Effects and Analysis of Minimum Pulse Width Limitation on Adaptive DC Voltage Control of Grid Converters

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1 Aalborg Univeritet Effect and Analyi of Minimum Pule Width Limitation on Adative DC Voltage Control of Grid Converter Sun, Bo; Trinti, Ionut; Munk-Nielen, Stig; Guerrero, Joe M. Publihed in: Proceeding of the 31t Annual IEEE Alied Power Electronic Conference and Exoition (APEC) DOI (link to ublication from Publiher): /APEC Publication date: 2016 Document Verion Publiher' PDF, alo known a Verion of record Link to ublication from Aalborg Univerity Citation for ublihed verion (APA): Sun, B., Trinti, I., Munk-Nielen, S., & Guerrero, J. M. (2016). Effect and Analyi of Minimum Pule Width Limitation on Adative DC Voltage Control of Grid Converter. In Proceeding of the 31t Annual IEEE Alied Power Electronic Conference and Exoition (APEC) ( ). IEEE Pre. htt://doi.org/ /apec General right Coyright and moral right for the ublication made acceible in the ublic ortal are retained by the author and/or other coyright owner and it i a condition of acceing ublication that uer recognie and abide by the legal requirement aociated with thee right.? Uer may download and rint one coy of any ublication from the ublic ortal for the uroe of rivate tudy or reearch.? You may not further ditribute the material or ue it for any rofit-making activity or commercial gain? You may freely ditribute the URL identifying the ublication in the ublic ortal? Take down olicy If you believe that thi document breache coyright leae contact u at vbn@aub.aau.dk roviding detail, and we will remove acce to the work immediately and invetigate your claim.

2 Effect and Analyi of Minimum Pule Width Limitation on Adative DC Voltage Control of Grid Converter Bo Sun, Ionut Trinti, Stig Munk-Nielen, Joe M. Guerrero Deartment of Energy Technology, Aalborg Univerity, Aalborg, Denmark {bo, itr, mn, Abtract Thi aer reent an adative dc-link voltage controller for the uroe of decreaing the oerating dc-link voltage in a back-to-back converter ytem. An outer loo to control the maximum modulation (Mmax) index i added to the conventional dc-link voltage controller, and hence the ytem can online adat it dc-link voltage reference baed on the ytem oerating tate. Furthermore, the relationhi between current THD, minimum ule width limitation and Mmax index reference i analyzed in order to obtain an otimized erformance of the ytem. Finally, the dynamic erformance and analyi of controller i invetigated by exerimental reult. eyword dc/ac converter; dc voltage adative control; Minimum Pule Width Limitation I. INTRODUCTION Wind energy i one of the fatet-emerging renewable alternative for the electrical uly, and wind turbine are intalled with exonential caacity over the at year around the world [1]. The general tate-of-the-art wind turbine ytem i hown in Figure 1. Back to back (B2B) converter are widely alied to realize the controllability and ower roceing. Two vector control tructure are imlemented on generatoride and grid-ide converter, reectively. The objective of the control cheme for the generator-ide converter i to obtain the otimal ower tracking for maximum energy cature from the wind by adjuting the eed of the wind turbine. The control of grid-ide converter i to coordinate the energy balance between grid and generator, and it can be alo reonible for controlling reactive ower flow between grid and converter [4]. The grid converter ynchronize with the grid voltage, and the control normally include an inner current loo and an outer dc-link voltage controller. Normally the dc-link voltage reference i fixed, and et to a level that allow the converter oeration over the entire ower and grid voltage range. However, decreaing the energy tored in the dc-link can benefit the efficiency of both grid and generator converter due to reduced witching loe [4]. Alo for ome alication that converter are intalled at high altitude, the effect of reduced energy tored in dc-link imrove the reliability by decreaing the failure rate baed on comic ray [5]. The energy torage in dc-link can be reduced by adatively adjuting the dc-link voltage reference according to different ytem tate. In [6], a method baed on calculation of an offline table for the dc-link voltage reference wa introduced. However, the erformance of thi offline calculation method deend on the accuracy of the ytem model, which i ractically affected by variou factor, uch a oerating temerature of the generator, grid voltage, grid imedance and grid filter imedance. Adatively adjuting the dc-link voltage reference online can eliminate the calculation error baed on the involved ytem modelling. In [4], an outer loo i added to the dc-link voltage controller, which adat the dc-link voltage reference by controlling the maximum modulation (Mmax) index. The controller how an imroved efficiency imlemented in a wind turbine converter. However the Mmax index i imly et a in that work. In a ractical ytem, Minimum ulewidth (MPW) limitation i normally imoed on ower converter for the uroe of rotection but introduce the current ditortion. It influence the modulation and need to be taken into account. Thi aer imrove the adative dc voltage controller rooed in [4] for grid converter, and further tudie the effect of MPW limitation in order to otimize the erformance of controller. The ret of aer i organized a follow: Section II demontrate the Mmax adative dc voltage control trategy Fig. 1 Wind turbine ytem with full cale converter

