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

Introduction 1
2 INTRODUCTION
This thesis is about the analysis and synthesis of ML-VSC (multi level voltage source
converters). New ML (multi level) topologies and topology specific control techniques for
converter internal quantities are introduced and evaluated, namely in the area of voltage control for
the DC link and flying capacitors. The main focus is on 3-L DC link converters, a family of
converters with good operational performance and reasonable resource requirements. The
operating ranges regarding modulation depth and load angle are investigated for various topologies
in this family and the impact of the new control and modulation schemes is demonstrated.
ML converters are used extensively in various industrial applications. They are the backbone of
a significant part of ABB’s (Asea Brown Boveri Corporation) business, namely in the area of MV
(medium voltage) and HV (high voltage) applications. It is of high business relevance to find the
best suitable topologies and secure the IP (intellectual property) in the field. Any given topology has
operation and control requirements. Limits of operation are not always evident for new circuits and
suitable control schemes need to be developed to make best use of the PE (power electronics)
hardware and push the operating range to the theoretical physical limits. Therefore, the synthesis of
new circuits and control schemes, the analysis of performance and operational limits, and the
comparison with the state of the art are crucial.
2.1 Thesis structure
The starting point and main problem to be studied in this work on ML converters is the NP
(neutral point) balancing problem of the 5-L ANPC. One of the key questions is, whether the 5-L
ANPC and the 3-L NPC (neutral point clamped) converter (referred to as NPC in the remainder of
the document) differ regarding NP controllability and operating range, or whether they behave the
same and thus one can make use of all concepts published for the NPC. The NP balancing
problem of the NPC has been investigated extensively by industry and academia. There are a large
number of publications on the NPC covering NP control methods, natural balance, stability
analysis as well as investigation of operational limitations. The 5-L ANPC (active neutral point
clamped) converter [7] has been investigated much less, due to its relatively recent introduction in
2005. Hardly any publications can be found in the scientific literature (Status of 2008).
Based on the main problem statement and the opportunities for original contribution
indentified in the first phase of the thesis work, the scope of the thesis has been widened to the
synthesis of new ML converter topologies and also to an investigation on the properties of 3-L DC
link converters in general including for example NPC and SMC (stacked multi cell converter). The
term 3-L DC link converters refers to a whole family of converters featuring a DC link supply with
3 levels (DC+, DC- and NP) and a converter stage generating 3 or more output levels, e.g. by the
use of flying capacitors.
2.1.1 Synthesis and analysis of ML converter topologies (chapter 3)
Many different ML topologies have been presented in the literature. There have been a few
publications putting different converters in a common framework (e.g. [8]), but with limited scope.
The approach in this thesis is to go one step further, unifying all major ML topologies in the same