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

18 ML Converter Topologies
3.4 Generic ML VSC framework
ML converters can be built based on multiple different switching cells, connecting them in
various different ways. A large number of different approaches are possible, leading to numerous
different topologies. Even though several different types of switching cells are used, it is possible to
present most ML-VSC topologies in one generic framework based on generically connected
switching cells. The starting point is the generalized multilevel converter that has been presented in
2001 by Fang Zheng Peng [8]. This converter topology contains a maximum of switching cells
connected in a triangle. A new way of representation is chosen in Figure 21 to improve the
readability and ease the systematic design of new topologies.
U DC+
U DC
U out
U DC-
Figure 21, generalized multilevel converter by Fang Zheng Peng
Even though the basic switching cell is the half bridge, it can be seen that the other two types
of switching cells are also hidden in that structure. Figure 22 shows the generalized ML converter
with highlighted switching cells:
- Half bridge cells (a)
- Flying capacitor cell (b)
- Diode clamped cell (c)
The green paths indicate positive current flow; the red ones indicate negative current flow. The
commutation takes place between diodes and IGBT’s of the same color. The diode clamped cell
does not have common current paths for positive and negative currents. Consequently, the
commutation loop is not the same for the two polarities. In fact, the size of the commutation loop
for negative current increases with the number of levels implemented in a MLDC converter.
Converters based on half bridge or flying cap cells are good in the respect that the switching cell