Overview and Comparison of Grid Harmonics and Conducted EMI Standards for LV Converters Connected to the MV Distribution System

Transcription

1 2012 Mesago PCIM GmbH Proeedings of the first Power Eletronis South Ameria 2012 Conferene and Exhibition (PCIM 2012), South Ameria, Saõ Paulo, Brazil, September 11-13, 2012 Overview and Comparison of Grid Harmonis and Conduted EMI Standards for LV Converters Conneted to the MV Distribution System R. Burkart, J. W. Kolar This material is published in order to provide aess to researh results of the Power Eletroni Systems Laboratory / D-ITET / ETH Zurih. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for reating new olletive works for resale or redistribution must be obtained from the opyright holder. By hoosing to view this doument, you agree to all provisions of the opyright laws proteting it.

2 Overview and Comparison of Grid Harmonis and Conduted EMI Standards for LV Converters Conneted to the MV Distribution System Ralph Burkart and Johann W. Kolar Power Eletroni Systems Laboratory, ETH Zurih, Switzerland, Abstrat Grid harmonis and onduted EMI standards pose fundamental onstraints on the design and optimization of ommerial and industrial power eletronis systems. The aim of this paper is to provide an overview of different suh standards whih apply to onverter modules in non-publi low voltage grids onneted to the medium voltage distribution system. The examinations over the IEEE 519, IEEE 1547, and CISPR 11 standards as well as the tehnial guideline Generating Plants Conneted to the Medium-Voltage Network from BDEW. In a seond step, a omparative analysis is performed whih identifies the most stringent limits in the onsidered frequeny range from 100 Hz to 30 MHz. The analysis shows that BDEW generally defines the lowest limits for frequenies below 9 khz, whereas for frequenies higher than 150 khz the most diffiult standard to omply with is CISPR 11. Additional limits are proposed for frequenies between 9 khz and 150 khz for whih urrently no appliable standards exist. 1 Introdution A fundamental requirement of ommerial and industrial power eletronis systems is the ompliane with grid harmonis and onduted EMI standards. In medium voltage (MV) distribution systems, typial loads are low voltage (LV) onverter systems (e.g. wind farms, fatories) omprising a multitude of individual (possibly low power) onsuming and/or generating LV onverter modules as shown in Fig. 1(a). In ontrast to LV grid harmonis standards whih are often defined for individual modules (e.g. IEC [1]), the MV standards are mostly defined for the point of ommon oupling (PCC, see [2] for a detailed definition) and hene for the overall onverter system. From the design point of view, it is hene of interest what limits the individual modules must omply with in order to guarantee overall system ompliane at the PCC. In this paper, an overview of MV grid harmonis standards is given. Based on these standards, limits for the individual modules are derived. Conduted EMI standards, whih typially apply to the individual modules and to higher frequenies (Fig. 1(b)), are inluded into the analysis as they are relevant for the proper funtioning of the onverter system. In a seond step, the standards are ompared to eah other to identify the most stringent limits in the frequeny range from 100 Hz to 30 MHz. Setion 2 gives a short summary of the standards onsidered in this paper. In Setion 3, the omparative analysis of the investigated standards is performed. Finally, Setion 4 gives a pratial example. RMS urrents and RMS line-to-line voltages are assumed throughout this paper. If not otherwise stated, SI units are used. EMI Standards GH Standards LV onverter modules LV onverter system (a) PCC MV distribution system Limits GH Standards EMI Standards Frequeny (b) Fig. 1: Power onsuming and generating onverter modules in a LV onverter system onneted to the MV distribution system at the PCC (a). Relationship between MV grid harmonis (GH) and EMI standards (b).

