig. 12. Spectrogram of EMI measured at low voltage connection point of 1.5 MW photovoltaic power plant.
Fig. 13. Waveforms of active power and 16 kHz voltage harmonic of EMI generated by PV power plant.
ics, introduced by an uninterruptible power supply (UPS) installed at a control station, prevail.
The spectra measured at point C, shown inFig. 16, consists all of
the main components, however the 16 kHz component remains dominant.
Using the possibilities offered by a digital EMI receiver the envel-opes of the main frequency components were presented in the form
of a waterfall spectrogram, shown inFig. 12for the attainable period
of 120 s. In order to evaluate the variation of the EMI levels over longer periods an investigation was made of the data containing maximum, average and minimum values, registered by PQ BOX
300 in 10 min time slots, based on 200 ms intervals.Fig. 17shows
the maximum, average and minimum values measured in 10 min time slots for the 16 kHz component, during a three-day analysis
of the PV power plant operation (Fig. 13). It should be emphasized
that in each 10 min slot, during the PV power plant operation, signif-icant differences between maximum, average and minimum values were registered. The differences for maximum and average values measured during the three days are also substantial and might influ-ence appropriate EMC assessment.
6. Conclusion
This paper has presented measurement difficulties accompany-ing EMC assessment in muliticonverter systems for conducted EMI frequency range. The cognitive aspects of the phenomena linked with aggregation of interference generated by a group of convert-ers seem to be crucial for proper EMC assessment in such systems, especially in the absence of related standards.
On the basis of theoretical considerations, supported by exper-imental results, it has been shown that low frequency envelopes, arising from frequency beating, are responsible for unacceptable dispersion of the results obtained with standardized EMI measur-ing techniques for a multiconverter system.
The rate of change of the envelope depends on the differences of a converters’ harmonic frequencies. Thus, in a real situation for multiconverter systems, based on the same converters, the rela-tively long period envelopes of interference should be considered during EMI measurements.
As it has been shown, the difference between aggregated com-ponent frequencies are multiplied by the number of harmonics, thus the analysis of the envelopes of higher harmonics might con-tribute to the assessment of the time required for proper evalua-tion of conducted electromagnetic interference.
The presented investigations indicate that the currently binding standards do not allow for objective EMC assessment in multicon-verter systems. According to the EMC directive recommendation an effort should be made to organize standardization so that stan-dards are developed which take into account the cumulative effects of electromagnetic phenomena. The statistical analysis based on multiple measurements, as well as on waterfall spectro-grams, might be useful for the proper evaluation of conducted EMI in multiconverter systems, which constitute a significant part of EMC assessment.
Fig. 14. Spectrum of conducted EMI generated by PV power plant for point A in Fig. 13.
Fig. 15. Spectrum of conducted EMI generated by PV power plant for point B in Fig. 13.
Fig. 16. Spectrum of conducted EMI generated by PV power plant for point C in Fig. 13.
Fig. 17. Waveforms of 16 kHz voltage harmonic of EMI generated by PV power plant.
In this paper the scope of investigation has covered theoretical description of the phenomenon accompanying aggregation of interference. Some of findings have mainly cognitive value, because in practice it is difficult to obtain the necessary parameters with the required accuracy, e.g. find the largest common divisor of all pairs of beating frequencies. However, most of the theoretical assumptions have been experimentally confirmed in arbitrarily selected multiconverter systems. Measurements have been taken in laboratory arrangement as well as at a 1 MW PV power plant. The future research will be focused on multipoint, simultaneous measurements to evaluate contribution of individual converters to aggregated interference and elaboration of generalized proce-dures for EMC assessment in multiconverter systems.
Declaration of Competing Interest
The authors declare that they have no known competing finan-cial interests or personal relationships that could have appeared to influence the work reported in this paper.
CRediT authorship contribution statement
Robert Smolenski: Conceptualization, Methodology, Writing -original draft. Piotr Lezynski: Methodology, Writing - -original draft. Jacek Bojarski: Conceptualization, Methodology. Wojciech Drozdz: Conceptualization, Methodology. Lok Choon Long: Data curation.
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R. Smolenski was born in 1973 in Krosno Odrzanskie, Poland. He obtained his M.Sc., Ph.D. and post-doctoral degrees in electrical engineering from the University of Zielona Gora. He is currently Associate Professor and Head of the Institute of Automatics, Electronics and Electrical Engineering at the University of Zielona Gora, and Deputy Chair of Joint IEEE IES/PELS Poland Chapter as well as Member of the Electromobility Board of the National Centre for Research and Development. Robert Smolenski has over twenty years of experience in the development of practical solutions to EMC and PQ related problems and has worked on many national and international research projects. His research focuses on issues linked with assur-ance of power quality as well as the interoperability of power systems involving power electronic interfaces and control arrangements.
P. Lezynski obtained his MSc in 2008 and Ph.D. in 2014 in electrical engineering from the University of Zielona Gora. In the years 2008–2010, he worked as a designer of power electronics at the company Metrol in Zielona Gora. Since 2011, he has worked at the Institute of Automatics, Electronics and Electrical Engineering at the University of Zielona Gora. In addition to scientific and didactic work, he conducts commercial EMC measurements in the EMC Laboratory of the Institute. He has been involved in worked on many research projects in the field of elec-trical engineering. His scientific interests include issues of EMC of power electronics devices.
J. Bojarski was born in 1968 in Kostrzyn, Poland. He was awarded an MSEE in 1996 and PhD in 2000 from the Technical University of Zielona Gora. Now he is a teacher in the Division of Mathematical Statistics and Econometrics at the University of Zielona Gora. His major research interests include statistical (especially Bayesian estimators) and uncertainty analysis. One of his research activities includes the utilization of advanced computer science technologies in mathemat-ics.
W. Drozdz is associate Professor, PhD in Economics and a graduate of the University of Szczecin, University of Economics in Poznan and University of Warsaw. He specializes in energy economics and management of energy security, international economic relations and international logistics. His scientific and research interests are focused on logistics, transport economics (including electromobility and aviation), energy man-agement, energy and international security as well as international economic relations. His research and development work is strengthened by membership of national and international institutions and associations (member of the management board of Enea Operator Ltd., as vice president for innovation and logistics, chairman of the Szczecin branch of the Polish Geopolitical Association, member of the Association for Economy and Energy for Poland (PAEE), a member of the International Association for Energy Economics (IAEE)).
C.L. Lok received his B. Eng. (Electrical) in 2011 from the Universiti Tun Hussein Onn Malaysia. From 2011 to 2019, he worked as an electrical engineer in various engineering industries. In 2017, he obtained his M.Sc. in Experimental Physics from the University of Malaya, Malaysia. In addition, he is a certified Registered Elec-trical Energy Manager (REEM) and certified Grid-Connected Photovoltaic (GCPV) engineer since year 2017. Currently, he is pursuing a Ph.D. in the ‘‘Smart Cities EMC Network for Training (SCENT)” project at the Institute of Automatics, Electronics and Electrical Engi-neering, University of Zielona Gora, Poland. His research interests include EMI propagation in power electronics, Smart Grids, power systems and photovoltaic systems.
