Christoph Haederli - Les thÃ¨ses en ligne de l'INP - Institut National ...

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3-L DC Link ML Converter Properties 59 The zero axis connects the output voltage vectors with zero output in αβ. A total number of 19 different states is possible for {000} 2D. All redundant states can be used for FC and NP control. If the redundant NP output states are considered too, the number of possible states is even higher. The red arrow in Figure 51 is pointing at the vector {131} 3D. This vector has redundant states in all three phases, resulting in eight available converter states. Figure 52 indicates the number of available redundant states with either differing CM voltage or differing NP or FC current in αβ. Obviously, there are many more redundant states available for the small vectors than for the large vectors. These numbers also show that the degree of freedom in defining a modulation and control scheme is siginificantly higher than for the NPC b β 1 2 1 2 1 2 6 6 6 6 2 1 6 10 8 10 6 1 2 6 8 12 12 8 6 2 a α 1 6 10 12 19 12 10 6 1 2 6 8 12 12 8 6 2 1 6 10 8 10 6 1 2 6 6 6 6 2 c 1 2 1 2 1 Figure 52, Redundant vectors with different FC or NP current in the 5-L ANPC1 (343 states) 4.4 Modulation This paragraph introduces the fundamental PWM methods according to the state of the art and relates them to the work in this thesis. 4.4.1 CB PWM Regarding CB (carrier based) PWM schemes, a lot of material has been published with respect to ML converters. Examples are the work by Holmes [35], McGrath [36] or Walker [37]. Li proposes overlapping carriers, which results in non-constant switching frequency, possibly non optimal modulation but reduces the output voltage distortion in the low modulation depth region [38]. PD (Phase Disposition) PWM with asymmetric sampling (constant reference for half a modulation period) is used in this thesis, as it is most favorable with respect to harmonics in 3 phase, 3 wire systems. The carriers can either be associated directly with a given phase and switching cell, or they can be generic (Figure 53) with a subsequent logic (state machine) to determine the actual states to be applied. Both approaches are used in this thesis.
60 3-L DC Link ML Converter Properties A CM voltage needs to be applied to make full use of the available output voltage range. 3 rd harmonic injection [39] is a good method to get smooth operation, CSPD PWM (Center Spaced PD PWM) yields best results regarding output harmonics. Both methods are applied in this thesis, both for simulation and experimental verification. State machine Gate signal generation 4.4.2 SVM Figure 53, Generic 5-L PD PWM modulator with subsequent state machine SVM (Space Vector Modulation) allows calculating states and their corresponding application times based on two or three dimensional vectors for all three phases at once, rather than per phase as in the carrier based PWM. SVM has been the subject of many publications (e.g. [40] for a basic 2-L performance analysis, [41] for ML SVM). Usually the NTV (Nearest Three Vectors) method is used for SVM to get optimum switching patterns. Other ways of SVM are possible. Fukuda is proposing to determine optimal switching sequences by means of optimal control [42, 43]. The optimal control approach is of specific interest if multiple quantities (like FC voltages in a ML converter) shall be controlled by the SVM modulator (e.g. [25]). Several authors also have proposed 3-D SVM ([44], [45]). In this case, 3-D reference vectors are used (see also Figure 52) rather than 2-D vectors in αβ. Consequently, the reference also contains a part for the CM. Control schemes acting on CM may be applied in this case. This approach results in very similar results as the carrier based modulation schemes, as both are using a reference defined in all three dimensions. In fact, the carriers are only a graphical representation of an algorithm that can be used to calculate states and their application times based on a given 3-D space vector. This is specifically true in the case of use of generic carriers as in Figure 53. CB PWM and SVM are closely related to each other and essentially represent two different mathematical methods to get the same (or similar) resulting pulse pattern. A number of publications analyze the relationship between CB PWM and SVM (e.g. [46], [47], [48],) or even propose hybrid CB / SVM schemes [49]. In this thesis an asymmetric SVM is used. Like in the case of PD PWM, there are two sampling periods per modulation period. 3 or 4 vectors are applied per sampling period. 4.4.3 Modulation by choice of “next best vector” In contrast to the PWM schemes presented in the previous paragraphs, “next best vector” schemes do not pre-determine a sequence of states but only apply one state at a time. The duration is not known at the application time but determined dynamically, for example by means of a switching surface controller (or hysteresis controller). Many different schemes of that kind are possible. Typical representatives are DTC (direct torque control) and DPC (direct power control), which also have been adapted for ML converters (e.g. by L. Serpa [50]). These schemes are typically implemented by means of look up tables, but other solutions are possible. For example MPC (Model Predictive Control) [51] is a very powerful approach for effective implementation of DTC
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THÈSE En vue de l'obtention du DOC
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Acknowledgements i ACKNOWLEDGEMENTS
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iv Table of Contents 3.4 Generic ML
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vi Table of Contents 7.1 General mo
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viii Résumé français (French sum
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x Résumé français (French summar
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xiv Résumé français (French summ
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xviii Résumé français (French su
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Introduction 1 2 INTRODUCTION This
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Introduction 3 tighter capacitor di
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Introduction 5 TABLE 1, OPERATING R
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Introduction 7 2.3 Glossary Note th
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Page 32 and 33: Introduction 11 DPC DSP DTC FC FPGA
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Page 46 and 47: ML Converter Topologies 25 (a) (b)
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NP Control with Carrier based PWM 1
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NP Control with Optimal Sequence SV
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Application and Verification 149 7
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Application and Verification 151 TA
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Application and Verification 155 7.
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170 Conclusions and Outlook 8.1.2 M
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172 Conclusions and Outlook 8.2 Out
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174 Conclusions and Outlook 8.3 Sum
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176 Appendix I = 1− ) (99) ( C 2
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178 Appendix TABLE 78, FLYING CAPAC
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180 Appendix 9.3 Single phase NP cu
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182 Appendix 9.4 States of the ANPC
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184 Appendix 9.5 NP currents as a f
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186 Appendix 9.5.3 Maximum and mini
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188 Appendix 9.6 NP current functio
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190 Appendix TABLE 91, NP CURRENT I
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192 Appendix TABLE 93, NP CURRENT I
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Bibliography 195 10 BIBLIOGRAPHY [1
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Bibliography 197 [33] P. Steimer,
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Bibliography 199 neutral point bala
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Bibliography 201 [88] J. Pou, J. Za
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Christoph Haederli - Les thÃ¨ses en ligne de l'INP - Institut National ...

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