3 Tab. 1: Overview of standards disussed in Setion 2. IEEE 519/1547 BDEW CISPR 11 A1 Appliation Systems of non-linear loads (IEEE 519), power generation equipment (IEEE 1547) Generating plants ISM equipment Grid type Distribution systems kv Distribution systems 1-60 kv General LV grids Frequeny Range Type of limits Measurement 100 Hz khz MHz Current harmonis limits at integer multiples of the fundamental frequeny, IEEE 519: SCRdependent At PCC of onsumer/distribution system interfae Integer and non-integer urrent harmonis limits, linearly dependent from SCR At PCC of onsumer/distribution system interfae, geometri summation of harmonis within a band of 200 Hz above 2 khz QP and AVG limits of mains terminal disturbane voltages LISN voltage measurement, QP- and AVG-evaluation of harmonis within a frequeny band of 9 khz 2 Overview This paper analyzes the IEEE 519 [3], IEEE 1547 [4], and CISPR 11 [5] standards as well as the tehnial guideline Generating Plants Conneted to the Medium-Voltage Network from BDEW [6] (hereafter referred to as BDEW ). In this setion, these standards are briefly introdued and disussed. A summary of the most important aspets is given in Tab IEEE 519 / IEEE 1547 The IEEE 519 standard defines reommended urrent harmonis limits I ν at the PCC for onverter systems omprising non-linear loads (e.g. stati power onverters, ar disharge devies and saturated magneti devies). The limits, whih are defined for multiples of the fundamental frequeny, are shown in Tab. 2. They are lower for even multiples and vary for different distribution system voltage levels U MV and short-iruit ratios (SCR). The limits were developed with the objetive of limiting the voltage harmonis at the PCC to a onstant perentage of the fundamental voltage and under the assumption that the distribution system an be haraterized by a purely indutive short-iruit impedane jωl SC. In addition to the individual harmonis I ν, IEEE 519 also defines limits for the total distortion over frequeny. The orresponding integral measure is defined as the total demand distortion TDD, Tab. 2: Maximum admissible urrent harmonis Iν (odd and even) and TDD in % of I L aording to IEEE 519 for distribution system voltages U MV = [0.12, 161] kv. I SC is the short-iruit urrent at the PCC, U I SC = 3 MV 2π50 HzLSC. Maximum harmoni urrent distortion I ν and TDD in % of I L for distribution systems kv Individual harmoni order ν (odd harmonis) 1) I SC /I L < ν < ν < ν < ν TDD 20 2) (20, 50] (50, 100] (100, 1000] > ) Even harmonis are limited to 25 % of the odd harmoni limits. 2) All power generation equipment is limited to these values of urrent distortion regardless of I SC /I L. For distribution systems in the voltage range kv the above limits redued by 50 % apply.

4 TDD = ν=2 I2 dist,ν I L, (1) where I L is the maximum fundamental load urrent and I dist,ν the generated urrent harmonis of the onverter system at the PCC. The IEEE 1547 standard applies to power generation equipment. Its limits are derived from to the IEEE 519 limits for SCR 20 (Tab. 2) and are independent from SCR. 2.2 BDEW The BDEW standard was defined by the German assoiation of energy and water industries. It is general guideline for the planning and operation of generating plants onneted to the MV distribution system. Tab. 3 shows the defined urrent limits applying to the PCC. The admissible harmoni urrents I ν,µ for module n within a system of N modules an be alulated by multipliation of the values in Tab. 3 with the short-iruit power S SC of the MV distribution system at the PCC, I ν,µ,n = I ν,µ [ A /MVA] S SC Sr,n S r,n, (2) where S r,n is the module rated power and S r,n the system rated power of all N modules. Lower limits than (2) apply in ase of more than one system onneted (at different PCCs) to the same MV distribution system. Note that the limits in Tab. 3 are inverse to the voltage levels and an be interpolated. Furthermore, in ontrast to the IEEE standards, BDEW also defines limits for non-integer multiples of the fundamental frequeny. For harmonis between 2 khz and 9 khz, the distortion levels I dist,ν,µ must be measured within a frequeny band of 200 Hz, entered around the onsidered harmoni, aording to EN , Annex B [7], I dist,ν,µ = 20 Idist,((ν,µ) 50+n 5)Hz 2. (3) n= 19 Tab. 3: Admissible integer urrent harmonis I ν and non-integer urrent harmonis I µ related to the short-iruit power S SC of the MV distribution system aording to BDEW. Admissible, related harmoni urrent I ν,µ in A/MVA Ordinal number ν, µ 10 kv grid 20 kv grid 30 kv grid < ν < 40 1) /ν /ν /ν even-numbered 0.06/ν 0.03/ν 0.02/ν µ < /µ 0.03/µ 0.02/µ 40 < ν, µ 180 2) 0.18/µ 0.09/µ 0.06/µ 1) Odd-numbered 2) Integer and non-integer within a range of 200 Hz, measurement aording to EN , Annex B [7]

