Christoph Haederli - Les thèses en ligne de l'INP - Institut National ...

Introduction 3
tighter capacitor dimensioning in the design phase. Charging and discharging of flying capacitors
may also be achieved with non-redundant states. This approach may lead to operating limitations,
but it can also be used systematically for new control schemes.
2.1.3 Synthesis and analysis of new NP control schemes (chapters 5 and 6)
ML converters need to control both internal and external quantities. External quantities like
output current, machine torque etc. can be controlled by appropriate modulation and feedback
control schemes. Such methods are application specific and are not treated in this thesis. On the
other hand, the control of internal quantities is very topology specific and requires topology specific
control schemes, which is the focus of this part of the thesis. Examples of internal quantities are
not only capacitor voltages in the DC link or flying capacitors but also include optimization
objectives like semiconductor losses (overall losses or loss distribution) or current THD in the
capacitors. Control also has an impact on the design of ML converters, e.g. on capacitor
dimensioning or required semiconductor blocking voltage.
The control issues dealt with in this thesis have a strong focus on NP control in 3-L DC link
ML converters. Chapter 5 introduces new NP control schemes with CB (carrier based) PWM (pulse
width modulation); chapter 6 introduces new NP control schemes based on SVM (space vector
modulation).
2.1.4 Application and experimental verification (chapter 7)
Selected findings throughout the thesis have been experimentally verified. The most important
topologies and control schemes have been tested with low power prototypes. Experimental
verification of the proposed concepts is important as simulation is not likely to capture all aspects
of a real world system. The following issues often differ between simulation and hardware
implementation:
- Delays in the control loop including current sensing and gate drivers
- A/D and D/A conversion, delays, noise, and resolution
- General disturbances and environmental noise
- Algorithm execution times in the CPU and FPGA
It is not obvious that more complex control algorithms can run on a given control platform,
especially in the case of algorithms requiring significant calculation time. Only prototyping can
prove feasibility of the implementation. Experimental verification of the new control schemes is
partially covered in chapters 5 and 6 where the basic waveforms are shown. Chapter 7 relates the
ML topologies and the control schemes to specific operating conditions. This chapter also
proposes a combination of several different control schemes introduced in this thesis within a
hysteresis controller. Experimental verification in various operating conditions demonstrates the
performance of the NP control schemes.