3 in detail, and give an exlanation on the Mmax index detection and effect of MPW limitation. Section III how the exerimental reult and the arameter guidance of Mmax controller on a 2 kw converter rototye. Finally, Section IV give the concluion and future reearch direction. II. ADAPTIVE DC VOLTAGE CONTROL To achieve an online adatation of the dc-link voltage reference, a control loo can be imlemented a hown in Fig. 2. Intead of etting a fixed dc-link voltage reference, the Mmax index reference i controlled taking the feedback from the inut modulation index to the modulator. M Max Mmax Control M Ma x DC Voltage Control VDC Current Control Modulation Fig. 2 Prooed adative dc-link voltage controller A. Control tructure The detailed control tructure for a tyical grid converter with LCL filter i hown in Fig. 3. The inner loo i current control loo which i decouled in ynchronou reference frame (d-q frame) [8]: R i d ω d v DC d id 1 vgd = L R (1) i d q L q i q Lline vgq ω L where i q and i d, i S d q and d d are DC-like current and duty ratio aligned with q and d rotating axe, reectively, while R and L are er hae reitance and inductance of the ac filter including line imedance. The tyical out loo contain only a dc voltage regulator with the maller bandwidth than current loo. Here, a Mmax index regulator i added to voltage control loo by detecting the Mmax index to modulator. Hence, the outut dc-link voltage reference will adat to the converter ytem tate, and at all ower level, the oerating dc-link voltage i reduced ignificantly. B. Mmax Index detection and reference According to the reult in [4], the Mmax index reference i direct roortional with the grid voltage, current, imedance and additional harmonic voltage reectively and invere roortional with the dc-link voltage. Becaue Mmax index i in relationhi with o many arameter, it i otimal to online detect it. In thi aer, the Mmax value of modulation duty ratio i udated in each fundamental eriod. In thi way, Mmax index i the maximum value of duty ratio of three hae. In [4], the reference of Mmax index i directly et a 1. However, the Mmax i alo affected by ytem tate, uch a grid condition, tranferred ower and MPW limitation. Among thoe factor, MPW limitation directly influence the maximum modulation index. Therefore, the Mmax index cannot be et a the ideal value of 1 in a ractical ytem. C. Minimum Pule Width limitation MPW limitation i uually imoed by the manufacturer of the active ower electronic device ued in converter. Such limitation ha to be enforced to revent the introduction of ower tre on the device, eecially in high ower alication [7]. Commonly there are three MPW method, a i hown in Fig. 4. Pule droed if they are lower than a minimum width Pule are hold a minimum width value if they are lower than minimum width. Pule are droed if they are lower than half MPW, but held to MPW value if they are lightly higher than the MPW. DC Voltage Control - δ i i i M Max Control dq abc θ i d i q M Max i d i q Current Control i ωl ω L i Current Control M Max v gd v d v q v gq dq abc θ Modulation v a v b v c SVPWM S a S b S c Fig. 3 Grid converter d-q adative control tructure

4 Thi aer the third condition i alied. Setting a maximum modulation index reference above the voltage reerve for the MPW filter can introduce additional ditortion on the outut current. Therefore the maximum Mmax reference hould be exerimentally determined in order to comly with the harmonic tandard. Finally, the modulation cheme and MPW limitation i reented in fig. 5. Fig. 4 minimum ule width limitation method Fig.6 Exerimental tet etu. V / V abc DC offet calculation Minimum ule width filter Maximum modulation index detector modulation M Ma x 1 Maximum ule width Minimum ule width 0 Fig. 5 Modulation and MPW limitation III. EXPERIMENTAL RESULTS The imulated control from Fig.3 wa imlemented in a dace1006 control latform. The converter rototye i a twolevel 2.2 kw Danfo converter connected to the 400 V grid. The tet etu i hown in Fig.6. The grid LCL filter had a total inductance of 3.6 mh and a delta connected caacitor bank of 9 μf. The converter i modulated with continuou ace vector ule width modulation, witching and amling at a frequency of 10 khz. A. Dynamic tet The dynamic oeration of rooed Mmax controller i howed in Fig.7. The converter tart oerating at a fixed dclink voltage reference of 600 V, oerating at light load in rectifying mode. The Mmax controller wa enabled at 1.8 with reference a The dc-link voltage i regulated at an adative reference of around 565 V with a reone of 1 and the dc-link voltage i reduced by 35V. B. MPW limitation tet To tudy and analyze the effect of MPW limitation on the Mmax controller, the exerimental tet are imlemented reectively when MPW value i between 1% and 4% of the witching eriod and Mmax ref i between 98% and 100%. The dc ower ource inut i ket contant at 2 kw to imulate the wind generator. Fig. 8 how the exerimental reult in MPW value i Fig.8a how the Mmax ref increae from 0.98 to 1 with a te of The MPW limitation can be oberved in Fig.8b when Mmax ref i Converter-ide current and zoomed ectrum are reented in Fig.8c. Fig. 7 dynamic oeration of Mmax controller a. Mmax reference in MPW=0.02

