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

ML Converter Topologies 49
3.8 Executive summary for chapter 3
A generic framework for the representation for ML VSC’s has been introduced (see Figure 21
and Figure 22). This framework proves to be helpful both for analysis and synthesis of ML
converters. Most ML VSC known today fit in that framework (Figure 23 to Figure 27). A
systematic approach to come up with new topologies has been introduced: Choice of basic concept,
positioning of flying capacitors, definition of suitable paths to operate converter. Several new
topologies could be introduced that way, leading to two patent applications (one published by 2008:
[4])
A number of different ML topologies have been compared and can be characterized as
follows:
1. The MC has the best properties regarding semiconductor parts count, but it
requires high flying capacitor energy.
2. MLDC and other topologies with more than 3 input levels require no or
significantly less flying capacitor energy, but they have a higher semiconductor parts
count (diodes) for higher number of levels and have serious operating range
limitations (regarding modulation depth and load angle).
3. 3-L DC link based converters with multiple internal MC circuits provide a good
trade off between semiconductor parts count, flying capacitor energy and
controllability. Important examples in this family of topologies are the SMC and the
ML ANPC.
4. M 2 LC has a high total semiconductor blocking voltage (twice the MC converter
rating) and high flying capacitor energy, but both are independent of the number of
levels. The M 2 LC requires less flying capacitor energy than MC based converters
starting from a certain number of levels, which makes it attractive for high voltage
applications. The M 2 LC is less suitable for low frequency applications as the module
capacitors scale reverse proportionally with the fundamental output frequency.
Based on those findings, we can state that 3-L DC link based topologies with internal MC
circuits are most attractive of all considered topologies for MV applications of any kind. This is in
line with [5], which concludes with the statement that the 3-L based inverters SMC and ANPC
offer a very good trade off between flying capacitor energy and total semiconductor blocking
voltage.
The remainder of this thesis focuses on 3-L DC link topologies for more in depth analysis.
Although, the balancing of the capacitors is inherently possible for those topologies, the limits of
operation for the different topologies is not obvious and the NP current control capacity with state
of the art control schemes is not good in certain operating points (very high and very low
modulation depth, reactive power operation). New schemes extending the feasible operating range
and improving the NP current control capacity are discussed in the following chapters.