5 Tab. 4: CISPR 11 limits U CISPR for lass A group 1 (A1) equipment. S r is the rated module power, I r the module rated phase urrents and IT grid refers to isolated neutral or high impedane earthed grids as defined in IEC [13]. Mains terminal disturbane voltage limits U CISPR for A1 equipment in db(µv) S r 20 kva S r > 20 kva 2) S r 75 kva, IT grid 3) Frequeny (MHz) 1) QP AVG QP AVG QP AVG ) ) ) At the transition frequeny, the more stringent limit shall apply. 2) Limits equivalent to: CISPR 11 A2, S r 20 kva; IEC C3, I r 100 A; IEC C3, I r = (16, 100] A 3) Limits equivalent to: CISPR 11 A2, S r > 20 kva; IEC C3, I r > 100 A; IEC C3, I r > 100 A 4) Dereasing linearly with logarithm of frequeny HF urrents a LF urrents L2 L1 L1 L2 L3 DUT b To test reeiver C 3 C 2 R 2 C 1 R 1 50 Ω terminations Three-phase LISN Fig. 2: Typial realization of a three-phase LISN aording to CISPR 16 [8]. L 1, L 2, C 1, C 2, R 1 and R 2 form a bidiretional filter. It defines a stable impedane seen from the DUT and filters inoming grid disturbanes. High-frequeny (HF) emissions from the DUT are oupled to the test reeiver, whereas the low-frequeny (LF) urrents an flow between the grid and DUT. 2.3 CISPR 11 CISPR 11 defines limits for onduted emissions (CE) in the frequeny range MHz for industrial, sientifi and medial (ISM) devies. In ontrast to the IEEE standards and BDEW, CISPR 11 is a LV standard applying to individual onverter modules. The emissions must be measured by means of a line impedane stabilization network (LISN) whih is plaed between the devie under test (DUT) and the LV grid (Fig. 2). In the relevant frequeny range, the LISN features a defined impedane of Z LISN 50 Ω between phases and ground [8]. A test reeiver is used to measure the emissions aross that impedane. The assoiated harmonis inside a moving frequeny band of 9 khz, entered around the analyzed frequeny, are then proessed either by means of an average (AVG) or quasi-peak (QP) detetor, whih both show non-linear input-output harateristis. However, both are tuned so as to indiate the RMS value in ase of only a single harmoni within the frequeny band. The interested reader is referred to [5, 8, 9] for more detailed information. Tab. 4 shows the emission limits U CISPR of lass A group 1 (A1) equipment, whih inludes amongst others semiondutor power onverters in ommerial and industrial environments. Note that the generi CISPR 11 lass A limits are idential to the devie speifi limits in IEC (industrial equipment) [10] and to those of IEC (UPS systems) [11] and IEC (adjustable speed eletrial power drive systems) [12] for ategory C3 (industrial environment) equipment. 3 Comparison In this setion, based on the investigated standards, limits are derived whih an be applied to an individual onverter module. The limits should guarantee overall system ompliane, provided that all modules omply with these limits. Furthermore, a omparative analysis of the derived limits is performed

6 I CISPR I IEEE I BDEW I IEEE IBDEW DUT S r={10,50} kva f s > 1 khz LISN U =0.4kV LV PCC SCR=20 U =10kV MV Fig. 3: Setup for omparison of standards and derivation of module-based limits. The onverter module (DUT), either in a grounded or isolated IT LV grid, is onneted to the MV distribution system by means of a transformer. The depited urrents denote the alulated admissible urrent harmonis. in order to identify the most stringent standards in the frequeny range from 100 Hz to 30 MHz. Swithed onverter modules with swithing frequenies f s in the kilo-hertz range are onsidered. 