5 b. modulation index in Mmax ref=0.995, MPW=0.02 Fig.9 Converter-ide Current THD in Different Sytem State Frequency 1e4 c. Converter-ide current and zoomed ectrum in Mmax ref=0.995, MPW=0.02 Fig.8. Exerimental reult of MPW=0.02 Finally 20 grou of data are obtained and imorted to MATLAB to analyze the relationhi between current THD, dc voltage, MPW limitation and Mmax ref. The current THD and voltage in different ytem tate are lited in Table I and II, and lotted in Fig. 9 and 10. It i oberved from Fig. 9 and 10 that the THD of converter-ide current increae lightly with the Mmax ref increaing when MPW value i below 0.03, while THD boot critically more than 3% when MPW value i over It can be alo oberved that, dc link voltage decreae with the Mmax ref increaing and reache it minimum value to when Mmax i 1. Fig.10 dc-link voltage in different ytem tate TABLE I. Converter-ide Current THD in Different Sytem State Mmax_ref MPW % 15.1% 15.3% 15.7% 16.3% % 15.2% 15.2% 15.4% 15.9% % 15.0% 15.5% 16.0% 16.5% % 15.8% 16.1% 16.5% 18.4% TABLE II. dc-link voltage in different ytem tate (V) Mmax _ref MPW

6 From the exerimental reult, it can be concluded that the MPW increae the harmonic and reduce the effective alied voltage, thu require higher dc link voltage for the given modulation. A a trade-off, it may be recommended that the Mmax ref i et a 1 to obtain the minimum dc link voltage when MPW i from 0.01 to 0.03; when the MPW value i larger than 0.03, the Mmax ref can be reduced to decreae the current ditortion. IV. CONCLUSION AND FUTURE WOR An imroved and imle controller that adatively regulate the dc-link voltage i ued for B2B converter in wind turbine ytem in thi aer.. The dynamic of oeration wa invetigated and the relationhi between current THD, dc voltage, MPW limitation and Mmax ref are analyzed in detail by exerimental reult on a 2kW rototye. The Mmax reference i choen in trade-off between minimum dc voltage and current THD. A future work, the Mmax controller oeration with dicontinuou PWM cheme will be invetigated, and tability of the rooed control trategy will be analyzed. REFERENCES [1] Blaabjerg, F.; e Ma, Future on Power Electronic for Wind Turbine Sytem, IEEE J. Emerg. Sel. Toic Power Electron., vol.1, no.3, , Set [2] Friedli, T.; olar, J.W.; Rodriguez, J.; Wheeler, P.W.;, Comarative Evaluation of Three-Phae ACAC Matrix Converter and Voltage DC- Link Back-to-Back Converter Sytem,, IEEE Tran. Ind. Electron., vol.59, no.12, , Dec [3] Maing, J.R.; Stefanello, M.; Grundling, H.A.; Pinheiro, H., Adative Current Control for Grid Connected Converter With LCL Filter, IEEE Tran. Ind. Electron., vol.59, no.12, , Dec [4] Trinti, I.; Munk-Nielen, S.; Abrahamen, F.; Thoegeren, P.B., Efficiency and reliability imrovement in wind turbine converter by grid converter adative control, Power Electronic and Alication (EPE), Lille, France [5] Conentino, G.; Laudani, M.; Privitera, G.; Pace, C.; Giordano, C.; Hernandez, J.; Mazzeo, M., Effect on ower tranitor of Terretrial Comic Ray: Study, exerimental reult and analyi, Alied Power Electronic Conference and Exoition (APEC), Fort worth, US, [6] Dayaratne, U.I.; Tennakoon, S.B.; Shamma, N.Y.A.; night, J.S., Invetigation of variable DC link voltage oeration of a PMSG baed wind turbine with fully rated converter at teady tate, Power Electronic and Alication (EPE),Birmingham, U, [7] B. Welchko, S. Schulz, and S. Hiti, Effect and comenation of deadtime and minimum ule-width limitation in two-level PWM voltage ource inverter, in Proc IEEE Ind. Al. Conf., 8 12 Oct. 2006, vol. 2, [8] V. Blako and V. aura, A new mathematical model and control of three-hae ac-dc voltage ource converter, Power Electron., IEEE Tran. on, vol. 12, no. 1, , 1997.