3.1 Methodology Although the IEEE and BDEW limits are formulated for the PCC and hene for onverter systems that possibly onsist of more than one module, equivalent limits an be alulated whih an be applied to the individual modules of the system. The translation from PCC-based limits to individual module-based limits requires, however, that the SCR at the PCC is given (see (2) and Tab. 2). Consequently, for the purpose of this omparison, a worst-ase short-iruit ratio of SCR = 20 has been assumed. A setup of an individual module in a LV onverter system (U LV = 0.4 kv) onneted to the MV distribution system (U MV = 10 kv) has been onsidered (Fig. 3) Equivalent IEEE 519/1547 Limits The equivalent IEEE 519/1547 admissible urrent harmonis I IEEE for an individual onverter module on the LV seondary side of the transformer an be alulated by I IEEE = I IEEE = I ν SCR = 20 [%] I L = I ν SCR = 20 [%] Ir = I ν SCR = 20 [%] I r, (4) where an idealized transformer with turns ratio = U MV U LV has been assumed and I r is the module rated phase urrent. The TDD limits will not be onsidered in this omparison Equivalent BDEW Limits The equivalent urrent limits I BDEW aording to BDEW an be alulated using (2), I BDEW = I BDEW = Iµ,ν = Iµ,ν 10 [A /MVA] kv 10 kv [A /MVA] S SC Sr,n = Iµ,ν S r,n 10 kv [A /MVA] SCR U MV Ir (5) SCR U MV I r, (6) Equation (6) is valid for single harmonis. However, BDEW onsiders a measurement band of 200 Hz for frequenies above 2 khz (3), whih may inlude multiple harmonis. Therefore, for the purpose of a more meaningful omparison, the typial sidebands at multiples of f s of swithed onverter systems (see Fig. 6(a)) will be aounted for by reduing Iν,µ 10 in (6) by a fator 2 (3 db). kv

7 r Equivalent CISPR 11 Limits The CISPR 11 equivalent admissible urrent harmonis I CISPR an be omputed by means of the LISN impedane Z LISN, I CISPR = 10 UCISPR [db(µv)] V Z LISN. (7) Again, (7) is only meaningful in ase of a single harmoni within the orresponding CISPR measurement band of 9 khz. In ase of multiple harmonis, it is not suffiient to guarantee (7) for all individual harmonis as the QP and AVG detetors will indiate higher emission levels [9]. Hene, U CISPR in (7) will be redued by 15 db(µv), whih is the worst ase differene between the individual harmonis and orresponding AVG emission levels observed in simulations of various 2- and 3-level voltage soure inverter (VSI) topologies with sinusoidal PWM and f s > 1 khz. Moreover, only AVG limits will be onsidered as they are generally more stringent than QP limits. This is due to the similar sensitivity of the QP and AVG detetor type for swithing frequenies in the kilo-hertz range ([14], Fig. 6())and the lower AVG emission limits (Tab. 4). Finally, in ontrast to (4) and (6), (7) does not depend on SCR but on S r and the grounding onditions of the LV grid. Therefore, a onverter with S r =10 kw in a grounded LV grid as well a onverter with S r =50 kw in both grounded and isolated IT LV grids have been onsidered. 3.2 Results Fig. 4 shows the alulated admissible urrent harmonis for the onsidered onverter module as shown in Fig. 3 in perent of I r. Furthermore, redued BDEW and CISPR limits are depited as disussed in the last setion in order to allow for a more meaningful omparison between limits based on different measurement frequeny bands. It an be seen that for frequenies below 150 khz, the BDEW standard defines more stringent limits than the IEEE standards. This holds true for SCRs up to approximately 40, where IEEE 1547 beomes the most stringent standard due to the inreasing limits of BDEW and IEEE 519 for growing SCRs. For frequenies above 150 khz, CISPR 11 is learly more stringent than IEEE, provided that S r is not too low Admissible urrents [% I ] a + a _ ~a _ b + b _ + _ d + d a a _ IEEE 519/1547 BDEW BDEW, extrapolated CISPR 11 A1 AVG S r = 50 kva, IT grid CISPR 11 A1 AVG S r = 50 kva CISPR 11 A1 S = 10 kva AVG r ~a b _ b d _ d _ Frequeny [Hz] Fig. 4: Comparison of the investigated standards of Setion 2 for SCR = 20, S r = {10, 50} kw, based on the setup as shown in Fig. 3 and (4), (6), (7). Squares and irles orrespond to limits for disrete frequenies, the lower IEEE 519/1547 and BDEW limits to even order harmonis. The measurement bands of 200 Hz for BDEW (above 2 khz) and 9 khz for CISPR 11 have been aounted for by lowering Iµ,v 10 in (6) by 3 db (urve a) and U CISPR in (7) by 15 db(µv) (urves b,, d), respetively. Curve ã kv represents the extrapolated BDEW limits proposed in Setion 3.3, whih lose the urrently existing gap of appliable limits from 9 khz to 150 khz.

8 r r U DC a b Lboost LDM L1 L2 L3 U LV C DM Fig. 5: Fuel ell system with boost onverter, 2-level VSI and LCL output filter in a LV grid with grounded MV transformer seondary star point as shown in Fig. 3, SCR = 20, S r = 50 kva, U DC = 650 V, f s = 8 khz, U LV = 400 V. 3.3 Proposed Limits for the Frequeny Range khz To the authors knowledge, there are no other existing standards than IEEE 519 and IEEE 1547 that omprise limits for the frequeny range khz. However, the IEEE urrent harmonis limits are mainly intended for line-ommutated and self-ommutated thyristor-based onverter topologies. As suh onverters typially generate only LF harmonis at integer multiples of the fundamental frequeny, IEEE 519/1547 do not define limits for non-integer harmonis and, moreover, the limits stop dereasing for harmonis with orders ν 35 (Tab. 3). On the one hand, this results in a large vertial gap between the admissible urrents when ompared to CISPR 11 (Fig. 4). On the other hand, a further derease of the admissible urrents would be neessary to limit the individual voltage harmonis to a onstant value due to ωl SC (see Setion 2.1). For these reasons, the IEEE limits are not well suited for modern swithed onverter modules with signifiant high-frequeny input harmonis, possibly loated at non-integer harmonis. Due to the inreasing penetration of systems omprising suh modules, more stringent standards for the frequeny range khz must be, however, expeted in the future. Consequently, it seems meaningful to apply alternative, more stringent limits than IEEE 519/1547 in the meantime. Based on the existing standards, it is proposed to extrapolate the BDEW limits intended for the frequeny range 2-9 khz up to 150 khz. As an be seen in Fig. 4, this is a simple yet effetive approah, whih loses the gap between 9 khz and 150 khz vertially and horizontally. 4 Example In this setion, the standards investigated in this paper are applied to a pratial example. The same setup as shown in Fig. 3 is onsidered. The onverter module (DUT) with S r = 50 kva omprises fuel ell stak with DC/DC boost onverter to stabilize the DC-link voltage U DC = 650 V, a 2-level VSI to onvert the DC to AC power and an LCL output filter (Fig. 5). The VSI operates at a swithing frequeny f s = 8 khz and employs sinusoidal PWM. The impat of BDEW and CISPR 11 to the filter design is now demonstrated. Only differential mode (DM) disturbanes are examined and ideal omponents are assumed. I [% I ] ν dist, Hz Frequeny [khz] Currents [% I ] a a _ CISPR 11 A1 QP S r = 50 kva 100 ~a 80 _ 60 U dist, ν I dist, 40 U ν dist,qp Frequeny [khz] Emissions [db(μv)] Frequeny [khz] (a) (b) () U dist,avg Fig. 6: Disturbane harmonis I dist,ν without the filter omponents L DM and C DM (a). I dist,ν after adding L DM and C DM (b). AVG and QP emission levels U dist, AVG, U dist, QP ompared to the appliable limits (). The labeling of the limits is taken from Fig. 3.

9 In a first, step, the boost indutor is hosen to be L boost = 0.6 mh in order to guarantee a maximum urrent ripple of approximately ±10 % of I r. Negleting L DM and C DM, the output urrent harmonis I dist,ν, whih are dominant over I dist,µ, an be obtained by means of simulations. Fig. 6(a) shows the harmonis around f s, whih are the most ritial in terms of damping. Applying BDEW (3) gives I dist,ν,µ 3.24 %. Comparison with the admissible magnitude of emission in Fig. 4 yields an additional required filter attenuation of a = 38 db at f s. In order to enable a lean voltage measurement with low ripples of less than ±1 % of U LV, C DM is hosen to be C DM =60 µf. Hene, the required indutane is L DM = a + 1 a ( (2πf s ) 2 C DM ) 0.53 mh. (8) In Fig. 6(b), the effetiveness of the designed DM filter an be seen. Besides ompliane with BDEW, Fig. 6(b) also implies ompliane with CISPR 11 as the harmonis at frequenies above 150 khz are largely attenuated. This is onfirmed by Fig. 6() for the onsidered DM disturbanes. Note, however, that for CM disturbanes, a more detailed filter design is required, where CISPR 11 beomes relevant. Finally, Fig. 6() also onfirms that the differenes in magnitude between the AVG and QP emission levels are only small (see Setion 3.1.3). 5 Conlusion In this paper, different MV grid harmonis and onduted EMI standards were investigated. Based on these standards, limits applying to LV onverter modules in LV onverter systems onneted to the MV distribution system were derived. A omparative analysis showed that the most stringent limits for frequenies below 9 khz and low SCRs is BDEW, whereas for frequenies higher than 150 khz CISPR 11 is the most stringent standard. Furthermore, the investigations found a lak of appliable standards in the frequeny range khz. However, standards overing this frequeny range an be expeted in the near future due to the growing deployment of swithed onverter systems. Meanwhile, it is thus proposed to apply extrapolated BDEW limits as a guideline for the design of onverters and its filters. Referenes [1] Eletromagneti ompatibility (EMC) Part 3-12: Limits Limits for harmoni urrents produed by equipment onneted to publi low-voltage systems with input urrent >16 A and 75 A per phase, IEC , [2] T. Hoevenaars, K. LeDoux, and M. Colosino, Interpreting IEEE STD 519 and meeting its harmoni limits in VFD appliations, in Petroleum and Chemial Industry Conferene, Reord of Conferene Papers. IEEE Industry Appliations Soiety 50th Annual, sept. 2003, pp [3] Reommended Praties and Requirements for Harmoni Control in Eletrial Power Systems, IEEE 519, [4] Standard for Interonneting Distributed Resoures with Eletri Power Systems, IEEE 1547, [5] Industrial, Sientifi and Medial (ISM) Radio-Frequeny Equipment Eletromagneti Disturbane Charateristis Limits and Methods of Measurement, CISPR 11, [6] Tehnial Guideline Generating Plants Conneted to the Medium-Voltage Network, BDEW Std., [7] Eletromagneti ompatibility (EMC) Part 4-7: Testing and measurement tehniques General guide on harmonis and interharmonis measurements and instrumentation, for power supply systems and equipment onneted thereto, IEC , [8] Speifiation for Radio Disturbane and Immunity Measuring Apparatus and Methods, CISPR 16, [9] M. L. Heldwein, EMC Filtering of Three-Phase PWM Converters, Ph.D. dissertation, ETH Zurih, [10] Eletromagneti ompatibility (EMC) Part 6-4: Generi standards Emission standard for industrial environments, IEC , [11] Uninterruptible power systems (UPS) Part 2: Eletromagneti ompatibility (EMC) requirements, IEC , [12] Adjustable speed eletrial power drive systems Part 3: EMC requirements and speifi test methods, IEC , [13] Low-voltage eletrial installations Part 1: Fundamental priniples, assessment of general harateristis, definitions, IEC , [14] J. S. Wilson et al., Test and Measurement. Elsevier Siene & Tehnology, 2009.